5BELLE2-REPORT-2023-001
DRAFT Version 2.1
22 December 2023
Submitted to BPAC

Belle

The Belle II Detector Upgrades
Conceptual Design Report
Belle II Collaboration

Executive Summary

5

10

15

20

25

30

35

The SuperKEKB accelerator and the Belle II detector started operation in 2019 witha steady increase in luminosity, reaching a peak instantaneous luminosity of Lpeak =4.65 × 1034 cm−2 s−1 in June 2022 and collecting an integrated luminosity of 424 fb−1 .
It has become clear that reaching the target peak luminosity of 6 × 1034 cm−2 s−1 andthe integrated luminosity 50 ab−1 will require an upgrade of the accelerator complex,
including potentially a redesign of the interaction region (IR).
A new IR would require a replacement of the vertex detector, and would imply theneed for a long shutdown (named LS2, since LS1 extended from 6/2022 to 12/2023),
providing also a window of opportunity for the installation of improvements in manydetector subsystems. The timescale for LS2 is placed after 2027–2028 but not welldefined yet, as it depends on the precise definition of the accelerator upgrade plans. It
should become clearer after the restart of the operation in 2024 and the full evaluationof the LS1 improvements. It should be added that the extrapolation of backgrounds
to full luminosity is affected by large uncertainties which may erode the Belle II safetymargins and reduce the overall operational efficiency.
The Belle II upgrade program is therefore motivated by the desire to improve theoverall physics performance of the experiment by: (i) improving the robustness against
backgrounds and the radiation resistance, providing larger safety factors to run athigh luminosity; (ii) developing the technology to cope with different future paths,
improving the detector performance, and also providing a safety net in case anything
goes wrong with the current detector.
This Conceptual Design Report (CDR) presents a number of possible detector improvements that can be implemented in the short or medium term, along with otherideas for longer term upgrades, that go beyond the currently planned program. The
various upgrades are largely independent of each other and can be selected individually on the basis of actual need, physics performance, technical readiness, and funding
availability. In this sense they do not form a single coherent picture of the future BelleII, but rather a menu of possibilities. A different kind of opportunity is explored by
the beam polarization option, which requires a SuperKEKB upgrade but no detector
modification, opening a new and unique program of precision physics. Only a briefsummary is presented in this CDR.
The various options are the result of an internal selection process over the course of
the past 2-3 years and are clearly at different levels of readiness. This CDR documents
the current state so that R&D and engineering activities can be properly organized.
The next step will be the preparation of Technical Design Reports for those options
that are worth pursuing, possibly on different timescales.

2Contents
1 Introduction
7
1.1 Belle II: status and perspective . . . . . . . . . . . . . . . . . . . . . . . 7
1.2 Belle II upgrades overview . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5

10

15

20

25

30

35

2 SuperKEKB
14
2.1 SuperKEKB status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1.1 Impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1.2 Sudden beam loss . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.1.3 Dynamic aperture and Touschek lifetime . . . . . . . . . . . . . . 21
2.2 SuperKEKB evolution and upgrade path . . . . . . . . . . . . . . . . . . 22
2.3 Future plans and prospect . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.4 Interaction region design . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.4.1 Introduction of the IR optics . . . . . . . . . . . . . . . . . . . . . 24
2.4.2 Characteristics of the new interaction region . . . . . . . . . . . . 25
2.4.3 Optics evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.4.4 R&D of Nb3 Sn cables and QC1P quadrupole . . . . . . . . . . . . 28
2.4.5 R&D program for the new IR . . . . . . . . . . . . . . . . . . . . 29
3 Machine Detector Interface
34
3.1 Beam backgrounds and countermeasures . . . . . . . . . . . . . . . . . . 34
3.1.1 Particle scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.1.2 Colliding beams . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.1.3 Synchrotron radiation (SR) . . . . . . . . . . . . . . . . . . . . . 35
3.1.4 Beam injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1.5 Sudden beam losses . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1.6 Head-tail instabilities . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1.7 Countermeasures . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.2 Background status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.3 Background simulation procedure . . . . . . . . . . . . . . . . . . . . . . 39
3.4 Background extrapolation . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.5 Radiation monitoring and beam abort system . . . . . . . . . . . . . . . 47
3.5.1 The present system and features to be preserved . . . . . . . . . 47
3.5.2 Limitations and possible improvements . . . . . . . . . . . . . . . 49
3.5.3 Specifications for the new electronics . . . . . . . . . . . . . . . . 51
3.5.4 Technical solutions . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.5.5 Radiation sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

3

5

10

15

20

25

30

35

40

45

4 Physics Performance55
4.1 Physics performance motivations . . . . . . . . . . . . . . . . . . . . . . 55
4.2 Key detector performance parameters for different options . . . . . . . . 58
4.2.1 Low momentum tracking efficiency and impact parameter resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.2.2 Momentum resolution . . . . . . . . . . . . . . . . . . . . . . . . 60
4.2.3 Vertexing resolution . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.2.4 KS0 reconstruction efficiency . . . . . . . . . . . . . . . . . . . . . 62
4.2.5 KL0 reconstruction efficiency . . . . . . . . . . . . . . . . . . . . . 64
4.3 Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.3.1 Machine learning . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.4 Physics benchmarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
5 Inner Detector
71
5.1 Vertex Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.1.1 VXD upgrade baseline . . . . . . . . . . . . . . . . . . . . . . . . 71
5.1.2 OBELIX sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5.1.2.1 Sensor technical specifications . . . . . . . . . . . . . . 72
5.1.2.2 TJ-Monopix2 as prototype . . . . . . . . . . . . . . . . . 76
5.1.2.3 Sensor technical implementation . . . . . . . . . . . . . 79
5.1.3 VTX structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.1.3.1 Ladder concepts . . . . . . . . . . . . . . . . . . . . . . 84
5.1.3.2 Thermomechanics, services and data acquisition . . . . 87
5.1.4 Roadmap towards realization . . . . . . . . . . . . . . . . . . . . 89
5.1.5 VTX cost estimate . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
5.1.6 R&D and technology options . . . . . . . . . . . . . . . . . . . . 90
5.1.6.1 DuTiP sensor . . . . . . . . . . . . . . . . . . . . . . . . 90
5.2 CDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
5.2.1 Upgrade of readout electronics . . . . . . . . . . . . . . . . . . . 96
5.2.1.1 New ASD-FADC ASIC . . . . . . . . . . . . . . . . . . . 97
5.2.1.2 Prototype of the upgraded module . . . . . . . . . . . . 97
5.2.1.3 Radiation hardness . . . . . . . . . . . . . . . . . . . . 100
5.2.1.4 Potentially interested community in Belle II . . . . . . . 101
5.2.2 Order of magnitude cost estimate . . . . . . . . . . . . . . . . . . 101
5.2.3 R&D and technology options . . . . . . . . . . . . . . . . . . . . 101
5.2.3.1 TPC-based tracking system . . . . . . . . . . . . . . . . 101
6 Outer Detector
103
6.1 TOP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
6.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
6.1.2 TOP program summary . . . . . . . . . . . . . . . . . . . . . . . 105
6.1.3 Upgrade proposal . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
6.1.4 Design development and human resources . . . . . . . . . . . . . 107
6.1.5 Cost and schedule . . . . . . . . . . . . . . . . . . . . . . . . . . 108
6.2 KLM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
6.2.1 KLM geometry and instrumentation . . . . . . . . . . . . . . . . 110
6.2.2 Upgrade path 1: precision timing for KL momentum . . . . . . . 111
4

5

10

15

20

25

30

6.2.2.1 New SiPM and preamplifier . . . . . . . . . . . . . . . . 112
6.2.2.2 Time resolution of long scintillator bar . . . . . . . . . . 112
6.2.2.3 Geant4 simulation for the KL momentum determination113
6.2.2.4 Replacement of the RPC layers in the barrel . . . . . . . 115
6.2.2.5 Improved scintillator front-end readout . . . . . . . . . 115
6.2.2.6 Materials cost estimate . . . . . . . . . . . . . . . . . . 116
6.2.2.7 Upgrade logistics . . . . . . . . . . . . . . . . . . . . . . 116
6.2.3 Upgrade path 2: RPC avalanche mode . . . . . . . . . . . . . . . 117
6.2.3.1 RPC response in streamer and avalanche modes . . . . 118
6.2.3.2 Gas mixture . . . . . . . . . . . . . . . . . . . . . . . . 119
6.2.3.3 Readout electronics . . . . . . . . . . . . . . . . . . . . 120
6.2.3.4 R&D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
6.2.3.5 Cost estimation and risk assessment . . . . . . . . . . . 122
7 Software, Trigger, DAQ
124
7.1 Software, machine learning, AI . . . . . . . . . . . . . . . . . . . . . . . 124
7.2 Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
7.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
7.2.2 Hardware upgrade . . . . . . . . . . . . . . . . . . . . . . . . . . 126
7.2.2.1 Universal trigger board . . . . . . . . . . . . . . . . . . 126
7.2.2.2 CDCTRG upgrade with CDC front end board . . . . . . 127
7.2.2.3 ECLTRG upgrade . . . . . . . . . . . . . . . . . . . . . . 127
7.2.2.4 TOPTRG upgrade . . . . . . . . . . . . . . . . . . . . . 128
7.2.2.5 VXDTRG with VXD detector upgrade . . . . . . . . . . . 128
7.2.3 Firmware upgrade . . . . . . . . . . . . . . . . . . . . . . . . . . 130
7.3 DAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
7.3.1 Optically connected trigger timing distribution system . . . . . . 132
7.3.2 Improvement of throughput in readout system . . . . . . . . . . 133
7.3.3 High-level trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
7.3.3.1 Increase of computational power in HLT system . . . . . 135
7.3.3.2 Study of data-handling framework for triggerless DAQ . 136
8 Beam Polarization and Chiral Belle139
8.1 Introduction and physics motivation . . . . . . . . . . . . . . . . . . . . 139
8.2 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
8.3 Implementation planning . . . . . . . . . . . . . . . . . . . . . . . . . . 1413540

9 Longer Term Upgrades
143
9.1 ARICH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
9.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
9.1.2 Photon detector candidates . . . . . . . . . . . . . . . . . . . . . 144
9.1.2.1 Silicon photo-multipliers . . . . . . . . . . . . . . . . . 144
9.1.2.2 Large area picosecond photo-detector . . . . . . . . . . 145
9.1.3 Sensor read-out electronics . . . . . . . . . . . . . . . . . . . . . 145
9.1.4 Radiator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
9.1.5 Potentially interested community in Belle II . . . . . . . . . . . . 146
9.1.6 Cost and schedule . . . . . . . . . . . . . . . . . . . . . . . . . . 146
5

5

10

9.2 ECL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
9.2.1 Anticipated performance limitations . . . . . . . . . . . . . . . . 148
9.2.2 Upgrade hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . 150
9.2.2.1 Replacement of CsI(Tl) crystals with pure CsI scintillator counters . . . . . . . . . . . . . . . . . . . . . . . . 150
9.2.2.2 A preshower detector in front of the ECL . . . . . . . . 150
9.2.2.3 Complementing the PiN diode photosensors with avalanche
photodiodes . . . . . . . . . . . . . . . . . . . . . . . . 151
9.2.2.4 Complementing the PiN diode photosensors with Silicon Photomultipliers (SiPM) . . . . . . . . . . . . . . . 152
9.2.3 Cost and schedule . . . . . . . . . . . . . . . . . . . . . . . . . . 154
9.2.4 Potentially interested community in Belle II . . . . . . . . . . . . 155

6

Chapter 1
Introduction
Editor: F.Forti

1.1
5

10

15

20

25

30

Belle II: status and perspective

The first generation asymmetric B-Factories experiments, BaBar at PEP-II and Belleat KEKB, discovered large CP violating effects in the B meson sector and provided
experimental verification of the Cabibbo-Kobayashi-Maskawa mechanism for quark
mixing and CP-violating phase in the Standard Model, along with a wealth of othermeasurements [1].
The SuperKEKB accelerator [2] and the Belle II detector [3, 4], shown in figure 1.1,
are a major upgrade of the Belle/KEKB complex. The nomenclature of the Belle II
subdetectors is the following: Pixel Detector (PXD), Silicon Vertex Detector (SVD),
forming together the VerteX Detector (VXD), Central Drift Chamber (CDC), Time of
Propagation Counter (TOP), Aerogel Ring Imaging Cherenkov Counter (ARICH), Electromagnetic Calorimeter (ECL), K-Long Muon System (KLM), Trigger and Data acquisition (TRG/DAQ), and High Level Trigger (HLT). A summary of the subdetector
technology is presented in table 1.1 [5, 4]
SuperKEKB and Belle II were initially commissioned with colliding beams in 2018
without the Vertex Detector VXD (Phase 2 operation). After establishing that the levelof machine background was acceptable, at the end of 2018 the VXD was installed,
but only with 2 out of 12 ladders in the PXD second layer, because of delays in theconstruction of the ladders.
In spring 2019 accelerator and detector operation started in earnest (Phase 3 operation), leading SuperKEKB to surpass the highest recorded instantaneous luminositiesof the previoius B factories and of the LHC, obtaining a peak instantaneous luminosityof Lpeak = 4.65 × 1034 cm−2 s−1 in June 2022.
In summer 2022 the accelerator and detector operations were suspended for theLong Shutdown 1 (LS1), used to perform a large number of technical improvements,
both on the accelerator side and the detector side, including the installation of a complete PXD detector. As shown in figure 1.2, in the first run period 2019–2022 a total
integrated luminosity of 427 fb−1 was collected, of which 424 fb−1 in good runs, allowing the publication of numerous measurements [6]. Belle II is an international
collaboration of 1000 members at more than 100 institutions in 28 countries/regions.
7

8

KLM

Muon ID

KLM

ECL

ARICH

TOP

CDC

Calorimetry

Particle ID

PXD

Tracking

SVD

Beryllium

Name

Beam pipe

PurposeConfiguration

Readoutnelschan- θ coverage

Cylindrical, inner radius 10 mm, 10µm Au, 0.6 mm Be, 1 mm paraffin, 0.4mm BeSiliconPixel Sensor size: 15×(L1 136, L2 170) 10M[17◦ ;150◦ ](DEPFET)
mm2 , Pixel size: 50×(L1a 50, L1b 60,
L2a 75, L2b 85) µm2 ; two layers at
radii: 14, 22 mm
Silicon Strip
Rectangular and trapezoidal, strip 245k
[17◦ ;150◦ ]pitch: 50(p)/160(n) - 75(p)/240(n)µm, with one floating intermediate
strip; four layers at radii: 38, 80, 115,
140 mmDrift Chamber 14336 wires in 56 layers, inner radius 14k[17◦ ;150◦ ]with He-C2 H6 of 160mm outer radius of 1130 mm
gas
RICH
with 16 segments in ϕ at r ∼ 120 cm, 275 8k[31◦ ;128◦ ]quartz radiatorcm long, 2cm thick quartz bars with
4×4 channel MCP PMTs
RICH with aero- 2×2 cm thick focusing radiators with 78k[14◦ ;30◦ ]gel radiator
different n, HAPD photodetectors
CsI(Tl)Barrel: r = 125 − 162cm, end-cap: z = 6624 (Barrel), [12.4◦ ;31.4◦ ],−102 − +196cm
1152(FWD), [32.2◦ ;128.7◦ ],960 (BWD)[130.7◦ ;155.1◦ ]barrel:RPCs
2 layers with scintillator strips and 12 θ 16k, ϕ 16k[40◦ ;129◦ ]and scintillator layers with 2 RPCs
stripsend-cap: scintil- 12 layers of (7-10)×40 mm2 strips
17k[25◦ ;40◦ ],lator strips[129◦ ;155◦ ]Component

Table 1.1: Summary of the detector components[4].

1

2

3

4

5

6

7

8

9

10

11

12

TOP VIEWA

Backward
NIKKO

BarrelBelle II

KLM

A

Forward
OHO
KLM
KLM

Super conducting coil
2980

B

B

1020(E-ECL)

1960(E-ECL)

940(TOP,ECL-flange)

1930(TOP,ECL-flange)
1670(ARICH)
B-ECL1590(CDC
w/o S.C:2325)

735(CDC w/o S.C.)

E-ECL

633.5(Cryostat)R1165(CDC IDS)
R1250.4(BECL)

700(Gap sensor)
2150

(20)

257.5

0.0415r

D

QCSR

e-

e+
E

Beam line axis
(After +25.95mrad. rotation )

10

(20)

E-ECL

(20)

E-ECL
(70)

(24
)

2650(CDC)
2870(ECL flange)

(4
7)

F

(22)

(116)

F

End cap shield

ARICH

628(Cryostat)1156.4

308.1

QC2RP

145

IP Chamber
311.7

QC2RE

ø1299

535(SVD)

R410

400(SVD)

(155)

(88)

R452

ø500

(213.3)

ø540

ø926.8

ø1650.7

25

Belle II solenoid axis

0.0085r

QC1RP

End cap shield
13

280

QC1RE

QC1LP
QC1LE

QC2LE
E

ARICH

Small cell chamber

ø742
ø694
ø734

570

ø530
ø873

R155(VXD)

310

R1145(ARICH)

PXD(2 layers)

SVD

R160(CDC)

Pre-amp1200(TOP)R1133.8(TOP inner most R.)

30º

CDC

0.083r

QC2LP

C

1650(CDC:2650)

0.0415r

QCSL

1000(CDC)

17º

D

E-ECL

TOP

R1130(CDC-SC outer most R.)

C

B-ECL
G

1490

KLM

2417.5

G

KLM

3920

3351

KLM

20130318 R1120120720 R1020120418 R9
20120327 R8
20120302 R7
20120111 R6
20110701 R5
20100808 R4
20100519 R3
20100330 R2
20100310 R1

20160303 R14•Update-QCSL(D5,E5)
DESIGNEDDRAWNCHECKED
Takashi Kohriki
Takashi Kohriki
20160225 R13
20100215
20100215•Updates
FINISHORIGINAL SCALE
SIZE
SHEET
REV.
-CDC,SVD,TOP->minor chages
CLEAN & DEGREASE-QCS vacuum vessel->QCS20160107
1/10
A1 of R14REMOVE ALL BURRS•Addition-Gap sensor(D4,D8)
TITLE
DRAWING NO.
-End cap shiels(D4,D9,E4,E9)
Belle-ll(Nano beam)
Belle-llTopview±41.5.vwx-RVC(D5,D7,E5,E7) not detailed
IR=±41.5 mrad.(Top view A)
20140213 R12
PROJECTPROJECTION-QCS20131203
MECHANICAL ENGINEERING GROUPINSTITUTE FOR PARTICLE AND NUCLEAR STUDIES-CDC covers
Belle IIHIGH ENERGY ACCELERATOR RESEARCH ORGANIZATION-B&FWD new pole pieces
OHO 1-1, TSUKUBA, IBARAKI 305-0801, JAPAN

H

H

KEK

1

2

3

4

5

6

7

8

9

10

11

12

Figure 1.1: Belle II detector top view.

5

10

15

20

The current program foresees the accumulation of an integrated luminosity of Lint
= 50 ab−1 by the first years of the next decade.
SuperKEKB is expected to be able to reach Lpeak of 2 × 1035 cm−2 s−1 with theexisting accelerator complex. However, in order to reach the target luminosity of
6 × 1035 cm−2 s−1 , an upgrade of the interaction region and the QCS superconducting
final focus will be required, as discussed in section 2. A second Long Shutdown (LS2)in a time frame after 2027–2028 will be required to install accelerator and detector
upgrades, although it is not clear at this time whether the interaction region can beredesigned in that time scale. A notional projection of the integrated luminosity forthe coming years is shown in figure 1.3. An International Task Force has been setup toprovide advice to SuperKEKB on the possible technical solutions [7].
The LS2 and the possibility of the SuperKEKB interaction region redesign provide
both a window of opportunity for the installation of detector improvements in many
subsystems, and the requirement for a vertex detector replacement, since even smallchanges in the interaction region would make it impossible to reuse the current VXD.
Even without an interaction region redesign, upgrading the VXD would provide better
physics performance as well as creating a safety net in case the current detector getsdamaged.
In section 1.2 the detector upgrade options are briefly summarized, while the restof the document provides a detailed description of the proposed upgrades and theirstatus of development. It should be recognized that at this time significant R&D is stillrequired to arrive at a final design, and that there are still large uncertainties on the9Figure 1.2: Belle II integrated luminosity in the first run period 2019 - 2022.
time frame and exact path of the upgrade project.
The detailed design of the detector upgrade will be prepared and documented ina future Technical Design Reports, to be prepared once the plans for the SuperKEKB
upgrade are clarified.

5

10

15

1.2

Belle II upgrades overview

Soon after the construction of the Belle II detector was completed (at the end of 2018)
an Upgrade Working Group (UWG) was established by the collaboration, with the
charge of examining the limits of the current detector and develop upgrade possibilities. Both short and long term options were to be considered, in order to ensure
successful completion of the existing physics program, and longer term options to copewith possible higher luminosity upgrades.
As Belle II started full operation in 2019, it became clear, not surprisingly, that machine background plays an important role in the safety, efficiency, and performance ofdata taking. SuperKEKB is a new and challenging accelerator with a relatively narrow
operating margin between high luminosity and high backgrounds. It was realized thatreaching the design 8 × 1035 cm−2 s−1 luminosity was probably too ambitious and an intermediate target of 6 × 1035 cm−2 s−1 was set, that in any case required some machine
improvement program with the possibility of an interaction region redesign.
In summer 2020 a MEXT roadmap 2020 was defined in order to reach the luminos10

Figure 1.3: Projected luminosity for SuperKEKB.

5

10

15

20

25

ity target of 50 ab−1 , calling for two long shutdowns in the lifetime of the experiment:
LS1 in 2022 for the replacement of PXD, and LS2 in 2026 or later for the upgrade of
the machine elements and the interaction region. LS2 provides a window of opportunity for a significant detector upgrade, possibly improving robustness against machine
backgrounds and physics performance.
The UWG has requested the submission of Expression of Interests from the various
detector groups documenting upgrade ideas, mainly focused on the LS2 time scale, butalso considering longer time scales and future ugprade paths. These internal EOIs werecollected in February 2021 and reviewed by an internal Upgrade Advisory Committee
formed at the end of 2020 to provide recommendations on the upgrade process. As aresult of this internal review the initial EOIs were adjusted and somewhat reduced inscope, resulting in the submission of a Belle II Upgrades White Paper to the Snowmass
process [8].
Belle II is designed to operate efficiently even with the background levels expected
at design luminosity, but safety margins are not large. It should be noted that the extrapolation of the level of backgrounds to full luminosity is affected by large uncertainties due to many elements, such as: limited accuracy of the present parameterization;lack of modeling for the injection background; lack of a specific design for the interaction region upgrade. The expected evolution of these backgrounds with luminosity
is described in [9] and [10]. The P5 report [11] considers the Belle II and SuperKEKB
upgrades as key elements to pursue the quantum imprints of new phenomena andrecommends supporting the program.
The upgrade program is therefore motivated by a number of considerations:• Improve detector robustness against backgrounds• Provide larger safety factors for running at higher luminosity• Increase longer term subdetector radiation resistance• Develop the technology to cope with different future paths, for instance if a majorinteraction region (IR) redesign is required to reach the target luminosity• Improve overall physics performance
11

The upgrade activities can be classified over three different time scales:

5

10

15

20

25

30

35

1. short term: Long Shutdown 1 (LS1) was started in July 2022, and concluded
at the end of 2023, with the main purpose of installing a complete pixel detector (PXD2). Many other minimal improvements on other subsystems were also
implemented.
2. medium term: Long Shutdown 2 (LS2), in a time frame after 2027–2028, willprobably be needed for the upgrade of the interaction region to reach Lpeak =6 × 1035 cm−2 s−1 . A new vertex detector might be required to accommodate
the new IR design, and other sub-detector upgrades are possible, with various
options being evaluated.
3. long term: Studies have started to explore upgrades beyond the currently planned
program, such as beam polarization and ultra-high luminosity, possibly Lpeak in
excess of 1 × 1036 cm−2 s−1 . While the beam polarization has a concrete proposal,
for ultra-high luminosity studies have just started.
LS1 is nearing completion at the time of writing, with a number of activities being
carried out:
• VXD: Extraction and re-installation of the VXD, with the complete PXD2 detector.
• CDC: Improvement in gas circulation, monitoring and modification of the highvoltage resistor chain.
• TOP: MCP-PMT replacement with ALD life extended PMTs
• ECL: Improvement in pedestal correction. Gain adjustment in the shaper DSP• KLM: Efficiency recovery of some damaged chambers. Reinforcement of monitoring system• TRG: Optimization of trigger veto.
• DAQ: New readout board PCIe40 long-term stability test with realistic highoccupancy data.
• MDI: Installation of additional loss monitors and speed-up of abort signal, as
well as of additional shields against neutrons and electromagnetic showers.
Table 1.2 shows the current ideas for the medium and long term Belle II detector
upgrade ideas, that evolved from the Snowmass White paper, reducing the number of
options and focusing on a concrete path as much as possible.
In the following sections a more detailed description of the various options is provided. Some approximate cost estimates are also indicated. The simulations reported
in the text, if not specified otherwise, have used the Belle II Analysis Software Framework basf2 [12, 13].

12

13

ECL

Trigger
DAQ
ARICH

KLM

Subdetector
MDI
VXD
CDC
TOP

Function
upgrade activity
time scale
RMBA
Faster and more performant electronics
medium-term
Vertex Detector all-pixels DMAPS CMOS sensors (VTX)
medium-term
Tracking
upgrade front end electronics
short/medium-term
PID, barrel
Replace not-life-extended ALD MCP-PMTs
medium-termFront end electronics upgrade
medium-term
KL , µ ID
replace 13 barrel layers of legacy RPCs with scintillators
medium/long-term
upgrade of electronics readout and proportional mode RPC readout medium/long-term
timing upgrade for K-long momentum measurement
medium/long-term
hardware and firmware improvements
continuous
add 1300-1900 cores to HLT
short/medium-term
PID, forward
replace HAPD with Silicon PhotoMultipliers
long-term
replace HAPD with Large Area Picosecond Photodetectors
long-termγ, e ID
Add pre-shower detector in front of ECL
long-term
Complement ECL PiN diodes with APDs or SiPM
long-term
Replace CsI(Tl) with pure CsI crystals
long-term

Table 1.2: Known short and medium-term Belle II subdetector upgrade plans, sorted by time scale. MDI is the Machine-DetectorInterface, while RMBA is Radiation Monitoring and Beam Abort system. Moving from inner to outer radius, the current Belle II
sub-detectors are: Silicon Pixel Detector (PXD), Silicon Strip Detector (SVD), forming the VerteX Detector (VXD), Central Drift
Chamber (CDC), Time of Propagation Counter (TOP), Aerogel Rich Counter (ARICH), Electromagnetic Calorimeter (ECL), K-Long
Muon System (KLM), Trigger and Data aquistion (TRG/DAQ), including the High Level Trigger (HLT).

Chapter 2
SuperKEKB
2.1
5

10

15

20

25

30

SuperKEKB status

Editor: Y. Ohnishi, M. Masuzawa
SuperKEKB is an energy asymmetry electron-positron collider aiming for a peakluminosity of 6 × 1035 cm−2 s−1 , the highest luminosity ever achieved by a collider, andan integral luminosity of 50 ab−1 . The adoption of a collision method based on the
nano-beam scheme[14] is expected to open up unexplored areas of luminosity. In the
nano-beam scheme, a sufficiently thin bunches collide with a large horizontal crossing
angle. The length of the region where bunches overlap in the direction of travel (σz,ef f )can be sufficiently shortened compared to the bunch length, as schematically shown in
Fig. 2.1. This makes it possible to reduce the vertical beta function at the interaction
point (IP), βy∗ , to a value as small as σz,ef f and luminosity increase is expected. In
SuperKEKB, βy∗ is squeezed down to about 0.3 mm, equivalent to about 1/20 of that of
KEKB, to achieve the target luminosity.
The SuperKEKB collider complex consists of a 7-GeV electron ring (the high-energy
ring, HER), a 4-GeV positron ring (the low-energy ring, LER), and an injector linear
accelerator (LINAC) with a 1.1-GeV positron damping ring (DR), as shown in Fig. 2.2.
The extremely high luminosity of SuperKEKB required significant upgrades to the HER,
LER, and final-focus system of KEKB. The injector linac also required significant upgrades for injection beams with high current and low emittance. A new RF-gun wasdeveloped and installed, and a new DR was designed and constructed for realizing
low-emittance electron and positron injection to the SuperKEKB Main Ring (MR).
Phase 1 commissioning was carried out in 2016 without beam collisions. After
that, the interaction region (IR) was completed by installing the final focusing superconducting quadrupole magnet system and the Belle II detector (without VXD), and
the Phase 2 commissioning started in March 2018 and a series of collision tuning was
performed. The first hadron event was observed by Belle II on April 26 of the same
year. Phase 3 commissioning started in March 2019 with the fully instrumented BelleII detector. A summary of the operation since 2019 is shown in Fig. 2.3.
Figure2.4 shows the evolution of βy∗ together with reference to values at otheraccelerators. By carefully looking at the relationship between the injection efficiency
and the background in the detector and adjusting the collimator opening, we wereable to perform the first collision experiment at βy∗ = 1.0 mm during the 2019 run. A
14

Figure 2.1: Schematic view of the nanobeam collision scheme.

Figure 2.2: Schematic view of SuperKEKB Main Ring. The electron and positron rings
have four straight sections named Tsukuba, Oho, Fuji, and Nikko. The electron andpositron beams collide at the interaction point in the Tsukuba straight section.

15

Figure 2.3: Operation history from 2019 to 2022: beam current in the HER, LER, daily
luminosity, and integrated luminosity.

5

10

test of squeezing βy∗ down to 0.8 mm was carried out for approximately one week in2020 and 2022.
The crab waist (CW) scheme [14, 15] was adopted for the both rings since the2020 spring run. The CW ratio, the ratio to the magnetic field required to align thewaist of the electron (positron) beam on the positron (electron) orbit, is 80% in the
LER and 40% in the HER, respectively at this point.
A peak luminosity of 4.65 × 1034 cm−2 s−1 was achieved with βy∗ = 1.0 mm in thephysics run, and 4.71 × 1034 cm−2 s−1 is the highest luminosity obtained without dataacquisition by the Belle II detector up to now. And this peak luminosity is a world
record in collider history.
The machine parameters that accomplished the peak luminosity are presented inTable 2.1 together with those valid for βy∗ = 0.8 mm. The luminosity formula in the
nano-beam scheme is expressed as
L=

15

N+ N− nb f0,
2πϕx Σz Σy∗(2.1)

where ± indicates positrons (+) or electrons (-), N is the number of particles in abunch, nb is the number of bunches, f0 is the revolution frequency, and ϕx is the half
16

Figure 2.4: History of βy∗crossing angle. Σz and Σy∗ are defined by
q
q
2
2
∗2
∗2
Σz = σz++ σz−
Σy∗ = σy+
+ σy−
,

5

10

(2.2)

where σy+ and σy− indicate the vertical beam sizes of the positron and electron beams
at the IP, respectively. In the case of the nano-beam scheme, a geometrical luminosity
loss due to an hourglass effect [16] is sufficiently small, even if the beta function at the
IP is squeezed down to the bunch length divided by the Piwinski angle, and is omitted
here. The vertical beam–beam parameter is expressed bysβy∗re N∓ 1
,
(2.3)
ξy± ≃
2πγ± ϕx σz εywhere re is the classical electron radius, γ is the Lorentz factor, and εy is the verticalemittance. Here, the beam spot-size at the IP between the LER and HER is assumed tobe equal. Alternatively, the vertical beam–beam parameter is expressed using Eq. (2.1)
and Eq. (2.3) asLβy∗
ξy± = 2ere
,
γ± I±

(2.4)

where e is the elementary charge, I+ and I− are LER beam current and HER beam
current, respectively. Therefore luminosity is proportional to ξy± and I± and inversely
proportional to βy∗ , as is shown by Eq. (2.5).
L∝

I± ξy±
βy∗
17

(2.5)

Table 2.1: Machine Parameters achieved at SuperKEKB

Parameters

June 8, 2022
LER
HER

I (A)
1.321
1.099
nb
2249
Ib (mA)
0.587
0.489
∗
βx (mm)8060
βy∗ (mm)1.0
ξy4.07 × 10−2 2.79 × 10−2εy (pm)
31.7
∗
0.252
Σy (µm)σz (mm)5.69
6.02
CW ratio (%)
80
40
−2 −1
34
L (cm s )4.65 × 10

5

10

20

25

0.744

0.600
1565

0.475
80

0.383
60

0.8
3.09 × 10
2.19 × 10−2
29.6
0.218
5.40
5.78
80
40
34
2.49 × 10−2

As the beam-beam parameter increases, the collision probability increases, but sodoes the instability. How high this value can be raised is the key to increasing luminosity.
Figure 2.5 shows the vertical beam–beam parameter defined by Eq. (2.4) for βy∗ =1.0 mm. When the ratio of beam currents in the LER to that of the HER is 7 to 4, it
becomes the energy transparent condition, γ+ I+ = γ− I− . However, the ratio of bunch
current in the LER to that of the HER is 5 to 4 in the physics run, which is the optimized
value based on the operational experiences. The maximum beam–beam parameter of
0.0565 (LER)/0.0434 (HER) at the bunch current of 1.1 mA in the LER was achievedby the ratio of 4 to 3 in the machine study called high bunch current (HBC) study. The
specific luminosity is defined as the total luminosity divided by the number of bunches
and by the bunch current product as Eq. (2.6).

Lsp =

15

May 22, 2022
LER
HER

1
L
∝
,
Ib+ Ib− nbΣz Σy∗(2.6)

where Ib is the bunch current.
Figure 2.6 shows the specific luminosity achieved during the 2022 physics run. The
specific luminosity decreased as the bunch current product increased, which implies
that the beam–beam blowup was observed [17]. The comparison between the CW
and no CW for βy∗ = 1.0 mm is also shown in the figure. The specific luminosity was
improved by utilizing the CW scheme [18].
The specific luminosity with βy∗ = 0.8 mm was higher than that of 1 mm by 12% at
0.18 mA2 . The specific luminosity is more than 20% at a lower bunch current productof less than 0.1 mA2 . The reason that the specific luminosity decreases quickly can be
the chromatic X–Y couplings at the IP for βy∗ = 0.8 mm.
In the case of βy∗ = 1.0 mm, two of the chromatic X–Y couplings (∂r1∗ /∂δ and ∂r2∗ /∂δ)in the LER are corrected by utilizing 24 rotating sextupole magnets [19]. The sextupole magnet is mounted on a rotating table to induce a skew sextupole field. Prior
18

Figure 2.5: Beam–beam parameters as a function of the bunch current. The positrons
in the LER are indicated by red, whereas blue is used for the electrons in the HER. The
crosses are data from the high bunch current (HBC) test.
to the correction of chromatic X–Y couplings, the machine error at the IP is locallycorrected using IP tuning knobs such as X–Y couplings (r1 , r2 , r3 , r4 ) and vertical dis∗) so as to maximize luminosity.
persions (ηy∗ , ηpy

2.1.1
5

10

15

Impedance

The collimators are important to reduce beam related backgrounds at SuperKEKB.
Movable collimators for the horizontal and vertical plane are installed in the LER and
HER, respectively as shown in Fig. 2.7 [20].
A kick factor from each collimator becomes large because the vertical half aperture of the collimator is very small, typically of approximately 1-2 mm. Therefore,transverse mode coupling instability (TMCI) might become serious. The TMCI, also
called strong head-tail instability, was first observed at PETRA [21]. For a large collider with a small chamber height and collimator aperture, the TMCI limits the performance. At SuperKEKB, we observed a vertical beam blowup at the bunch current ofapproximately 0.8 mA, which is much smaller than the TMCI threshold Ith expressedas Eq. (2.7)[20].

19Figure 2.6: Specific luminosity for βy∗ = 0.8 mm and 1 mm. Comparison between the
CW and no CW for βy∗ = 1 mm.

Ith =

Cfs E
.
Σi βy,i ky,i (σz )

(2.7)

5

Here, C, fs , E, βy,i , ky,i , and σz are constants, the synchrotron frequency, beamenergy, vertical beta function at a component i, vertical kick factor at a component
i, and longitudinal bunch length, respectively. The expected TMCI threshold from
Eq. (2.7) is about 2 mA, while the observed value is 0.8 mA. We call this phenomena“-1 mode instability” because the coherent-mode frequency corresponding to νy − νsappears when the beam blowup occurs. Recently, it was found that a tuning of thevertical bunch-by-bunch feedback system [22] affects the -1 mode instability with ainterplay of the vertical short-range wake field due to the collimators. Optimization of
the bunch-by-bunch feedback system is necessary to suppress the -1 mode instability.

10

2.1.2

15

Sudden beam loss

A fast and large beam loss has been observed frequently when the bunch current
increases over 0.7 mA [23]. We call this fast beam loss “sudden beam loss (SBL).”
More than half of the beam current was lost within a few turns (20-30 µs). The SBL
occurs in the both rings, but it is more serious in the LER. When a SBL occurs, thevertical collimators become damaged and sometimes QCS quenches occur.
The mechanism of SBL is not understood yet. The highest and earliest beam losses
during the SBL event are usually detected near the narrowest vertical collimators in20

Figure 2.7: Collimator Map in 2022.

5

the LER (e.g., D06V1, see Fig.2.7). A hypothesis of “fireball” has been proposed to
explain the SBL, which is based on a RF cavity breakdown mechanism [24]. In thefireball, isolated micro particles with higher sublimation point such as tungsten andtantalum become fireballs heated by the beam-induced field. The fireball touches amaterial surface with lower sublimation point, and then plasma is generated aroundthe fireball. The plasma grows and might lead to significant interactions with thebeam. We will apply the copper-coated collimator head to mitigate the fireball in theoperation after LS1.

2.1.3
10

15

Dynamic aperture and Touschek lifetime

The dynamic aperture (DA), the stable region in phase space for a beam that canorbit stably, is estimated by a particle tracking simulation code such as SAD[25] withcomparison of the measured beam lifetime. The measured lifetime at βy∗ = 1.0 mm
was 8 minutes for Ib+ = 0.62 mA with εy = 40 pm in the LER and 23 minutes for Ib− =0.5 mA with εy = 35 pm in the HER. The simulation can almost reproduce the Touschek
lifetime in the LER. However the HER simulation overestimates the measured DA by afactor of 2, consequently there is a larger machine error on the understanding of the
HER. As smaller DA will become serious as one squeezes βy∗ and increases the bunch
current, and for beam injections[26].
21

2.2

5

10

SuperKEKB evolution and upgrade path

The SuperKEKB physics run began in March 2019, with a fully instrumented Belle IIdetector. The crab waist scheme, which was not included in the original SuperKEKB
design plan, was implemented in April 2020 to enhance beam performance. In 2020,
SuperKEKB surpassed the luminosity record of 2.11 × 1034 cm−2 s−1 achieved by its
predecessor, KEKB, with vertical and horizontal beta functions at the IP set at 1.0 mm
and 80 mm (LER)/60 mm (HER), respectively. Thanks to the nano-beam scheme, thepeak luminosity record was surpassed with smaller beam currents than those used at
KEKB. In June 2022, SuperKEKB achieved a new world record for a peak luminosityof 4.7 × 1034 cm−2 s−1 with a βy∗ of 1.0 mm at the collision point.
Table 2.2: Machine Parameters in the Two-stage Improvement
First stage
Parameters

LER

Second stage

HER

I (A)
2.08
1.48
nb
2345
Ib (mA)
0.89
0.63∗
βy (mm)0.8−2
ξy4.44 × 10
3.56 × 10−2εy (pm)30∗
2.18 × 10−1Σy (µm)σz (mm)6.49
6.35
L (cm−2 s−1 )1 × 1035

15

20

25

30

LER

HER

2.75

2.20
2345

1.17

0.938

0.6
6.04 × 10
4.31 × 10−221
1.60 × 10−1
7.23
7.05
2.4 × 1035
−2

The operation of SuperKEKB was halted for one year and a half since the summerof 2022 to replace some components of the Belle II detector and upgrade the machine.
Installation of a Nonlinear collimation (NLC) scheme is one among many other accelerator upgrades during LS1. The NLC is installed in the OHO section of the LER. A
pair of skew sextupole magnets are installed to create an angular kick to deflect mainly
halo particles. Deflected particles can then be stopped by a regular vertical collimator placed in between the two skew sextupole magnets. The vertical collimator canthen be opened much wider than typical vertical collimators. Since the betatron function at this collimator is small, the wakefield contribution of the NLC to the machine
impedance, and hence, to the TMCI threshold is negligible. We can tighten this collimator and relax other vertical collimators to increase the TMCI limit while keeping
the IR beam backgrounds at acceptable levels. The idea of NLC was initially proposedin the 1990s [27, 28], but SuperKEKB is the world’s first collider to install NLC andtest it.
The initial target of the luminosity is 1×1035 cm−2 s−1 after LS1. By increasing beam
currents and squeezing βy∗ down to 0.6 mm while keeping the specific luminosity high,we plan to increase the luminosity up to 2.4 × 1035 cm−2 s−1 as shown in Fig. 2.8 by thesecond long shutdown (LS2), when further upgrade work is required to achieve morethan 6×1035 cm−2 s−1 . Table 2.2 lists the machine parameters for these milestones after
the LS1. We have achieved the designed number of bunches. Therefore, we attempt22Figure 2.8: Achieved specific luminosity as a function of beam current product (circles)
an its targeted development between LS1 and LS2.
to increase the bunch current toward the target luminosity while suppressing beam
blowup.

2.3
5

10

15

Future plans and prospect

During the next long shutdown, several major modifications are considered to further
improve the machine’s performance. Modification of the IR, an upgrade of the injection complex, a new HER beam transport (BT) line (Fig. 2.9), an increase of the HER
RF stations, and replacement of various aging components are among modification
candidate items. A new BT line aims at keeping the emittance growth by coherent
synchrotron radiation (CSR) and incoherent synchrotron radiation (ISR) under control. Additional RF stations are needed to increase the stored beam to the design
current in the HER.
While discussions on when to start LS2 are underway, consideration has alreadybegun regarding modifications during the LS2 period. The modifications must be
effective enough that the integrated luminosity lost during LS2 is recovered quickly
afterwards. Design and evaluation progress of the IR upgrade, which is one of the
upgrade items, are described in the following subsection.

23

Figure 2.9: The new beam transport line and the present beam transport line for the
HER.

2.4

5

Editor: M. Masuzawa, N. Ohuchi
It should be noted that this is not a completely new design from a blank slate. We
have begun to reconsider the issues left unfinished at the SuperKEKB IR design stage.
We are currently considering items that would be possible to renovate over several
years. Below is a status report about the new IR design.

2.4.1

10

Interaction region design

Introduction of the IR optics

The SuperKEKB interaction region (IR) is designed to achieve extremely small beta
functions at the IP, βy∗ in the vertical direction, and βx∗ in the horizontal direction. Superconducting magnets QC1s and QC2s provide the focusing magnetic field required
to squeeze down βy∗ and βx∗ , respectively [29].
The solenoid field of Belle II is fully compensated with the superconducting compensation solenoids (anti-solenoid) on each side of the IP, thereforeZ
Bz (s)ds = 0,

(2.8)

IP

15

where Bz is magnetic field along the solenoid axis and s is the distance from the IP. The
crossing angle between the Belle II solenoid axis and each beam axis of the LER andHER is ±41.5 mrad. The X–Y coupling, and the horizontal and vertical dispersions arecorrected to be zero at the IP by skew quadrupole magnets and horizontal and vertical24

5dipole magnets on each side of the IP. In the LER, the chromatic X–Y coupling arisingfrom solenoid field overlapping on QC1s are corrected by using rotatable sextupole
magnets.
The finite crossing angle between both beam orbits and the solenoid axis causes
vertical dispersion. This results in a vertical emittance that is strongly dependent on
the transverse component of the magnetic field along the orbit represented by Bx (s),sϕx
Bx (s) ∼= − Bz′ (s) = − Bz′ (s).
22

10

A half crossing angle ϕ or/and Bz′ (s) should be decreased to reduce the verticalemittance. Because decreasing half crossing angle ϕ requires a complete change of theIR, we focus on modifying Bz (s) such that its differential along beam trajectory Bz′ (s)becomes smaller than that in the present system.

2.4.2

15

20

25

30

(2.9)

Characteristics of the new interaction region

The new design concept of the IR upgrade is to make the beam trajectory as parallel to the QC1 magnet axis as possible to cancel the X–Y coupling and chromaticity
between the IP and QC1 as close to zero as possible and minimize ϵy by redesigning
the Bz (s) profile. The vertical offsets and the rotations of QC1P and QC2P and thehorizontal offsets of QC1E and QC2E are expected to be considerably smaller than thepresent values with the new anti-solenoid field design and with incorporating the magnetic yokes on QC1P. The design concepts of the new IR are listed below and shown
schematically in Fig. 2.10.
• Separation of the solenoid field and QC1P
• QC1P relocation, closer to the IP by 100 mm (from the present L∗ = 935 mm to
835 mm)
The magnetic field from the line current distribution was evaluated using Opera3D[30], where the magnetic field by the Belle II solenoid coil is included. As is shown in
Fig. 2.11, the peak field becomes lower with the new IR, resulting in thinner coil. The
detector solenoid field is cancelled between IP and QC1P. The cryostats are includedin Fig. 2.12 and Fig. 2.13 for the current and new QCS system, respectively.
Some parts of the cryostat and the Belle II detector (e.g., the vertex detector support structure) interfere, on the forward side. A different solution for connecting thevacuum systems have to be developed. The thinner solenoid coil creates more spacein the radial direction between the detector and the cryostat, which makes cablingeasier around the cryostat. It should be noted though that these are very simplified
preliminary drawings. Drawings with final dimensions cannot be produced until themagnet design and detailed design of each part are finalized.

25

Current ver.

ForwardBackward
QC1P

QC1P
0

Upgraded ver.

0.5

Forward1m

Backward
QC1P

Nb3Sn

Nb3Sn

QC1P

0

0.5

1m

Figure 2.10: Comparison between the current QCS configuration (top) and the new
QCS configuration (bottom). Anti-solenoid coils and iron (or permendur) shield are
indicated in red and green, respectively.

Figure 2.11: Comparison between the current and the new anti-solenoid field profile
of Bx and Bz for the magnet configurations in Fig. 2.10 for the LER. The anti-solenoid
coil cancels the magnetic field generated by the Belle II detector solenoid before QC1P.

26

Figure 2.12: The current QCS system with cryostat. The red, brown, yellow, blue, cyan
and green shaded areas corresponds to the vacuum vessels of the cryostat, the liquid
helium vessels, the tungsten radiation shields, the anti-solenoid coils, the magnetic
yokes/shields, and the QC1P magnets, respectively. The left side corresponds to thebackward side.

Figure 2.13: The new QCS system with cryostat. The red, brown, yellow, blue, cyan
and green shaded areas corresponds to the vacuum vessels of the cryostat, the liquid
helium vessels, the tungsten radiation shields, the anti-solenoid coils, the magnetic
yokes/shields, and the QC1P magnets, respectively. The left side corresponds to thebackward side.

27

2.4.35

10

15

Optics evaluation

Optics evaluation was carried out focusing on the effects on the dynamic aperture andTouschek lifetime, chromatic coupling, and the vertical emittance. Neither Beam-beameffects nor octupole correction are included in this evaluation. The R-side backward
compensation solenoid is not implemented in the optical evaluation model being discussed. The upstream vertical orbit and vertical emittance are provisional values.
Fig. 2.14 and Fig. 2.15 compare the dynamic aperture and Touschek lifetime of thepresent IR lattice (sler 1704) and the new IR lattice (V21-r0g0) for LER, respectively.
The design parameters, such as 0.27 mm for βy∗ , were used as input parameters forthe simulation. By relocating the QC1P 100 mm closer to the IP, Touschek lifetime
increases from 261 seconds to 424 seconds.
Table 2.3 summarizes chromatic X–Y coupling for the present and new IR latticedesigns. It is shown that the chromatic X–Y coupling parameters, ∂R1 /∂δ, ∂R2 /∂δ,
∂R3 /∂δ and ∂R4 /∂δ, are few orders of magnitude smaller with the new IR with/without
the sextupole correction on. A clear improvement is seen with the new IR lattice.
Table 2.3: Chromatic Coupling comparison between the design lattice and the newlattice with (ON) and without (OFF) chromatic coupling correction by rotatable sextupole magnets.

20

25

Lattice

Sext.cor∂R1 /∂δ∂R2 /∂δ∂R3 /∂δ∂R4 /∂δsler 1704
sler 1704
V21-r0g0
V21-r0g0

ON
OFF
ON
OFF

−8.9 × 10−3
−2.3
2.3 × 10−5
1.1 × 10−1

4.0 × 10−3
−1.0 × 10−2
−6.0 × 10−6
2.8 × 10−4

−5.0 × 101−4.2 × 102−4.4 × 10−2
8.1

2.9
−6.1 × 102
5.5 × 10−3
2.6 × 101

The present and new IR orbit and the beta functions are shown in Fig. 2.16 andFig. 2.17, respectively. The orbit displacement is ∼10 µm at QC1, while it is about
1 mm with the present lattice. The dispersion functions of the present and new lattice
are shown in Fig. 2.18 and Fig. 2.19, respectively. The vertical emittance from the IR
is calculated to be 14 fm, which is negligibly small.
To summarize, 1) dynamic aperture and Touschek life time is improved by relocating QC1 closer to the IP, 2) chromatic X–Y coupling improves significantly, 3) thecontribution to the vertical emittance from the IR is in an order of several tens of femtometers. The new IR removes the difficulties in luminosity tuning that we are facing
now. Luminosity tuning is expected to become easier with the new IR.

2.4.4

30

R&D of Nb3 Sn cables and QC1P quadrupole

QC1P must be designed more compactly so that it can fit into the smaller space when
moved 100 mm closer to the IP than its current position. Therefore, it is necessary
to develop superconducting cables that can operate at higher current densities. The
design current density of the new QC1P superconducting cable is twice that of thecurrent QC1P.
Nb3 Sn is known to have a higher critical current density and critical temperaturethan NbTi, which is commonly used as an accelerator magnet material [31]. In case of
28

5

10

15

Nb3 Sn quadrupole magnet, the temperature margin for keeping the superconducting
condition of the magnet can be doubled compared to the current NbTi quadrupole
magnet [32]. This effect is very important for stable beam operation at larger beam
currents (twice larger than the present) after LS2. The development of superconducting magnets using Nb3 Sn cables to produce a required magnetic field with a quite
high current density in the superconducting coil has been carried out though there
are many items that need to be considered and investigated to be used for QC1P. The
items include issues such as countermeasures against quenches caused by magnetic
flux jumps within a superconductor, and time decay of the electromagnet magnetic
field due to magnetic flux creep. Fabrication of a small coil using Nb3 Sn also needs
to be confirmed. Nb3 Sn magnet fabrication is quite difficult because of the brittlenessof the material compared to NbTi. To produce Nb3 Sn as an A15 compound, the coilneeds to be heated over 650 degrees Celsius for tens of hours after completing thecoil winding. The heat-treated coils are very brittle and need to be impregnated andassembled into the magnet very carefully; the higher multipole fields in the magnet
must be at the level of a few 10−4 of the main quadrupole field.
At present, we have the following development schedule:• Study of the Nb3 Sn cable for the new QC1P magnet.
• Production of the Nb3 Sn cable, and design of the prototype QC1P and
the R&D coil.

20• Construction of the first R&D coil and a mirror magnet [33], followed
by a test of the coil in the mirror magnet configuration.
• Review of the proto-type QC1P magnet design based on the test resultsof the mirror magnet and confirmation of the design.
• Start and completion of the construction of the first Nb3 Sn prototype QC1P magnet after fabrication of 4+1 coils. The magnet will
be tested, and the magnetic field measurements will be performed.

25

We will proceed with feasibility studies of Nb3 Sn cables for QC1P and possibly forthe anti-solenoid coils.
30

35

40

2.4.5

R&D program for the new IR

A detailed machine optics study, including beam pipe geometry optimization, is essential to identify the narrowest machine aperture, where the majority of IR beam loss
concentration is expected based on our operation experience. The new outcome ofthis study might require additional detector shielding installation around the final focusing magnets with potential impact on the vertex detector envelope.The remaining
tasks related to beam optics and lattice design are listed below.
Regarding the second compensation solenoid:• Optical modeling, evaluation of effects on trajectory and optics optimization of LER R side (upstream side of IP) trajectory with consideration of the effect on the detector background.
29

For reviewing and evaluating machine performance:

5

10

15

• Consistency checks of machine parameters, such as– Are the assumed collision parameters, lifetime, stored beam current, and injector performance consistent?
– Is it necessary to construct an evaluation model for detector background estimate?
* The difficulties of this scheme of large Piwinski angle collisionat βy∗ of 0.3 mm with Crab Waist are, narrow dynamic apertureand short Touschek lifetime. The maximum stored beam current is determined by the balance between injection capacity
and lifetime while controlling the background to the detector.
• Evaluation with beam-beam, wake field and machine errors, which isnecessary to estimate actual performance.
– We need to identify the causes of the current machine performance defects and establish reproduction techniques and evaluation models on simulation.
Tasks required for construction:

20

25• Completion of the HER IR optics– Review of horizontal orbit design and correction of optics due tointroduction of magnetic shielding to QC1P.
• Redesign of outside QCS region (IP ±4 m) for both LER and HER– The model being considered in the LER has not been tested to see
if it will physically fit in the existing tunnel.This point of view also
needs to be considered in the actual construction of beamlines.
– Add and relocate elements needed for matching/adjusting luminosity tuning knobs, resolve interference with tunnel boundaryand HER.
– HER also needs to reflect IR horizontal orbit change.

30Figure 2.14: The present lattice (βy∗ = 0.27 mm)

Figure 2.15: The new lattice (βy∗ = 0.27 mm)

31

Figure 2.16: The present orbit and beta functions for βy∗ = 0.27 mm.The horizontal andvertical beta functions are indicated by red and blue lines, respectively.

Figure 2.17: The new orbit and beta functions for βy∗ = 0.27 mm.The horizontal andvertical beta functions are indicated by red and blue lines, respectively.

32

Figure 2.18: The present dispersion functions for βy∗ = 0.27 mm.The horizontal andvertical beta functions are indicated by red and blue lines, respectively.

Figure 2.19: The new dispersion functions for βy∗ = 0.27 mm.The horizontal and vertical beta functions are indicated by red and blue lines, respectively.

33

Chapter 3
Machine Detector Interface
Editor: H. Nakayama, A. Natochii

3.1
5

10

15

Editor: H.Nakayama, A.Natochii
Here, we briefly review the main sources of beam-induced backgrounds, usuallysimply referred to as beam backgrounds for short. Beam backgrounds can arise fromparticle losses from the beam during machine operation, leading to electromagnetic(EM) showers. Secondary particles from such showers then cause higher hit rates andother detrimental effects in Belle II. While this is the typical example, any aspect ofmachine operation, even the intentional collision of beams, which results in higher
Belle II occupancy or radiation dose, can be considered a beam background. Since
SuperKEKB is designed to operate at much higher beam currents and collision luminosities than its predecessor KEKB, the IR backgrounds are also expected to be muchhigher.

3.1.1

20

25

Beam backgrounds and countermeasures

Particle scattering

Beam particles circulating in SuperKEKB may undergo single-beam processes causedby i) scattering with residual gas molecules due to Bremsstrahlung and Coulomb interactions and ii) Coulomb scattering between particles in the same beam bunch, commonly referred to as the Touschek effect. These processes lead to the scattered particles falling out of the stable orbit and reaching the physical or dynamic aperture of themachine. This creates EM showers when these stray particles hit the beam pipe walls.
The beam-gas background rate is proportional to the vacuum pressure and the beam
current, while the Touschek background is proportional to the beam current squared
and inversely proportional to the number of bunches, beam size, and the third powerof the beam energy.

3.1.2

Colliding beams

While particle collisions at the IP are the goal of SuperKEKB, there are several undesirable collision processes that have very high cross sections but are of little interest for34

physics measurements. Examples are radiative Bhabha scattering (e+ e− → e+ e− γ) andtwo-photon processes (e+ e− → e+ e− e+ e− ). These processes increase the Belle II occupancy and radiation dose, and we refer to these increases as luminosity background.
The rate of this background component is proportional to the luminosity.
5

3.1.3

10

X-rays are emitted when electrons or positrons pass through the strong magnetic fieldof the QCS or dipole magnets upstream and downstream of the IP. SR can potentiallydamage the inner layers of the vertex detector. Because SR power is proportional tothe beam energy squared and magnetic field strength squared, X-rays from the HER
are the main concern.

3.1.4

15

25

35

Sudden beam losses

During otherwise stable machine operation, unexplained beam instabilities and beamlosses may occasionally occur in one of the rings. Such losses can occur in one shotat a specific location around the ring due to machine element failure or due to thebeam interacting with dust particles in the beam pipe. In extreme cases, high-intensity
EM showers can lead to detector or collimator damage or superconducting magnet
quenches. For example, eight such LER beam losses led to QCS quenching in the
February – June 2022 run period.

3.1.6

30

Beam injection

Since the beam lifetime, which is limited mainly by Touschek losses, is of order 10–
60 min, SuperKEKB is operated using a so-called top-up injection scheme where thebunches are continuously refilled to keep the current stable. Due to optics mismatches
between the injection chain and the main ring, injected particle bunches do not enter
the aperture of the machine perfectly, resulting in higher particle losses from injectedbunches until they stabilize (∼ 10 ms).

3.1.5
20

Synchrotron radiation (SR)Head-tail instabilities

In linear accelerators (linacs), when a dense bunch of charged particles travels throughthe accelerator structure, the front part of the bunch (head) induces an EM wakefield
which deflects the rear part of the bunch (tail). After some time, the tail oscillates
around the head trajectory, which remains unperturbed. These oscillations may cause
particle losses on the aperture of the machine. This process is referred to as Beam
Break-Up (BBU) instability. In a synchrotron machine like SuperKEKB, the RF field
induces a longitudinal focusing of the bunch, creating a collective transverse motion
with head-tail modes of betatron and synchrotron oscillation coupling. Below a certain
bunch current, this oscillation remains stable while at the threshold value, the growthrate of the tail is faster than the synchrotron period so that the head-tail modes aredestroyed, and the beam becomes unstable. This process is referred to as the Transverse Mode Coupling Instability (TMCI) [34]. Since the TMCI threshold depends on35

the most narrow and steep aperture in the ring, the bunch current threshold is usuallycalculated considering only collimators. It can be defined as follows [34, 35]:
C1 fs E/e,
Ith = Pj βj k j(3.1)

where Ith is the upper limit on the bunch current, fs = 2.13 kHz or fs = 2.80 kHz is
the synchrotron frequency for the LER or HER, respectively, E is the beam energy, e
−3

5

10

is the unit charge, βj and kj ∼ dj 2 are the beta function and kick factor, as a functionof the aperture dj [36], for the jth collimator, respectively. Usually, the constant C1
is equal to 8 or 2π [35]; however, for SuperKEKB, C1 ≈ 4π is found to be a more
realistic value based on machine studies [37] conducted in 2021 – 2022. Additional
machine elements with a variable aperture, such as the IR beam pipe, can be includedin Eq. 3.1 by extending the sum over βj kj to include terms both for collimators andthese additional elements.

3.1.7

15

20

25

30

35

40

Countermeasures

In the last decade, we have developed a comprehensive set of countermeasures against
each of these background sources. We use collimators at specific locations to suppress
single-beam and injection backgrounds by stopping stray particles before they reach
the IR. Vacuum scrubbing helps us reduce each ring’s vacuum pressure, thus lowering
the beam-gas scattering rate [38]. A heavy metal shield was installed to protect the
inner detectors against EM showers from beam losses in the IR. A polyethylene+leadshield inside the ECL [39] protects the ECL crystals and photodiodes from neutrons.
Similarly, the inner layers of the ARICH were replaced with a boron-doped polyethylene shield against neutrons. The incoming beam pipes of the IR collimate most of theSR photons and suppress their reflections thanks to the design geometry and a ridge
structure on the inner beam pipe surface [40]. To protect the vertex detector against
residual SR, the inner surface of the IP beryllium beam pipe is coated with a 10 µm
thick gold layer. The newly constructed Damping Ring (DR) reduces the emittanceof injected positrons, thereby suppressing injection beam losses. We apply an injection veto on the Belle II Level-1 trigger, to avoid recording high beam losses when theinjection bunch crosses the IR, for about 10 ms after each injection. In addition, we
continuously perform injection chain tuning to improve injection efficiency and reduce
injection background. To monitor the beam losses in the IR, a system based on singlecrystal synthetic diamond sensors, so-called Diamonds, is used since the start of theSuperKEKB operation. The system records the radiation dose-rates in positions close to
the inner detectors of the Belle II experiment, and protects both the detector and accelerator components against destructive beam losses, by participating in the beam-abortsystem [41]. It also provides complementary information for the dedicated studies ofbeam-related backgrounds [42]. To protect Belle II against abnormal beam losses, thebeam abort system has been complemented by two new systems based on sCintillation Light And Waveform Sensors (CLAWS) detectors [43] near the IR and CsI-crystals
around each ring. The CLAWS system provides an additional fast beam abort signal,
while the CsI-based sensors help to locate the initial beam loss locations along the ring.

36

5

Furthermore, as discussed in Ref. [44], during LS1, the machine and detector protection system is planned to be upgraded. This includes additional neutron and EMshielding around QCS magnets and Belle II, heavy-metal shielding around IR bellows,
new collimators, and a new IP beam pipe. So far, all mentioned upgrades have notbeen implemented in the official MC sample production code, leading to presumably
overestimated detector BG rates at higher luminosity.

3.2

10

15

20

25

30

35

40

Background status

Since the start of the SuperKEKB commissioning with a fully assembled Belle II detector in 2019, we have performed several dedicated beam background studies. Usually,
the measurement starts from a single-beam configuration where only one beam isfilled with a beam. Changing the number of bunches allows us to disentangle beamgas and Touschek backgrounds during beam decay measurements when the injection is stopped. Repeating this measurement for the LER and HER, we can extract
background sensitivities for each sub-detector separately. Then, by extrapolating thesingle-beam backgrounds to the collision study when both beams are filled, we subtract the single-beam contribution from the observable and estimate the luminosity
background sensitivity. A detailed procedure of Belle II background measurements is
discussed elsewhere [10]. For example, Belle II background rates measured in June
2021 are shown in Fig. 3.1. The data was recorded with a betatron function at the
IP of βy∗ = 1 mm, beam currents of ILER = 732.6 mA and IHER = 647.2 mA, withring-averaged beam pipe pressures of P LER = 88 nPa and P HER = 24 nPa, nb = 1174
bunches in each ring, and a luminosity of L = 2.6 × 1034 cm−2 s−1 . Current backgroundrates in Belle II are acceptable and, in most cases, well below the limits listed in Table 3.1. The TOP limit before LS1 is related to the replacement of TOP conventional
PMTs planned for LS1. At the same time, the limit after LS1 is associated with the replacement of ALD PMTs in LS2 and the longevity of life-extended ALD PMTs. The TOP
luminosity background is assumed to be 0.925 MHz/PMT per 1 × 1035 cm−2 s−1 . The
upper background rate limit quoted for the Diamond read-out electronics can be increased by selecting a lower signal amplification. The KLM detector limit corresponds
to the muon reconstruction efficiency drop of about 10%. Rates in the KLM appear
high, but this is an analysis artifact: the barrel RPCs have a high pedestal rate associated with the noise floor. The KLM is a rather robust detector, with gradual, temporary
efficiency reduction but no permanent damage at high rates. A dedicated analysis thatsuppresses the pedestal in KLM-RPC layers is in progress.
Figure 3.1 shows that dominant backgrounds for all sub-systems are the LER singlebeam (beam-gas and Touschek) and luminosity backgrounds. The HER single-beambackground is at the level of 10 % for most of the detectors except for the ARICH,
which is placed on the FWD side of the Belle II and hence sees more forward-directedparticle losses in the IR from the electron beam. QCS-BWD-315, BP-FWD-325, and
QCS-FWD-225 indicate backward QCS, beam pipe, and forward QCS Diamond detectors, respectively, with the higher dose rate. Barrel KLM L3 corresponds to theinnermost RPC layer in the barrel region of the KLM detector. TOP ALD shows theaveraged background over ALD-type MCP-PMTs, slots from 3 to 9.
The current level of the SR is of no concern in terms of occupancy for the innermost
37

Table 3.1: Background rate limits for different Belle II detector sub-systems. The
third column shows the total measured background rate in June 2021, excluding thepedestal rate.
Detector
Diamonds
PXD
SVD L3, L4, L5, L6
CDC
ARICH
Barrel KLM L3

BG rate limit
1–2 rad/s
3%
4.7 %, 2.4 %, 1.8 %, 1.2 %200 kHz/wire
10 MHz/HAPD
50 MHznon-luminosity BG
Before LS1
After LS1
3 MHz/PMT 5 MHz/PMT+ luminosity BG

TOP ALD

Measured BG< 132 mrad/s
0.1 %< 0.22 %22.3 kHz/wire
0.5 MHz/HAPD
4 MHz

1.8 MHz/PMT

layers of the vertex detector. However, in the case of a large increase, SR may cause
inhomogeneities in PXD module irradiation, which would make it more difficult tocompensate by adjusting the operation voltages of the affected modules.
HER SyncRad BG

0.04
0.03

20
0

PXD L10

QCS-FWD-225

BP-FWD-325

QCS-BWD-315

0.01
0.005

0.01
0

0.015

0

20

0.18

18

0.16
0.14
0.12
0.1
0.08
0.06

16
14
12
10
8
6

1.8

0.45

1.6

0.4

Photon rate [MHz/HAPD]

40

0.05

0.02

0.02

2

2
0

4

60

0.06

0.025

22

0.2

1.4
1.2
1
0.8
0.6
0.4

6

0.35

5

0.3
0.25
0.2
0.15

4
3
2

0.1

0.04

4

0.02

2

0.2

0.05

0

0

0

0

TOP ALD

4

80

0.07

PXD L2

6

6

100

0.08

Mean occupancy [%]88Mean occupancy [%]10

Dose rate [mrad/s]

12

Dose rate [mrad/s]

Dose rate [mrad/s]

10

Mean occupancy [%]

0.03

0.22

CDC

14

0.09

Hit rate [kHz/wire]

120

SVD L30.035

12Pedestal (Noise Floor)

10Barrel KLM L3

Luminosity BG

HER Injection BG

Hit rate [MHz]LER Injection BG

HER Touschek BG

ARICH

LER Touschek BG

HER Beam-gas BG

Hit rate [MHz/PMT]LER Beam-gas BG

Figure 3.1: Extrapolated Belle II backgrounds from measurements in June 2021. Eachcolumn is a stacked histogram. From [10].

510The high injection background observed in 2022b led to a visible degradation of
CDC performance shortly after injection, caused by a voltage drop in the HV resistor
chain and space charge effects.
Although currently at an acceptable level, some detectors started to see singleevent upsets (SEU) of FPGAs electronics boards in 2021 and 2022. These SEUs arepresumably from neutrons created in EM showers. Therefore, during LS1, we plan to
introduce an additional Belle II shielding against neutrons and EM showers originatingfrom the accelerator tunnel and final focusing magnets [44].
38

5

10

15

20

25

30

35

40

During the same running period, studies revealed single-beam and single-bunch
instabilities that cause vertical emittance blow-up in the LER. This occurred at lower
bunch currents (∼ 1 mA/bunch) than predicted by a standard TMCI theory (∼ 2 mA/bunch),
limiting the bunch current at the time. Machine studies showed that the instability
threshold relates to the collimator settings and the vertical beam size inside the QCS
beam pipe structure. This observation implies that the beam instability observed in
the LER is not simply a TMCI phenomenon and might force us to eventually open collimators at higher beam currents and accept higher detector backgrounds. Moreover,dedicated studies held in 2022 show that the bunch current threshold before the instabilities occur also depends on the interplay between the machine impedance, which ismainly dictated by collimator settings and the IR beam pipe aperture, and the bunchby-bunch feedback system [45] adjusted to dump the frequency of the 0 mode (verticalbetatron tune: νy ), while the vertical beam size blow-up is attributed to oscillations
caused by the –1 mode (νy − νs , where νs is a synchrotron tune) [46].
In order to reach the luminosity of 4.7 × 1034 cm−2 s−1 in 2022, we gradually increased beam currents above 1 A. This consequently induced more frequent sudden
beam losses (SBLs) in both rings. The SBLs usually appeared at the bunch current
above 0.7 mA/bunch, limiting the maximum beam currents and hence the collision
luminosity. Some of the larger beam loss events caused severe damage to collimators, which made it challenging to effectively control beam backgrounds in Belle II.
Moreover, some events also induced high radiation doses in the IR, damaging 4–5 % ofswitcher channels in the PXD detector and causing QCS quenches.
Figure 3.2 illustrates the impact of the damaged horizontal collimator in the LER(D06H3) on Belle II backgrounds. The collimator surfaces closest to the beam weredamaged due to the injection kicker failure steering some part of the main beam horizontally toward the first horizontal collimator D06H3, which is located about 1.2 km
from the IP. By increasing the collimator aperture (black, solid line), which is definedas the distance between collimator jaws, the measured background for the most vulnerable Belle II sub-systems, CDC (red, dashed line), and TOP (blue, dot-dashed line),
dropped by about 40%.
Although the D06H3 collimator was not damaged in SBL events, Fig. 3.2 clearly
demonstrates that opening the damaged collimator helps reduce Belle II backgrounds.
Therefore, in 2022 after more than 50 SBLs, several damaged collimators were opened,
which led to a significant Belle II background increase.

3.3

Background simulation procedure

We use several dedicated Monte-Carlo (MC) simulation tools to predict beam backgrounds at different machine settings and investigate possible mitigation measures. In
addition, the prepared MC samples, stored in so-called BG overlay files, are used forsoftware training and detector performance studies.
For the simulation of beam-gas and Touschek scattering, we first use the Strategic
Accelerator Design (SAD) [25] software developed at KEK. Bunches of scattered particles (e+ or e− ) are generated at different locations around the ring and tracked for1000 machine turns. The code collects the coordinates of particles, which reach theaperture of the machine, i.e., the inner beam pipe wall or collimators. These particles
39

D06H3 aperture [mm]10D06H3 aperture
CDC BGTOP BG

9.8
9.6

1.4

90

1.3

80
70

9.4
60
9.2
50

9

1.1
1
0.9
0.8

8.8
8.6

1.2

40
18:55

19:00

19:05

TOP ALD average rate [MHz/MCP-PMT]

10.2

100

CDC average current [µA]

10.4

0.7

Time [HH:MM]

Figure 3.2: The measured on December 2, 2021 (JST) Belle II background as a function of the LER horizontal collimator aperture. The data were taken during LER singlebeam operation at the beam current of 750 mA with 1174 bunches. The periodic structure of the background observables is due to top-up injection.

5

10

15

are then considered as lost. We recently improved the simulation of how beam particles interact with collimators. We have implemented a more realistic collimator profileand now include particle scattering off collimator materials. The measured vacuum
pressure distribution around the ring is now also used to improve the beam-gas background simulation. Here, the average ring residual gas pressure at the center of thebeam pipe is defined as:P̄ = P̄0 + 3 × dP̄ /dI × I,
(3.2)
where the base pressure, P̄0 , is the average ring pressure when there is no beam (I =0 A), and the dynamic pressure, dP̄ /dI × I, is the average pressure increase per unit
current, physically caused by gas molecules being released from the inner beam pipe
walls. Both the base and dynamic pressures are estimated from vacuum pressure
measurements with cold cathode gauges (CCG) installed around each ring. The factor
3 in the ring pressure formula is used to take into account the vacuum conductance
between the beam pipe and CCGs, while the measured base pressure is assumed to be
the same as seen by the beam. Further details about the multi-turn particle tracking
in SAD for SuperKEKB can be found in Ref. [10, 47].
At the next step, the lost particles in SAD are passed to Geant4 [48, 49, 50] for fur40

5

10

15

20

25

ther simulation. We only keep particles lost near the interaction region, out to ±30 m
from the IP. The Geant4 software is embedded into the Belle II Analysis Software
Framework (basf2) [12, 13]. A realistic description of the geometry of the detector
and accelerator tunnel is used to simulate the resulting electromagnetic showers andsecondary particles produced inside and outside the detector. Energy deposits in detectors are digitized and can then be compared against dedicated single-beam andluminosity background measurements.
To produce the MC samples for the luminosity background due to radiative Bhabha
scattering and the two-photon process, we use the same Geant4-based software andgeometry, but SAD is not required.
The current accuracy of our beam background simulation is illustrated by Fig. 3.3,
which shows ratios of measured to simulated background rates, so-called Data/MC
ratios. The ratios are shown separately for different background components. These
ratios are generally within one order of magnitude from unity, except for the shown
innermost Barrel KLM RPC layer, where the simulated BG is still vastly underestimated.
This is a substantial improvement compared to the early SuperKEKB commissioning
phases in 2016 [51] and 2018 [52].
Since the neutron shielding around Belle II is not ideal, neutrons can penetrate
the detector and cause performance degradation. Our MC simulation predicts fastneutrons traveling from the accelerator tunnel towards Belle II due to i) single-beamlosses at the collimators closest to the IR (∼ 15 m from the IP) and ii) radiative Bhabha
scattering producing photons parallel to the two beams at the IP. These photons follow
the beams out of Belle II, resulting in localized hot spots in the beam pipes just outsideof the detector (∼ 10 m from the IP) [53]. However, as discussed in Ref. [44], during
LS1, we plan to install additional concrete and polyethylene shielding around Belle II,
which according to preliminary simulation, should reduce the neutron flux from theaccelerator tunnel by up to a factor of 2.

3.4
30

35

40

Background extrapolation

In this section, we discuss the Monte-Carlo simulation setup to predict beam backgrounds in Belle II at the luminosity above 3 × 1035 cm−2 s−1 , so-called after LS2 operation. The outcome of this exercise is the predicted Belle II background rate in different
sub-detectors at higher luminosity with expected detector safety factors1 .
Currently, the target instantaneous luminosity is 6.0×1035 cm−2 s−1 . It should allow
us to collect 50 ab−1 of data within the next decade. However, at the time we arepreparing this CDR, we do not have a so-called working machine lattice to reach thetarget at the design optics with βy∗ = 0.3 mm, see Table 3.2 (Target column). Moreover,the final design of the IR magnet system and beam pipes is not completed, and it isstill under discussion; see Section 2.4. According to the dedicated studies discussed
in Refs. [44, 10], the highest possible luminosity that could be achieved at the current
SuperKEKB is 2.8 × 1035 cm−2 s−1 at βy∗ = 0.6 mm, see Table 3.2 (Before LS2 column).
Despite the marginal discrepancy between the machine parameters before LS2, as
1

The safety factor is defined as a ratio between the detector limit and predicted background rate. Itshows how much the background level can increase before reaching the detector limit.

41

Syst. unc.

Beam-gas

102

10

10

ARICH

Barrel KLM L3

TOP ALDCDC

SVD L3

PXD L2PXD L1

Barrel KLM L3

ARICH

TOP ALDCDC

SVD L3

PXD L1PXD L2

10−2

BP-FWD-325

10−2

QCS-FWD-225

10−1

QCS-BWD-315

Syst. unc.

Stat. unc.

1

10−1

Luminosity

Touschek

QCS-FWD-225

1

BP-FWD-325

Stat. unc.

QCS-BWD-315

Touschek

Data/MC

Data/MC

Beam-gas102

Syst. unc.

Stat. unc.

102

Data/MC

10

1Barrel KLM L3

ARICH

TOP ALDCDC

SVD L3

PXD L2PXD L1

QCS-FWD-225

BP-FWD-325

10−2

QCS-BWD-315

10−1

Figure 3.3: Combined Data/MC ratios for LER (top, left), HER (top, right) single-beam
and Luminosity (bottom) backgrounds.

5

10

outlined in Table 3.2 proposed long ago for the detector background study, and thosedefined by the SuperKEKB team in Table 2.2 (second stage) based on the most recent machine operation experience, we maintain the perspective that these disparities
are relatively minor. We are of the opinion that these variations do not significantly
impact the outcomes of our study or alter our final conclusions. However, the useof a more aggressive scenario with L = 2.8 × 1035 cm−2 s−1 (Table 3.2) instead of
L = 2.4 × 1035 cm−2 s−1 (Table 2.2) might be considered as a conservative approach forthe detector backgrounds simulation in the CDR.
The prediction for the BG rates before LS2, with known machine configuration,
is made with the full BG simulation described before, re-scaling each BG component
with the measured average Data/MC ratios, shown in first column of Table 3.3. Belle II
background rates expected with these conditions, shown in Fig. 3.4, are high but acceptably below detector limits listed in Table 3.1.
42

Table 3.2: SuperKEKB parameters used for the background extrapolation study. β ∗ , L,
I, P̄eff. , nb , ε, σz , and CW stand for the betatron function at the IP, luminosity, beam
current, average beam pipe pressure, number of bunches, equilibrium beam emittance,bunch length, and Crab-Waist sextupoles, respectively. The numbers in columns Before
LS2 and Target are from Ref. [10], while Design column numbers are adopted from
Ref. [40].
Setup
Before LS2TargetDesign0.6/0.6
0.27/0.3
0.27/0.3βy∗ (LER/HER) [mm]βx∗ (LER/HER) [mm]
60/60
32/25
32/25
35
−2 −1
L [×10 cm s ]
2.8
6.0
8.0
I(LER/HER) [A]
2.52/1.82 2.80/2.00
3.6/2.6P̄eff. (LER/HER) [nPa]
48/17
52/18
133/133nb [bunches]
1576
1761
2500
εx (LER/HER) [nm]4.6/4.5
3.2/4.6
3.2/4.6
εy /εx (LER/HER) [%]1/1
0.27/0.28 0.27/0.28
σz (LER/HER) [mm]8.27/7.60 8.25/7.58
6.0/6.0
CW
ON
OFF
OFF

5

10

15

20

25

On the contrary, given all the mentioned uncertainties in the machine configuration after LS2 (i.e., machine-detector protection upgrade during LS1, final focusing
magnet redesign during LS2, collimator relocation, and the injection chain tuning), an
alternative solution to roughly estimate background rates in the detector was used forthis extrapolation. The background predicted for conditions before LS2 are used as astarting point and scaling factors are applied to the various components to extrapolatethem to after LS2 in the following way. In addition to Data/MC ratios, the following
three scaling factors are considered for single-beam background components: i) x2 –optimistic Scenario-1, comes from a simple scaling of all single-beam background components from before LS2 to target beam parameters (see Table 3.2) following heuristic
fit formulas discussed in Ref. [10], ii) x5 – nominal Scenario-2, and iii) x10 – conservative Scenario-3, an arbitrary factor assuming that all single-beam backgrounds
will be elevated by order of magnitude after LS2. Luminosity BG terms are insteadcorrectly simulated for the target luminosity after LS2 over the luminosity before LS2,
applying the measured Data/MC ratio.
Figure 3.5 shows background rates in the crucial sub-detectors estimated withbasf2 (prerelease-07-00-00d) for after LS2 conditions at the luminosity of 6.0×1035 cm−2 s−1 ,for the three different Scenarios described previously. Table 3.3 summarizes the various scaling factors applied to produce BG overlay files for after LS2 extrapolations
starting from the before LS2 setup.
We note that there are a number of issues related to the scaling factors listed inTable 3.3 and BG rates shown in Fig. 3.5 that must be taken into account:• Data/MC ratios
After producing BG overlay MC samples with the scaling factors listed in Table 3.3, we found a bug in the basf2 code used for the PXD occupancy calculation. This bug affected how particles interact with the detector, and it led to the
43

0.02

0.2

10

0

0

0

0.5
0

0.4
0.3
0.2
0.1
0

0.8
0.6
0.4
0.2
0

0.5
0.4
0.3
0.2
0.1
0

7
6
5
4
3
2
1
0

♦ Barrel KLM L3

20

1

0.5

1

0.6

Background rate [MHz]

30

1.5

0.6

1.2

Barrel KLM L3

0.4

40

2

0.7

8.1 +−0.9
0.7
Background rate [MHz]

0.04

0.6

50

2.5

0.8

♦ ARICH

0.06

0.8

60

3

Total BG
Syst. unc.

9.0 +−1.0
0.9
Photon rate [MHz/HAPD]

0.05

0.1
0.08

1

CDC

0.1

0.12

Hit rate [kHz/wire]

0.15

0.14

SVD L30.2

Mean occupancy [%]

0.25

70
1.2

2.8 +−0.2
0.2

ARICH

1.4

0.16

Luminosity BG
Stat. unc.

Photon rate [MHz/HAPD]

0.18

2.9 +−0.1
0.1
Background rate [MHz/PMT]

4.3 +−0.9
0.7

PXD L20.3

019.3 +−1.2
1.2

Mean occupancy [%]0.35

PXD L1

Mean occupancy [%]9.0 +−0.5
0.5

LER Touschek BG
HER Touschek BG

TOP ALDLER Beam-gas BG
HER Beam-gas BG

Figure 3.4: Estimated Belle II background composition for predicted beam parameters before LS2. Each column is a stacked histogram of BG rates from dedicated MC
samples produced with FTFP BERT HP Geant4 Physics List and scaled with average
Data/MC ratios listed in Table 3.3, except for the column marker with a ♦ symbol,
where individual Data/MC ratios from Fig. 3.3 are used instead. The red numbers inrectangles are detector safety factors, showing that Belle II should be able to operatesafely until a luminosity of 2.8 × 1035 cm−2 s−1 , with some important caveats, discussed
in the text.

5

10

15

20

overestimation of simulated BG rates used for the Data/MC analysis. In addition, there was a bug related to the heuristic fit formula used for the estimationof the measured BG components in the Belle II sub-systems. Instead of using
estimated base pressure in the ring (7–15 nPa [10]), the code used a default
value (10 nPa). Thus, fixing the mentioned bugs results in a set of new, slightly
different Data/MC ratios shown in Table 3.4, which should be used for future
campaigns of BG overlay files production. In Table 3.4, we assume the luminosity Data/MC ratio equals one based on our observation for the PXD detector
since, applying all the mentioned bugfixes here, the luminosity ratio tends to
be closer to unity. Moreover, the PXD BG is dominated by the luminosity component, while in outer sub-detectors, the relative luminosity contribution is lesssignificant. Therefore, applying the average sub-detector ratio for the luminosity
BG, which is 0.81 (before bugfixes, see Table 3.3) and 0.82 (after bugfixes, see
Ref. [10]), we could largely underestimate the PXD BG. While the conservative
ratio equal to unity only slightly affects outer detector BGs.
• Geant4 Physics List
There is a difference in calculated BG rates using MC samples produced with
Default Belle II Physics List [54], which is used for the BG overlay files production, and FTFP BERT HP Physics List which is recommended by Geant4 for collider physics applications [55] and used by the beam background group for the
Data/MC analysis [44, 10]. The origin of this difference is unknown and under44

Scenario-1

BG rates

2.9

Scenario-2

5.9

1.1

Scenario-3

1.0

1.3

3.3

2.2

10
9
8
7
6
5
4
3
2
1
0
PXD

%]
cy [

pan

ccu

L1 O

PXD

%]

cy [

pan

ccu
L2 O

SVD

]

%]
cy [

pan

ccu
L3 O

CDC

Hit

wire
Hz/

50 k
e [x
Rat
TOP

ALD

Hit

T]
PM

PD]

Hz/

2M

e [x
Rat

CH

ARI

to
pho

te
n ra

[MH
Bar

A
z/H

LM

rel K

Hz]

5M

te [x

it ra
L3 h

Figure 3.5: Estimated beam background rates in Belle II for after LS2 operation atluminosity of 6.0 × 1035 cm−2 s−1 . Solid bars represent BG overlay results with average
Data/MC ratio and default Belle II Physics List, while hatched bars for ARICH andKLM stand for the BG levels with accurate neutron physics and individual Data/MC
ratios. The histograms are not stacked. The numbers in rectangles are detector safety
factors for Scenario-2: PXD, SVD, CDC, and TOP factors are calculated using solid
bar numbers, while ARICH and KLM factors are shown as hatched bars. The units inbrackets contain scaling factors used for better visualization, fitting all bars within thesame vertical axis range; for example, the estimated nominal (green, Scenario-2) CDC
Hit Rate should be read as 4 × 50 = 200 kHz/wire.

5

10

15

investigation. This issue leads to at least an order of magnitude higher neutron
rate in the KLM for FTFP BERT HP compared to Default Belle II Physics List.
• Beam-gas BG
An additional bug in the BG overlay files production scripts was found. The codethat mixes base and dynamic components of the beam-gas BG in proportions of
1:3 (see Eq. 3.2) failed. This led to the underestimation of beam-gas BGs in
Belle II by up to a factor of 4, with the fraction of the beam-gas BG in the total
BG of 10–20 %.
The listed issues impact our estimation of beam-induced BGs in Belle II at the levelof 30%, except for the ARICH and KLM, where the effect is much stronger. Due toi) the Geant4 Physics List issues mentioned above and ii) the usage of the average
Data/MC ratios instead of individual ratios, the KLM BG rates are presumably underestimated by about two orders of magnitude in BG overlay MC files. In addition, theaverage Data/MC ratios underestimate the ARICH BG estimation by a factor of three.
Therefore, for accurate safety factor estimation, in addition to BG overlay levels shown
as solid bars in Fig. 3.4, we added BG levels in hatched bars calculated with accurate
neutron physics FTFP BERT HP and individual Data/MC ratios.
Nevertheless, the extrapolated BG rates for the three scenarios are still justifiablegiven the uncertainties associated with i) unknown machine and beam parameters at
45

Table 3.3: Scaling factors used for the BG overlay MC sample production after LS2 atthe luminosity of 6.0 × 1035 cm−2 s−1 for three Scenarios (Sc.).
Backgroundcomponent
Beam-gas LER
Beam-gas HER
Touschek LER
Touschek HER
Luminosity

5

Average
Data/MC
3.46
0.63
3.44
0.18
0.81

Single-beam scaling
Total scaling
Sc.-1 Sc.-2 Sc.-3 Sc.-1 Sc.-2 Sc.-3
2
5
10
6.92 17.30 34.60
25
10
1.26 3.15
6.30
2
5
10
6.88 17.20 34.40
25
10
0.36 0.90
1.80
1
1
1
0.81 0.81
0.81

the target luminosity and ii) the set of assumptions regarding the usage of average
Data/MC ratios (Table 3.3), instead of individual ratios (Fig. 3.3), for all detectors
in addition to empirical scaling factors for single-beam components of 2, 5, and 10.
Therefore, we use the currently available BG overlay MC samples, which correspondto BG rates shown in Fig. 3.5 at the luminosity of 6.0 × 1035 cm−2 s−1 , for the detector performance study. Moreover, we note that previously mentioned machine anddetector protection system upgrades planned for LS1 are not included in the current
background simulation, which presumably leads to overestimated detector BG rates athigher luminosity.
Table 3.4: Updated average Belle II Data/MC ratios after the basf2 code bug fixing.
Background
LER
HER
+0.92
Beam-Gas 3.94−0.74 0.59+0.25−0.18
Touschek
3.67+1.22
0.21+0.10
−0.92
−0.07
Luminosity
1.00

10

15

20

25

Comment
From Ref. [10]From Ref. [10]Assumption

According to the BG rates estimation for Scenario-2, which is a compromise between optimistic Scenario-1 and conservative Scenario-3, Belle II backgrounds for the
PXD, ARICH, and KLM are expected to stay well below detector limits (i.e., safety
factors > 1), while the SVD, CDC, and TOP are expected to be closer to the detector limits listed in Table 3.1 (i.e., safety factors ∼ 1). Although the TOP background isclose to its limit, it will not constrain the luminosity increase but shorten the lifetime of
MCP-PMTs, which in principle can be replaced during a normal summer shutdown. In
addition, even without considering possible performance degradation due to high injection background, the CDC background level is estimated to be at the detector limit,
which was defined based on Belle/KEKB operation and potentially is not applicable forBelle II/SuperKEKB operation. Therefore, we plan to study the latest MC samples toaccurately update the limit for the CDC. Moreover, we still need to investigate the injection and SR BGs at higher luminosities by developing dedicated simulation models
and conducting machine studies. Therefore, at the current stage of our understanding,we prefer to keep both injection and SR BG rates not exceeding the measured level inJune 2021 (see Fig. 3.1), which corresponds to 10–20 % and 5–10 % of the total BG,
respectively.
Furthermore, for more accurate background estimation and Belle II performance46

study at the target luminosity of 6.0 × 1035 cm−2 s−1 , the working machine lattice atβy∗ = 0.3 mm is essential. Here is a list of known shortcomings and missing ingredientsordered by importance of the future BG estimation at the target luminosity after LS2:

510

• Lack of working machine lattice at βy∗ = 0.3 mm;• Unsimulated detector shielding planned for LS1;
• Unsimulated collimation system after LS1 upgrades;• Unclear vacuum pressure evolution at higher beam doses;• Lack of accurate SR and injection background simulations.
We are working on improving Belle II background measurements and predictions
and closely collaborating with other accelerator laboratories around the globe on optimizing SuperKEKB in order to reach the target luminosity.

3.5

15

20

25

Editor: L.Vitale
In this section we discuss the potential upgrades of the radiation monitoring andbeam abort system around the interaction region. We first describe the present system,
based on 28 single crystal CVD diamond sensors and its custom read-out electronics
and briefly recall its excellent performance. We underline the lessons learned from thefirst five years of operations and notable features to be preserved in future, in view ofpossible medium and long term upgrade. Then we critically review its main limitations
and possible improvements. Finally we discuss the specifications and technical solutions for an upgraded electronics and for the sensors. The present beam abort system
has been also complemented in 2021 by the CLAW scintillator system. This systemcannot provide a steady monitoring and recording of the dose rate, but it can detectwith a very good time resolution sudden beam losses. It was thus optimized using the
diamond data to provide a faster beam abort signal. Its possible upgrade will also bementioned in the sensors part.

3.5.1

30

35

40

Radiation monitoring and beam abort system

The present system and features to be preserved

The present Belle II radiation monitoring and beam abort system is based on singlecrystal diamond sensors, grown by chemical vapour deposition (sCVD sensors), andread-out by custom-designed electronics [56, 57].
Diamond is known as an extremely radiation-resistant material since a very longtime. Among the synthetic diamonds now available, the electronic grade sCVD diamond is the one with the highest purity, presenting unique characteristics that makeit an exceptional material for radiation detectors, such as its efficiency, stability, risetime, temperature-independent response [58, 59].
Our detectors are made of electronic grade sCVD sensors with a size 4.5 × 4.5 ×
0.5 mm2 and two (Ti + Pt + Au), (100 + 120 + 250) nm electrodes, mounted on acompact Rogers printed circuit board, providing both the mechanical support and theelectrical screening, completed by a thin aluminium cover. The final package of thedetectors has a size of 18.0 × 12.0 × 3.1 mm2 .
47

Table 3.5: Selectable current-measurement ranges of diamond control units, with thecorresponding rms noise values, measured in 100 kHz data (third column) and in
10 Hz data (fifth column). The typical corresponding ranges and rms noise values
in dose-rate units can be obtained multiplying by the average calibration factor k =35 (mrad/s)/nA. The effective dynamic ranges expressed in decibel at 100 kHz and
10 Hz are also shown in the fourth and sixth columns.

5

10

15

20

25

30

Range
index

Current
range

Rms noise@100 kHz

0
1
2

36 nA
9 µA
4.5 mA

0.23 nA
3 nA
0.22 µA

dyn.range Rms noise dyn.range@100 kHz@10 Hz@10 Hz
44 dB
70 dB
86 dB

0.8 pA
70 pA
40 nA

93 dB
102 dB
101 dBFor Phase 3 a total 28 radiation detectors were mounted in several key points ofthe vertex detector and of the beam pipe (BP). In particular 8 detectors were mounted
around the cooling blocks of the BP to be as close as possible to the interaction point(IP) and at about the same radius of the PXD, 12 detectors on the SVD support cones,
and the remaining 8 detectors on the BP bellows, close to the QCS magnets.
Two coaxial cables, one for the signal current and the other for the bias voltage,
are routed for about 25 m to the diamond electronics system. The electronics consistsof seven purpose-designed modules, Diamond Control Unit (DCU). Each DCU controls
and reads out groups of four diamond detectors, moreover it provides the bias voltages
individually to the four diamonds.
The diamond currents are amplified by trans-impedance amplifiers, digitized by
a 16-bit ADC and processed by FPGAs in the DCUs. Three amplifier gain values canbe selected by resistors in the feedback loop of the front-end operational amplifier,to provide three different current measurement ranges, indexed as 0, 1, and 2 in thefollowing, as shown in Table 3.5. The analogue bandwidth at the lowest gain (range
2) is about 10 MHz, matched to large and fast signals; it is reduced to the order of 10
kHz at the highest gain (range 0) used for monitoring smaller signals at 10 Hz. Morethan 10 orders of magnitude for signal currents can be covered in the three ranges, but
the switching between ranges can be done by a slow control procedure that requires
rather long times (order of few minutes).
The electronics is located in the Electronic Hut, outside of the radiation controlledarea of the Belle II detector, and accessible even during the collisions, in case of maintenance, replacements or even in case of a full upgrade of the system itself.
This system can provide a steady monitoring of the radiation dose during initial
injection, collisions and continuous injections with a running efficiency close to 100%.
In case of sudden beam losses the system can trigger a fast beam abort signal on atime scale shorter than the revolution time (10 µs). Moreover the system records thedose rate with a 2.5 µs granularity before the abort, for post-abort analysis.
In Phase 3 run 1 (2019-2022), four of the eight detectors mounted on the beam
pipe were dedicated for the abort function, with range 2 selected in the corresponding DCU; the remaining 24 diamond detectors were dedicated to monitoring, and thecorresponding DCUs preset on the most sensitive current measurement range 0. Oc48

5

10

15

20

25

casionally four of the eight QCS bellows diamonds were used for injection and beamabort studies and their corresponding DCU set to range 1 or 2 respectively.
In this way this system provides a substantial protection not only for the vertex
detector, but for the full Belle II detector and also for some accelerator components.
The experience of the first years of running for SKB and Belle II showed that sudden
beam losses can for instance damage vertex PXD detector parts and collimator jaws.
They can also quench the super-conducting magnets of the QCS. An early and efficientbeam abort helps to prevent or at least reduce impact and the risk for such potentiallycatastrophic occurrences [57].
The compact size of this detectors and their radiation hardness at least up to integrated doses of the order of 0.1 − 0.2 MGy are well-suited for radiation monitoringin the interaction region both for the present Belle II detector and for its future upgrades; moreover their temperature-independent response allows operation without
additional temperature monitoring and related corrections, as for example needed forsilicon diodes.
In summary the notable features of the present system to be preserved also for
future upgrades are:• Fast beam abort cycle of the order of 2.5 µs or better.
• Recording of pre-abort dose rates with same time resolution.
• Large dynamic range of dose rate measurement over at least 10 orders of magnitude (now limited by the slow switch among the three available ranges).
• Flexibility, several configurable options, separate thresholds for beam abort andmasking for each channel.
• Performance of the sensors in terms of stability, fast response and linearity overa wide range of stationary and transient radiation.
• Radiation hardness of the diamond sensors to the future integrated doses withlarge margins.
• Overall reliability of the full system.

3.5.2
30

35

40

Limitations and possible improvements

As underlined in the previous section, the present system performed very well, andfor many aspects even better than in the original specifications [56]. However some
limitations have also emerged, mainly in the read-out system.
a) Dynamic range:
The DCU dynamic range (Table 3.5) is not sufficient to accommodate all needs.
In particular: (a) accelerator tuning needs measurements of dose-rates down tothe order of 1 mrad/s, with a 10% precision of about 0.1 mrad/s, corresponding
on average to about 3 pA; (b) beam losses correlated with aborts can reach orexceed 10 − 100 krad/s, corresponding to currents of the order of several milliampere; (c) in continuous injection mode, the beam-losses peak during shorttime intervals at values exceeding the average losses by more than two orders ofmagnitude. The separation of the functions of 24 detectors (6 DCUs) dedicated tomonitoring smaller currents in range 0, and four detectors (one DCU) dedicatedto generate beam aborts on large signals in range 2, is a compromise solution,
49

5

10

15

20

25

30

35

40

45

working only to some extent; in particular, injection spikes still saturate range 0.
A wider dynamic range would be desirable.
b) Common mode noise spikes:
The noise quoted in Table 3.5 does not take into account some high-frequency
common-mode noise, not completely filtered out by averaging the 50 MHz ADC
data at 10 Hz. The origin of this effect has been studied; its mitigation has been
discussed here [60].
c) Memory read:
The reading of the DCU internal buffer memory, containing the 400 kHz data, isdesigned as a memory dump after the arrival of an abort signal from SuperKEKB.
The abort “freezes” the memory by stopping the increase of the memory pointer,and allows the subsequent read-out of the appropriate memory section by EPICS.
This mechanism is too rigid and has some drawbacks. During the readout timeinterval the abort function (computation of moving sums and comparison withabort thresholds) is suspended: memory read and abort are incompatible. This is
not a problem after simultaneous aborts of both beams, as those generated now
by the diamond-based system. However it makes it impossible, for VXD detector
safety reasons, to read the memory after a single-beam abort or during normal
operations with circulating beams, while periodically obtaining the informationof dose-rates from diamond detectors with full 2.5 µs time resolution might berelevant (for instance to study the injection losses with such time resolution, or
during background studies).
d) Synchronization:
The synchronization with SuperKEKB is limited to the exchange of abort signals:
abort requests from DCUs to SuperKEKB and the abort confirmation timing signals from SuperKEKB. A rough alignment of 400 kHz data recorded by the DCUs
with SuperKEKB data and events is only possible through the SuperKEKB abort
timing signal. A tighter synchronization using an accelerator clock and some sortof time stamps from SuperKEKB had been considered as possible part of the initial specifications, but was abandoned due to lack of feedback from SuperKEKB
and for the sake of simplicity. For the future, at least an additional timing signal
from SuperKEKB injection should be considered. In this case, a precision of theorder of 100 ns should be sufficient.
e) Injection-related dose rates:
As mentioned previously, injection-related dose-rate peaks are clearly visible inthe 400 kHz data, read out after aborts or for some special studies of range 0
saturation effects. The time resolution of 2.5 µs is largely sufficient to observe
the beam-loss increase and oscillation patterns following injection, studied by
CLAWS in Phase 1 [61]. Upgraded electronics should be able both to avoid
saturation and to integrate the radiation dose due to injection separately, using
SuperKEKB injection timing signals as time reference, for proper monitoring of
injection quality.
f) HV control:
The high voltage (HV) modules included in the DCUs are quite stable and reliable. However an internal reading of the output HV value is not available atpresent; a manual measurement of the voltage at the output connector is needed
50

5

10

to cross-check the voltage setting. In one occasion, an EPICS software bug resulted in a wrong HV setting for a substantial period of time, during which diamond detector signals were reduced by about a factor two. Recorded data could
be corrected for this effect afterwards, but clearly a better protection against
such accidents is needed.
g) External logics:
The type conversion and distribution of SuperKEKB abort signals to DCUs and the
fan-out of DCU abort requests to SuperKEKB, CLAWS and PXD are performed by
NIM modules hosted in a NIM crate. This solution is simple, flexible and reliable,
but the availability of spares for the NIM crate and modules might be questioned
in the long term.

3.5.3

15

20

25

30

35

40

Specifications for the new electronics

To avoid confusion deriving from different calibration factors for the diamond detectors, we will use current units everywhere, keeping in mind the average conversion
factor 35 (mrad/s)/nA to dose rate units, and the relative variation of individual factors of up to ±50% with respect to the average.
The new electronics for the readout and control of diamond detectors will have thesame functions as the DCUs, in particular: (1) individual HV control for the diamonddetectors; (2) amplification and digitization of current (dose-rate) signals; (3) integration and recording of charge (dose) samples in a ring buffer memory at 400 kHz;(4) computation of moving sums of programmable numbers for the samples recorded
in memory; (5) comparison with abort thresholds to generate logical signals; (6) generation of abort requests, using programmable logical combinations of these logicalsignals, and programmable masks; (7) read-out of ring buffer memory; (8) computation of 10 Hz monitoring data from sums of samples recorded in memory, and theirread-out; (9) initialization of parameters such as abort thresholds etc.
The DCU limitations listed in the previous sections will be overcome by complying with the requirements listed here; the corresponding numbers are summarized inTable 3.6.
The crucial reliability of this system requires the introduction of self-test featuresin the electronics, generating a SuperKEKB interlock (beam abort request) if faults aredetected.
The present method of generating two beam abort request signals for HER and LER(computation of moving sums from 400 kHz data, comparison with abort thresholds,
channel masking and combinatorial logics) has proven to be adequate and does notneed to be changed or upgraded. The operating experience shows that the useful timeintervals for the computation of the moving sums do not exceed about 1 ms, corresponding to 400 buffer memory locations at 400 kHz. The main possible modification
concerns the limited dynamic range, which does not allow the use of each individual diamond detector for both monitoring (smaller signals) and beam abort (higher
signals), as explained below.
a) Dynamic range:
The total dynamic range required to accept the input current signals exceeds
about 180 dB, that is more than nine decades: from a minimum of about 3 pA,
51

Table 3.6: Summary of dynamic range parameters, assuming an average current todose-rate conversion factor k = 35 (mrad/s)/nA. Explanations are given in the text.
Parameter

value

Dose rate monitoring
Current (dose-rate) rms noise
3 pA (0.1 mrad/s)
Max. current (dose-rate)
5 mA (175 krad/s)
Total dynamic range
184 dB
Max. injection peaks
20 µA (0.7 krad/s)
Continuous injection: dynamic range
136 dB
Beam abort generation
Current (dose-rate) rms noise
0.2 µA (7 rad/s)
Max. current (dose-rate)
5 mA (175 krad/s)
Dynamic range
88 dB

5

10

15

20

25

sampling
10 Hz
10 Hz
10 Hz
10 Hz
10 Hz
400 kHz400 kHz400 kHzset by rms noise and requested precision on small signals, to about 5 mA, whichshould be measurable in the 10 Hz data. Present measurements of beam-loss
spikes corresponding to injection, extrapolated to future injection conditions,indicate that a lower dynamic range of about 136 dB would be needed to covercurrents from the minimum set by noise up to the about 20 µA (0.7 krad/s) ofprojected injection peaks. To keep the present flexibility for abort thresholds
settings, a dynamic range of at least 88 dB is needed in the 400 kHz data: fromabout 0.2 µA up to 5 mA. These numbers are summarized in Table 3.6.
b) Bandwidth and noise rejection:
The accepted signal bandwidth should be of the order of 1 MHz to match the
400 kHz data sampling in the ring buffer memory and the radiation dose integration requirements. A reduction in common-mode noise pick-up might beobtained by an optimization of cabling and of the grounding scheme. Moreover the introduction of bi-polar ranges in the sampling of the analogue currents
should be considered since in the present system, the choice of uni-polar ranges
increased the noise, due to the lack cancellation symmetric noise in the averagingof the over-sampled currents.
c) Memory read:
The internal ring buffer memory should store data at 400 kHz, and have a depth
corresponding to at least 600 ms (the present one has a depth corresponding to85 s). It must be readable at any time, without interruption of the computationof the moving sums and the generation of abort signals. This may be obtainedby writing the 400 kHz in two separate memories, one of which must be reserved
for the beam abort function. The other memory can then be read out at any time,in particular: (a) after a beam abort the reading is triggered by the SuperKEKB
abort timing confirmation; (b) for injection background studies, the reading mustbe triggered by the SuperKEKB injection timing signal (to be added as an input);(c) for other purposes, an operator should be allowed to initiate the reading of amemory section any time.

52

5

10

15

20

25

d) Synchronization:
At present only a rough synchronization with SuperKEKB is obtained by the SuperKEKB abort timing confirmation signal. At least another synchronization signal must be available as input, the SuperKEKB injection timing. This signal initiates an adjustable veto in the Belle II trigger system; the information on the vetowidth must also be available as a hardware signal or via programming of slow
controls. It should be clarified if a deeper integration with the SuperKEKB control system is needed and possible, in particular if the accelerator clock should be
used and if some time-stamp mechanism could be used to identify local eventsin the SuperKEKB context.
e) Injection monitoring:
The numerical integration of digitized charge (dose) data by the FPGA firmware
should take advantage of the improved synchronization including injection timing. In particular, the doses in the injection veto time intervals and in the complementary time intervals should be separately available.
f) HV control:
At present, all diamond detectors are biased at the same HV value: 100 V. In
principle, the requirements of separate HV adjustment for each detector might
be relaxed. However, an important feature must be added: the possibility ofreading back the bias voltage value without manually measuring it at the output
HV connector.
g) External logics
The new system design should minimize the amount of external logics needed
to interface the system with SuperKEKB and should avoid the use of NIM crates
and modules, which may pose problems for long-term availability.

3.5.4

30

35

40

Technical solutions

The basic building blocks of an upgraded system will be similar to those of the existing
electronics, described in [57, 60] (a) a digital core based on a FPGA complemented
by external memory; (b) an analog front-end with amplifiers and analog-to-digitalconversion; (c) HV modules; (d) communication interface for commands exchange,
parameters initialization, and for readout of 10 Hz and 400 kHz data.
A preliminary technical choice concerns the modularity and form factor of the system.
An approach based on some standard crate with power supply, hosting modules
that exchange signals via a back-plane, has some advantages. In particular, it may
allow the use of existing modules restricting the new design to some custom part. External cabling is minimized in this approach. It is also well suited to large systems withmany channels, where the management of repairs and spare modules may be simplified. However, the overhead of crate infrastructure for test stations may significantly
increase the cost for systems with a limited number of channels.
The approach of the present system may be advantageous for a relatively small
number of readout channels: the self-contained DCUs, each dealing with four detectors, include all the components (power supply, FPGA board, analog front-end, HV
modules, communications board). It is easy to set up a stand-alone bench for tests

53

5

10

15

and maintenance; the drawback in this case is the need of some external cabling andinterfacing to exchange signals among DCUs and between SuperKEKB and DCUs.
The major challenge in the upgrade project is undoubtedly the re-design of the analog front-end with a wider dynamic range, so R&D is primarily required to investigate a
viable solution. Some preliminary R&D work showed that the required dynamic range
can be achieved by the simultaneous introduction of a fourth range and a controlled
system of fast switching devices that commute from one range to the other in about
1 µs taking into account also noise introduced by the commutation itself. What seems
still difficult to achieve is an overall fast commutation in the lower current ranges dueto their slow rise-time for a sudden increase of current (intrinsic bandwidth limitation for the required accuracy of the lower ranges). A workaround must be found to
overcome this limitation. Also the digital part would need some preliminary R&D, inparticular the management of two memories by the FPGA, to fulfil the requirement ofcompatibility between memory-read and beam-abort.
A continuation of the collaboration with Elettra, initiated with the development
and construction of DCUs, has been established in 2022 mainly because this could
speed up the R&D and design phase, since some parts of the present design could be
re-used.

3.5.5
20

25

30

35

40

Radiation sensors

Radiation detectors made with electron grade single crystal CVD diamond sensors
have proven to be very good solution for the current systems both for the radiation
monitoring and for the beam abort functionalities. This solution seems to be adequate
for the upgrade. Moreover the present detectors can be used for a short and medium
term upgrade or even be re-used for a longer term one depending on the actual integrated dose up to the upgrade time and the expected dose for the new usage. The
present irradiation studies show no damage up to 0.1 MGy, at least with electrons andphotons.
However more studies could be useful such as:• radiation hardness studies above 0.1 MGy with different radiation sources, sincelarge beam losses can occur any time on some spots.
• calibration with steady radiation over several order of magnitudes• short and intense pulses to study the response of sensor in such transients wherelarge saturation effects may occur.
Their main drawback are the costs, the few available providers for the sensors andthe metallization, and the long calibration procedure.
If more detector would be needed for instrument other Belle II sub-detectors, theQCS or accelerator other options should be considered such as SiC or other radiation
hard materials.
For the beam abort only functionality, the CLAW scintillator system has proven to
be very effective.

54

Chapter 4
Physics Performance
Editor: A. Gaz

4.1
5

10

15

20

25

30

Physics performance motivations

The physics case of Belle II is extensively presented in Refs. [5] and [62]. It covers
a large number of precision measurements on observables of B- and D-meson and τ lepton decays that are related to fundamental parameters of the standard model andcould be sensitive to the existence of particles and couplings not envisioned by thetheory. Another part of the physics program consists of searches for exotic multiquark
states that challenge the traditional QCD models and of fundamentally new particles
that are connected to a dark sector extension of the standard model.
In order to fully complete this ambitious project and to probe new areas of researchthat might become interesting in the near future, Belle II must ensure that the detector
performance matches or exceeds the expectations drawn at the start of the experiment.
Unfortunately, the machine related background levels proved to be significantly
more severe than expected in the first years of data taking, so the focus of this section
will be mostly on the ways to ensure that the main physics goals of the experimentwill be achieved despite the harsher than anticipated data taking conditions.
From the point of view of the tracking of charged particles (see e.g. Ref. [63] for an
overview of the current implementation and performance), the CDC is the sub-detector
that has exhibited the highest sensitivity to machine related background during thefirst years of data taking. The most evident effect has been a loss of reconstruction
efficiency for the tracks that primarily rely on the CDC (most notably the pions originating from the KS0 that decay beyond the fourth layer of the VXD) due to the large
increase in chamber current during and just after injection, resulting in a voltage dropof the HV resistor and space charge effects. Other detrimental issues have been adegradation of the dE/dx measurement accuracy and loss of trigger efficiency.
We do not plan any modification of the CDC hardware in the near future (while
technically feasible, the replacement of the innermost layers of the chamber is veryrisky and will not be further considered), so the interventions to keep the tracking
efficiency high must be concentrated on:a) upgrades in the readout electronics (see Sec. 5.2);
55

5

10

b) developments for the tracking software. As of now, we are considering someideas, such as inverting the tracking chain (so that we start from the silicon
tracker and extrapolate outward to the CDC), filtering the CDC hits with small
electronic signal (and possibly add them back to the reconstructed track, to improve the dE/dx measurement), and activating a local CDC finder, to recover thereconstruction efficiency for very displaced tracks. These ideas are not mutually
exclusive and will be tested in the near future.
It will be critical in any case to monitor the background conditions and make surethat thresholds motivated by physics performance are not exceeded for long periodsof time.

Figure 4.1: Occupancies (colored bars) of the current and planned tracking devices
as a function of the level of beam background and corresponding tracking efficiencies(colored markers) for the SVD+CDC, VTX+CDC and CDC-only detector configuration.
It is possible to see that the VTX can almost completely compensate the dramatic lossof efficiency of the CDC-only case (which the current SVD can only partially mitigate).

15

20

Tracking efficiency, particularly for low momentum tracks, can be improved witha new silicon vertex detector with finer granularity both in space and in time. This
new concept would ensure higher efficiency at low momentum, better impact parameter resolution, and more importantly more robustness against machine backgrounds.
Given the limitations set by the CDC, the interplay between the gas chamber and thesilicon tracking device will be of paramount importance to maintain the tracking efficiency (at least for the charged particles that do not originate far from the interaction
region), as shown in Fig. 4.1. Advanced stage designs for such a sub-detector alreadyexist, see e.g. Sec. 5.1.3, and the replacement of the currently installed VXD detector
will be one of the priorities of the next stage of the Belle II upgrade, especially if restrictions in the detector envelope will be required by the upgrades in the accelerator.
Improvements in the vertexing capabilities do not only mean better precision on timedependent analyses of B and D mesons, but provide also handles for discriminating
56

5

10

15

20

25

30

35

40

45

among different decay topologies. Moreover, the parameters at the point of closest
approach of the tracks in the rest of the event enter the most refined flavor tagging
algorithms, giving significant contributions to their performance, see e.g. Ref. [64].
For the Particle IDentification (PID), no major hardware improvements are foreseen
for the ARICH and TOP, if we exclude regular maintenance and possible replacementof photo-detectors. Also in this area the CDC is the most vulnerable sub-detector as
in high background conditions the measurement of the specific energy loss (dE/dx)
is affected by a bias that has a dependence on the time that elapsed since the laste+ /e− bunch injection into the main ring. This effect can be partially corrected with adedicated procedure (currently under test) aimed at determining calibration constants
that depend on the time since the last injection. Further improvements in the PID
performance will be achieved thanks to a more pervasive use of machine learningtechniques applied to the reconstruction software.
For the reconstruction of the neutral particles, we do not plan any significant hardware modifications of the ECL, while we are considering an upgrade of the KLM subdetector, replacing the resistive plate chambers in the barrel section with scintillators,
adding also time of flight capabilities in the readout, so that a (10 – 20%) measurement of the momentum of the neutral particles detected in the KLM can be achieved.
While there are no serious concerns about the muon reconstruction efficiency and theperformance stability of the current detector, the upgraded version of the KLM could
achieve better KL0 reconstruction efficiency. This would be very relevant not only toincrease the signal yield of modes with KL0 ’s in the final state (most notably the eigenstates studied in time dependent CP violation analyses, such as J/ψKL0 , η ′ KL0 , etc.),
but also to improve the KL0 veto capabilities for analyses with neutrinos or other particles escaping detection in the final state (an undetected KL0 would behave like aneutrino, and thus increasing the KL0 veto efficiencies would translate in a reductionof the physics background). Examples of modes that would benefit from this include
the Belle II flagship analyses B → K (∗) ν ν̄, B → D(⋆) ℓν, B → τ ν, and inclusive |Vub |measurements.
In Sec. 4.2 we will discuss the expected improvements to key physics performanceparameters that we foresee from some of the upgrade scenarios that are currently
under consideration.
One of the areas in which Belle II needs to quickly find strategies to improve performance is that of trigger. With the current background conditions and luminosity scenarios, the experiment might find itself in the difficult position of having tosacrifice bandwidth for important physics channels, first of all those characterized
by low-multiplicity final states (very few tracks/photons with possibly missing energy/momentum and/or displaced vertices). Under extreme background conditions
even some low charged-multiplicity BB events (e.g. those in which one B 0 decays
to π 0 π 0 ) will suffer from reduced trigger efficiency. Short-term measures will allow
us to survive for a while longer, but for the long term we must radically change ourapproach. In Sec. 4.3 we present examples of radically new approaches, in whichmachine learning is expected to play a decisive role.
In Sec. 4.4 we will conclude this chapter presenting the improvements in the performance that we expect in some key areas of our Physics program.

57

4.2

5

10

15

For the VTX upgrade, we study the performance of the upgrade geometries, and compare them to that of the current Belle II detector, utilizing a full Geant4 simulation of
the detector in the study of specific decay modes of interest. This approach, coupledwith the state-of-the-art predictions of machine backgrounds presented in Chapter 3.1,
ensures that all the effects and correlations are properly taken into account (with the
exception of possible correlated losses due to the unsimulated injection background).
We investigate two different VTX geometries, which differ only in the location of
the third layer. The nominal geometry has the third layer placed at a distance of
3.9 cm from the interaction point (IP), and is expected to maximize the track impact
parameter resolution, while in the alternative geometry, the third layer is displaced at
6.9 cm from the IP, in order to improve the KS0 reconstruction efficiency. The results
are presented in Sec. 4.2.1-4.2.4.
For the KLM upgrade scenario, we do not have yet a full Geant4 geometry implemented in the Belle II software framework, thus we will limit ourselves to somequalitative statements in Sec. 4.2.5.

4.2.1
20

25

Key detector performance parameters for different
options

Low momentum tracking efficiency and impact parameter
resolution

One of the areas in which we see a striking improvement of the VTX upgrade scenarios
over the nominal Belle II detector is the tracking of low-momentum particles, mostnotably the soft pions from the decays of D∗± mesons.
We study the low momentum tracking efficiency by reconstructing the decay chain0 −00
and D → K + π − or K + π − π + π − . We report
B → D∗− ℓ+ ν, with D∗− → D πsoft
00here the results only for the D → K + π − decay mode, the performance with D →K + π − π + π − is perfectly consistent.
Table 4.1: Summary of the reconstruction efficiency and purity for the B 0 → D∗− ℓ+ ν0 −0decay chain, with D∗− → D πsoft
and D → K + π − , for the nominal Belle II geometry
with the intermediate v2 background scenario and the nominal VTX configuration in
all three background scenarios.

Generated events
Correctly reconstructed signal
Self-crossfeed
Efficiency
PurityBelle II (v2)
32533
10059
28495
30.9%
26.1%

VTX (v1)
32559
16913
51375
51.9%
24.8%

VTX (v2)
32559
16848
51826
51.7%
24.5%

VTX (v3)
30255
15583
47527
51.5%
24.7%

Table 4.1 shows the yields for correctly reconstructed signal and self-crossfeed (i.e.
58

5

10

15

signal events in which some of the particles assigned to the signal candidate are actually decay products of the other B meson in the event) after the application of minimal
analysis cuts on the quality of the reconstructed tracks, masses of the resonances involved, and overall vertex probability of the reconstructed chain. The nominal Belle II
geometry with intermediate v2 beam backgrounds scenarios is compared to the nominal VTX configuration (the one with Layer3 closest to the beam line) in all threebackground hypotheses.
We see a remarkable improvement (of almost a factor 1.7) in the reconstruction
efficiency for the VTX geometry over nominal Belle II, with comparable purity (definedas the ratio between the number of correctly reconstructed signal events and the total
number of candidates). It is also apparent that the performance is very stable against
the increasing background conditions.
The much better reconstruction efficiency is mostly due to the enhanced tracking−
efficiency for the πsoft
candidates, as can be seen in Fig. 4.2, where we see a dramatic
improvement for transverse momenta below 0.05 GeV/c and only for pT ≳ 0.2 GeV/c
we see the reconstruction efficiency of the nominal Belle II detector approach that of
the VTX upgrade scenario.
Kπ - Efficiency vs true θπT,πsoft

Efficiency

Efficiency

Kπ - Efficiency vs true pMCnominal Belle II (v2)
nominal VTX (v1)nominal VTX (v2)
nominal VTX (v3)

1

0.5

20
1.5
1
0.5
00

soft

MC
nominal Belle II (v2)
nominal VTX (v1)nominal VTX (v2)
nominal VTX (v3)

1

0.5

0.05

0.1
0.15
Belle II (v2) / VTX (v2)

0.05

0.1
0.15
pT (πsoft) [GeV/c]

0.2

True pT,π
0.2

0.25

20
1.5
1
0.5
00

[GeV/c]
soft

0.25

20

40

60

80
100 120 140
Belle II (v2) / VTX (v2)

20

40

60

80
100 120θ (π ) [deg]

160

180

True θπ [°]soft
140 160 180

soft

Figure 4.2: Reconstruction efficiency of B 0 → D∗− ℓ+ ν as a function of the transverse
0 −−momentum of the πsoft
decay (left plot) and cosine of the polar
from the D∗− → D πsoft−
angle of the πsoft track (right). We compare the performance of the nominal Belle IIdetector (black dots) with medium v2 background and the nominal VTX configurationin the optimistic v1 background scenario (green squares), medium v2 (blue upward
pointing triangles), and pessimistic v3 (red downward pointing triangles). The shaded−
blue histograms display the momentum spectrum of the πsoft. The bottom plots show
the ratio between nominal Belle II and nominal VTX in the medium v2 background
scenario.

20

The improvement in the tracking quality of the soft pions can also be seen from
Fig.4.3, where we compare the width of the distributions of the residuals of d0 , thesigned distance from the z axis of the Point Of Closest Approach (POCA), and z0 , thez-coordinate of the POCA. The VTX geometry improves by a factor of up to ∼ 4 overthe nominal Belle II and does not show any significant sensitivity to the backgroundconditions.

59

Kπ - z0,πresidualsoftMCnominal Belle II (v2)
nominal VTX (v1)nominal VTX (v2)
nominal VTX (v3)3

10residualsoft

σ68 [µ m]

σ68 [µ m]

Kπ - d0,πMCnominal Belle II (v2)
nominal VTX (v1)nominal VTX (v2)
nominal VTX (v3)

3

10

102

0
4

102

0.05

0.1
0.15
Belle II (v2) / VTX (v2)

0.2

0.25

0
4

2
0
0

0.05

0.1
0.15
Belle II (v2) / VTX (v2)

0.2

0.05

0.1
0.15
pT (πsoft) [GeV/c]

0.2

0.25

2

pT,π
0.05

0.1
0.15
pT (πsoft) [GeV/c]

0.2[GeV/c]
soft

00

0.25

pT,π

[GeV/c]soft

0.25

Figure 4.3: Width (σ68 ) of the distribution of the residuals of d0 (left plot) and z0−(right) as a function of the πsoft
momentum. We compare the performance of the nominal Belle II detector (black dots) with medium v2 background and the nominal VTX
configuration in the optimistic v1 background scenario (green squares), medium v2(blue upward pointing triangles), and pessimistic v3 (red downward pointing trian−gles). The shaded blue histograms display the momentum spectrum of the πsoft. Thebottom plots show the ratio between nominal Belle II and nominal VTX in the medium
v2 background scenario.

4.2.2

5

For tracks with transverse momentum greater than ∼ 0.3 GeV/c, the momentum resolution is dominated by the CDC, so it is not surprising that no appreciable improvementin the track momentum resolution is observed for the daughter leptons of the J/ψ inthe above mentioned sample.
A moderate improvement (a few %) is seen for the momentum resolution of the0
KS candidates, for the VTX upgrade scenarios, mostly due to the improved vertexing
capabilities.

4.2.3
10

15

20

Momentum resolution

Vertexing resolution

We study the vertexing performance of the upgrade scenarios by analyzing samples
of one million B 0 → J/ψKS0 events generated and reconstructed with all the possiblecombinations of detector geometries and background scenarios.
Figure 4.4 shows the distribution of the decay vertex resolution σz = σ(zmeas −ztrue ) (the width of the distribution obtained by taking the difference between themeasured and the true positions) along the z axis of the signal B decay for the threedetector configurations in the intermediate background scenario and for the nominal
VTX geometry in the three background scenarios. The results are summarized also in
Table 4.2 and show that the upgraded geometries achieve a ∼ 35% better resolution
on the B decay vertex and, contrary to the nominal Belle II detector, they do not show
any significant degradation as a function of the background conditions.
Similar results (not shown) are obtained for the vertexing resolution along the x
and y directions: the VTX upgrade guarantees significantly better performance and isalmost insensitive to the levels of machine background.
60

nominal VTX
alternative VTX

0.15
0.15
0.1

0.1
0.1

0.1
0.1

0.05

0.05

0.05
0.05

0.05
0.05

0
−0.01
−0.004 −0.005−0.002
Ratios(a. u.)

nominal Belle II02
nominal VTX / Belle II
alternative VTX / Belle II

1

0
−0.01
−0.004 −0.005−0.002

0

(v1)
(v2)
(v3)

0
−0.01
−0.004 −0.005−0.0020.002
0.005 0.004 0.01
Bsig σzyx [cm]Ratio wrt (v1)

(a. u.)

0.15
0.15
0.1

0.002
0.005 0.004 0.01
Bsig σzyx [cm]1(v2) / (v1)(v3) / (v1)

0.5
−0.01
−0.004 −0.005−0.0020.002
0.005 0.004 0.01
Bsig σyxz [cm]

0

0

0.002
0.005 0.004 0.01
Bsig σyxz [cm]Figure 4.4: Left figure: comparison of the B decay vertex resolution along the z axis in
B 0 → J/ψKS0 events for the nominal Belle II detector (solid line), nominal VTX geometry (dotted line) and alternative VTX geometry (filled grey histogram). The bottom
plot shows the ratio between the VTX geometries (empty squares for the nominal andfilled triangles for the alternative) and nominal Belle II. Right figure: B decay vertex
resolution along the z axis for the nominal VTX geometry in the three background scenarios: optimistic v1 scenario (green solid line), intermediate v2 scenario (blue dottedline), and pessimistic v3 scenario (red filled histogram). The bottom plot shows theratio between the two higher background scenarios and the optimistic one.

Table 4.2: Signal B vertex resolution along the z axis (in µm) for the three detector
geometries and three background scenarios investigated.
Bsig z vertex resolution (µm)
Belle II
Nominal VTX
Alternative VTX

Bkg (v1)
21.9
14.5
14.4

61

Bkg (v2)
23.0
14.4
14.3

Bkg (v3)
24.9
14.1
14.0

(a. u.)(a. u.)

5

Analogous studies for the KS0 decay vertex resolution are displayed in Fig. 4.5. Alsoin this case, the upgraded geometries show a better vertexing resolution compared to
the nominal Belle II detector and do not appear to suffer any degradation in performance due to the increasing machine background. The apparent improvement of thevertexing resolution with the harsher background scenarios that is visible in the rightplot of Fig. 4.5 is a spurious effect that is due to the loss of reconstruction efficiency
for candidates with large flight distance (that are thus affected by poorer vertex resolution) in the higher background scenarios.

nominal Belle II

0.2

nominal VTX

20000

(v1)
(v2)
(v3)

0.2

alternative VTX

0.15
0.15
0.1
10000

0.1

0.05

0.05

0 −0.02

0

−0.01
10
0
20
0.01
0.02 30
KS transverse flight distanceKS σz z [cm]2

Ratio wrt (v1)

Ratios0

nominal VTX / Belle II
alternative VTX / Belle II10

0 −0.02

−0.01
10

−0.01

0

0.01

0.02
KS σz [cm]

0.01

0.02
KS σz [cm]1(v2) / (v1)(v3) / (v1)

0.5

0
20
0.01
0.02 30
KS transverse flight distanceKS σz z [cm]

−0.02

−0.02

−0.01

0

Figure 4.5: Left figure: comparison of the KS0 decay vertex resolution along the zaxis in B 0 → J/ψKS0 events for the nominal Belle II detector (solid line), nominal
VTX geometry (dotted line) and alternative VTX geometry (filled grey histogram).
The bottom plot shows the ratio between the VTX geometries (empty squares for thenominal and filled triangles for the alternative) and nominal Belle II. Right figure:KS0 decay vertex resolution along the z axis for the nominal VTX geometry in thethree background scenarios: optimistic v1 scenario (green solid line), intermediate v2
scenario (blue dotted line), and pessimistic v3 scenario (red filled histogram). The
bottom plot shows the ratio between the two higher background scenarios and the
optimistic one.

4.2.4
10

KS0 reconstruction efficiency

We study the KS0 reconstruction efficiency using the B 0 → J/ψKS0 samples defined
above. Figure 4.6 shows the distributions of the measured flight length, projected onthe (x, y) plane, of the KS0 candidates reconstructed with the three different detector
configurations under study. We only consider candidates that are matched with thegenerated KS0 particles.
62

5Standalone VXD/VTX tracks require at least 2.5 3-dimensional hits (a hit on eachside of a double-sided sensor counts as 0.5) in the detector: due to this requirement theefficiency rapidly falls as a function of the transverse flight distance. The nominal VTX
geometry has a more pronounced falloff (compared to the others) in correspondenceof the position of the third layer. For the alternative geometry, a similar feature appears
a few cm further from the z axis of the detector (and thus the overall impact on theefficiency is smaller). Beyond ∼ 10 cm, the KS0 reconstruction is dominated by the
CDC, so no significant differences between the geometries can be seen.

nominal Belle IInominal VTX

20000

alternative VTX

10000

Ratios

0

0

10

20
30
KS transverse flight distance [cm]1
nominal VTX / Belle II
alternative VTX / Belle II

0.5
0

10

20
30
KS transverse flight distance [cm]Figure 4.6: Distributions of the flight length, projected on the (x, y) plane, of KS0 candidates from B 0 → J/ψKS0 events reconstructed with the nominal Belle II detector (solidline), nominal VTX geometry (dotted line) and alternative VTX geometry (filled grey
histogram). The bottom panel shows the ratio between the nominal (empty squares)or alternative (black triangles) VTX geometry and the nominal Belle II configuration.
The intermediate (v2) background hypothesis has been utilized for these plots.

10

15

Table 4.3 summarizes the reconstructed yields in all the investigated configurations. Besides some common selection criteria on the properties of the B and J/ψ
candidates, we require that the three-dimensional flight length of the KS0 candites begreater than 1 cm: this is applied to reduce the contribution from random combinations of tracks giving an invariant mass similar to that of the KS0 . Furthermore, inorder to make the comparison more meaningful, we require that the projection of theflight length on the (x, y) plane be less than 14 cm, so that the decay vertex of the KS0
candidate is within the volume of the silicon vertex detector.
The nominal Belle II configuration shows the best performance in terms of overall
reconstruction efficiency and also in terms of resilience to the increasing background63

conditions. The alternative VTX geometry, having one less layer compared to the current VXD detector, shows only slightly worse resilience against beam background andexhibits a ∼ 5% lower reconstruction efficiency. Significantly worse results are obtained with the nominal VTX geometry, especially in the higher background scenarios.
Table 4.3: Number of reconstructed KS0 candidates for the three detector geometries
and background scenarios that we are considering. For each of the nine configurations,
we report the yield obtained by requiring that the three-dimensional flight length begreater than 1 cm (this is typically done to reduce the combinatorics) and by requiring in addition that the flight length in the transverse plane be less than 14 cm (torestrict to the candidates that decay within the volume of the silicon vertex detector).
The percentages in parentheses under the yields in the last two columns indicate theloss of efficiency of the intermediate (v2) and pessimistic (v3) background hypotheses
compared to the optimistic (v1) scenario.
BG (v1) BG (v2)
Nominal Belle II
flLen > 1 cm
272 845 243 396(-10.8%)flLenxy < 14 cm 237 021 217 761(-8.1%)
Nominal VTX
flLen > 1 cm
254 357 222 243(-12.6%)flLenxy < 14 cm 219 103 196 974(-10.0%)
Alternative VTX
flLen > 1 cm
259 563 230 514(-11.2%)flLenxy < 14 cm 225 559 206 002(-8.7%)

5

10

15

4.2.5

BG (v3)
181 395(-33.5%)
169 687(-28.4%)154 258(-39.4%)143 109(-34.7%)
171 998(-33.7%)160 890(-28.7%)

KL0 reconstruction efficiency

Neutral K particles enter many channels of the Belle II Physics program. Most channels containing one K 0 in the final state can be studied by reconstructing only K 0 →KS0 → ππ candidates and then assuming that the same results hold true for K 0 → KL0 .
Notable exceptions are the channels in which we have multiple K 0 ’s in the final
state (where we cannot assume that e.g. KS0 KL0 X behaves as KS0 KS0 X) and channels utilized in analyses of time-dependent CP violation, for example B 0 → J/ψK 0 ,which are still limited by statistical power and thus the inclusion of KL0 ’s candidates,
which typically account for ∼ 30% of the total yield, improves the precision on the CP
violation parameters. The intended KLM upgrade is expected to improve the reconstruction efficiency and will also allow for a measurement of the KL0 momentum with
64

5

10

15

20

25

30

35

40

a resolution of ∼ 13% for the momentum range relevant for CP violation measurements. Traditional B-factory analyses of final states containing KL0 ’s apply a kinematicconstraint (for example imposing that the beam-constrained mass of the candidate coincides with the mass of the B meson, or that the difference between the measured
and expected energy of the B candidate ∆E is null) to supply for the missing information on the KL0 momentum. This kinematic constraint allows to obtain a narrow signal
peak over a much broader background component. The additional constraint derivingfrom the direct measurement of the KL0 momentum will allow to better characterize
the signal component and to significantly reduce the combinatorial backgrounds, but
the final impact on the sensitivity to the CP violation parameter will likely be small.
More promising results could be obtained in analyses in which KL0 ’s are produced
as part of a background process that must be vetoed. These analyses typically target
final states with one or more neutrinos in the final state, and are thus among the golden
channels of the Belle II physics program. Events with unreconstructed KL0 ’s (that would
mimic the behavior of a neutrino) constitute one of the major sources of backgroundin these studies.
Example analyses include the inclusive measurement of |Vub |, in which decays originating from b → c → KL0 transitions must be kept under control. In Belle and BaBar
analyses (see e.g. [65] and [66]) uncertainties related to the KL0 reconstruction efficiency are among the leading sources of systematic errors. Another class of analyses
that might benefit from increased KL0 veto capabilities is that of (semi)tauonic B decays, such as B → τ ν or B → D(∗) τ ν. The Belle analysis [67] explicitly applies a KL0veto, which results in a 7.3% uncertainty out of 14.7% total systematics.
Also in the B → K (∗) νν and D → πνν analyses, the ability to veto KL0 candidates
is critical to control peaking background. In the recent B + → K + νν analysis [68]from Belle II a sizable fraction of the backgrounds, for both the inclusive-tag andthe hadronic-tag approaches, contains KL0 ’s in the final state. Moreover, the B + →K + KL0 KL0 process, is very difficult to distinguish from the expected standard model
signal in case both KL0 ’s are unreconstructed. For all the analyses mentioned above,
having an increased KL0 detection efficiency would not only reduce the backgroundcontamination, but it would allow also to gather higher-statistics and more reliabledata control samples, further reducing our systematic errors.
A higher KLM efficiency will also translate into more effective photon/neutron veto,which is currently a limiting factor for some dark sector searches, most notably thesearch for dark photons in the single photon final state, which is severely limited by
the inadequate γ-veto capabilities of the current detector. Searches for specific dark
photon final states with long-lived particles that can be detected in the KLM and whose
momentum can be measured with time-of-flight capabilities would also benefit froman upgraded detector.
At the moment we do not have a full simulation of the Belle II detector incorporating the upgraded KLM, thus we cannot provide quantitative statements on the
expected improvement on the physics reach in the channels mentioned above.

65

5

10

15

20

25

30

35

40

4.3

Trigger

4.3.1

Machine learning

Editor: T.Ferber
The existing Belle II trigger system mostly relies on conventional reconstruction algorithms that utilize local information and rather simple algorithms to fit the hardware
constraints of currently used FPGAs. One notable exception is the NeuroZ track trigger
at Belle II that uses conventionally constructed track segments as inputs for a simple
fully-connected neural network to infer transverse momentum and z-impact parameter. With the advent of significantly more powerful FPGAs and more tools to simplify
the translation of Python-level ML solutions to FPGAs, more complicated network architectures and higher dimensional inputs are of interest for real-time applications.
Track reconstruction is a crucial part of the L1 trigger in the Belle II experiment.
Track-finding algorithms are designed to balance the requirements of high efficiency infinding charged particles with good track parameter resolution, a low rate of spurious
tracks, and reasonable demand of resources. We started development of a complete
Graph Neural Network track finding and fitting algorithm. A graph representation of
CDC detector hits, including the 2D position, timing information and pulse height, isused to find tracks and infer their track parameters. In order to identify the varying
number of tracks per event, we use GNN-based object condensation for track finding. While in principle the inference of the algorithm aims at low latency single-step
inference using for example GravNet layers, the large number of CDC-hits per event
prevents the building of a single event-wide graph. Instead, we plan to first run acleaning step using smaller GNNs that utilize local graphs in 2-3 sectors per superlayer
on about 20 individual UT4 FPGAs in parallel (see Fig. 4.7). For simulated beam backgrounds for final luminosity conditions, we find that 99% of all generic BB events
contain less than 300 hits per sector. These GNNs will classify hits into signal- andbackground-like and reject background-like hits based on locally built graphs usingedge classifiers. This clean-up step will also be responsible to provide data that israther robust against changes of beam background conditions for the final tracking
step. The cleaned output is passed to a single large FPGA on which a global graphis built, which is then passed to a GNN with object condensation loss [69] for trackfinding and parameter inference. We have demonstrated that real-time graph building
on FPGAs is possible within the existing throughput and latency requirements of theBelle II experiment’s hardware-based trigger system for the CDC [70]. First studies
show that the large field-of-view of a global GNN provides superior performance forsignatures with shallow track angles, tracks with low transverse momentum, overlapping tracks, and tracks originating from displaced vertices [71]. A full implementation
requires a co-design of graph building, cleanup GNN, and final track finding GNN to
optimize efficiency and resolution metrics. The current trigger system does not provide
pulse height information which proved powerful for background rejection. Conceptually, it is also possible to include a vertexing step by changing the object condensation
loss targets.
ECL clustering is another area where GNNs can be used to exploit information fromlarge fields of view. The main interests for possible improvements are better spatial

66Figure 4.7: Schematic representation of the Graph Neural Network based track finding
and fitting algorithm.

5

10

15

20

25

resolution, separation of overlapping clusters, and low energy cluster identification.
The current ECL trigger algorithm is based on so-called trigger cells (TCs) that contain about 4x4 crystals, depending on their location. The energy of all TC crystals
is combined into an analog sum and read out if it exceeds a threshold of 100 MeV.
The actual clustering algorithm combines TCs into clusters but is not able to separateneighbouring TCs into separate clusters or provide actual spatial reconstruction. For
simulated beam backgrounds for final luminosity conditions, we find that 99% of all
generic BB events contain less than 89 TCs. This number is small enough to directly
feed it into a global GNN: a graph representation of ECL TCs, including the 2D position, timing information and energy, is used to find clusters and infer their energy
and cluster centroid position. In order to identify the varying number of clusters per
event, we use GNN-based object condensation. First studies show comparable efficiency and fake rate for photons with E > 100 MeV, but a better energy and position
resolution [72]. The latter can be used to improve Bhabha-vetoes that rely on angulardeviations from back-to-back topologies of ECL clusters. Using the full informationof nearby energy depositions, we have shown that local GNNs are able to separate
overlapping energy depositions [73] that are a signature of axionlike particles (ALPs)
or high energy neutral pions even in very high background environments. In particular the ALPs a produced in ee → γa(→ γγ) are reconstructed as ee → γγ eventsfor low mass ALPs and are prescaled on trigger level if the decay photons are not isolated. In addition to replacing the current clustering algorithm with a GNN approach,
two hardware upgrades are promising to provide a robust calorimeter trigger at highbeam background levels: first, the readout threshold is limited by electronics noisethat could be improved by dedicated redesign of the Shaper-DSP boards. The GNN
performance generally improves for lower readout thresholds and shows an improved
energy and position resolution. Second, the large size of 4-by-4 crystals in each TC lim67

5

10

its the position resolution and the capability to resolve overlapping clusters. We study
a Shaper-DSP upgrade with reduced TC size of about 2-by-2 crystals with 25 MeV
readout threshold. This increases the number of input nodes for the GNN but provides
a handle to run neutral particle identification on trigger level exploiting shower shapes
as additional object condensation loss targets.
The GNNs that show the best robustness against beam backgrounds for both tracking and clustering relied on learning a latent space representation, followed by a knearest neighbour algorithm to select graph nodes of interest. kNN implementations
on FPGAs are currently not able to cope with the large number of input nodes and additional studies to either replace kNNs or to implement approximate kNNs on FPGAs
are ongoing.

4.4

15

20

25

30

35

40

Physics benchmarks

In Section 4.2, we showed that the area in which we expect the biggest impact inphysics performance is the improvement in reconstruction efficiency and quality forthe πsoft produced in the decay of charged D∗ mesons.
This improvement will be very beneficial in at least two major areas of the Belle II+
physics program. In the first place, the D∗+ → D0 πsoft
decay is the main process0
through which we reconstruct and determine the flavor (D0 or D ) of the neutral
D mesons that are utilized in measurements of the DD mixing and in searches for
mixing-induced CP violation in the charm sector, which are still very much limitedby the statistical uncertainties and for which Belle II will have unique sensitivity indecay modes containing one or more π 0 ’s. Secondly, the reconstruction of D∗ candidates is of the utmost importance for B-physics channels exploiting the Full Event
Interpretation (FEI) [74]. In these analyses, one of the two B mesons in the event isreconstructed through one of many hadronic or semileptonic decay chains. Charged
D∗ mesons feature in about ∼ 50% of the reconstructed B mesons in the tag side, thus
an improvement in the πsoft reconstruction efficiency would immediately translate in
an increased sensitivity for the channels that need FEI (most of which are still severely
limited by statistical uncertainties). These analyses typically target final states containing one or more neutrinos and are in many cases among the golden channels ofBelle II.
Significant improvements over Belle II will be seen also on the vertexing precision.
These will translate into better sensitivity to time dependent analyses in the B andcharm sectors. As an example, we report the estimated sensitivity to the time dependent CP asymmetry in the B → KS0 π + π − γ channel, which is potentially sensitive toright-handed contributions from physics beyond the standard model.
We estimate the sensitivity of the analysis by generating 1000 toy experiments
for each combination of detector geometry and background scenario, assuming an
integrated luminosity of 50 ab−1 . The critical observable for this class of analyses
is the proper decay time difference between the two B mesons in the event: ∆t =t(Bsig ) − t(Btag ), where t(Bsig ) is the decay time of the signal B candidate (the one
decaying to KS0 π + π − γ in this case) and t(Btag ) is the decay time of the other B meson
produced in the event. The decay time difference ∆t is determined from the spatial

68Figure 4.8: Left figure: ∆t resolution for B 0 → KS0 π + π − γ events for the nominal
Belle II detector (solid line), nominal VTX geometry (dotted line) and alternative VTX
geometry (filled grey histogram). The bottom plot shows the ratio between the VTX
geometries (empty squares for the nominal and filled triangles for the alternative) andnominal Belle II. Right figure: ∆t resolution for the nominal VTX geometry in thethree background scenarios: optimistic v1 scenario (green solid line), intermediate v2
scenario (blue dotted line), and pessimistic v3 scenario (red filled histogram). The
bottom plot shows the ratio between the two higher background scenarios and the
optimistic one.

69

5

10

15

displacement along the boost direction of the two decay vertices. Fig. 4.8 shows the ∆tresolution obtained for the different vertex detector geometries and same intermediate(v2) background scenario (left plot) and the resolution for the nominal VTX geometry
and different background levels (right plot). It is apparent that the performance of the
VTX is significantly superior to that of the current detector and shows little sensitivity
to the background conditions.
The events in the toy experiments are generated with a smearing on the generated
value of ∆t determined by a fit to the distributions of the residuals for each configuration; the fitting function we utilize is the sum of three gaussians. The much better∆t resolution of the VTX configurations more than compensates the small loss of efficiency in the KS0 reconstruction that we discussed in Sec. 4.2.4. The final expected
uncertainty on the time-dependent CP violation parameter S is 0.028 for the current
Belle II detector, 0.023 for the nominal VTX geometry, and 0.025 for the alternative
VTX. The differences between those results might seem unimpressive, but it is worth
noting that the current Belle II detector would need a ∼ 40% increase in the available
integrated luminosity in order to match the result of the nominal VTX upgrade, i.e.
the upgrade is estimated to be worth the equivalent of 20 ab−1 of data on the design
luminosity.

70

Chapter 5
Inner Detector
Editor: C. Marinas

5

10

15

20

25

30

5.1

Vertex Detector

5.1.1

VXD upgrade baseline

While a number of scenarios are being considered as a part of the machine upgrade
for achieving the intended final peak luminosity, several of these options result in
modifying the spatial envelope of the interaction region. This step would require
a complete overhaul of the Belle II innermost subdetectors. Preparing for a vertex
detector upgrade also in the scenario in which the interaction region is maintained
has also strong strategic value, for example to replace the current VXD in case ofa severe accident or to exploit the advantages of having a better performing innerdetector system in certain physics channels.
Among the various possibilities considered by the Belle II upgrade group, the development of a CMOS-based successor to the PXD and SVD detectors has emerged as
the most promising solution. The monolithic active pixel sensor CMOS technology has
the potential for providing higher granularity, thinner, intelligent detectors at a lower
overall cost. The monolithic CMOS option is supported as the baseline of the VXD
upgrade proposal and is implemented through the development of the OBELIX sensor
discussed in section 5.1.2.
This detector concept would improve the tracking robustness against large machine
backgrounds and allow the detector envelope to be adapted to possible changes to the
QCS systems. In addition, track reconstruction for low momentum particles and vertex position resolution are expected to improve. Other performance for physics should
stay at the level of the present detector. Quantitative estimates obtained with baseline
design described in this chapter are presented and discussed in section 4.2.
The studies presented here assume the new vertex detector would occupy the same
volume of the current VXD (between 14 mm and 135 mm in radius), although the system is being designed flexibly to adapt to any possible modifications of the machinedetector boundaries. A good particle hit spatial resolution below 15 µm is required,along with a low material budget around 0.2%X0 per layer for the inner radii and
0.8%X0 per layer for the outer radii. To ensure good long term performance the ref71erence hit rate and radiation levels used in the design of the system are the following
values for the innermost layer:

5

10

15

• Hit rate capability: 120 MHz/cm2• Total ionizing dose: 100 Mrad
• NIEL fluence: 5 × 1014 neq /cm2
The current VTX baseline design consists of five detection layers, all equipped withthe same monolithic sensor. Nevertheless, the design and integration of the layers
depends on the radius and two separate concepts have emerged. Layers below 30 mm
radius are grouped into the so-called iVTX design, while layers at outer radii follow
the oVTX configuration.
Section 5.1.2 describes the main sensor option, which relies on the CMOS technology
benefiting from a large expertise and support in the R&D community. The VTX structure, including ladder design and service concepts, is detailed in section 5.1.3. Finally,schedule and cost estimate for the baseline option are discussed respectively in 5.1.4
and sections 5.1.5.

5.1.2

20

25

OBELIX stands for Optimised BELle II monolithic pIXel sensor and is the name of
the monolithic active pixel CMOS sensor (or MAPS) proposed as the main option toequip all VTX layers. The introductory section 5.1 reviews the main requirementson the detector layer in terms of detector performance and the practical constraints
related to the integration of the sensor into detection modules. These requirements
and constraints lead to the sensor specifications, as explained in section 5.1.2.1. The
evaluation of the forerunner sensor, TJ-Monopix2, comes next in section 5.1.2.2. Thenthe sensor implementation is presented in section 5.1.2.3 along with the development
schedule.
5.1.2.1

30

35

40

OBELIX sensor

Sensor technical specifications

The sensor technical specifications are reviewed in the following order: the pixel matrix, the trigger logic and track-triggering, the single event effect tolerance, the power
dissipation, the sensor interface with the outside world and finally the sensor dimensions. All specifications are summarized in table 5.1.
The design of the pixel matrix of monolithic sensors drives several key performances. The position resolution, detection efficiency and radiation tolerance dependon the depletion and depth of the sensitive volume, the pixel size, the analogue preamplification and detection threshold settings. A spatial resolution below 15 µm typically requires a pixel pitch below 40 µm. In order to match the material budget between 0.2%X0 and 0.8%X0 per layer and taking into account support structures andcables, the sensor thickness in the fiducial volume has to be below 100 µm. Consequently the depleted sensitive thickness should be lower than 50 µm, well in line withavailable thickness of MAPS. Such thickness will lead to MIP signals on pixels as low
as few thousand electrons and calls for equivalent detection thresholds in the range
100-200 electrons to ensure detection efficiency close to 100%.
72

5

10

15

20

25

30

35

40

45

The overall hit rate capability of the sensor results from the combination of two
factors. First comes the rate of fired pixel addresses and associated timestamps thatthe read-out architecture in the pixel matrix can actually reconstruct efficiently. The
second factor is the performance of the logic in the matrix periphery that correlates intime these fired pixel with the triggers available to the sensor.
The hit rate value of 120 MHz/cm2 quoted in section 5.1.1 corresponds to an averagerate exceeding by a safety factor of 4 the expectation for the innermost layer presented
as the conservative scenario 3 in section 3.4. This average value sets the requirements
for the overall hit rate, encompassing the performance of the pixel matrix and the periphery treatment. Hence, the corresponding hit rate requirement is referred to as Hit
rate capability at the sensor output in table 5.1.
During SuperKEKB injections, short spikes of very high rates are expected. A maximum rate is presently set to be 600 MHz/cm2 for a period of 0.5 µs every 10 µs, thebeam revolution period in the collider. Only the pixel matrix should sustain this maximum value during periods where no triggers are expected, leading to a requirement
labeled as Hit rate capability in the matrix in table 5.1.
The trigger logic storing the hit information coming from the matrix in order to compare their timestamp to the arrival times of triggers, should offer a memory large
enough to contain the hit information up to a maximal trigger delay of 10 µs (extending the current limit of 5 µs) with a hit rate of 120 MHz/cm2 . In addition, the logic
should handle trigger rates up to 30 kHz, which is the unchanged maximal L1 trigger
rate set in the Belle II technical design report [3].
A key requirement for the new sensor consists in achieving a high granularity simultaneously in space and time. While the pixel pitch limit fixed previously addresses
the former, the latter leads to a maximum integration time of 100 ns from the trigger arrival time. This limit on the overall integration time actually applies to the
combination of the timestamp precision with any fluctuation over the matrix of thetimestamp reference.
Providing better position information on tracks than the main tracker (central drift
chamber, CDC) within a time useful for the trigger system would improve the Belle II
track-trigger capability. Setting precise specifications for the space-time granularityof such information is not possible without a detailed study of the trigger algorithm,
which is not yet available. As an starting point, the sensor will be divided azimuthally
into 10 segments. The information about which of these regions are hit by particles
within about 10 nanoseconds should then be continuously transmitted out of the sensor.
Tolerance to Total Ionizing Dose (TID) and Non Ionizing Energy Loss (NIEL) fluence have been discussed already. But Single Event Effects (SEE) generated by radiation might affect the sensor. Since no Single Event Latchup (SEL) were observed in
the past operation years of Belle II, no specification is given. However Single Event
Upsets (SEU) were observed and will be a concern to some extent, but are not yet
quantified. Consequently, the first specification is to ensure that the control systemallows resetting the sensor registers to operational values at least every few minutes.
The actual frequency will be chosen after the measurement of the SEU cross section73with the OBELIX sensor and the comparison to the occurrence distribution of large
linear energy transfer in the experiment.

5

10

15

20

25

30

35

40

The material budget per layer does not only depend on the sensor thickness but
mainly on the services and support structures required to operate the sensor. The
chip should then feature a pad ring facilitating the connection with a single light
cable. The sensor power dissipation is also desired to be as low as possible and constrained within 200 mW cm−2 , a value which is anticipated to allow air-cooling for thesmall area of the two inner layers and single phase liquid coolant with small radiuspolyamide pipes for the outer layers.
For the sake of the simplicity of the system integration, it is also specified that thesensor is powered through on-chip voltage regulators and features a single rather low
data bandwidth (<200 Mbps).
The sensor overall dimensions follow from various constraints. First, the chosen
CMOS technology offers a maximal reticule size of 30 × 25 mm2 . Considering the needto cover a large area (about 1 m2 for the outer layers) with a minimal number of chips,
matching this size for OBELIX seems benefical. However, the iVTX concepts uses 4
consecutive chips simultaneously diced out of the wafer to make a ladder. Taking
into account an expected yield of 60% for individual die, it is critical to maximise thenumber of such potential 4 consecutive dies on the wafer in order to minimise thenumber of wafers needed to obtain the 20 iVTX ladders. The result of the optimisation
is displayed in figure 5.1 and leads to a chip dimension of 30 × 19 mm2 .
Assuming a 3 mm width for the digital periphery, as an educated guess from previous
sensors in the same technology, the sensitive area or pixel matrix size reaches 30 ×
16 mm2 .
The specifications regarding the pixel matrix established for the Belle II sensor
overlap significantly with the ones used for the development of monolithic pixel sensors targeting the upgrade of the inner tracker of the ATLAS experiment. Within
this program, the TJ-Monopix family sensors, depleted MAPS designed in the TowerJazz 180nm process, match particularly well the Belle II requirements as summarizedin table 5.1. The first sensor, TJ-Monopix1 has already shown promising detection
performance. The second version TJ-Monopix2, featuring a larger matrix and comingwith a number of improvements, was fabricated in 2021.
It was then decided to use the pixel matrix of TJ-Monopix2 as a starting point
for the OBELIX sensor in order to inherit from the results obtained during the development of the TJ-Monopix series. An additional benefit from this choice is that
TJ-Monopix2 offers all the functionalities needed for an application-ready sensor: fast
output, triggering, slow-control interface. Consequently, TJ-Monopix2 can be considered as the first large size prototype for the VTX detector concept. As a consequence,
the test system of TJ-Monopix2 will be re-used for the later OBELIX sensor.

74

75

Specification
Pixel pitch< 40 µm
Sensitive layer thickness< 50 µm
Sensor thickness< 100 µm
Hit rate capability in the matrix> 600 MHz cm−2
Hit rate capability at the sensor output> 120 MHz cm−2
Trigger delay> 10 µs
Trigger rate
30 kHzOverall integration time< 100 ns
(optional) Time precision< 50 ns
Total ionizing dose tolerance
100 MradNIEL fluence tolerance
5 × 1014 neq /cm2
SEU tolerance
frequently ( min−1 ) flash configuration
Matrix dimensions
around 30 × 16 mm2
Overall sensor dimensions
around 30 × 19 mm2Powering
through voltage regulators
Outputs
one at < 200MHz

TJ-Monopix2< 33 µm
30 µm and 100 µm> 600 MHz cm−2
≫ 100 MHz cm−2
15
1.5 × 10 neq /cm2
19 × 19 mm2
20 × 19 mm2
one at 160 MHz

Table 5.1: OBELIX sensor specifications, compared to the relevant specification of the TJ-Monopix2 sensor.

Figure 5.1: Sketch of the OBELIX reticule occupancy over an 8 inch diameter wafer.
Red rectangles indicate possible dicing area including 4 consecutive sensors for iVTX
ladders, while blue rectangles stand for individual chip dicing for oVTX stave population.
5.1.2.2

TJ-Monopix2 as prototype

Sensor description

5

10

15

20

The TJ-Monopix2 general view is depicted in figure 5.2. The in-pixel collection nodeand front-end of the TJ-Monopix pixel matrices inherit from the ALPIDE sensor concept [75], featuring a small diode and low-threshold discriminator. The fabrication
process from Tower Semiconductor, CMOS Image Sensor 180 nm, has been further
modified [76] in order to enhance the radiation tolerance beyond 1013 neq cm−2 . The
discrimination threshold can be adjusted for each pixel through a 3-bit DAC and any
individual pixel can be masked from the read-out.
Four front-end flavours are implemented in four different regions of the pixel matrix[77], all aiming for enhanced radiation tolerance compared to the original ALPIDE
design. The standard or normal version follows the ALPIDE implementation with onlytransistor size modification. A version targeting higher gain uses a cascode scheme
amplification. The two last versions correspond to the introduction of capacitive coupling between the charge-collection electrode and the pre-amplification, normal or
cascode. The capacitor allows biasing the collection node at a few tens of volts, way
beyond the 1.8 V supply of the technology. These so-called HV-normal and HV-cascodeflavours are expected to increase the depletion of the sensitive volume compared to
the non-HV flavours. Without the AC coupling, the depletion growth depends on thevoltages applied on the p-type wells surrounding the collection diode and on the substrate, which have limited excursions.
The read-out architecture implemented within the matrix uses the established synchronous column-drain mechanism developed for the ATLAS FE-I3 readout chip [78].
76

(a) General layout of the sensor functional
blocks.

(b) Photograph of the sensor bonded on a test
board.

Figure 5.2: Two different views of the TJ-Monopix2 sensor.
For each fired pixels, the leading and trailing edge times are stored in two 7-bit counters clocked at 40 MHz. The end-of-column logic uses these counters to quantify the
charge collected by the pixel with the time-over-threshold and to time-stamp the signal arrival with a precision of 25 ns.
5The digital logic located at the periphery of the matrix contains an integrated readout controller, LVDS drivers and receivers as well as the command decoder compatible
with the RD53B readout chip [79], which allows configuring all the chip registers via
only four differential lines.
10

Evaluation

15

20

The detailed evaluation of the TJ-Monopix2 sensor is currently being performed [80].
Operation of selected parts of the pixel matrix have been demonstrated with detection
threshold from 100 e− a temporal noise around 6 e− equivalent noise charge and afixed pattern noise (taken as the standard deviation of the threshold distribution) corresponding to about 3 e− equivalent noise charge after tuning the pixels. Both results
match with the design values.
The VTX collaboration has also conducted optimization of the TJ-Monopix2 chip[81, 82, 83] to acquire experience with operating this sensor technology and find themost performance sensitive parameters.
Control and data acquisition system is based on the BDAQ53 setup. Since the OBELIX
sensor will embed the same control interface, see 5.1.2.3, this system will require minimal adaptation for its use for OBELIX characterization. The firmware will essentially77Figure 5.3: Test setup for TJ-Monopix2 using the BDAQ53 system.

(a) Noise distribution.

(b) Threshold distribution.

Figure 5.4: Threshold and noise distribution obtained on the normal front-end of the
TJ-Monopix2 sensor, from [83].

5

10

15

20

need an update to take into account the larger number of pixels and potentially slightly
different format for the pixel data.
Typical results after tuning the matrix are displayed in figure 5.4 where the distributions of the thresholds and temporal noise for the normal front-end are presented.
Considering all pixels of each sub-matrix and for thresholds between 200 to 300 e− ,the average temporal noise varies from 7 to 8 e− equivalent noise charge dependingon the front-end flavor and the temporal noise varies from 10 to 17 e− .
Two test campaigns at the DESY 3-5 GeV electron beam were conducted. Soon
after the initial commissioning of TJ-Monopix2 in the laboratory, first detection performance with minimum ionising particles were explored in June 2022 and using an
EUDET-type telescope [84]. Due to the early stage of the sensor tuning at that time,
only high-threshold operation was established. Figure 5.5 illustrates the typical cluster charge distribution and residuals. Still, with a threshold set at 500 e− equivalentcharge and an unirradiated sensor featuring a high resistivity Czochralski layer, it waspossible to obtain above 99.9% detection efficiency.
The spatial resolution was estimated to 9.15 µm, slightly better than the digital resolution expected from the 33 µm pixel pitch and consistent with the small cluster size
induced by the relatively high detection threshold. We determined the sensitive depthof the sensor to be 30 µm (systematic uncertainty 2 µm) via an angular study. Thisobservation justifies the base choice for the starting material of OBELIX-1, namely an78

(a) Cluster charge.

(b) Residual between the track extrapolationand the measured position by TJ-Monopix2.

Figure 5.5: Distributions obtained with the TJ-Monopix2 sensors recording data froma 4 GeV e- beam at normal incidence, with a threshold of 500 e− , from [83].
epitaxial layer of 30 µm thickness.
These results were used to implement a realistic sensor response into the Belle II software (basf2). The existing model reproducing the digitized signals on each pixel firedby a particle has been tuned to reproduce these beam test results [85].
510In July 2023, the second test period with beam occurred after the tuning of the
TJ-Monopix2 sensor was refined. This campaign focused on sensors featuring a 30 µm
epitaxial layer and included a chip irradiated to a fluence1 of 5 × 1014 neq cm−2 . The
detection thresholds were tuned in a range from 200 to 300 e− .
The data analysis is on-going while this text is being finalised. Only early results arebriefly quoted below. The detection efficiency was measured to exceed 99.9% for all
un-irradiated front-end flavours. For the irradiated sample, 99.5% detection efficiency
can be reached for the AC-coupled front-end when the high voltage is raised to 30 V
and the detection threshold lowered to about 200 e− .

15This test beam campaign is the last one before the submission of the Belle II dedicated sensor OBELIX-1 based on the TJ-Monopix2 matrix. The early results so far
confirms the role of TJ-Monopix2 as a forerunner.
20

The careful evaluation of the various front-end performances from the last beam
test forms the basis for limiting the number of front-end variants (possibly to only 2
versions) in OBELIX-1.
5.1.2.3

25

Sensor technical implementation

This section describes the technical implementation of the OBELIX sensor in order tomatch all specifications listed in table 5.1.
1

Irradiation with 24 MeV proton beam

79

Figure 5.6: Layout of the OBELIX sensors, right: functional layout, left: physical layout
with the various blocks described in the text.
Pixel matrix
As explained previously, the pixel matrix in OBELIX reuses the architecture developed
for the TJ-Monopix2 sensor described in detail in reference [86]. Only the main features are underlined here as well as modifications when appropriate.
5

10

15

20

25

The pixel conserves exactly the same pitch, 33 × 33 µm2 , with the same layout for
the analogue and digital parts. Based on current characterization results, we decided
to choose the Cascode front end flavor and for the next iteration we will keep DC and
AC coupling versions on equal area for final verification on the full size prototype.
Pixels are still powered from horizontal rails in order to prevent performance gradients throughout the matrix, inducing the need for power pads on the short side of thesensor.
The signal digitization over 7 bits, through the Time-Over-Threshold method, andthe column-drain read-out architecture implemented for pairs of columns stay unchanged. Also, the 3-bit register to trim the threshold value at the individual pixel
level is still included, but the range of the possible correction is enlarged. However,each column-pair is equipped with its individual End-of-Column logic, which actually transmits ToT and timestamp information from each fired pixel to the memory
stage, which is discussed in the trigger logic paragraph just below. Following the TJMonopix2 nomenclature adapted to LHC, the timestamp is also named BCID for bunch
crossing identification. The clock frequency driving the precision of the ToT and the
BCID changes to a lower value in order to reflect the targeted integration time, typically from 40 down to 20 MHz. The current baseline for OBELIX, is then to timestamp
fired pixels with a precision of 50 ns.
The matrix read-out logic has been demonstrated [86] to sustain a hit-rate beyond
600 MHz cm−2 , covering all the currently expected use-cases in Belle II operation2 .
The sensor peripheral logic is designed to transmit data to the outside world up to a2

Additional simulations and test with the fabricated OBELIX-1 sensor will be conducted to estimate
the hit-rate limit beyond which the hit detection efficiency for subsequent triggers might be affected
and at which level.

80Figure 5.7: Schematic illustrating the principle of operation of the trigger logic unit
(TRU).
hit-rate of 120 MHz cm−2 , as explained in the TRU module description.

Trigger logic (TRU, TXU, TTT)

5

10

15

20

Two entirely new modules are related to the Belle II trigger. First the trigger logic unit
or TRU has the key task to select among the fired pixel information coming from thematrix, those related in-time with the triggers sent by the Belle II system. A second
unit, track trigger transmitter (TTT), is in charge to transmit to the trigger system useful information to build a trigger decision based on tracks.
The TRU module handles the incoming data from the pixel matrix with a twostages of memory as depicted in figure 5.7. The first stage memory simply buffers the
pixel information, during the trigger latency. In the second stage memory, the fired
pixel BCIDs are compared to each trigger time present in a dedicated global memory
keeping track of received triggers. When matched, the pixel information is passed tothe transmission unit (TXU). The size of both TRU memories have been optimized inorder to ensure full efficiency at the maximum hit-rate of 120 MHz cm−2 , taking into
account the 30 kHz trigger rate and 10 µs latency and to minimize the power dissipated. It was verified that the memory size can also absorb the spikes of excess hit
rate at 600 MHz cm−2 .
The matching criteria associates to a single trigger fired pixels with a BCID overlapping with the trigger time and also with the next BCID. Since the BCID precision is
50 ns, the duration of the integration window of OBELIX is then 100 ns. This strategy
accounts for the possibility that a physics hit associated to a trigger is timestampedwith a later BCID due for instance to timewalk effect.

25

30

The TTT module profits from the presence of a fast signal for each column of thematrix, corresponding to the OR of all pixel outputs along the column. Trading the
spatial granularity for speed, the TTT divides the matrix into 8 (still to be confirmed)
regions and produces a one byte word indicating which of these regions is fired. The
light load represented by a single byte (or even two if the granularity needs to be extended) allows to transmit this information within typically 100 ns on a line parallelto the main data output of the sensor. The ensemble of such lines can then be input81

5

to the trigger system for elaborating a decision based on the vertex information, ofcourse with coarse position precision.
This functionality is rather exploratory in its current state and may only be used on
the oVTX layers due to power budget constraints. It still requires further simulations
in correlation with the trigger and full VTX system behaviours, which might lead tofurther optimisation.

Control (CRU, synchro)

10

15

20

25

The control of the sensor is implemented in the control unit (CRU) exploiting the protocol developed for the RD53 ASICs [79], as in the case of TJ-Monopix2. It allows
receiving the configuration instructions (i.e. list of masked pixels, threshold settings,etc.) and the in-coming trigger information as well as sending out the data preparedby the TXU module.
The anticipated data throughput per sensor is estimated to not exceed 100 Mbps
in the innermost layer, allowing to use a single physical output line clocked at about
160 MHz.
The OBELIX sensor is configured by setting a number of registers, which define thesensor operation and necessary biases. The protocol is fast enough to allow for reconfiguration of the sensor on a regular basis mitigating potential mis-configurations fromsingle radiation events.
Complementing the control unit, the synchronisation unit uses the SuperKEKB
radio-frequency clock (509 MHz), which is provided to one of the sensor input pads,
to derive all other needed clocks. The most important ones are the 21.2 MHz clock for
the BCID (meaning that the timestamp precision is actually 47 ns) and the 169.7 MHz
clock for the data output (allowing a 340 Mbps throughput).

30

Powering and dissipation estimate

35

To simplify the power distribution along a single ladder, which may contain up to 24
sensors (see table 5.2), a low dropout (LDO) voltage regulator is included in eachsensor. As depicted in the left part of figure 5.6, one pad-ring and one associated LDOregulator dedicated to powering the chip is located on each side of the pixel matrix.
That arrangement aims to insure uniform bias supplies throughout the matrix.

40

The existing description of the various units composing the sensor allows to estimate the power dissipation with the tools provided by the electronic design automation software (EDA). Three main parameters drive the chip current consumption; thecurrent flowing into the front-end, the BCID clock frequency, which drives the timestamping precision and the hit rate. The estimated dependence is showed by figure 5.8.
This evaluation comes with a 20% uncertainty, which reflects the current status of the82

Figure 5.8: Estimated power of the OBELIX sensor depending on the main front end
current (Ibias), timestamping precision (tbin) and hit-rate.
sensor design.

5

10

It appears that the power budget of 200 mW cm−2 is exceeded by 10 % at the
maximum average hit-rate of 120 MHz cm−2 , corresponding to the most pessimistic
scenario, and for the best timestamping precision of 50 ns. The power can be recovered within budget by compromising the timing precision either by worsening thetime binning to 100 ns or with lower biasing current to the in-pixel amplifier, which
degrades the time walk figure.

Monitoring
The temperature and supplied voltages within the sensor are monitored through dedicated thermal probes and an analogue to digital converter featuring 12 bits (10 bits
resolution targeted) and a sampling rate of about 1.7 kHz.

15

OBELIX schedule
The first version of the sensor, OBELIX-1, is currently being designed by a team composed of six full time designers (about ten people spread over CPPM-Marseille, Dortmund University, HEPHY-Vienna, IFIC-Valencia, IPHC-Strasbourg and SiLab-Bonn) un-

83

LayerRadius (mm)# Ladders# Sensors/ladderMat. budget (% X0 )

1
14.1
6
4
0.2

2
22.1
10
4
0.2

3a
39.1
17
7
0.3

4
89.5
40
16
0.5

5
140.0
31
2 × 24
0.8

Total
104
2311
2.0

3b
69.1
30
12
0.4

Total
117
2552
2.1

Table 5.2: The VTX detector main parameters with the two geometrical options (3aand 3b) for the radius of the middle layer.

5

der the coordination of the IPHC-C4Pi3 , which takes in charge the ASIC overall integration and submission. The OBELIX-1 chip will be submitted for fabrication in thefirst quarter of 2024. It is expected for such a complex ASIC that a single submission
is not enough to reach the sensor quality required for building an experiment. Consequently a second improved version, OBELIX-2, will become the production version.

10

OBELIX-1 is expected to return from fabrication in the middle of 2024 and theevaluation of its main performances will require about 6 months. OBELIX-1 featuring
the already final size of the VTX sensor, the chip will also be used to prototype final
detection modules and layers.
While the design of OBELIX-2 will start in 2024, its submission can only occur after
the characterisation of OBELIX-1, hence at the earliest after Q1-2025.

5.1.3
15

20

25

The VTX proposal consists of five barrel-shaped layers, equipped with the same sensor.
The design of the ladders differs due to their active length, varying from 12 to 70 cm
to cover the required acceptance (17◦ < θ < 150◦ ). The two innermost layers (iVTX)
follow an all-silicon ladder concept, the outermost (oVTX) layers require carbon fiber
support structures and flex print cables. The VTX targets an overall material budgetof about 2.0% X0 and will be the first light and compact MAPS-based vertex-tracker
operating at a high-rate e+ e− collider.
Table 5.2 details the main structural parameters of the VTX detector. As discussed in
the physics performance section 4.2.4, the two options currently under consideration
for the radius of the middle layer appear explicitly.
One additional option is being considered, although not explicitly described in thisdocument, to replace the entire VXD with CMOS sensors keeping current machinedetector boundaries as backup option should fatal accidents happen.
5.1.3.1

30

VTX structure

Ladder concepts

The two innermost layers of iVTX are placed at radii 14 and 22 mm respectively. Theytarget an individual material budget of about 0.2% X0 per layer. Such light layer ispossible due to the moderate overall surface of these two layers, below 400 cm2 , the3c4Pi is a facility with 22 engineers devoted to the development of CMOS pixel sensors.

84

(a) Cut 3D view.

(b) End view.

Figure 5.9: Two different views of the VTX detector layout for version 3a. The fivelayers are represented with their support structure, their radii are: 14 , 22 , 39 , 89 ,140 mm.

5

10

15

20

25

low sensor power dissipation and the few connections needed for the sensor operations. Such conditions make air cooling a viable solution. The monolithic structure isessentially made of four contiguous OBELIX sensor blocks diced out of the production
wafers and thinned to 50 µm except in some border areas, that are a few hundred µm
thick to insure mechanical stability.
In more detail, a post-processing step etches additional metal strips on a so called
redistribution layer (RDL) to interconnect sensors along the ladder and to provide a
unique connector at the ladder backward end. The RDL will be produced in a thinfilm process with photo lithographic structuring of polymer dielectric and metal layers.
Vias connect the sensor bond pads to the metal traces, which route power and data via
impedance-controlled transmission lines to the end of the ladder, where a flex cable
is connected. Exposed pads are created on top of the RDL to mount surface mounted
components (SMD) like bypass capacitors. After the RDL has been processed, thebackside of the ladder will be thinned selectively. A thickness of 40-100 µm is foreseen underneath the active sensor area, the outer perimeter of the ladder remains
unthinned for mechanical rigidity. Mounting holes will be added via laser-cutting. The
first ladder demonstrator is currently under production (see figure 5.10), to explore
the feasibility of the two techniques, a large size signal RDL on top of the sensors andthe selective backside thinning of the ladder. The demonstrator is based on a blankwafer with dummy heater structures in place of the sensors, allowing to characterize
the electrical, mechanical and thermal performance of the ladder design. A working
plan, increasing complexity in several steps, is being developed together with IZM(Berlin).
A more traditional approach drives the oVTX ladder concept for the outer layers,
strongly inspired by the design successfully developed for the ALICE ITS2 [87]. Eachladder (see figure 5.11) is made of a light carbon fiber support structure, a cold plateincluding tubes for liquid coolant circulation by negative pressure, a row of sensors
85

Figure 5.10: Schematic view of the iVTX ladder design.

5

glued on the cold plate and finally two flex print cables (see figure 5.13) connecting
each half ladder to a connector. For Layer 3 a single flex exiting on the backward sideof the detector can be foreseen in order to allow more room for machine components
on the forward. Depending on their radius, the material budget of individual oVTX
ladders ranges from 0.3 to 0.8% X0 , matching the requirements set in section 5.1.1.

10

A drawing of the parts composing the Layer 5 ladder is reported in figure 5.12.
The r−ϕ overlap between adjacent ladders is ensured to be at least 10 pixel pitches for
all the ladders, with a clearance between the ladders in radial direction of the order of
1 mm.

Figure 5.11: Schematic view of the oVTX ladders: for Layer 3 there are the two solutions at radius 39 mm and 69 mm. Layer 4 and Layer 5 are placed at radii 89 and 140
mm respectively. The green region on the chip corresponds to the periphery, i.e. notsensitive, while the yellow region indicates the sensitive area.

86

Figure 5.12: Exploded view of the Layer 5 ladder.
5.1.3.2

5

10

15

20

25

Thermomechanics, services and data acquisition

A specific power density baseline of 200 mW/cm2 is assumed in the thermo-mechanicalstudies. For iVTX the power could be anyway reduced by operating the front-end at alower current and/or lower frequency, in case the extreme hit rate of 120 MHz/cm2 isreached in the innermost layer, while power dissipation could be even lower for oVTX,
due to the significant lower hit rate in the external layers.
The proposed VTX detector is expected to be operated at room temperature. The
sensor temperature will be monitored both internally, see section 5.1.2.3, and withadditional thermal sensors as implemented in the current VXD [88]. The usage ofthe same sensor all over the detector allows a unique control and power system. An
important reduction of services with respect to the current VXD is expected, due tosmaller cross sections of the data cables with the extensive use of optical fibers, and aless complex cooling (i.e. single phase) system. Consequently, simpler overall mechanical support, cabling and acquisition system are required. In particular, the standard
PCIe40 [89] acquisition boards used in Belle II match well the data throughput requirement.
The iVTX full-silicon concept is currently being assessed with industrial partners,
first using dummy silicon wafers and in future with real sensors. Thermal studies,considering power is uniformly dissipated on the sensor surface, demonstrated that
air at 15◦ C flowing at a speed of 10 m/s is effective to cool a single inner module. The
maximum temperature reaches about 20◦ C.
At system level, the bottleneck is the space to pass the tubes to bring the air to the IP
region: a rough estimate shows that an equivalent cross section of 6 tubes of 10 mm
in diameter is needed to evacuate the 65 W of heat source of the inner layers. An
experimental verification of such studies on a dedicated thermal test bench is foreseen
87

5

10

15

in order to produce a mechanical design compatible with the new interaction region.
Prototypes of key elements of the oVTX ladder, the longest carbon fiber support
structure and flex cable have been designed and fabricated. For Layer 4 and Layer
5 services (electrical connections and cooling) can be provided both on forward andbackward sides, naturally dividing the ladders in two electrical units. To limit the
space required on the forward side, giving more room for the accelerator components,
for the Layer 3 ladder also the solution with the only access on backward has been
investigated. Traditional flex circuits can be used to distribute power and for data
output for the oVTX. A multiline power bus has been designed and realized first on
four-layer process with copper in order to have each OBELIX chip along the ladder
powered by a dedicated positive voltage (VDD) and ground (GND) line pair. Tracewidths are trimmed to fulfill the same maximum voltage drop requirement (200 mV)
for all the chips. The electrical tests on signal integrity performed on the prototypes
demonstrated that the bandwidth is large enough to allow more than the needed 160
Mbps baseline data throughput.
For the flexible printed circuit in aluminum, the design (see figure 5.13) is following the floorplan of the chip, to allow, after the characterization, also a final test by
eventually connecting the final chip.

Figure 5.13: Drawing of the prototype Al flex for the layer 5 half-ladders.
20

25

30

A full scale Layer 5 prototype, with kapton heaters mimicking the power dissipation from the chips, has been assembled and characterized on the thermofluidynamic
bench. We set the coolant (demineralized water) temperature at 10◦ C at the inlet, theenviromnent at 20◦ C and a negative pressure of 0.2 bar. Three flow configurations(monodirectional, bi-directional and with an U-turn at one end) have been tested andthe results have shown an average temperature of 24◦ C with a maximum gradient∆T ≈ 4◦ C along the full length of the ladder, matching the specifications required tosafely operate the chip.
Results of the FEA studies on the longest Layer 5 ladder have been confirmed bymechanical measurements on the sagitta distorsion. Also vibrational FEM analysis
matched the experimental tests on the truss, showing the first resonance frequency ofthe structure at 200 Hz, safely far from the typical earthquake frequency. All these tests
allowed a validation of design of the longest (i.e. most challenging) Layer 5 ladder.

88

5.1.4

5

10

15

20

25

30

35

40

Roadmap towards realization

The previous sections 5.1.2 and 5.1.3 described the intensive R&D program to establish the technical solutions for the sensor and the detection element structures (electric, mechanical and thermal). Both are expected to converge at the end of 2024.
Additional developments for the systems to control and acquire the data will start andadvance in 2024 in order to identify the main solutions, but not to complete the entire
development which may be finalised during the detector production phase .
It is hence expected that the VTX critical R&D phase will close at the end of 2024,
allowing to enter a production and construction phase should the project be accepted.
The production of all the VTX parts is foreseen to last for slightly more than two
years. This estimation stems from the following assumptions for the needed part quantities. All parts are produced in quantities including roughly 20% spares. A sixth layers might be added which would typically correspond to retain both options 3a and3b from table 5.2. the VTX geometry requires then the assembly of 20 iVTX ladders
and 140 oVTX ladders, which correspond to about 3000 sensors. The ASIC fabrication
yield is taken as 50 % and 30 dies are expected per wafer, hence the total number
wafers to produce is 195.
It is also considered that among the 9 Belle II groups currently involved in the development of the VTX design (Austria: Vienna; France: Marseille, Orsay, Strasbourg;
Germany: Bonn, Göttingen; Italy: Pisa; Japan: KEK, Spain: Valencia) and a few
additional groups that may join, about 8 laboratories will join production activities.
Finally we elaborate the schedule assuming 4 work days per week and 3 work weeks
per month.
• 6 month for the wafer production, followed by 6 months for the probe testing(the integration of sensors onto modules can obviously start before the full probing step is finished). The probing occur a minimum of 2 sites, with 1 wafer tested
per day.
• 10 months for the iVTX ladder production, assuming 2 ladders per month at asingle production site (IZM, Berlin).
• 24 months for the oVTX ladder production, assuming 2 ladders per month at 3
sites. It has to be noted that a potential intermediate step will most probably
needed, i.e. a ladder being composed of modules. In this case, 2 additional sites
will be needed for the module production.
• 12 month for the mounting of the ladders onto partial detector structures, likesemi-cylindrical shells representing half-layers, which are then assembled aroundthe beam-pipe outside the Belle II detector. This operation might be done entirely
in Japan at KEK or partially in one laboratory in Europe.
iVTX and oVTX production can be conducted in parallel and start mid-way (after 3
months) of the wafer testing phase. The shell production can begin after 6 months
from the oVTX production start. This very notional plan leads to a total production
length of: 6 (wafer prod) + 3 (delay oVTX prod) + 6 (delay full assembly) + 12 (fullassembly) = 27 months.

45

89

From the point where the VTX is attached to the beam pipe, an additional 6 to 12
month period is needed for installing and commissioning the detector in the Belle IIexperiment.
5

Under this preliminary schedule, the VTX can be ready to take data between 2.5
to 3 years after the final sensor (OBELIX-2) is submitted to fabrication and without
contingency.

5.1.5
10

VTX cost estimate

The VTX cost is estimated from various sources, first the experience of the previous
ALICE-ITS2 detector cost, then the part prices actually found during the R&D phase(typically for the cost of the sensors, the ladder structure and flex cables). Crosschecking the estimates obtained from these two sources, the cost breakdown is provided in table ??.
Table 5.3: Breakdown of the current cost estimate of the VTX in kEUR without any
contingency.
Component
Sensors
Ladders
Assembly
DAQ & services
Installation
Total

15

20

Development
380
120
130
280
910

Production
1130
1400
630
1060
100
4320

Total (kEUR)
1510
1520
760
1340
100
5230

Considering the development stage of the project and the possibility of large pricefluctuations from the semiconductor industry, it is reasonable to apply a rough 50 %contingency on the production cost and estimate that an envelope of 7800 kEUR will
cover the project finalisation.

5.1.6

R&D and technology options

5.1.6.1

DuTiP sensor

One of the promising technology options is a monolithic sensor using Silicon-OnInsulator (SOI) technology. The sensor concept optimized to Belle II upgrade, called
DuTip (Dual Timer Pixel), was invented [90] based on PIXOR striplet detector [91].
The DuTiP prototypes are fabricated using the SOI technology.
25

General concept of DuTiP
Under large track density environments, a global shutter readout mode based onlevel 1 trigger would be adopted to realize a fast readout with low occupancy allowing
90

5

10

15

for small data transfer rates. Hit information is to be stored in the pixel detector during the trigger latency. Even if a second hit arrives at the same pixel during this period,
this information would be also kept. To satisfy these requirements, DuTiP concept wasinvented.
Fig. 5.14 shows the block diagram of the DuTiP. The analog part consists of a
pre-amplifier, shaper, and comparator which are the usual configuration for a binary
detector. The digital circuit is the most important part of the DuTiP concept. Whenthe binary hit signal is sent to the digital part, one of the timers starts counting down.
The starting time is set as trigger latency plus one clock cycle. If the trigger signal
is received when the time is 1 (2/0), the signal is readout as current (next/previous)
timing (Previous/Current/Next timings are denoted as PCN timings). If trigger signal
is not received at the PCN timings in the pixel, the timer is reset. To take into account the second hit in a pixel during trigger latency, a sequencer and two timers areequipped.
The concept does not specify the technology however the complicated digital circuit
should be fabricated on each pixel, thus an SOI technology is chosen as a baseline forthe development of DuTiP sensor.

Figure 5.14: Analog, Digital and Scan blocks for DuTiP detector. The analog anddigital circuits are on a pixel while the Scan block is on a periphery.

SOI Technology

20

25

30

SOI technology has the potential to build thin and low power monolithic sensors for its
use in pixel vertex detectors in particle physics experiments. Figure 5.15 (Left) showsa schematic view of the SOI detector. We choose the Lapis semiconductor 0.2 µm
FD-SOI CMOS technology for the development since many pixel detectors are alreadydeveloped, for example, SOFIST for the ILC [92], and pixel detectors for CEPC [93]and CLIC [94, 95].
To fabricate a complicated circuit, reduction of the circuit area is important evenfor the SOI. Since active regions of NMOS and PMOS can be merged and the contacts
can be shared with the SOI [96], the circuit area can be smaller than bulk CMOS withthe same process rule (Fig. 5.15 (Right)). While the SOI implementation is tolerant
against neutrons and single event effects, the most important concern is related to surface damage. Charged particles create holes in the SiO2 insulator layer which causes
91the backgate effect. Nevertheless, this had been solved incorporating a double SOI
structure and test structures have shown radiation tolerance up to 1 MGy [97].

Figure 5.15: (Left) Schematic view of SOI. The circuit silicon layer (light yellow) ison the insulator SiO2 layer (grey). The sensor silicon layer (sky blue layer) is electrically connected with the circuit layer with through-silicon vias. (Right) Active merge
technique to merge PMOS and NMOS.

DuTiP sensor technical specifications

5

10

15

20

Table 5.4 shows DuTiP sensor specification. To achieve an intrinsic resolution better
than 15 µm, pixel pitch is chosen to be 45 µm (Figure 5.16). The sensor active
thickness define the amount of generated charge carriers in the bulk. We choose 50 µm
as a baseline however up to 80 µm meets the requirement for intrinsic resolution. The
clock speed is 62.9 ns which is determined by SuperKEKB clock 509 MHz divided by
32. For analog circuits, ALPIDE analog circuit with some modifications for optimizing
to the SOI is adopted [98]. To achieve faster shaping time, the supply current isincreased to 200 nA which gives about 500 ns pulse width for the output signal. The
equivalent noise charge (ENC) is expected to be less than 100 electrons.
The DuTiP sensor is equipped with a 300 MHz LVDS interface which has a datatransfer rate of 600 Mbps. Since DuTiP sensor transfers only hit information synchronized with trigger timing, the data rate is expected to be low even for the innermost
layer. The sensor size is determined by the reticle size for the SOI process. The Lapis
Semiconductor SOI process has a successfully demonstrated stitching technique. Three
sensors for X-ray imaging were combined into single chip with the stitching technique.
DuTiP can also adopt the same technique to produce larger chips which should be
more mechanically stable when making ladders.
DuTiP performance with GEANT MC

25

The basic performance of the DuTiP sensor is evaluated with simple GEANT MC. The
PXD is replaced by two layers of DuTiP sensors, while keeping the beam pipe and
SVD untouched. The impact parameter resolution is studied using pions with different momentum (Figure 5.17). The fit results for incident angle of 90 degree are92

Table 5.4: DuTiP sensor specifications.
DuTiP L Sensor (S Sensor)
Pixel pitch
45 µm
Sensitive layer thickness
50 ∼ 80 µm
Sensor thickness< 100 µm
Clock speed
62.9 ns
Trigger latency
8 µs
Trigger rate
< 30 kHzTime precision< 16 ns
data transfer rate
600 Mbps
Pixel array size
21.6 × 28.8 mm2 (14.4 × 28.8 mm2 )Sensor size
24.4 × 29.6 mm2 (17.2 × 29.6 mm2 )Figure 5.16: Intrinsic resolutions as a function of incident angle with different (left)
pitch and (right) thickness using GEANT MC.

93

σr−ϕ (µm) = 13.5⊕11.4/p(GeV) sin3/2 θ and σZ (µm) = 14.2⊕11.4/p(GeV) sin5/2 θ which
are enough good to measure the B meson decay vertices.

Figure 5.17: Impact parameter resolutions for (left) r − ϕ and (right) Z directions.

DuTiP Prototype Sensors

5

10

15

20

The first prototype DuTiP1 were delivered in 2021 (Fig. 5.18 left). This prototype wasdeveloped for demonstration of DuTiP concept thus all functionalities in pixel were
fabricated while scan block and fast readout system were not implemented. The chipsize is 6×6mm2 with a pixel array of 64×64. The active thickness is 300 µm whichis larger than the baseline. The digital circuit worked using a test pulse with 25 MHz
clock. The time resolution of DuTiP is checked with test pulse. About 5000 electrons
are injected to the analog circuit and check the time resolution with faster 50 MHz
clock (Figure 5.19). The time resolution is evaluated as 11 ns which is enough smaller
than the time bucket of 62.9 ns. The efficiency is checked using a 90 Sr radioactive
source with a scintillation counter as the reference. The efficiency is measured as
98 ± 2%. The large uncertainty is associated with systematic effect of the configuration
for the scintillation counter. The efficiency and position resolution will be measured
with a electron beam at KEK with reference telescope.
The second prototype DuTiP2 had been delivered in 2022 (Fig. 5.18 right). This
prototype has all functionalities except for fast hit data collection to periphery, and isfull size in column direction, chip size of 17.2×6mm2 with a pixel array of 32×320,
thus we can perform the full characterization needed for operation (extending the chipsize to row direction is trivial). The evaluation is now on-going.
The third prototype DuTiP3 had been submitted in 2023 and will be delivered in
942024 (Figure 5.20). This prototype is based on DuTiP1 and equips the optimized
amplifier and LVDS and PLL circuits.
We will submit the fourth prototype having all functionalities needed for real DuTiP
in 2024.

Figure 5.18: (Left) Picture of the DuTiP1 (6×6mm2 ). (Right) Picture of the DuTiP2 (6×17.2mm2 ).

Figure 5.19: Time resolution of DuTiP. The fit result of 12.6 ns includes a finite binningeffect.

5

10

5.2

CDC

Assuming that the degradation in CDC performance associated with the injection background observed in 2022 can be mitigated by the SuperKEKB improvements implemented in LS1, the CDC detector is expected to withstand the background conditions
after the Belle II upgrade through Runs 2 and 3. However there are concerns regarding the radiation hardness of components of the CDC readout. While dedicated ageing
studies are being prepared to understand possible degradation of the chamber during
its lifetime, this section discusses the upgrade path for the electronic components.

95

Figure 5.20: Layout of the DuTiP3 (6×6mm2 ).

5.2.1

Upgrade of readout electronics

The front-end electronics located inside the detector read 14336 signal lines from the
CDC. Figure 5.21 shows a photograph of the current CDC front-end module.
signal
ASD
ADC

FPGA

TRG DAQ
power

FPGA Clock/Trigger
programing
signalFigure 5.21: Photograph of the present CDC front-end module.

5

10

15

With the design luminosity being an order of magnitude higher than the currently
achieved, the operating environment is expected to become much harsher, especiallyin terms of radiation damage. The expected final dose and fluence at the end of thephysics run are, respectively, 1 kGy of gamma rays and 1.0 × 1012 neq /cm2 of neutrons,
being larger than initially expected at the time of the design of the current front-end
modules.
In this respect, we plan to upgrade the front-end electronics to operate CDC readout system until the end of the the Belle II experiment. The main critical item on theboards are the optical transceivers, that are very sensitive to the total ionizing dose.
Therefore, these components will be replaced with a higher radiation tolerant opticaltransceivers. The increase in beam currents and luminosity also demands the trigger
system to effectively classify physics phenomena of interest under high backgroundconditions. To achieve this, the CDC readout boards are required to transmit moredata about particle information while suppressing electrical noise such as on-board
cross-talk. A newly developed ASIC chip would enable to operate under such background levels. The expected radiation tolerance of the new optical transceiver and96

5

ASIC are O(1 kGy) and over 1 MGy, respectively. Thus, this upgrade plan will provide
solutions to the above requirements for radiation hardness, cross-talk suppression, andfaster data transfer.
The development of the ASIC has already been completed, and the first prototype
board has been produced. We are currently conducting performance tests on the prototype, including radiation tests. In the following section, we give details about the
new components and present the results of the performance tests of the prototypes.
5.2.1.1

10

15

20

25

The new ASIC chip has been designed at KEK by the electronics system group using a 65-nm CMOS process. This chip consists of eight sets of Amplifier-ShaperDiscriminator (ASD) and Flash Analog-to-Digital Converter (FADC) circuits. In the
ASD sector, a double-threshold technique is applied, where the lower threshold makes
the time-walk effect small and the higher threshold cuts off electrical noise. Charged
particles that create large energy loss can induce smaller electrical signals, known ascross-talk, in the surrounding channels within an ASIC chip, compared to signals associated with tracks. Cross-talk signals increase the fake rate of the track trigger asbackground levels increase. The higher threshold suppresses cross-talk contaminationin data, leading to improvements in both online and offline tracking reconstruction
performance. Data handling of FADC has about 300 ns delay from timing informationin the present electronics. FADC gives charge (dE/dx) information, but this is not
used for the level-1 trigger decision due to the unacceptable long delay. A triggerperformance study shows the use of charge information in the tracking trigger improves the trigger performance. The FADC of the new ASIC reduces the delay due
to data processing to about 100 ns and enables the use of charge information in thetrigger system. The estimated power consumption of 41.3 mW/ch is low compared tothe total power consumption of the ASIC and ADC on the current board.
5.2.1.2

30

35

40

New ASD-FADC ASIC

Prototype of the upgraded module

We have developed a prototype of the upgraded CDC front-end module that has thesame functionality as the current module. To test both, the current and upgraded functionalities, two attachment boards have been produced. Figure 5.22 shows a pictureof the prototype of the new CDC front-end module along with the two attachment
boards. The module incorporates the following parts:• Six ASIC chips to measure signal timing and digitize waveform information• A Field Programmable Gate Array (FPGA) for online data processing for trigger
and data acquisition systems• A Quad Small Form-factor Pluggable (QSFP) for data transfer to the trigger anddata acquisition systems• SFP connector for standalone performance tests (to be removed in future iterations)• A connector for the attachment boards to operate B2TT and B2JTAG
One attachment board is designed for the traditional slow control and time/trigger
distribution operations with two RJ45 sockets, while the other one is designed to
97

FPGA

ASIC x6

QSFP
SFP

for attachment

Figure 5.22: Prototype of the new CDC front-end module (Left) and the two attachment boards (Right).

5

operate over three lanes using a QSFP module. Firmware development is ongoing forthe event-data format and link, trigger and timing separately, and the former is alreadyavailable.
The basic performance of the new module, specifically its linearity and gain, hasbeen evaluated using a charge input board and a waveform generator. The gain of
the ASD component was measured by using a charge input board. We measured thedetection efficiency defined as follows:
Detection Efficiency =# of Events Exceeding Threshold.
# of Input Events(5.1)

We varied the thresholds and calculated the detection efficiency, subsequently determining the pulse height by fitting it with an error function:


Z x
2
−(t − µ)2
erf(x) = √
exp
dt
(5.2)
2σ 2
π 0
10

15

20

where µ represents the wave height and σ denotes the uncertainty in wave height. The
gain was determined by fitting relationship between wave height and input charge.
Figure 5.23 illustrates the relationship between wave height and input charge andgain values for each channel. The mean gain value is approximately 0.83 V/pC, slightly
lower than the designed value of 0.9 V/pC.
We assessed the time resolution using a CDC test chamber during a dedicated testbeam campaign. The campaign was conducted at the AR Test Beam Line, KEK, in
November 2022 with a ∼3 GeV electron beam. Figure 5.24 provides an illustration of
the beam test setup. The chamber configuration is outlined as follows:• Cell: 15 mm × 15 mm• Structure: 8 layers and 44 wires• Gas mixture: He:C2 H6 =50:50• High Voltage: 2.37 kV

98

gain [V/pC]

wave height [V]

1.4
1.2
1
0.8
0.6

CH0

0.4

slope: 0.842 ± 0.003

00

0.2 0.4 0.6 0.81

1.2 1.4 1.6 1.8

0.85

0.8

intercept: 0.000 ± 0.001

0.2

0.9

0.75
0

2

input charge [pC]

5

10

15

20

25

30

35

40

45

channelFigure 5.23: (Left) Relationship between wave height and input charge of channel
0. The red line represents the results of linear fitting. (Right) Gain values for eachchannel. The red solid and dashed lines represent the mean and mean±RMS valuesof the gain.

e-

Figure 5.24: Photograph from the electron-beam test.
Time resolution is determined through a fitting process applied to the residual distribution, which is calculated as:
residual = (tA + tC ) ×

5

10

1
− tB
2

(5.3)

where ti represents the timing of particle detection, and i corresponds to the wire
ID, as explained in Fig. 5.25. For performance comparison between the new moduleand the existing one, data was also collected using the same setup with the current
module. The residual distributions are displayed in Fig. 5.26 The measured time resolution of the new module is 4.23 ns. This is slightly worse than the current module’s
performance, but since there are more charged particles at ∼1 GeV and multiple scattering dominates, the impact of the CDC performance is negligible. Further detailed
performance evaluations are still ongoing.

99

30
20
10

wire Awire Bwire C0e- track−10−20 Figure 5.25: Wire configuration. Black dots represent wires in the chamber and aresurrounded by drift circles, with diameter equal to the drift time (blue circumfer−30 ences). The black line indicates the reconstructed trajectory of an electron.

−50

0

20

entry

−40

×103

preliminary

3.5

40 New module
60

80 Current module
100
Entry: 36089
Entry: 36089 x [mm]
FRONT-END MODULENEW FRONT-END MODULE

Entry: 39297
3 Entry:
39297
Const.: 3325.54± 23.63

2.5

Mean: -0.25 0.02 cnt
Mean: -0.25± 0.02 cnt
Sigma: 4.23 0.02 cnt

2Sigma: 4.32± 0.02 cnt

Const.: 3184.85±23.84

Mean: 0.89 0.02 cnt
Mean: 0.89±0.02 cnt
Sigma: 4.07 0.02 cnt
Sigma: 4.07±0.02 cnt

1.5
1
0.5
0
−40

−30

−20

−10

0
10
20
30
40
residual [TDC count (0.98 ns)]

Figure 5.26: Residual distributions obtained at beam test. The red line corresponds
to the Gaussian fit result of the black histogram representing the new module TDC
counts.
5.2.1.3

5

10

15

20

Radiation hardness

The radiation tolerance of the front-end module and related optical modules should
be examined through gamma-ray and neutron irradiation. The required tolerance is a
total dose of 1 kGy and a neutron fluence of 1.0 × 1012 n/cm2 over the 10 years Belle
II data acquisition period. Some of the integrated circuits (ICs), including voltage regulators, are identical to those employed in the current module, and their radiation
tolerance has already been verified. The new components to be tested are the ASIC,QSFP, and FPGA. The semiconductor technology integrated into the ASIC is acknowledged for its robust radiation resistance, adequately enduring the anticipated radiationlevels of Belle II.
Regarding the QSFP, typical optical module failures are generated by the total ionizing dose; therefore, we have to find QSFP candidates with sufficient radiation tolerance. To this end, two gamma-ray irradiation tests were conducted at Tokyo Institute of Technology, in March 2022 and February 2023. During these tests, radiation
hardness was evaluated by monitoring errors during the high-speed communications
including also variations between individual modules. As a result, we found three
QSFP candidates that meet the required radiation tolerance. These candidates werealso qualified against neutron damage, as described below.
As for the FPGA, neutrons can introduce soft errors, leading to unintended logicchanges - a phenomenon termed single event upsets (SEUs). The implemented firmware

100

5

10

automatically corrects SEUs using a soft-error mitigation scheme provided from Xilinx
Inc., but it cannot correct multi-bit upsets (MBUs). Therefore, when MBUs are detected, data acquisition is temporarily stopped to reprogram the FPGA. The duration ofthis MBU-induced reprogramming interval potentially constitutes a significant portionof data-acquisition downtime. We conducted a neutron irradiation test at Kobe University using a TANDEM accelerator providing 2 MeV neutrons in March and September
2023. The soft-error mitigation scheme successfully detected soft errors and corrected
SEUs and no permanent damage was observed. In the new module, the soft-error
rate has reduced to 2/3 while the number of logic blocks is twice that of the current
module. This is thought to be attributed to the transition from 65-nm technology to a
28-nm technology FPGA.
5.2.1.4

15

The design of the new ASIC and layout of the prototype board have been done byexperts in KEK Esys group. The mass production of the present readout modules
has been conducted in Taiwan. National Taiwan University (NTU) has experiencesin performing comprehensive quality assessments across all of these modules. It is
anticipated that KEK and NTU will contribute on this upgrade.

5.2.2
20

Potentially interested community in Belle II

Order of magnitude cost estimate

An estimation of the main cost is shown in Table. 5.5. We include 10% spare. Cost formass production of electronics is based on the cost of FPGA. Mass production of the
ASIC chip will be completed by the end of JFY2023 (March 2024).
Table 5.5: Estimated cost for mass production of ASD ASIC and readout electronics
board.
item
mass production of ASIC
mass production of FE board

25

30

number
∼ 3,000 chips∼330 FEs (including spare)

5.2.3

R&D and technology options

5.2.3.1

TPC-based tracking systemcost
25 M JPY
100 M JPY

At sufficiently high luminosity drift chambers will suffer from high occupancy due totheir two-dimensional nature. A highly segmented three-dimensional tracker could
allow tracking at far higher luminosities. A natural solution would be a gas Time Projection Chamber (TPC) with micropattern gaseous detector readout, combining low
material budget with fine three-dimensional segmentation. With existing technologies, the O(104 ) CDC cells could be replaced with O(1012 ) effective voxels. Accounting
for the O(102 )-times longer drift time for primary electrons, such a TPC therefore
promises an occupancy reduction of O(106 ) relative to the CDC.
A preliminary conceptual design for a TPC-based tracking system for a longer-term
Belle II upgrade was described in Ref. [99]. The envisioned TPC used an integrated
101

5

10

15

20

25

30

35

grid (InGrid) pixel readout with 200 µm pixel pitch and continuous binary readout. In
this concept, the VTX system is extended to to r = 44 cm and optional fast-silicon timing layers at r = 25 or 45 cm provide a trigger and low-momentum track PID via timeof-flight. The preliminary simulation-based studies suggest that such a system could
provide comparable tracking performance to the current CDC even in the pessimistic
background scenarios investigated, though considerable open questions remain.
Ions generated during amplification drift 100 times slower than the primary elections, so ion backflow (IBF) could lead to performance-degrading space-charge buildup.
This is potentially most problematic with injection backgrounds due to the 50 Hz repetition rate of SuperKEKB’s top-up scheme. The ion drift time of 3 ms is comparable
to the stabalization time of the beam after injection, so the TPC integrates injection
backgrounds over bunch revolutions but not over injections. It is not possible to add
an active gating layer above the amplification without unacceptable dead-time. The
preliminary results suggest that the ion density would be comparable to that of current
or planned tracking TPCs due to the intrinsically low IBF of InGrid sensors. However,no injection background simulation was available.
Without a modification of the injection scheme, it would be prudent to minimize
the IBF intrinsic to the amplification. One option is to add a second integrated amplification layer that also acts to screen backflowing ions. Such a TwinGrid device [100]could suppress IBF by a factor up to two orders of magnitude relative to InGrid sensors, to the level of about half an ion per primary electron [101]. However, injectionbackground simulation followed by testbeam campaigns with prototypes would beneeded to confirm acceptable space-charge densities.
Regardless of the source, the overwhelming majority of hits are generated by lowenergy electrons spiraling in the gas toward the readout plane. These microcurlers arise
mainly from Compton scattering of low-energy beam-induced background photons in
the target gas. While these hits are not an occupancy concern, they could overwhelm
the throughput of the system with useless data and obscure hits associated with tracks
from collisions. However, microcurlers are very distinctive, and it is likely that they
could be identified and filtered out at the front end using embedded machine learning
without loss of tracking information.
Although preliminary and not conclusive, these early findings suggest that a TPCbased tracking system combined with a radially extended vertex detector compared tothe current one and the implementation of dedicated trigger and PID detector layers
could be viable at luminosities beyond the limits of the CDC, and are promising enoughto motivate a dedicated R&D program to develop the concept further.

102

Chapter 6
Outer Detector
6.1

TOP

Editor: E.Torassa
5

6.1.1

Introduction

With four years of running experience at Belle II, the world-record luminosity of SuperKEKB has been accompanied by higher than anticipated backgrounds. For the TOP

Figure 6.1: Illustration of the back side of one TOP module where electronic boards andphotodetectors are interfaced with the quartz prism and quartz bar.

10

subsystem (Fig. 6.1) this implies that the hit rate and the quantum efficiency degradation of the Micro-Channel Plate Photomultiplier Tubes (MCP-PMTs) must be monitored
to assess the risk that even the last generation, the life-extended atomic layer deposition ALD MCP-PMTs, may not survive until the target integrated luminosity.
R&D has started on a possible silicon photomultiplier (SiPM) replacement option.
This option looks promising, although the required cooling will reduce the space available for the readout electronics and limit its acceptable heat load. Thermal gradients103

5and potential thermal and mechanical stresses need to be carefully evaluated.
Moreover, the projected high rates of Single Event Upsets (SEUs) is problematic, assome number of them are not recoverable and will lead to increasing data taking loss.
There are concerns about the radiation tolerance of the fiber optic transceiver modules
connecting the front-end to the back-end electronic boards. Simulations indicated afairly low radiation of 10 Gy/year with a corresponding expected lifetime of 20 years
larger than intrinsic lifetime, but the real background can be largely underestimated.

Figure 6.2: Expected QE degradation for the three different generations of MCP-PMTs
installed in the TOP detector: conventional, ALD and life-extended ALD. Horizontal
axis shows the year and vertical axis is the relative QE.

10

15

20

The timescale for replacement of the TOP photosensors is related to the level ofbackground during the next years. Fig. 6.2 shows a projection of the expected PMT
quantum-efficiency (QE) degradation. The Monte Carlo expected background wasconsidered for the components related to the luminosity (Bhabha scattering, twophoton processes) and a maximum limit of 5 MHz/PMT for the other components(beam-gas, Touschek). The expected gradual increase in peak luminosity describedin [102] was taken into account. In this case, the QE of normal ALD type PMTs will
drop below 80 % starting from 2027 and all PMTs will reach this level of QE degradation at around the current luminosity target. Some MCP-PMTs showed a fasterthan expected reduction in QE considering their accumulated charge. One possible
explanation is that they operated at a higher temperature for some time due to a temporary overheating of the electronics. Quantum-efficiency measurements performedin the laboratory as a function of the accumulated charge show a faster degradation
at higher temperatures (Fig. 6.3). New readout electronics with reduced power consumption can help to increase the MCP-PMT’s lifetime.
Planning has started for these upgrades, which could take place during a long104

Figure 6.3: Relative QE degradation as a function of the accumulated charge at 20◦ C
and 40◦ C for two conventional MCP-PMTs.

shutdown to rebuild the interaction region on the timescale of roughly 5 years from
now.

6.1.2
5

10

15

20

25

TOP program summary

The rudimentary schedule and cost estimates for the electronic readout are based upon
production and delivery of the 8k channels of TOP readout. This endeavor leverages
heavily two sizeable past and new investments: re-use of existing infrastructure andfirmware, wherever possible and development of ASICs appropriate to the TOP upgrade. In both cases the pre-production plus production timescale is approximately 2
years, with 6-9 months for installation and commissioning. Based upon the preliminary prototyping being performed and costed separately from this, the total project
cost is estimated to be 3.1 M$ for upgrades to the TOP readout systems. For the photosensors the primary option is the replacement of normal ALD PMTs with life extended
ALD MCP-PMTs once the installed PMTs approach the end of their lifetime. With thisoption, we need 220 new MCP-PMTs to replace normal ALD type PMTs, at an estimated costs of 1.6 M$. The production of life-extended ALD MCP-PMT has alreadyrestarted, 90 MCP-PMTs have been ordered, 30 of them have been delivered. A further
improvement in the ALD technology and the MCP-PMTs replacement with cold SiPMs
are the possible solutions if the increase of luminosity leads to a rapid loss of efficiencyof the life extended ALD MCP-PMTs.

6.1.3

Upgrade proposal

The goals of the baseline upgrade to the readout electronics are twofold: reduce significantly their size, as well as the power required. Both goals facilitate the implementation of the requisite cooling needed to operate the SiPMs in single-photon detection(though with significantly increased dark-count) mode.
In order to reduce cost and development time, much of the existing software,
firmware and hardware infrastructure can be retained. The boundary conditions con105

sidered for this costing exercise are:

5

10

15

20

25

30

35

40

45

• retain the Standard Control Read-Out Data (SCROD) board, the existing cables,
low-voltage power system and the existing infrastructure for the TOP-based online event time logic of the L1 trigger in TOP FEE;
• remove the HV divider chain/delivery board and associated cables;• employ multiple Analog to digital converter System-on-Chip (ASoC) developed
by Nalu Scientific.
Nalu Scientific, LLC (NSL) is a small business in Honolulu, HI and was originallya spin-off of the University of Hawaii’s High Energy Physics group back in 2016. NSL
works closely with US DOE laboratories and universities. It has developed commercially available variations of the IRSX and TARGETX chips that had been used for TOP
and KLM readout. Further development and refinement of the IRSX and TARGETX
ASICs has led to a new generation of chips: the ASoC (Analog to digital converter System on a Chip) [103] and the HDSoC (High Density digitizer System on a Chip) [104].
The ASoC and HDSoC include on chip circuitry and functionality to perform sampling,
digitization, triggering, region of interest extraction operations. Essentially, an analog
channel enters the chip and packetized data exits the chip in high speed serial form.
The chips operate with a high level of autonomy, which reduces the need for high-end
FPGAs on the readout board. The ASoCv3 is a 4 channel digitizer with 16k samplesof buffer per channel operating at 2.5-4 GSa/s channel. It is commercially available
with a suite of test hardware, firmware, software and documentation. Version 3 (afour-channel chip) has been tested and has improved power consumption comparedto the IRSX. It can be used for bench testing and development purposes starting immediately. A custom version of the ASoC (ASoCv4) is proposed for the TOP readout
upgrade with 8 channels per chip, which is matched to the existing readout infrastructure and has improved speed and power consumption. This new ASIC (ASoCv4) would
be developed for the TOP upgrade. The ASoC can communicate with less complex FPGAs (compared to IRSX) which will reduce size, cost, power consumption, and SEU
issues while maintaining overall firmware backwards compatibility with the current
IRSX based system.
For the photosensors, the Hamamatsu production line of life-extended ALD MCPPMT is still operating with an expected output of 10 MCP-PMTs/month. A possible
further increase of lifetime can be investigated for example by an optimization of theatomic layer thickness.
The option of MCP-PMT replacement requires several studies and possibly further
SiPM developments with the goal to reduce the dark count rate. It is important to findthe best balance among different SiPM’s characteristics: single device size, single celldimension, operating temperature. Considering the expected neutron rate near thephotodetector region at the target instantaneous luminosity of 6×1035 cm−2 s−1 is about
2.0 × 1010 n/(cm2 · year), we need to be able to distinguish single photons at least up toan irradiation of 1×1011 n/cm2 , even more for higher luminosity. We need also to retain
photon efficiency, a good time resolution and have acceptable thermal and mechanicalstresses for the detector. The single channel active area of the MCP-PMT with 16
channels and 1 in2 size is about 6 × 6 mm2 . Smaller SiPMs have been considered tolimit the impact of the dark count rate to the single photoelectron spectrum. Sixteen
106

Hamamtsu SiPMs 1.3 × 1.3 mm2 size and 50 µm cells have been irradiated from 1.03 ×
109 n/cm2 to 5.07 × 1011 n/cm2 . The single photon spectra are shown in Fig. 6.4 for5.07 x 1010 neutrons/cm2

5.07 x 1011 neutrons/cm2

SiPM #3

7)# entries

7)

0 °C

# entries

20 °C

3)

1)

ADC
5) counts

5)

2)- 20 °C# entries# entries

SiPM #0

6)

7)

6)

3)

4)

5)

ADC counts
5)

ADC counts# entries# entries

ADC counts
4)2)# entries# entries

0 °C- 20 °C

ADC counts

ADC counts- 40 °C

20 °C

7)

ADC counts- 40 °C

ADC counts

Figure 6.4: Single photoelectron spectrum (number of entries vs signal amplitude in
ADC counts) for an irradiation of 5.07 × 1011 n/cm2 (left) and 5.07 × 1010 n/cm2 (right)
at different temperatures. Signal peaks have been fitted with the convolution of a
Poisson distribution with some double Gaussian functions (dashed lines), a Gaussian
background (yellow) has been added to the fit.

5

10

15

two different irradiations at different temperatures. In the most critical condition with
5.07 × 1011 n/cm2 and 20 °C the single photon peak cannot be distinguished from thedark count background. From this preliminary test SiPMs operating at -20 °C is apossible solution also with a factor of 5 safety margin. However it should be notedthat using 1.3 × 1.3 mm2 SiPMs will require 20 times more electronic channels or amultiplexing downstream of the SiPM amplification.
The expected improvements in the SuperKEK instantaneous luminosity will require
to tighten the readout timing windows in most detector subsystems. As the result, theTOP-based contribution to the online event time logic of the L1 trigger will become
even more important. Significant backgrounds expected in the future call for novel
machine learning and artificial intelligence algorithms to be implemented in the trigger
firmware logic of the existing trigger boards UT4. We anticipate also to take fulladvantage of the new trigger board UT5 which is based on Versal FPGA architecture
and is being developed at KEK. This work will also require to upgrade firmware of thetrigger path in TOP FEE. Backend TOPTRG L1 work is further discussed in the TRG
section of this CDR.

6.1.4
20

Design development and human resources

The design of the electronics for the readout of all the channels digitized by the new
ASoCv4 is still to be defined. To properly estimate the power consumption reduction107

5full details of the new boards are needed.
The primary option for the photosensors is to replace the older ALD MCP-PMTs,
when they reach their lifetime limit, with life-extended ALD MCP-PMTs.
It is anticipated that Nagoya U., INFN/U. Padova and INFN/U. Torino will work on
the photosensor upgrade, while Indiana U, U. Hawaii, U. Pittsburgh and Nalu Scientificpersonnel will contribute to the electronics upgrade.
The TOP-based L1 trigger work in TOP FEE and in the TOP TRG system will be
performed by U. Hawaii and U. Pittsburgh with (being discussed) contributions fromother universities and laboratories.

10

6.1.5

15

20

Cost and schedule

It is anticipated that 1.5 FTE of engineering effort is required for the design of thecarrier card board in years 1 and 2. In addition 1 postdoc and 1 graduate student willassist with assembly, test and documentation under the oversight of the U. Hawaii instrumentation frontier professor. Board fabrication and assembly costs are based upon
those realized during earlier prototyping. After the pre-production a yearly firmware
development engineering support is costed during the 2 years of production, and at a
reduced level during commissioning. The estimated costs of the electronics upgrade
are described in Table 6.1. The estimated costs for the replacement of the 220 ALD
MCP-PMTs are described in Table 6.2 for the life-extended ALD MCP-PMT option andin Table 6.3 for the SiPM option. The estimated cost of human resources for TOP-based
TOP FEE and L1 trigger work is 0.5 FTE (from base funding). Estimates of the costs
associated with the new trigger boards are discussed in the TRG section of this CDR.
Table 6.1: TOP readout electronics upgrade cost estimate.
Phase
Pre-production
Production
System test
Commissioning
SubtotalContingency (20 %)Total

ASoC ASIC readout Upgrade
Resource
Basis of estimate
Engineering & test
1.5 FTE
Parts procur. & assembly cost summaryEngineering & test
1.5 FTE
Parts procur. & assembly cost summary
Beam test
past experience with
IRSX
Integration & Installation 1.0 FTE

108

Cost (k$)
610
120
610
800
500
445
3085
617
3702

Table 6.2: Life-extended ALD MCP-PMT upgrade cost estimate
Phase
Production

Commissioning
Total

Resource
MCP-PMT Production
MCP-PMT Test
Assembly parts
Integration

Basis of est.
Engr. est.
Engr. est.
Engr. est.
0.5 FTE

Cost (k$)
1500
15
50
50
1615

Table 6.3: SiPM upgrade cost estimate
Phase
Pre-production

Production

Commissioning
Contingency (20 %)Total

Resource
Engineering
SiPM R&D
Cooling R&D
Engineering
SiPM + assembly parts
Cooling system
Integration

109

Basis of est.
0.5 FTE
Engr. est.
Engr. est.
0.5 FTE
Engr. est.
Engr. est.
0.5 FTE

Cost (k$)
50
10
50
50
700
100
50
202
1212

6.2

5

10

15

Editor: L.Piilonen, X.Wang, G.Varner, R. Peschke
Two independent upgrades have been proposed for the Belle II KLM subdetector,
which identifies KL mesons and muons. The KLM is described briefly in Sec. 6.2.1.
The first upgrade adds the capability of energy measurement for the KL mesons
via a time-of-flight measurement in new scintillator bars. This upgrade would rebuild each detector panel and replace all existing resistive plate chambers (RPCs) inthe barrel with scintillators. The precision-timing aspect of this upgrade is describedin Sec. 6.2.2. At the same time, the existing scintillator readout would be replacedby moving the external analog and analog-to-digital readout elements into the detector panels, simplifying immensely the external connections. This is described inSec. 6.2.2.5.
The second upgrade continues to use the existing RPCs, rather than replacing themwith scintillators, and operates them at somewhat lower voltage in avalanche mode
rather than in the extant streamer mode. The motivation here is to improve the immunity to the ambient neutron background that would otherwise lower the instantaneous
efficiency of the RPCs. This upgrade is described in Sec. 6.2.3.

6.2.1
20

25

30

35

40

KLM

KLM geometry and instrumentation

The KLM subdetector occupies the volume outside the superconducting solenoid, spanning radii of 200 − 340 cm in the octagonal barrel and 130 − 340 cm in the forward
and backward endcaps. Large-surface-area detector panels of about 3.1-cm thickness
are sandwiched in the gaps between the steel plates of the solenoid’s magnetic-field
flux return (Fig. 6.5). Each panel contains either RPCs (barrel layers 3–15) or scintillator strips (barrel layers 1–2, forward-endcap layers 1–14 and backward-endcap
layers 1–12). Each readout strip is about 4 cm wide, measured orthogonal to a linefrom the SuperKEKB e+ e− collider’s interaction point (IP); this matches the Molièreradius of muons originating at the interaction point and traversing the electromagnetic calorimeter located just inside the solenoid. Each strip can be up to 2.7 m long.
Each detector panel provides a three-dimensional measurement of a through-goingparticle’s position via hits on two orthogonal readout strips within the panel, constrained by the spatial location of the panel itself. There are about 38,000 readout
channels in the KLM, almost equally split between RPCs and scintillators.
Each RPC has two orthogonal planes of copper strips (on kapton sheets) that image
the streamer that forms within either—or both—of the two independently operated
RPCs within the detector panel when a charged particle passes through either RPC gas
gap. Each strip acts as a transmission line that delivers the imaged streamer-mode
pulse (amplitude ∼ 200 mV) to the panel’s readout end.
The cross section of each barrel scintillator strip is 4 cm × 1 cm, so each panel in layers 1–2 contains 54 z-readout strips and between 38 and 45 ϕ-readout strips, for a totalof 92 to 99 strips. The cross section of each endcap scintillator strip is 4 cm × 0.7 cm;
each panel contains 75 x-readout strips and 75 y-readout strips for a total of 150
strips. Photons from each scintillator strip are collected by an embedded wavelengthshifting optical fiber (Kuraray Y11(200)MSJ) and detected by a Silicon PhotoMultiplier1101616.0
101.6
Layer 2 (RPCs)
Kite

Bar

Kite

Solenoid

2202.02153.02109.02060.02019.0Layer 1
(scintillators)
2000.0Layer 0(scintillators)

1533.91708.2

Figure 6.5: End view of one octant of the KLM barrel, showing the thin detector
panels (light gray) sandwiched between the steel plates (dark gray) of the solenoid’s
flux return. The kites and bar are removable steel plates.

5(SiPM), also known as a Multi-Pixel Photon Counter (MPPC)—an avalanche photodiode operated in Geiger mode. This solid-state device, the Hamamatsu S10362-13050C, is affixed directly to the scintillator strip at one end of the WLS fiber. The fiber’sother end is mirrored.
The RPC-strip and scintillator-MPPC signals (preamplified internally by a factor of∼10 for the scintillators) are routed via ribbon cables to external readout electronics
mounted on the magnet yoke.

6.2.2
10

15

20

25

Upgrade path 1: precision timing for KL momentum

The motivations for pursuing the precision-timing upgrade path are twofold. First, it
improves our ability to use timing information to reject out-of-time hits in the KLM arising from the ambient neutron flux that would otherwise contribute to the backgroundin any B-physics or dark-matter related searches. Second, it adds the capability tomeasure the four-momentum of KL candidates—not just the direction as is presently
done—so that B or D decay modes with this particle in the final state can be addedwith a quality similar to that attained in the modes with the KS meson. (In certain
CP -sensitive studies, the opposite CP value for KL vs KS bolsters our confidence inthe validity of the measurements with the KS meson.)
There is no marked difference in the muon- or KL -identification performance measures for the existing RPCs and scintillators. Both are able to measure the direction of
the line joining the interaction point (IP) to a reconstructed hadronic cluster to about2◦ in the polar and azimuthal angles (assuming that the cluster was created by a parent
KL meson that originated at the IP).
The precise hit-time measurement can be used to veto any hit that is more than
±1 ns out of coincidence with the event’s electron-positron collision time, which would
be an improvement over the present ±10 ns coincidence-window capability. This would
permit better rejection of out-of-time background hits that would otherwise contaminate the prompt KL -candidate population or any dark-matter-like event.
111

5

10

15

20

25

Additionally, better scintillators and readout electronics would provide a precise
time-of-flight measurement for the KLM clusters that arise from hadronic interactionsof a KL meson in either the yoke steel or in the ECL (with shower leakage into theKLM). This time-of-flight measurement requires both the start time of the electronpositron collision at the IP (known to better than 32 ps) and the stop time at the pointof the KL hadronic interaction (or debris) in the KLM. This flight time and the flight
length from the IP can be converted to the candidate KL meson’s momentum magnitude, which is not measurable in the present KLM. Combining this with the alreadymeasurable direction, the four-momentum of this hadron can be determined. A stoptime resolution of δt = 100 ps and a flight length of 2 m from the IP provides a momentum resolution of δp = 0.19 GeV/c for a KL with a momentum of 1.5 GeV/c, i.e., afractional resolution of δp/p ≈ 13%. The new ability to measure the four-momentum
improves the momentum resolution of the parent particle in physics modes with a KL
meson in the final state, such as B 0 → J/ψKL , and reduces the accidental backgroundin such modes.
With several years of R&D in hand, it is found that the combination of pure scintillators with long attenuation length and without an embedded wavelength-shifting fiber,large-area photosensors, and custom preamplifiers with improved timing can achieve
the aforementioned stop-time resolution. This R&D program, elaborated in the following subsections, encompasses the evaluation of new large-area photosensors from
Hamamatsu and NDL (Beijing) coupled to new preamplifiers tailored for SiPM readout, with one or more SiPMs attached to a single scintillator strip (up to 1 m in length);
the Geant4 simulation of each geometry variant (for different scintillator thicknesses
and arrangements); the assessment of various algorithms to extract the stop time of
the collection of hits within a cluster; and the resultant timing and momentum resolution for the associated KL meson.
6.2.2.1

30

35

The new 6 × 6 mm2 Hamamatsu and NDL SiPMs (S14610-6050HS and EQR15 116060D-S, respectively) are combined with a novel high-speed and low-noise preamplifier [105]. This preamplifier has good gain stability, a bandwidth of 426 MHz, abaseline noise of ∼0.6 mV, and a dynamic range for the input signal of up to 170 mV.
Figure 6.6 shows the R&D setup for studying the intrinsic time resolution for various
SiPM and preamplifier combinations illuminated directly by short laser pulses. A time
resolution of about 20 ps per cluster has been achieved for clusters with 50 or more
pixels (i.e., detected photons). Pole-zero cancellation in the preamplifier shortens thelong tail of a typical SiPM pulse to about 5 ns so that no pile-up is observed for laserpulse frequencies up to 20 MHz.
6.2.2.2

40

New SiPM and preamplifier

Time resolution of long scintillator bar

The precision of the time-of-flight measurement for a KL candidate is improved withmore struck scintillators in its hadronic cluster. This requires multiple layers with purescintillators having very long attenuation length, rather than the extruded scintillatorswith embedded wavelength-shifting fibers presently in use in the KLM.

112

Figure 6.6: Measurement of the intrinsic time resolution for SiPM-preamplifier combinations. The top schematic shows the diagram of the apparatus while panel (a) is
a photograph of the interior of the dark box containing the laser, the SiPMs and thepreamplifiers. Graph (b) shows the measured time resolution versus the number ofdetected photons for several SiPMs.

5

10

15

Pure-scintillator samples from Bicron (BC420) and GaoNengKeDi are tested. Figure 6.7 shows one test with two GaoNengKeDi strips (2 cm × 5 cm × 1 m) and arrays of
12 SiPMs at the two ends of each strip. The 12 SiPMs are arranged in parallel (“12P”)
or parallel+series (“3P4S”) on a printed circuit board. Two short scintillator strips(10 cm length) are used to generate a cosmic-ray trigger with a detection efficiency
in the long strip of nearly 100%. SiPMs in series show a better time resolution of
about 70 ps but with a smaller pulse height compared to SiPMs in parallel. A hybridarrangement of the SiPMs shows a good balance between time resolution and pulse
height.
To maintain the required timing precision over time, a robust calibration protocol
with frequent updates each day will be necessary for the TOF-like KLM. This requires
an integrated laser-calibration infrastructure. A cosmic ray test with six strips (δt ∼130 ps per strip), calibrated with a laser split among the six channels, yields a speed
distribution of (30.0 ± 3.4) cm/ ns or 11% for throughgoing cosmic rays (moving at
almost the speed of light).
6.2.2.3

20

Geant4 simulation for the KL momentum determination

One barrel octant of an upgraded KLM with 15 layers of precision-timing scintillator
panels is simulated with Geant4, as shown in Fig. 6.8. The active components are
pure scintillators with a long attenuation length and SiPM arrays at both ends of eachscintillator strip. The ECL is not included in this simulation. KL mesons and muons
with a momentum of 1 GeV/c are generated at the IP using the Geant4 particle gun.
The figure shows the responses for a muon (clean track) and a KL meson (hadronic113

𝜎 = 104𝑝𝑠
𝜎= 56𝑝𝑠𝜎long = 87𝑝𝑠

𝜎 = 90𝑝𝑠
𝜎Trg = 56𝑝𝑠

𝜎long = 70𝑝𝑠

Figure 6.7: Cosmic-ray test of two GaoNengKeDi 1 m long scintillators. Top left shows
the photograph and schematic of the apparatus, with a 3 × 4 array of SiPMs at eachend of a scintillator. Top right shows typical signal pulses for this (3P4S) and theparallel (12P) SiPM configurations. Bottom left graphs show the time distributions
and extracted resolutions for these two configurations. After unfolding the triggerscintillator resolution, the time resolutions are 70 ps (3P4S) and 87 ps (12P). Bottom
right scatterplot shows the range of observed pulse heights at the two ends of a scintillator.

114

cluster). A preliminary algorithm is developed to extract the KL arrival time fromselected (earliest) hit times in the cluster and has a FWHM resolution of about 240 ps.
A Gaussian fit to this leading-edge time distribution yields σ = (95 ± 4) ps, consistent
with the FWHM measure.

Figure 6.8: Geant4 simulation results for one barrel octant of an upgraded KLM with
precision time-of-flight scintillators. Top left: response for one 1 GeV/c muon. top
right: response for one 1 GeV/c KL meson. Bottom left: time distribution of hits ineach layer and cumulative for the KL cluster; the resolution of (95 ± 4) ps is extracted
from a fit to the leading-edge (earliest) times in this graph. Bottom right: population
distribution of scintillator signals for the KL cluster.

5

6.2.2.4

Replacement of the RPC layers in the barrel

10

Concurrently with the upgrade of the innermost three barrel layers for the precisiontiming capability, it would be prudent to take this opportunity to replace the RPCs in
the outer twelve barrel layers with present-design scintillator panels. The design ofeach panel would match that of the existing scintillator panels in the two innermost
barrel layers: each 4 cm × 1 cm scintillator strip is extruded with a central bore fora wavelength-shifting optical fiber that captures the light generated within the scintillator and transmits the wavelength-shifted light via total internal reflection to one
end that is instrumented with a single SiPM. A protective and reflective TiO2 -loadedpolystyrene jacket is co-extruded with the scintillator strip.

15

6.2.2.5

Improved scintillator front-end readout

Leveraging experience from the deployment and operation of the endcap and innerbarrel KLM scintillators, there is a compelling reason to refactor the readout in a waythat eliminates the many kilometers of twisted-pair ribbon cables. We propose a morecompact readout placed inside each detector panel that interfaces with the external
115

5

10

DAQ system via a few optical fibers. This readout upgrade would be implemented inconjunction with the TOF-like KLM upgrade described earlier.
The new readout embeds the readout ASICs on circuit boards inside each newdetector panel, and incorporates a newly developed more compact SCROD (Control
and Readout) card that also resides inside the detector panel. We propose to use the
HDSoC ASIC, which has four times higher channel density than the existing TARGETX.
The HDSoC ASIC utilizes a high-speed serial connection to the new SCROD FPGA. For
the barrel KLM (15 layers), this new readout system will consist of 480 HDSoC ASICs
and 240 SCRODs. For the endcap KLM (five new precise-timing layers at each end),
this system will contain 120 HDSoC ASICs and 40 SCRODs.
6.2.2.6

15

20

25

30

35

This cost estimate is based on a scenario in which the three innermost barrel layers andfive innermost layers in each endcap are replaced with precision-timing scintillators
while the RPCs in the outermost twelve barrel layers are replaced with standard-design
scintillators and SiPMs (Sec. 6.2.2.4). All new detector panels would be instrumented
with the new readout described in Sec. 6.2.2.5. Labor cost is not included here sincesome of the personnel might already be on an institution’s payroll.
Table 6.4 itemizes the estimated material costs for each element of this upgrade.
The precision-timing scintillators would cost $0.3M for the three barrel layers and$0.5M for the ten endcap layers, based on material costs in China. With an array ofeight NDL SiPMs at each end of a scintillator strip, the photosensors would cost an
additional $0.8M for the barrel and $1.0M for the endcaps. Customized precisiontiming preamplifiers (one preamplifier at each end of a scintillator strip) would costan additional $0.3M for the barrel and $0.4M for the endcaps. Contingency is notincluded here.
The replacement of the RPCs in the outermost twelve barrel layers with currentdesign scintillator panels would cost about $3.4M, based on the actual costs for allof the components (strips, WLS fiber, photosensors, frame and passive materials, HV,shipping) and escalation for the greater geometric size and inflation at 3% per year.
This includes a 20% contingency.
The associated front-end readout upgrade is estimated to cost between $1.4M (utilizing the HDSoC ASIC with internal amplification) and $1.8M (if discrete amplifiers
are deployed instead). Development and prototyping of the new ASIC and SCROD
would add an estimated $1.6M to the total readout project cost.
In summary, the rough estimate of the material cost of this upgrade is $9.6M.
6.2.2.7

40

Materials cost estimate

Upgrade logistics

This upgrade requires the roll-out of Belle II from the interaction region and the opening of the forward and backward endcap doors to gain access to the barrel and endcap KLM detector panels. In the roll-in position, the base of the beamline-supporting
concrete shield prevents access to the bottom three sectors of the barrel KLM at theforward and backward ends.
If all existing barrel detector panels (backward and forward) are removed from theforward end and the new panels are inserted from this end, then all other-subsystem
116

Table 6.4: KLM Upgrade material cost estimates. This assumes that the items for the
TOF upgrade would be procured in China while the items for the RPC-replacement
upgrade and the readout upgrade would be procured in the US.

Barrel TOF Upgrade (layers 1–3)
Scintillator volume
Photosensors (16/scintillator strip)
Preamplifiers (2/scintillator strip)
Endcap TOF Upgrade (layers 1–5)x2
Scintillator volume
Photosensors (16/scintillator strip)
Preamplifiers (2/scintillator strip)
Barrel RPC-to-scint (layers 4–15)
Scintillator volume
WLS fiber length
Photosensors (1/scintillator strip)
Al frames, HDPE sheets, etc
Shipping (US to KEK)
HV modules (CAEN A1510)
HV mainframes (CAEN SY2527)
Contingency (20%)
Readout Upgrade
Parts for above layers
HDSoC-related R&D
ESTIMATED MATERIAL COST

5

10Unit cost

Cost (k$)

5.724 m3
75008
9376

$55000/m3$10/item$30/item

315
750
281

9.302 m3
96000
12000

$55000/m3$10/item$30/item

315
960
360

15.686 m32900 m22856
192 panels
28 crates
16 units
2 units$34000/m3$133/m$44/item$2700/item$7000/item$10750/item$20000/item

535
386
1005
520
196
172
40
571
1400
1600
9603

cables and services that reside in the service channels at the forward face of the barrel
yoke would have to be peeled away to permit these panel insertion/extraction operations, and then re-installed at their conclusion. In this forward-access scenario,
the other-subsystem cables and services at the backward barrel yoke would not be
disrupted. The KLM cables and services at both ends of the barrel yoke can be disconnected and reconnected without touching the other subsystems’ cables and services.
The endcap KLM sectors can be accessed without disturbing the other subsystems’cables and services.
We estimate that it would take one calendar year to accomplish these steps, including the time needed during the new-panel installation for testing and, if necessary,
remediation.

6.2.3

15

Quantity

Upgrade path 2: RPC avalanche mode

The second upgrade path does not replace any of the existing KLM detector panels
but instead addresses the inherent rate limitation of the barrel RPCs and their related
blinding by the ambient neutron flux in the KLM during high-luminosity operation in
the coming years. In this upgrade, the present streamer mode of RPC operation is replaced with the avalanche mode. In this way, the blinding from the localized discharge
117

5

10

of the glass-plate electrodes by the streamer is avoided, since the gas-gap electric field
will not be reduced so dramatically by the avalanche induced by an ambient neutron’sinteractions with the nearby material. Since the imaged pulse height is reduced by afactor of about 10, the avalanche-mode pulses will require new in-line preamplifiers
at the detector-panel faces at the forward and backward barrel yoke.
There are significant uncertainties in the expected particle flux at the design luminosity and in the RPC efficiency as a function of particle flux. Operating the RPCs inavalanche mode mitigates the dual risks of a higher-than-expected background neutron flux and/or a worse-than-expected rolloff of the RPC efficiency due to blindness
from the high flux.
6.2.3.1

RPC response in streamer and avalanche modes

Here, we explore the shifts in detector response when operating the KLM RPCs inavalanche mode. We also address the necessary modifications of the front-end electronics, gas mixture, and operating voltage.
The RPC rate capability is inversely proportional to the average charge ⟨Q⟩ produced in the gas gap by an incident particle [106]:
Rate Capability ∝
15

20

25

30

35

40

1
ρt⟨Q⟩

where ρ is the bulk resistivity and t is the total thickness of the electrodes. Given thatthe glass electrodes will not be replaced, this rate capability can only be increased
by reducing ⟨Q⟩. This approach is constrained by the existing noise levels within thesystem and the sensitivity of the readout electronics. We note that the charge received
by the front-end electronics is significantly smaller than the ⟨Q⟩ formed in the gas gap.
In our RPCs, the far end of each cathode strip is terminated to avoid pulse reflections:this implies a factor of two reduction in the signal at the preamplifier.
The KLM’s glass-electrode RPCs, originally built for the first-generation Belle experiment, have undergone thorough examination and analysis [107, 108, 109, 110, 111].
The electrodes surround a 2 mm gas gap. The ⟨Q⟩ values are depicted in Fig. 6.9 forstreamer-mode operation. In these papers, it was established that the typical streamer
discharge has ⟨Q⟩ ∼ O(100 pC).
This value is comparable to those in ATLAS for 2 mm gas gap RPCs with Bakelite
electrodes operating in streamer mode with various gas mixtures and a range of highvoltages: ⟨Q⟩ ∼ 30 pC [112], 70 − 120 pC [113] and 50 − 60 pC [114].
In the past two decades, it has become a prevailing practice to operate RPCs
in avalanche mode, wherein charge generation and amplification in the gas gap isquenched during the avalanche phase, avoiding progression to the streamer phase.
This can be achieved through the use of an electronegative gas like SF6 as a quenching agent and by reducing the operating voltage. As a result, avalanche events are
distinguished from streamer events by their notably smaller ⟨Q⟩ and distinct timing
characteristics. For the ATLAS RPCs operated in avalanche mode [112], a range ofvoltages with high efficiency and low streamer probability (for each gas mixture) canbe seen in Fig. 6.10.
The mean charge in the ATLAS RPCs operated in proportional mode is ⟨Q⟩ ∼ 9 pC
with these gas-mixture and HV modifications. This has been reduced further to ⟨Q⟩ ∼118

Figure 6.9: Mean charge ⟨Q⟩ for the Belle-style RPCs operated in streamer mode withvarious gas mixtures and over a range of high-voltage settings.
2 pC by the deployment of a novel preamplifier, as detailed in Sec. 6.2.3.3.
6.2.3.2

5

10

15

20

Gas mixture

Currently, the RPCs use a gas mixture consisting of 30% argon, 8% butane, and 62%
HFC-134a. Various studies have explored alternative gas mixtures with similar glass
RPCs, revealing a noteworthy shift toward lower signal charge while maintaining highefficiency. In particular, the introduction of SF6 , characterized by its high electron
affinity of 1.05 eV, appears to play a crucial role in avoiding streamer formation.
Ref. [107] primarily focuses on glass RPCs that closely resemble ours. It conducts
a comparative analysis of alternative gas mixtures, each of which contains SF6 andexcludes HFC-134a. This study reveals that the overall efficiency is only slightly lower
than that of the current mixture, and, notably, indicates a reduction in ⟨Q⟩ from 80 pC
to 25 − 40 pC. Notably, these test RPCs had a single gas gap, in contrast to the two
independently-operated RPCs within one KLM detector panel. Thus, the equivalent
two-gap efficiency of the test RPCs would have been ε2 = 1−(1−ε1 )2 . Recent advancements in front-end electronics might enhance the overall performance significantly.
The R&D task here is to find a suitable gas mixture and operating point for the highvoltage for our RPCs in avalanche mode to ensure high efficiency and low streamer
probability.
A key obstacle in identifying the suitable gas mixture revolves around environmental considerations, with particular attention paid to the potential introduction of SF6,
boasting a significantly higher GWP of 23,500 in contrast to HFC-134a, which has a
GWP of 1,300 [115]. Nonetheless, the research presented in Ref. [107] proposes a
gas mixture that would replace 62% of HFC-134a with just 2% SF6, resulting in a
noteworthy reduction in GWP emissions to the atmosphere. Regardless of the specific119Figure 6.10: Efficiency (solid) and streamer-probability (dashed) curves for the standard gas mixture and two selected HFO-based gas mixtures in ATLAS RPCs operated
in avalanche mode with a muon beam and no background radiation.
gas mixture employed, the collaboration is actively exploring the implementation of agas recirculation system.
6.2.3.3
5

10

15

20

Readout electronics

A corollary of the switch to avalanche-mode operation, with its much smaller ⟨Q⟩,
is the need for front-end preamplification of the signals as close as feasible to theirsources. The new preamplifiers would be placed at or near the ribbon-cable connectors
at the forward and backward ends of the barrel yoke. The preamplified signals would
then be routed along the existing meters-long ribbon cables to the digitization frontend electronics located on the magnet-yoke periphery.
The readout end of the ϕ-measuring strips inside the detector panel are very close
to these ribbon-cable connectors. In contrast, the z-measuring strips are located between 4 and 220 cm from these connectors and so their signals are propagated alonginterior ribbon cables to the external connectors. Any noise pickup and/or crosstalkon these internal cables would be unavoidable.
The initial focus of our R&D will involve measuring the existing electronic noise levels for the ϕ- and z-measuring strips when operating selected in situ RPCs in avalanche
mode. The signal height is expected to be approximately four to five times smaller thanthat in streamer mode.
With a robust characterization of the avalanche-mode signal and noise pulses, subsequent investigations will determine the options for the signal-processing chain, including at least a preamplifier stage but also, perhaps, incorporating the digitization
step into a unified readout solution. Several options are under consideration: pream120

510

plification plus continued use of the existing binary digitizers; integrated preamplification and binary digitization; preamplification and fine-grained pulse-height digitization (either integrated or separate); and pramplification with waveform digitization
as is done in the present scintillator front-end readout (either integrated or separate).
The challenge in each option is to find sufficient volume in the constricted and fullyallocated space at each end of the barrel yoke to add the preamplifiers.
Fig. 6.11 from the ATLAS collaboration demonstrates the performance of their
newly designed SiGe preamplifier [116], indicating that it achieves a 95% efficiency
with a signal height as low as ⟨Q⟩ = 2 pC, significantly improving upon the earlier
solution that required 9 pC and resulting in a markedly higher rate capability.

Figure 6.11: Total Charge versus Effective High Voltage (HVeff), highlighting the pointat 90% efficiency. Depicted in the graph are different data sets: black represents No
Front End (FE), blue represents Silicon Front End (Si FE), and red represents Silicon
Germanium Front End (SiGe FE). [116]15

The KLM RPCs exhibit a bulk resistivity approximately three orders of magnitude
higher than the ATLAS RPCs, resulting in a rate capability roughly three orders ofmagnitude lower. Nevertheless, with a similar upgrade approach, we have the potential to increase the KLM rate capability to approximately 10 Hz/cm2 , a level that
adequately accommodates the expected background rates at design luminosity, evenwhen considering the uncertainties discussed in the introduction.
6.2.3.4

20

R&D

Upgrading the current RPC system of the KLM detector presents a significant challenge, particularly as numerous detector components are immovable and unalterable.
Consequently, it is essential to characterize the existing electronic noise within the system to determine the detailed design of a preamplifier that would operate well with
121

5

10

15

20

the RPCs in avalanche mode. A few spare unused full-size RPC detector panels areavailable for these R&D studies, including investigations of optimal gas mixtures withlower-voltage operating points and evaluation of prototype pre-amplifiers. If necessary, we could use compact radioactive sources to create a radiation environment withan incident flux of 1 − 10 Hz/cm2 . Nevertheless, dedicated test-beam studies will be
required for comprehensive assessments.
The RPC avalanche mode upgrade encompasses several key areas of research anddevelopment (R&D). Firstly, the investigation into the detector itself is crucial, involving studies on various gas mixtures to validate results referenced in literature. To
accomplish this, a dedicated laboratory setup using spare RPCs is essential, allowing
comprehensive testing under diverse gas mixtures, bias voltages, and radiation environments. Another significant area of study focuses on the front-end electronics, as
reducing the charge generated per event might render signals too weak for current digitization cards. Therefore, exploration of low-noise preamplifiers becomes necessary,
inspired by successful implementations within the ATLAS community. Determining theparameters for future RPC preamplifiers is pivotal in ensuring compatibility with existing digitization cards. Additionally, investigating the detector’s gas system is vital,
considering the current venting of detector gas into the atmosphere. Developing a gas
recirculation system, potentially inspired by practices in the ATLAS community, might
be necessary based on the determined gas mixture, aiming to enhance operational
efficiency and mitigate environmental concerns.
6.2.3.5

25

30

35

40

45

Cost estimation and risk assessment

At its early stages, this upgrade proposal heavily relies on substantial R&D efforts
for viability. Compared to the alternative proposal focusing on entirely new BKLM
detectors, this approach primarily emphasizes upgrading the existing BKLM detector,
making it notably more cost-effective. Precise cost estimations are challenging without
further R&D, but significant material costs outlined in the previous chapter, around
9.6 M$, would be eliminated. The proposed hardware additions primarily include
preamplifiers and potentially a gas recirculation system, only if it proves more costeffective than the current venting setup. (We vent all RPC gas to the atmosphere:
during beam operation, this includes 270 kg/month of HFC-134a, a greenhouse gas
with a global warming potential of 1300, compared to 1 for CO2 and 23500 for SF6 .)This indicates potential operational cost reduction compared to the current detector
configuration, making it a promising avenue for upgrade.
For a preliminary cost estimate, it is assumed that each preamplifier costs approximately $30, and there is an estimated need for preamplifying roughly 16,000 channels.
This results in a cost estimate of around $0.5 million.
The risks associated with this upgrade can be categorized into two parts. Firstly,
there’s the concern regarding the feasibility of the upgrade itself. Early-stage researchand development (R&D) can help determine the feasibility of the upgrade. In case it
is not achievable, the possibility remains to continue operating with the current setup
or, if there’s sufficient time, switch to a full scintillator readout.
Secondly, the risk of damaging the subdetector or adjacent subdetectors exists.
Access to the detector itself is minimal, reducing the likelihood of damaging adjacentsubdetectors. Introducing a new gas mixture to the RPC chambers poses a potential122risk, but this can be mitigated by careful consideration during the R&D phase.
Overall, the risk of ending up with a non-functional detector after the upgrade isminimal. Additionally, the risk of damaging surrounding detectors is low since thedetector remains stationary during the upgrade process.

123

Chapter 7
Software, Trigger, DAQ
7.1
5

10

15

20

25

30

Software, machine learning, AI

Editor: T.Kuhr
The software for the simulation, reconstruction, and analysis of the data from theupgraded detector will be an evolution of the current Belle II software, basf2 [12,13]. Nevertheless, substantial improvements are required to cope with the increased
data rate, occupancy, and background level. On the one hand this challenge has tobe addressed by algorithmic improvements that find a good balance between computing and physics performance. On the other hand new technologies provide various
opportunities for software enhancements.
So far the Belle II software only runs on x86 architectures. First attempts weremade to port it to the ARM architecture which may result in a more energy efficient
execution. A large potential is expected from delegating compute-intense tasks to GPU
resources. This requires a porting of algorithms to GPU architectures, an adjustmentof the software framework to run on heterogeneous computing resources, and theirintegration in the distributed computing system. GPU resources are also essential forthe training of large machine learning models and can facilitate their application.
Machine Learning (ML) and Artificial Intelligence (AI) are another prime example
of a new technology with huge potential. Already now AI algorithms are used at
Belle II or are under development. ML applications at trigger level are described in
Section 4.3.1. A prominent ML application example in the offline software is the Full
Event Interpretation [74], see also Section 4.4.
Generative ML models are a promising technology to reduce the resource demandof simulations. Currently a non-negligible part of storage resources is needed for an
accurate simulation of beam background hits in the detector. For this task random trigger data is taken and overlaid to simulated signal hits. It requires DAQ bandwidth, disk
space, and a distribution to MC production sites. This can be avoided by generating
the background hits on-the-fly with a generative model.
Such an approach is most beneficial for detectors with large readout windows andthus large beam background overlay event sizes. This is currently the case for the PXD
and the ECL. It could be shown, that the effect of PXD background on the track reconstruction can be reproduced with background generated by a Generative Adversarial
Network (GAN) architecture with newly developed extensions [117]. GANs are also
124

5

10

15

studied for the simulation of ECL waveforms. A challenge that still has to be addressedis the generation of correlated background in different parts of the detector.
A complementary approach to the beam background is to reduce it as much as
possible (see also Section 3.1). This requires a good understanding of the accelerator
conditions that lead to the various types backgrounds. Such an understanding wasachieved by a neural network trained on diagnostic data from SuperKEK and BelleII [118]. The model can be used to analyze the background composition of recorded
data and to predict background levels. The latter provides the potential to optimize
the running conditions.
In general, AI has revolutionized how things are done in many areas. Problems ofclassification, optimization, and generation are nowadays often tackled by AI methods.
Their potential is not yet fully exploited in particle physics in general and Belle II inparticular. And the development is still ongoing rapidly. For example, Large Language
Models (LLMs) may help to develop better physics algorithms, design more robust
analyses, or write documentation and papers with fewer effort. These are examples ofareas that should be explored in the context of a Belle II Detector Upgrade.

7.2

Trigger

Editor: T.Koga

7.2.1
20

25

30

35

Introduction

The hardware trigger system (TRG) was designed to satisfy the following requirements
at the nominal design luminosity:• high efficiency for hadronic events from Υ (4S) → B B̄ and from continuum• a maximum average trigger rate of 30 kHz• a fixed latency of 4.5 µs• a timing precision of less than 10 ns• a trigger configuration that is flexible and robust
Figure 7.1 shows an overview of the system. The CDC and ECL take a major role totrigger charged particles and photons. The KLM triggers muons and the TOP measures
event timing precisely. The information from sub detector triggers are send to the
Global Reconstruction Logic (GRL) for the matching and the final trigger decision ismade at the Global Decision Logic (GDL). Although TRG has been developed following
the original design, further upgrades are needed to satisfy the requirements with thenominal design luminosity, due to the higher beam originated background rate than
the initial expectation. In addition, it is important to keep high efficiency of the nonB physics with low multiplicity of the final state particles, such as physics with tau,
dilepton, and dark/new particles searches. In order to achieve that, several upgrades
are planned for both hardware and firmware with various time schedules.

125Figure 7.1: Schematic view of the hardware trigger system

5

10

15

7.2.2

Hardware upgrade

7.2.2.1

Universal trigger board

TRG consists of several kinds of FPGA-based electronic circuit boards. Each board
has optical transceivers to communicate with other boards and the detector front-end
boards. The universal trigger board (UT) has been commonly developed for all sub
trigger systems to implement core trigger logic, as shown in Table 7.1. Upgrade of the
UT is on-going to improve the resource size of the FPGA and the optical transmissionrate, in order to implement more complicated trigger logic with increased informationfrom the detector. At present, the 3rd (UT3) and 4th (UT4) generation boards areused for the physics operation. The number of UT4 boards is planned to be increasedin the short and medium term. Development and production of the next generation of
the UT (UT5) are scheduled in the medium and long term. A high-end FPGA, Xilinx
Versal, is used for UT5 to implement machine learning based logic with large DSP and
AI engine. UT5 is being developed at KEK with cooperation of Belle II, ATLAS in Japan,
and KEK electronics group via collider electronics forum, for not only Belle II but also
for any collider experiments.
Table 7.1: Specification of the universal trigger boards.
UT generation
Main FPGA (Xilinx)
Sub FPGA (Xilinx)# Logic gate
Optical transmission rate
RAM
# UT boards
Cost per a board (k$)
Time schedule

UT3
Virtex6
XC6VHX380-565−−
500k
8 Gbps
−−
30
15
2014-

126

UT4
UT5
Virtex Ultrascale
VersalXCVU080-190
Artex7
Artex7, Zynq
2000k
8000k
25 Gbps
58 Gbps
DDR4
DDR4, UltraRAM
30
10
30
50
2019-2026
2024-2032

Figure 7.2: Wires used for present(red) and new(yellow) tracking algorithm.
7.2.2.2

5

10

CDCTRG upgrade with CDC front end board

CDC front end board and CDCTRG system will be upgraded around 2026. Bandwidthof the optical transceiver of the front end board is increased from 3Gbps to 10Gbps inorder to send TDC and ADC of all wires to the trigger system, as shown in Table 7.2
and Fig/7.2. The most upstream board of CDCTRG, Merger board, is upgraded to UT4
to increase bandwidth of the receiver. All CDCTRG firmware will be updated with the
TDC and ADC, in order to improve tracking and background rejection performances.
The expected performance improvement is studied by simulation, including upgrade of
tracking logic with machine learning. The vertex resolution is expected to be improved
from 10 cm to 5 cm, with 50% trigger rate reduction[119], as shown in Fig. 7.3.
Production of UT4 has been started.
Table 7.2: Information send from CDC front end board to TRG.

existence of wire hit
hit timing (TDC)
charge (ADC)

7.2.2.3

15

20

present
new
80% of all wires all wires
17% of all wires all wires
not sent
all wires

ECLTRG upgrade

At present, analogue sum of 16 crystals (Trigger cell, TC) are used as a minimum unitof ECLTRG to measure energy deposit in the calorimeter. Upgrade of shaper+DSP
and ECLTRG boards is considered in order to divide the analogue readout from eachcrystal. Background rejection performance is expected to be improved by measuringvertex of the photon with the cluster shape, like ratio of the energy deposit between
centered and outer crystals in a cluster. At offline analysis, 30% background photon
rejection has been achieved by using the cluster shape, and similar improvement is
expected to the ECLTRG trigger rate. In addition, signal efficiency is expected to be
improved by reducing pile up effects, when two daughter particles are close to eachothers within 20–40 cm on the ECL surface. Simulation study is on-going to evaluate
the expected performance quantitatively and fix the initial design.
127

Figure 7.3: Expected improvements of signal efficiency and background rejection rate.
Blue is present logic and the others are new logic with several machine learning architecture.
7.2.2.4

5

10

In principle, only TOP-based trigger could potentially provide online event time resolution of better than 6 ns. However, to achieve this goal (while also making TOPTRG
operate with a high efficiency) requires to implement a reliable way of estimating thelocation of the source of optical photons inside the quartz bars of the TOP detector.
Another challenge is posed by the beam-related background which severely affects the
TOP detector. To solve these problems and make TOP-based online event time usefulfor purposes of L1 trigger, our main focus will be in two areas: matching TOP trigger
information with CDC 3D trigger and, also, taking advantage of continuing progressin the area of ML/AI, especially with the future trigger board UT5 with Versal FPGA
architecture.
7.2.2.5

15

20

25

TOPTRG upgrade

VXDTRG with VXD detector upgrade

At present, PXD and SVD have no functionality to provide trigger signal. With the VXD
upgrade, it might be possible to implement a vertex trigger to reject beam backgrounds
originanting outside the interaction region, reducing the trigger rate. An initial evaluation of the new trigger logic performance was done based on simulation with a Thin
Fine-Pitch SVD sensor scenario [120], which is no longer an upgrade option. In that
hypothesis, OR hits in multiple channels (0.6-1.0 cm area with 50-80 µm strip pitches)
are sent from the front-end to the UT4 via a merger board as shown in Fig. 7.4. On
the UT4, track candidates from interaction points are selected, by requiring 3 of 4
layer hits with expected hit pattern saved on FPGA with a look up table. As shown in
Fig. 7.5, 1-2 cm vertex resolution is achieved with 95% tracking efficiency and a few
percent fake trigger probability. By taking matching with CDCTRG track, trigger rate
coming from beam background can be rejected more than 90%. Similar studies areon-going for the CMOS MAPS VTX scenario, using the trigger logic under development
for the Obelix chip (see Section 5.1.2.3).
128

Figure 7.4: Over view of the VXD trigger system with a Thin Fine-Pitch SVD sensor
scenario.

Figure 7.5: Expected trigger efficiency of VXDTRG along with vertex Z position ofsimulated particles.

1297.2.3

5

Firmware upgrade

In order to improve signal efficiency and background rejection, several firmware upgrades are planned with the upgraded hardware, as shown in Table 7.3. Because thefirmware modification can be done at anytime with FPGA, the plan can be changed
flexibly depending on superkekb machine condition, physics discovery, performanceof new logic, new idea of core logic and human resources etc. For each upgrade component, one or two full time workers are needed as the human resource.

130

Table 7.3: TRG firmware upgrade plan.
Component
CDC cluster finder

FeatureImprovementtransmit TDC and ADC beamBG rejecfrom all wires with the tion
new CDC front end
CDC 2Dtrack finder
use full wire hit patterns increase
occuinside clustered hit
pancy limit
CDC 3Dtrack finder
add stereo wires to track enlarge θ angle
finding
acceptance
CDC 3Dtrack fitter (1)increase the number of beamBG rejecwires for neural net train- tion
ing
CDC 3Dtrack fitter (2)improve fitting algorithm beamBG rejecwith quantum annealing tion
methodDisplaced vertex finderfind track outside IP orig- LLP search
inated from long loved
particle
ECL waveform fitter
improve crystal waveform resolution
fitter to get energy andtiming
ECL cluster finder
improve clustering algo- beamBG rejecrithm with higher BG con- tion
dition
KLM track finder
improve track finder with beamBG rejec2D information of hitting tion
layers
TOP-CDC track matching
include 3D information
event time resolution
TOPTRG ML-based algorithms implement
ML-based beamBG rejecsignal/background sepa- tion
rationVXD trigger
add VXD to TRG system BG rejection
with new detector andfront end
GRL event identification
implement neural net signal efficiency
based event identification
algorithm
GDL injection vetoimprove algorithm to veto DAQ efficiency
beam injection BG

131

Time #UT
2026 10

2022

4

2022

4

2025

4

2025

4

2025

1

2026

–

2026

1

2024

–

2024

–

2026

2

2032

–

2025

1

2024

–

7.3

DAQ

Editor: S.Yamada

7.3.1
5

10

15

20

25

30

35

40

Optically connected trigger timing distribution system

The Belle II data acquisition system incorporates a trigger and timing distribution(TTD) system [121], which distributes the system clock, trigger signal, and related
signals for synchronization and control to both front-end electronics and readout electronics boards. This system is designed to collect error, busy with data-processing, andother status signals from these electronics devices to monitor the data taking status andpause the trigger distribution when necessary to prevent recording corrupted data. A
schematic view of the TTD system as a part of the DAQ system is shown in Fig. 7.6.
The TTD system consists of a hierarchical structure of signal distribution modules of
the two-slot-wide 6U VME form factor called FTSW (Front-end Timing Switch) [122].
For the communication between modules, a custom protocol called b2tt was developed and utilized. The current connections between FTSW modules and between the
FTSW and front-end / readout devices are mostly made over a CAT7 network cables
using the RJ45 ports of the FTSW. For longer transfer distances, optical fiber connections with a limited number of SFP transceivers are also used with a daughter card
attached onto the FTSW module. In the operation of the Belle II experiment since2019, however, we experienced that sometimes the b2tt(Belle II trigger and timing)
link via some of the CAT7 cables between FTSWs and electronics modules gets lost
due to signal glitches, most likely due to external electromagnetic interference (EMI).
The problem does not happen in the connection between FTSWs within the same VME
crate, or in the optical connections. When the b2tt link is lost, data acquisition stops,
causing a non-negligible loss in the data acquisition efficiency until the error state isreset by starting a new run
In order to prevent this issue and establish a more EMI-tolerant TTD system, atransition to more extensive use of the optical interconnection is planned. To increase
the number of optical connections, it is essential to develop a new version of the
FTSW module (FTSW4), which is capable to connect to a sufficiently large number ofdestinations via optical fibers. The new FTSW4 module is also designed for the trigger
timing distribution to the newly designed CDC front-end boards described in Sec. 5.2,
which will receive the b2tt signals over optical fibers. The large number of the CDC
boards demand the FTSW4 to have a high channel density. Among the possible choicesof the high density transceivers, we decided to adopt the QSFP transceivers, which has
a moderate channel density but is highly cost-effective. The board design we adopt is
a two-slot-wide 6U VME module on which QSFP transceivers are mounted as shown in
Fig. 7.7. Equipped with 12 QSFP transceivers for trigger timing distribution and two
more QSFP transceivers for uplink and other purposes, the FTSW4 module is capable
to connect up to 24 front-end electronics or readout boards, with each connection
using two pairs of optical fibers. Additionally, it has six RJ45 ports for conventional
CAT7 cable connections. The trigger timing distribution is controlled using a costeffective Artix 7 FPGA.
The upgrade project is scheduled as follows: The first two prototype modules will132

Figure 7.6: Schematic view of the Belle II DAQ system
Table 7.4: Schedule of the production of the FTSW4 modules
FY2023
2 modules

FY2024FY2025

prototypemass production
20 modules 20 modules
Table 7.5: Schedule of the production of the FTSW4 modules

be manufactured in FY2023. Following successful testing of the prototype module,
mass production is set to occur in FY2024 and FY2025, aiming to produce about 40
modules, to cover the connections inside the E-hut and the connections to the newly
designed CDC front-end readout boards (Table. 7.5).
5

10

7.3.2

Improvement of throughput in readout system

The functionalities of the readout system [123] are receiving data from front-end electronics boards and performing partial event-building and a basic sanity check of data.
The Belle II DAQ system has a common readout interface with front-end electronics (FEE) of all sub-detectors except for PXD, which needs special data-treatment for
online data-reduction.
At the beginning of the Belle II experiment, we used a COPPER board [124], on
which we attached newly developed receiver cards, a CPU card, and an interface board
with the TTD system. However, since it becomes difficult to maintain a COPPER board
133

Figure 7.7: Conceptual drawing of the FTSW4 module

5

10

15

20

25

itself, which has been used from the previous Belle experiment and its technology, PCI
bus and Gigabit Ethernet aresomewhat obsolete. Sowe decided to replace the current
readout system with state-of-the-art technology. Since electronics boards with suchfunctionalities were already under development in other physics experiments for thebasically same purpose, we decided to use a PCIe40 board [89], which was originallydeveloped for the LHCb and ALICE experiments.
A PCIe40 board is equipped with Intel Arria10 FPGA with 48 bi-directional serial
links and 16 lanes of PCI Express 3.0 for I/O interfaces. With the much larger numberof 15 links per board compared with a COPPER board, which has 4, we can makea more compact readout system with PCIe40 boards. We then concentrated on thedevelopments of firmware and software to make the hardware fit in with the Belle II
DAQ system. The firmware of the new PCIe40-based system takes care of formatting
and data checks which has been done by the software on CPU board in the COPPER
system. Those formatted data are read by software running on the host server of a
PCIe40 board and are distributed to different HLT units for online event selection.
After implementing those functionalities in the new hardware, we performed a hightrigger-rate test with dummy data and achieved the throughput of 3.4GB/s per one
PCIe40 board up to a readout PC.
The replacement has been completed in the LS1. Although the upgraded system
can handle the 30kHz trigger rate, which is the designed rate for Belle II, in case of
unexpectedly high beam background occurs as luminosity increases in Run2, we willconsider further upgrade of the readout hardware. One candidate for the hardware is
PCIe400 board [125], which is an upgrade version of the PCIe40 board, where development is ongoing in the LHCb collaboration targeting the LHC-LS3 upgrade. Whilestill in the design phase, the new board will provide enhanced capabilities, supporting
134

5

10

15

20

25

30

35

40

higher bandwidth with a host server adopting newer technologies such as PCI Express
Gen 5.0 and the latest FPGA. This will enable greater data-processing capacity andtransceiver capabilities with increased bandwidth. Similar to the approach taken with
the implementation of the PCIe40 board, our focus will be on firmware and softwaredevelopment to integrate it with the Belle II DAQ system.

7.3.3

High-level trigger

7.3.3.1

Increase of computational power in HLT system

The HLT system [126] of the Belle II DAQ system performs an online event reconstruction and issues a software trigger to reduce the number of events to be recorded on
disks, which is necessary to save offline computing resources with the minimum effecton signal efficiency. The HLT system also sends region-of-interest (ROI) information
to the PXD readout system, which is calculated by extrapolating a track obtained from
CDC and SVD data. PXD data outside of the ROI are discarded, as most of themdo
not contain hits of signal tracks. For these purposes, the HLT system collects eventfragments from all-subdetectors except for PXD and performs online event reconstruction after event-building [daq:be] is done, which requires high computational powerand high network throughput. Like the readout system, the throughput requirement
for the HLT system also gradually increases with the luminosity of the SuperKEKB accelerator. In addition to the luminosity, the beam background, which is difficult toprecisely predict for increased luminosity in future, can also affect trigger rate andevent size. Considering that, we adopted a staging plan of the reinforcement of the
HLT system to keep up with increasing trigger rate and event size. With this staging plan, we can also benefit from the evolution of information technology to prepare
computational resources with higher performance at a later stage.
To make the staging plan possible, the HLT system was designed to be a scalable
system, consisting of multiple units, which can work in parallel, and adding otherunits will increase the total throughput of the system linearly. An HLT unit consistingof servers and network switches is stored in one rack and performs event building
and online event reduction independently of other units. In the HLT unit, there are
an input server, worker nodes and an output server. The input server receives event
fragments from the read-out system, builds events, and sends them to worker nodes,
which perform online event reconstruction. Since the online reconstruction of an event
can be done independently of other events, events which are built in the input server
are distributed to different worker servers to perform online reconstruction in parallel. On each worker server, there are multiple reconstruction processes running forparallel data-processing to make use of multiple cores in the server. Event selection is
performed here with reconstructed information and events which do not meet highlevel trigger conditions are discarded. The output server then sends raw data withtrack and cluster information to a storage system, which is dedicated for recording
data only from a specific HLT unit.
From the start of the Belle II experiment, we gradually increased the number of
HLT units and number of CPU cores in each HLT unit as shown in Table 7.6. In 2019,
10 HLT units were in operation with 2880 CPU cores in total and in the winter shutdown in 2020-2021, the total number of CPU cores was increased to 4800. With this135

Table 7.6: Schedule of the reinforcement of HLT system
2019 2020 2023 2024 2025
number of CPU cores 2880 4800 6212 6692 7172

5

10

15

computational power, reconstruction software can currently handle data at the Level-1
trigger rate of around 15 kHz before LS1. In the LS1 period, we installed three more
HLT units and the total number of CPU cores in the HLT system reached 6212, weestimate that the HLT system can process data up to the trigger rate of 20kHz.
Since the current performance does not reach the designed rate of 30kHz for the
Belle II DAQ system, we will continue adding more CPUs to the HLT system. The
next reinforcement plan is to add another HLT unit in 2024. After the Run2 starts,
we will closely monitor the increase in trigger rates and event size,which requires alonger time for reconstruction, as the luminosity increases and add more HLT units
if necessary. In addition to simply scaling up the CPU processing power, we plan to
explore the implementation of heterogeneous computing in the online reconstruction
within HLT. Currently, the same software used for offline analysis has been used for the
online reconstruction, which is running on CPU but offloading tracking or clustering
algorithm to GPU or FPGA could be a viable option to improve the performance of
HLT.
7.3.3.2

20

25

30

35

40

Study of data-handling framework for triggerless DAQ

In this sub-section, we describe an idea of a triggerless DAQ as a potential upgrade to
the Belle II DAQ framework in the future, where all data from front-end electronics aresent to the back-end DAQ system without the Level-1 trigger and the event selection issolely performed by the software trigger in HLT. This upgrade could require substantial
modifications not only in the DAQ system but also in the front-end electronics of eachsub-detector and the reconstruction software used by HLT. Therefore, it should be
noted that this is an initial idea and has not been verified with either the detector
group or the software group.
Currently, the front-end electronics of each sub-detector receives Level-1 triggers
from the TTD system to determine which data to send to the backend DAQ system.
This scheme can control the throughput from the front-end electronics and reduce
the workload of the back-end DAQ system. However, since the Level-1 trigger of theBelle II system is produced by an FPGA-based system with limited information, there ispotential to enhance the event selection efficiency by sending all data without Level-1
trigger to the backend system to execute event selection using comprehensive detector
information and sophisticated reconstruction algorithm primarily handled by software.
While the Belle II Level-1 trigger achieves nearly 100% efficiency for a BB̄ event, The
potential for improvement by introducing triggerless DAQ could lie in the search formore exotic events, such as those with a displaced vertex and low multiplicity.
If we adopt a trigerless DAQ scheme, besides handling the increased throughput
resulting from the removal of the Level-1 trigger, which requires reinforcements inboth the readout and HLT systems, a substantial upgrade in the software frameworkof the DAQ system is also necessary as shown in Fig. 7.8. Since no trigger information
is available, timestamp information becomes essential for event-building without the136

Figure 7.8: Data handling in triggerless DAQ

5

10

15

20

25

Level-1 trigger. For this purpose, the front-end electronics are required to add thetiming information to hits or waveform data. This information should consist of coarse
timing distributed by the TTD system and fine timing information generated internallyby the front-end electronics. Subsequently, the front-end electronics needs to transmit
all hits or waveform data to backend PC farms with the timing information. At thebackend DAQ system, software should be employed to merge data from different frontend electronics boards by using the timing information attached in data. The software
trigger could first examine a potential event time window using algorithms like trackfinding. Once the time window is determined, a detailed reconstruction process cantake place. The selected events will subsequently be stored in a storage system foroffline analysis. In this section, we have exclusively discussed the upgrade of DAQ
software framework. However, the reconstruction software developed by the Belle II
software group also requires a significant upgrade for this new DAQ scheme. Detailed
discussion and collaboration with the software group are necessary for the feasibility
study of this aspect.
Eliminating the Level-1 trigger and sending all data to the back-end DAQ system
also requires increased throughput capabilities for front-end electronics boards of eachsub-detector. With the exception of the current PXD readout system, the front-end electronics for other sub-detectors find it difficult to adopt trigger-less readout. Therefore,
if we proceed with the implementation of triggerless DAQ, we should consider a staging approach, such as introducing triggerless readout first for specific sub-detectors,
which has enough capability of the increase throughput. In such a scenario, in addition to the normal data processing using the Level-1 trigger, additional online analysis
will be conducted on data obtained from certain sub-detector’s data with the triggerless readouts. In addition to the study of the software framework for the trigerless137

scheme, a feasibility study for the front-end electronics and back-end DAQ for higher
throughput is ongoing.

138

Chapter 8
Beam Polarization and Chiral Belle
Editor: J.M. Roney

8.1
5

10

15

20

25

30

Introduction and physics motivation

Upgrading SuperKEKB with polarized electron beams in the high energy ring (HER)
opens a new and unique program of precision physics at a center-of-mass energy of
10.58 GeV. We summarize here the physics potential of this ‘Chiral Belle’ program andcorresponding technical requirements needed to achieve the goals and a brief summary of the conceptual design of the requisite new hardware needed for the upgrade.
These were introduced in the Snowmass White Paper on upgrading SuperKEKB witha polarized electron beam [127]. A separate Conceptual Design Report on the Chiral
Belle Beam Polarization Upgrade Project is being written and this section summarizes
that work.
Unique and powerful sensitivities to new physics via precision neutral current measurements at 10 GeV are enabled by upgrading SuperKEKB to have polarized electron
beams. With a measurement of sin2 θW having a precision comparable to the current
world-average Z 0 -pole value, but at 10 GeV, Chiral Belle will be uniquely sensitive tothe presence of dark sector parity violating bosons with masses below the Z0 . Moreover, it would be the only facility able to probe neutral current universality relations
between charm and beauty quarks and all three charged leptons at energies below
the Z 0 pole and would measure ratios of neutral current couplings with unprecedentedly high precision. As all of these measurements are accurately predicted in theStandard Model, any deviations would be signatures of physics beyond the Standard
Model to which only Chiral Belle would be sensitive for the foreseeable future. Theτ -pairs produced with polarized beams will also provide the only means to measure
the third-generation g − 2 at a precision that can begin to approach the O(10−6 ) level,
the equivalent of the muon g − 2 anomaly scaled by (mτ /mµ )2 as expected in Minimal
Flavor Violation scenarios. With 40 ab−1 , a measurement at the 10−5 level is possible
and to reach O(10−6 ) will require more statistics as well as improved measurementsof mτ and mΥ (1S) . The Chiral Belle physics program also includes precision τ EDM
measurements and QCD hadronization studies as well as substantial improvements tothe sensitivities of lepton flavor violation searches in τ decays, and τ Michel parametermeasurements.
139

8.2

5

10

Requirements

In order to implement e− beam polarization in the SuperKEKB HER, three hardware
upgrades are required:1) introduction of a low-emittance polarized source that supplies SuperKEKB withtransversely polarized electrons with separate data sets having opposite polarization
states;2) a system of spin rotator magnets that rotate the spin of the electrons in the beam tobe longitudinal before the interaction point (IP) where the Belle II detector is located,
and then back to transversely polarized after the IP, as depicted in Figure 8.1; and3) a Compton polarimeter that provides online measurements of the beam polarization
at a location between the first spin rotator and the IP. A precision measurement of thepolarization is also made at the IP by analysing the spin-dependent decay kinematicsof τ leptons produced in an e+ e− → τ + τ − data set, which gives Chiral Belle a unique
way of controlling the systematic uncertainties on the beam polarization.

Figure 8.1: The spin motion of the electron in the HER with the spin rotators installed.
15

20

25

The development work on a polarized source is underway and is a hardware projectthat has many synergies with the polarized source work currently being undertaken
for the Electron Ion Collider project at Brookhaven National Lab in the United States.
The helicity of the electrons is to be changed for different bunch trains by controlling the circular polarization of the source laser illuminating a GaAs photocathode,similar to that which was used for the SLC polarized source. The electrons circulating in the HER are required to be transversely (i.e. vertically) polarized, as depicted
in Figure 8.1. Since the electrons emitted from the photocathode are longitudinallypolarized, there must be a spin rotator immediately after photocathode, such as thatprovided by a Wien filter, that ensures transversely polarized electrons are injected
into the linac supplying the HER beam. Spin tracking studies from the source, throughthe linac and injector lattice, show that the transversely polarized state of the electrons will be preserved when injected into the HER. Multiparticle long term tracking
studies have shown that electrons injected with transverse polarization will maintain
140

5

10

15

that polarization in the HER with a polarization lifetime of many hours.
The challenging problem of providing a spin rotator in the SuperKEKB HER that istransparent to the rest of the HER lattice can be solved in a manner that minimizes
disruptions to the existing lattice and that easily permits the possible operation of the
HER without any spin rotator magnet fields. The solution, proposed by Uli Wienands(Argonne National Laboratory), uses compact combined function magnets consistingof a dipole, a solenoid and a system of six skew-quadrupole fields, as depicted in Figure 8.2. It can be implemented in the HER by using four compact combined function
magnets, two on either side of the IP. These would simply physically replace existing dipole magnets currently in the present lattice. Frequency Map Analysis dynamic
aperture studies, Figure 8.3, demonstrate the feasibility of this design. Furthermore,long term tracking studies, which include radiation damping and radiation fluctuations/quantum excitation effects, have explored non-linear features of beam lifetimeand polarization lifetime. These studies demonstrate that sufficiently long beam andpolarization lifetimes can be achieved and confirm the viability of this particular spinrotator concept.

Figure 8.2: The Wienands concept for a compact combined function spin rotator unit
with overlaid dipole, solenoid and skew-quadrupole superconducting coil fields.

20

25

30

Solutions for implementing a Compton polarimeter have been found where thepolarimeter is located in a place that minimizes any disruptions to the existing lattice as described in reference [128] and can achieve polarization measurements withsystematic uncertainties at the 0.5% level. Unique to the Chiral Belle program is the
additional means of measuring the beam polarization at the IP using τ pair events witha relative systematic uncertainty of 0.4%, as described in [129], a paper describing thetechnique and evaluating the statistical and systematic uncertainties using PEP-II e+ e−
collision data collected with the BaBar detector at the Υ (4S) center-of-mass energy.

8.3

Implementation planning

The next steps in the Chiral Belle project involve completing the Conceptual Design
Report on the Chiral Belle Beam Polarization Upgrade Project after which R&D on the
hardware components, including prototypes, will converge on the Technical Designand cost estimates. The capital costs for the polarization upgrade are expected tobe substantially less than half of the annual power costs of operating the SuperKEKB
accelerator. It can also be expected that a significant fraction of those capital costs will
be provided by non-Japanese groups on Belle II.

141

Figure 8.3: Frequency Map Analysis for the current HER lattice (left) and for the HER
lattice with four compact combined function spin rotator magnets installed (right)
demonstrate that spin rotators can be implemented in a manner that is transparent to
the rest of the lattice. This is further confirmed with long term tracking studies, as
described in the text.

5

10

A proposal to measure the lifetime of the transverse polarization in the HER is
currently being prepared, where a polarized source will inject transversely polarized
electrons and the polarization lifetime measured using the Touschek effect, as is donein resonant depolarization measurements. This would be a dedicated experiment of afew days duration and serve to validate the long term tracking studies that predict aspin lifetime that is much greater than the HER top-up rate. This Touschek-Polarization
Experiment would be conducted in parallel with the other work required for the Technical Design Report.
It is feasible to plan for the upgrade to commence towards the end of this decadeusing the next long shutdown and possibly a number of summer shutdowns. In sucha scenario, the polarization program could begin while SuperKEKB completes its program of delivering 50 ab−1 of data to Belle II and continued beyond that program.

142

Chapter 9Longer Term Upgrades
9.1

ARICH

Editor: R.Pestotnik
5

10

15

20

25

30

9.1.1

Introduction

The Aerogel RICH particle identification subsystem (ARICH) located in a perpendicular 1.5 T magnetic field in front of an electromagnetic calorimeter consists of three key
elements: aerogel radiator plane, photon detector plane with single photon sensors,and front-end readout electronics. Cherenkov photons emitted in the 4 cm aerogel
radiator are propagated through a 16 cm expansion gap and detected on the photon
detector consisting of Hybrid Avalanche Photo-detectors (HAPD) produced by Hamamatsu [130]. The HAPD is a hybrid vacuum photon detector consisting of an entrance
window with a bi-alkali photo-cathode with a 28% peak quantum efficiency and four
segmented avalanche photo-diodes in proximity focusing configuration. 420 HAPD
sensors with 144 channels are mounted in seven concentric rings surrounding thebeam pipe in the forward end-cap region of the spectrometer. The ARICH was designed to operate up to the nominal design luminosity of 8×1035 cm−2 s−1 . Due tohigh background environments, its operation poses new challenges when the BelleII operation is extended beyond the design luminosity. At high luminosity, the neutron background radiation and gamma radiation levels increase. They can affect thefunctionality of all detector components.
The silica aerogel is produced in a supercritical drying process and is not affectedby the irradiation [131]. Its performance was measured during the irradiation with adose of 98 kGy, and no significant degradation was found.
HAPDs, however, are sensitive to gamma and neutron irradiation. Different modifications in the APD structure have been implemented during the sensor’s development.
Different designs have been tested, including the insertion of a film on the active areaand different thicknesses of the p+ layer. The design with an intermediate electrode, athin p+ layer, and a film on the active area was finally selected for ARICH. The HAPD
was demonstrated to work up to the design luminosity but not after its 5 × longerterm upgrade [132]. Also, due to its high Boron content introduced during the chipproduction, a Xilinx Spartan-6 FPGA, part of the front-end electronics board, is prone143to single event upsets. By using a custom FPGA memory scrubbing [133] and triple
redundancy logic, we can operate the electronics in the short and mid-term.
We have already started investigating different possible upgrade scenarios for the
long-term upgrade, focusing on the possible photon detector candidates.
5

9.1.2

10

The choice of photon detectors capable of operating in the magnetic field of 1.5 T islimited to several different technologies, which we already considered during the R&Dof the current photo-detector [134, 135]. The HAPD, produced by Hamamatsu Photonics, is no longer a viable option as the HAPD production line has been disassembled
after the sensor production.
9.1.2.1

15

20

25

30

35

40

Photon detector candidates

Silicon photo-multipliers

The first technology is silicon photomultipliers, which have several advantages overhybrid avalanche photo-detectors. They have a very high photon detector efficiency,
reaching 60% at the peak. They are very easy to operate as they only require reverse
biasing from 30 to 70 V, much less than standard or hybrid vacuum detectors. In
addition, they have a very good timing resolution, which opens new possibilities forparticle identification in ARICH. As semiconductor devices, they are inherently insensitive to magnetic fields.
They have, however, several drawbacks. First is the large dark count rate. Even
more problematic is their sensitivity to radiation, which was a limiting factor a decadeago when selecting the photon detector candidates for the current ARICH photon detector.
The radiation damage increases the sensor currents, affects its breakdown voltage,and increases the dark count rate. The dark counts affect the measurement in two
ways. First, their signals cannot be distinguished from single photon ones. As a result,they produce a high background on the detector’s plane. The resulting effect on the
kaon identification capabilities for different background levels can be seen in Fig. 9.1.
Second, the increase of the dark count rate, proportional to the size of the device,is also responsible for the sensor baseline loss, which happens above 1011 neq /cm2
or less for small sensitive area (1 mm2 ) devices. The limit depends on many factors
related to the SiPM design and the operation conditions. The dark count rate increases
exponentially with the operating temperature, and the usability of the silicon photomultipliers should be tested for each specific application. The baseline loss affects
single photon counting much more than silicon photomultipliers in the multi-photon
regime, e.g., in calorimetry.
We are following the evolution of the technology and evaluating currently available
samples. We are also working with the producers to reduce the neutron sensitivity
and find the operation parameters where the damage might be under control. We willconsider different engineering changes in the electric field design and sensor SPAD
size [136].
Several techniques can contribute to the operation of Silicon photo-multipliers:
the reduction of their operating temperature, the use of timing information for background rejection, the use of a light collection system to reduce the sensor-sensitive
144

5

10

surface, and the high-temperature annealing. We are studying the response of the silicon photomultipliers at low temperatures and developing multi-channel timing electronics to read out the sensors. These developments in silicon photomultipliers aim
to demonstrate their capability to measure single photons and reduce the dark countrate to an acceptable level.
In the case of the RICH with the aerogel radiator where neutron fluences of up to
5 × 1012 n/cm2 are expected, the operation at lower temperatures requires the use ofan additional cooling system, which introduces more material in front of the electromagnetic calorimeter impacting its performance. The effect must be carefully studied
by performing detector response simulations to understand its consequences.
9.1.2.2

15

20

25

30

35

The second possible photon detector technology we are considering for the long-term
upgrade is micro-channel plate photo-multipliers (MCP-PMT). Compared to silicon
photo-multipliers, such sensors have several drawbacks. As they are based on bi-alkali
photo-cathodes and suffer from additional collection inefficiency, their photon detection efficiency is similar to the current HAPDs, much lower than that of SiPMs. Asa vacuum detector with an internal amplification structure, they also suffer from thegain drop in the magnetic field and lifetime issues due to charge collection on thephotocathode. In the case of Aerogel RICH, the above drawbacks can be, to a certain
extent, mitigated. In the current configuration, there are gaps between the HAPD detector modules, which can be reduced during the design of the photodetector layout.
Also, the gain drop in the case of the perpendicular magnetic field of ARICH is relatively small. Finally, the MCP-PMT lifetime can be extended by operating sensors at alower gain and by the latest technological advances (atomic layer deposition treatmentof the MCP structures). Compared to silicon-photomultipliers, the dark count rate of
MCP-PMTs is much lower, especially after exposure to neutron irradiation; thus, considering them as a candidate for the future ARICH photon detector is very justified.
MCP-PMT production is, in general, very expensive, apart from the case of the Large
Area Picosecond Photodetector (LAPPD) technology, where only inexpensive glass isused in the production process, leading to a significantly lower volume pricing. The
LAPPDs, produced by Incom USA, come in two sizes, 20×20 cm2 and 10×10 cm2 .
The large size reduces the complexity of the photon detector. Unfortunately, it would
introduce additional inactive areas on the photon detector. A conceptual design layout of the ARICH photon detector plane equipped with LAPPDs is shown in the rightplot of Fig. 9.1. A version of the devices with capacitively coupled electrodes enables
the design of custom readout granularities. We are optimizing the size of the pads
and studying possible charge sharing between several pixels, which may impact theoperation when signals from photons start to overlap.

9.1.3
40

Large area picosecond photo-detector

Sensor read-out electronics

Both possible sensors require reading out the signals in a narrow time window ofseveral ns concerning the trigger signal, requiring optimized internal design and highintegration with the read-out electronics at the back side of the sensor. We are studying
two potential front-end chip options. The first is based on upgrading the current
145

K eff.

1

0.95

0.9

0.85

0.8

1xBG, 4.0 GeV
2xBG, 4.0 GeV
5xBG, 4.0 GeV
10xBG, 4.0 GeV

0.75

0.7

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

π missid. prob.

Figure 9.1: (left) Simulated Kaon identification efficiency versus pion misidentification
probability at the momentum of 4 GeV/c for various levels of nominal background.
Bialkali photocathode was assumed during the simulation. (right) Conceptual layoutof ARICH photon detector plane populated with 20×20 cm2 and 10×10 cm2 LAPPDs.

5

ASIC used for Hybrid avalanche photo-diodes [137]. The chip has been designed;
the first samples are currently under functional tests. This option allows the use of aproven technology; however, it is not yet demonstrated how it will handle high darkcount rates. The second option uses novel FastIC ASIC, featuring low noise fast timing
capabilities, developed in collaboration with CERN and the University of Barcelona[138]. As the new ASIC will be used in the coming upgrades of the LHC experiments,
the synergies in common development (e.g., with LHCb RICH) are expected to reducethe development costs and commissioning time.

9.1.4
10

15

Radiator

A study has already been launched to estimate the possible gains of using more transparent aerogel tiles and possible higher refractive index. The aerogel production technology has been transferred from Chiba University to a local company. We are evaluating the quality of aerogel after the technology transfer by measuring the characteristicsof new samples.

9.1.5

Potentially interested community in Belle II

The following collaborators expressed their interest in the ARICH hardware upgrade
activities: KEK, Tokyo Metropolitan University, and Niigata University, all from Japan;
the University of Naples, Italy; and Jožef Stefan Institute, Slovenia.

9.1.6
20

Cost and schedule

The current Aerogel RICH can operate efficiently until the design luminosity is reached.
Therefore, all the sub-detector changes considered above are projected for the longterm upgrade. Preliminary cost estimates are given for two photodetector options:silicon photomultipliers with 250,000 1 mm2 channels and 64 20×20 cm2 and 16
146

Table 9.1: Preliminary cost estimates in kUSD for photon detector upgrade based on
SiPM and LAPPD. These include the costs for the associate R&D.
OptionDescription
Photo-sensor
Research and development
Production
Electronics
ASIC including TDC
Front-end board
Merger boards
Services and detector infrastructure
Photon detector mechanical frame
Cooling system
Power supplies
Radiator
Aerogel upgrade
Total
Contingency
Grand total

SiPM
k$

LAPPD
k$

500
1.500

500
2.300

700
200
200

700
200
200

200
300
50

200
re-use current system
re-use current system

700
4.350
+15%
5000

700
4.800
5520

10×10 cm2 LAPPDs. The estimates are based on the current understanding of thetechnologies under development.

147

9.2

5

Editor: C.Cecchi
This section contains a summary of the upgrade of the Belle II Electromagnetic
Calorimeter (ECL). The current detector limitations are due to insensitivity to the incident photon direction and to possible deterioration of the energy resolution due tothe signals pileup that can be produced under severe background conditions. Someupgrade options have been discussed in order to cope with the high occupancy andbackground rate and to maintain good particle reconstruction performances.

9.2.1
10

15

20

25

ECL

Anticipated performance limitations

Since CsI(Tl) crystals scintillation decay time is relatively long, ∼ 1 µs, the increaseof beam background would results in a larger pileup noise that would deteriorate thedetector energy resolution. This effect is particularly relevant for neutral particles withrespect to electron and positrons, as they are measured only in the ECL, without taking advantage of a good precise momentum measurement performed in the tracking
detector. Simulated beam background has been superimposed on generic Monte Carlo(MC) B B̄ events, and studies on the π 0 mass resolution have been done for the earlyphase-3 luminosity (L = 1×1034 cm−2 s−1 ) with nominal, ×2 and ×5 beam backgroundas well as the ultimate phase-3 luminosity (L = 8 × 1034 cm−2 s−1 ) with nominal beam
background cases. The γγ invariant mass spectrum is fitted with a Novosibirsk and a
Chebichev functions for the π 0 signal and background, respectively. Resultant distributions are shown in Figure 9.2, early phase-3 luminosity with nominal background(left) and ultimate phase-3 luminosity (right). In the early phase-3 case, the π 0 mass
resolution and reconstruction efficiency are found to be 6 MeV and 30%, while they
become 9 MeV and 15% in phase-3 case. The ultimate phase-3 environment withhigher background deteriorates the matching of the ECL deposit with the true photon because of the higher combinatorial in the number of ECL clusters. This could
represent a strong limiting factor for physics analysis with rare B decays where highefficiency is required.

Figure 9.2: Reconstructed π 0 mass for the early phase-3 luminosity with BGx1 background simulation (left) and for the ultimate phase-3 BGx1 background simulation(right). The signal peak has been fitted with a Novosibirsk function while the background is described by a Chebichev distribution.
In the B → τ ν analysis, one of the B mesons in the event is fully reconstructed148

Figure 9.3: π 0 mass distribution in B → τ ν events for ultimate phase-3 luminosity(green) compared to the early phase-3 luminosity with nominal BGX1 background
distribution (blue).

5

10

and the rest of the event is required to have τ daughters. Since substantial fraction ofτ decay modes contain π 0 , a good reconstruction of its invariant mass is crucial. The
γγ invariant mass distribution is shown in Figure 9.3. In Phase 3, the π 0 signal peak isunseen, thus indicating a serious limit in reconstructing fundamental physics objects.
To search for dark-sector particles in Belle II data, importance to search for a longlived particle is increasing. A very interesting process it is the production of an axionlike-particle (ALP), a, in the B decay of B → Ka. Such ALP is likely to be a longlived particle decaying into a photon pair. Since current ECL configuration has very
little sensitivity to incident photon direction whose reconstruction relies only on subtle
shower shape difference, its mass determination for the long-lived ALP can only bedone at the edge of the reconstructed invariant mass distribution when its lifetime (τ )is several tens of cm in cτ , as shown in Figure 9.4. Adding an extra device sensitive toincident photon direction is expected to significantly increase the mass determination
power for such physics case.

Figure 9.4: The reconstructed photon pair’s invariant mass distribution in B → Ka,a → γγ MC sample for the axion-like-particle (ALP), a mass ma = 1 GeV/c2 , lifetimecτ = 1 cm (left) and 40 cm (right) cases.
149

Figure 9.5: Concept of the preshower detector. It is placed just in front of ECL anddetects shower position and energy deposition, thus being sensitive to the photon
incident direction.

5

10

15

20

9.2.2

Upgrade hypothesis

9.2.2.1

Replacement of CsI(Tl) crystals with pure CsI scintillator counters

In order to eliminate pile up noise, faster scintillation crystal gives a primary solution. Pure CsI is suitable for this purpose because its fast light component decay time
is about 30 ns, a factor of 40 faster than CsI(Tl). However pure CsI light output isabout 20 times smaller than CsI(Tl) and its peak emission of 310 nm is in the near-UV
region where photosensors are less sensitive. That is why the use of CsI crystals for
the Belle II calorimetry is very challenging and requires the identification of performing photodetectors in the near-UV region. Front end electronics should provide a fast
integration and a high amplification for the signal readout. To match this condition,
avalanche photodiode (APD) readout based on the Hamamatsu APD S8664-55 [139]and S8664-1010 [140] has been studied. The S8664 series APD is usually operated
at the gain ×50, using it at higher gain of ×100 is found to be still stably operational.
Attaching to the photosensor a wavelength shifter (WLS) to emit about 500 nm wavelength light is also effective to get larger signal pulse by a factor 2∼3 [141]. The
best solution is then to combine these two approaches by placing 4 Hamamatsu APD
S8664-55 placed on the edge of the crystal with the photodetector coupled to a WLS.
Simulation studies have been carried out to compare energy resolution of the current
detector with a pure CsI calorimeter and it turned out that to have a resolution of theorder of 5% at 100 MeV energy we should have an Equivalent Noise Energy (ENC) nothigher than 1 MeV. The described configuration with pure CsI crystal connected to a
WLS and 4 APD’s has shown to satisfy the requirement on the ENC at sub MeV level.
9.2.2.2

25

A preshower detector in front of the ECL

A second option considered for the ECL upgrade is a small detector system just in frontof the ECL acting as a preshower detector as shown in Fig.9.5. That has a sensitivity forphoton incident direction, thanks to an excellent incident position resolution. Superior timing resolution can be featured. One layer consists of an active absorber with 1
150

5

10

15

20

radiation length thick crystal scintillator and a Si pixel detector to detect electron andpositron passage when producing an electromagnetic shower. A GEANT4 based simulation with BGO/LYSO as the active absorber material and 1 mm2 silicon pixels showsthat the incident direction can be reconstructed for 80% of the 1 GeV photons because
they produce hits on two or more Si pixel detector layers. The position and angularresolutions are found to be 1.9 mm and 0.08 rad., respectively, for normal incidence.
For the long-lived ALP decay to a photon pair with ma = 2 GeV/c2 and 40 cm flight
distance, the photons from IP can be separated from those produced by the ALP decay
by about 2σ. We also see that significant portion of events’ energy deposition is a few
hundred MeV, thus the absorber part must be active with a reasonably good energy
resolution. As for introduction of such a preshower detector, (1) it can act as a shield
to stop significant fraction of beam background soft photons before hitting into themain ECL, thus mitigating pile up noise effect, (2) longitudinal sampling may help todistinguish photons and neutral hadrons by their different shower development characteristics, such possibility is worth to pursuit in addition to the already implemented
pulse shape discrimination (PSD) [142] technique to improve particle identification
capability inside ECL, (3) it is interesting to identify physics cases due to better photon
incident position resolution, timing resolution and capability to reconstruct incident
photon direction, and (4) geometrical activity match between the preshower and themain ECL also may help to reduce backgrounds.
9.2.2.3

25

Complementing the PiN diode photosensors with avalanche photodiodes

In this section the possibility of complementing the Belle photodiodes readout with two
high-gain avalanche photodiodes (APDs) for the to readout of the CsI(Tl) crystals will
be described. The APDs were readout with either transimpedence amplifiers (TZA)[13] or with the charge-integrating CR-110 amplifiers. The APD+TZA signal is shownin Figure 9.6 (left) while the signal amplitude, after the CR − (RC)4 filter, from APDs
and the charge-integrating CR-110 is shown on the right side of the same Figure.

Figure 9.6: Signal amplitude for the APD plus transimpedence preamplifier configuration, the red curve represents the filtered waveform with a τ of 200 ns (left). Signal
amplitude after shaping in the case of APD plus CR110 preamplifier (right). Notice
the difference in the timescale on the horizontal axis for the two plots.
The Equivalent Noise Energy (the input-referred noise calculated from the signal
151

to noise ratio at the shaper output, indicated as ENE) of the sum of two APDs with
CR-110 readout, compared to the pin diodes on the same data run, is shown in Figure
9.7 with and without background overlay.

Figure 9.7: Equivalent Noise Energy as a function of the shaping time comparing pin
diodes with APD with CR110 amplifier without (left) and with (right) 60 Co source.

5

The large amplitude signal grant to APD’s a very small electronic noise and a reduced pile-up noise with respect to the pin diodes.
9.2.2.4

10

15

20

25

30

Complementing the PiN diode photosensors with Silicon Photomultipliers (SiPM)

To control the pile-up in condition of high background it is possible to use a high gain
photodetector to perform an accurate measurement of the timing exploiting the large
amount of light emitted by the CsI(Tl) crystals. SiPM’s are a very good option thanks
to their fast response, high gain and simplicity in use. Some preliminary studies havebeen done trying to understand the actual possibility of using these photodetectors fortiming measurement, keeping the PiN diodes for the signal amplitude measurement.
To start the R&D with SiPMs as the readout system for CsI(Tl), crystals from Belle anda prototype SiPM by FBK (Fondazione Bruno Kessler) [143], developed for the CTA(Cherenkov Telescope Array) experiment, have been used. This prototype SiPM is not
optimized to match the emission peak (550 nm) of the light emitted by CsI(Tl) counter,but it is good enough for our test presenting 25% of photo detection efficiency (PDE)at the above mentioned wavelength. Figure 9.8 (left) shows the PDE as a function of
the wavelength while on the right panel the PDE as a function of the overvoltage isshown.
The SiPM has an active area of 6x6 mm2 (effective area 80% of the active region),breakdown voltage of 26.5 V and a nominal gain at breakdown of 1.1 × 106 electrons/p.e. which increases linearly with the overvoltage (2 − 3 × 106 at +5 OV). The
signal produced by the SiPM has been readout with a board from AdvanSid [144]which contains a transimpedance amplifier with output impedance of Z=500 Ω corresponding to a gain of a factor 103 . The integral output signal is proportional to thedetected charge. To calibrate the system of CsI(Tl) plus SiPM the response to radioactive sources signal has been studied with a 137 Cs source emitting a photon at 700 KeV
and with a 60 Co source emitting two photons of energy 1.17 MeV and 1.33 MeV. The
calibration has been done with a sample of LaBr crystal which has high light output
152

Figure 9.8: Photodetection efficiency (PDE) as a function of the wavelength (left).
Photo detection efficiency (PDE) as a function of the overvoltage (right).

5(70000 photons/MeV), a very short decay time (20 ns) and the emission peak at about
360 nm which perfectly matches the region of the highest PDE of the FBK SiPM. In Figure 9.9 the measured spectrum for the two sources, on the left 137 Cs and on the right60
Co, is shown. The second peak of the 60 Co spectrum is fainter than the first one, as
expected, and in both distributions we can observe the contribution of the Compton
edge.

Figure 9.9: 137 Cs spectrum measured with the FBK SiPM (left). 60 Co spectrum measured with the FBK SiPM (right).

10

15

Using the measurements of the integrated charge performed with this setup, thegain of the Advansid board has been derived to be 2×105 , 1.3×106 and 3×106 for27, 29 and 31 bias voltages respectively. To evaluate the impact of the noise comingfrom the SiPM plus the Advansid board a measurement with and without radioactive
sources on a Belle CsI(Tl) crystal has been performed. Figure 9.10 shows the signal
peak amplitude with (blue distribution) and without source (orange distribution). The
distribution of the signal of the CsI(Tl), when the Cs137 source is present, is much larger
than the one observed with the LaBr crystal, furthermore the noise (orange distribution) is not well separated from the signal preventing the possibility of subtracting itin order to extract the signal.
Others SiPM’s better matching the CsI(Tl) emission spectrum as well as a customized low noise front end board will be tested to improve the separation between
signal and noise.
153

140

SIPM signal- confO mean • 5 49 mV\ConfO nstd • 2 37 mV- confl mean • 3 26 mV\Coofl nstd • 1 05 mV

120

100

..
lO

•

10

Peak - Pedestal [mVJ

12

14

Figure 9.10: Peak amplitude measured with the FBK SiPM with (blue) and without(orange) Cs137 source.

9.2.3

Cost and schedule

An estimation of the main cost for the Pure CsI crystals with APD readout + WLS
option is reported in Table 9.2.

Crystals (8736)
APD’s (4/crystal)
WLS (1/crystal)FE (4/crystal)
TOTAL

Cost/unit (ke)
4.9
0.1
0.05
0.3

Total cost (Me)
42.5
3.5
0.4
2.6
49

Table 9.2: Estimated cost for the Pure CsI crystal calorimeter with APD + WLS readout.

5

A rough estimation of the main cost for the pre-shower option has been evaluated(see Table 9.3) considering the following dimensions: each endcap has 4.4 m2 (Outerradius 1.25 m, Inner radius 0.42 m), the Barrel Inner cylinder surface is 11.7 m2 which
results in 20 m2 in total.

Si sensors
LYSO
Photosensor (3 layers, 2pc, 8736 crystal)
Readout
Mechanics
TOTAL

Cost/unit (ke)100/m2600/m2
100

Total cost(Me)
2
12
5.2
2
2
23.2

Table 9.3: Estimated cost for the pre-shower calorimeter.
Finally, an estimation of the main cost for the CsI(Tl) crystals with LAAPD readout
option is reported in Table 9.4.

154

APD’s (2/crystal)FE (2/crystal)
ECL extraction
TOTAL

Cost/unit (ke)0.5
0.2

Total cost (Me)
4.4
1.7
2.0
8.1

Table 9.4: Estimated cost for the CsI(Tl) crystal calorimeter with APD readout.

9.2.4

5

Potentially interested community in Belle II

The following collaborators expressed their interest in the hardware upgrade activities: Budker Institute of Nuclear Physics (BINP), Russia, KEK, Japan, Nara Women’s
University, Japan, University of Naples and INFN-NA, Italy, University of Perugia and
INFN-PG, Italy.

155

Bibliography[1] A. J. Bevan et al. “The Physics of the B Factories”. In: Eur. Phys. J. C 74
(2014), p. 3026. DOI: 10.1140/epjc/s10052-014-3026-9. arXiv: 1406.6311[hep-ex].
5[2] T. Abe et al. Technical Design Report of SuperKEKB. URL: https://kds.kek.
jp/indico/event/15914/.
[3] T. Abe et al. “Belle II Technical Design Report”. In: (Nov. 2010). arXiv: 1011.
0352 [physics.ins-det].

10[4] I. Adachi et al. “Detectors for extreme luminosity: Belle II”. In: Nucl. Instrum.
Meth. A 907 (2018), pp. 46–59. DOI: 10.1016/j.nima.2018.03.068.
[5] E. Kou et al. “The Belle II Physics Book”. In: PTEP 2019.12 (2019). Ed. by E.
Kou and P. Urquijo. [Erratum: PTEP 2020, 029201 (2020)], p. 123C01. DOI:
10.1093/ptep/ptz106. arXiv: 1808.10567 [hep-ex].
[6]

15

URL : https://confluence.desy.de/display/BI/Belle+II+Publications(visited on 10/24/2023).
[7] SuperKEKB International Task Force reports. https : / / www - linac . kek . jp /linac-com/skb-itf/ml/.
[8] F. Forti et al. “Snowmass Whitepaper: The Belle II Detector Upgrade Program”.
In: Snowmass 2021. Mar. 2022. arXiv: 2203.11349 [hep-ex].

2025[9] A. Natochii et al. “Beam background expectations for Belle II at SuperKEKB”.
In: Snowmass 2021. Mar. 2022. arXiv: 2203.05731 [hep-ex].
[10] A. Natochii et al. “Measured and projected beam backgrounds in the Belle IIexperiment at the SuperKEKB collider”. In: Nucl. Instrum. Meth. A 1055 (2023),
p. 168550. ISSN: 0168-9002. DOI: https://doi.org/10.1016/j.nima.2023.
168550. URL: https : / / www . sciencedirect . com / science / article / pii /S0168900223005405.
[11] Pathways to Innovation and Discovery in Particle Physics, Report of the 2023 Particle Physics Project Prioritization Panel. URL: https://www.usparticlephysics.
org/2023-p5-report/.

30[12] T. Kuhr et al. “The Belle II Core Software”. In: Comput. Softw. Big Sci. 3.1(2019), p. 1. DOI: 10.1007/s41781-018-0017-9. arXiv: 1809.04299 [physics.comp-ph].
[13] Belle II Analysis Software Framework (basf2). https : / / doi . org / 10 . 5281 /zenodo.5574115. DOI: 10.5281/zenodo.5574115.

156

[14] P. Raimondi. Status on SuperB Effort. Presentation at 2nd SuperB Meeting,
Frascati, Italy. 2006.
[15] M. Bona et al. “SuperB: A High-Luminosity Asymmetric e+ e− Super Flavor Factory. Conceptual Design Report”. In: (May 2007). arXiv: 0709.0451 [hep-ex].
5[16] Miguel A. Furman. “Hourglass effects for asymmetric colliders”. In: Conf. Proc.
C 910506 (1991). Ed. by Loretta Lizama and Joe Chew, pp. 422–424.
[17] D. Zhou et al. “Interplay of Beam-Beam, Lattice Nonlinearity, and Space Charge
Effects in the SuperKEKB Collider”. In: 6th International Particle Accelerator
Conference. 2015, WEYB3. DOI: 10.18429/JACoW-IPAC2015-WEYB3.

10[18] M. Zobov et al. “Test of crab-waist collisions at DAFNE Phi factory”. In: Phys.
Rev. Lett. 104 (2010), p. 174801. DOI: 10.1103/PhysRevLett.104.174801.
[19] M. Masuzawa et al. “Chromatic X-Y Coupling Correction by Tilting Sextupole
Magnets in the SuperKEKB Positron Ring” (Bangkok, Thailand). presented at
IPAC’22, unpublished. July 2022.

15[20] T. Ishibashi et al. “Movable collimator system for SuperKEKB”. In: Phys. Rev.
Accel. Beams 23.5 (2020), p. 053501. DOI: 10.1103/PhysRevAccelBeams.23.
053501.
[21] G. Besnier, D. Brandt, and B. Zotter. “THE TRANSVERSE MODE COUPLING
INSTABILITY IN LARGE STORAGE RINGS”. In: Part. Accel. 17 (1985), p. 51.

20

25

30

[22] M. Tobiyama, J. W. Flanagan, and A. Drago. “Bunch by bunch feedback systems for SuperKEKB rings”. In: Proceedings of the 13th annual meeting of Particle Accelerator Society of Japan. 2016, pp. 144–148. URL: https://www.pasj.
jp/web_publish/pasj2016/proceedings/PDF/TUOM/TUOM06.pdf.
[23] H. Ikeda et al. “Observation of sudden beam loss in SuperKEKB”. In: Proc.
IPAC’23 (Venice, Italy). IPAC’23 - 14th International Particle Accelerator Conference 14. May 2023, pp. 716–719. ISBN: 978-3-95450-231-8. DOI: 10.18429/
JACoW - IPAC2023 - MOPL072. URL: https : / / indico . jacow . org / event / 41 /contributions/1252.
[24] T. Abe et al. “Direct observation of breakdown trigger seeds in a normalconducting rf accelerating cavity”. In: Phys. Rev. Accel. Beams 21.12 (2018),
p. 122002. DOI: 10.1103/PhysRevAccelBeams.21.122002.
[25] SAD Home Page. Strategic Accelerator Design. http://acc-physics.kek.jp/
SAD/. 1986 (accessed December 2022).

35[26] N. Iida et al. “Beam injection issues at SuperKEKB”. In: Proc. IPAC’23 (Venice,
Italy). IPAC’23 - 14th International Particle Accelerator Conference 14. May
2023, pp. 832–835. ISBN: 978-3-95450-231-8. DOI: 10.18429/JACoW-IPAC2023MOPL120. URL: https://indico.jacow.org/event/41/contributions/1308.
[27] JLC Group. JLC-I. https://catalogue.library.cern/literature/2mzjdnyx42. Dec. 1992.

40[28] L. Merminga et al. “Collimation systems for a TeV linear collider”. In: Part.
Accel. 48 (1994), pp. 85–108.

157

[29] N. Ohuchi et al. “SuperKEKB beam final focus superconducting magnet system”. In: Nucl. Instrum. Meth. Phys. Res., Sect. A 1021 (2022), p. 165930.
[30] Opera-3d—SIMULIA. Dassault Systèmes. Kidlington, Oxfordshire, UK. URL: https://www.3ds.com/products-services/simulia/products/opera/.
510[31] Emanuela Barzi and Alexander V. Zlobin. “Nb3 Sn Wires and Cables for HighField Accelerator Magnets”. In: Nb3 Sn Accelerator Magnets. Ed. by Daniel Schoerling and Alexander V. Zlobin. 2019, pp. 23–51. DOI: 10.1007/978-3-03016118-7_2.
[32] A. Devred. “Practical low-temperature superconductors for electromagnets”.
In: (July 2004). DOI: 10.5170/CERN-2004-006.
[33] N. Andreev et al. “Magnetic mirror structure for testing shell-type quadrupole
coils”. In: IEEE Trans. Appl. Supercond. 20.3 (2010), pp. 288–291. DOI: 10 .
1109/TASC.2009.2039704.

15[34] J. Gareyte. “Transverse Mode Coupling Instabilities”. In: AIP Conference Proceedings 592 (2001), pp. 260–278. DOI: https : / / doi . org / 10 . 1063 / 1 .
1420419.
[35] A. W. Chao and M. Tigner. Handbook of accelerator physics and engineering.
World Scientific, 1999.

20[36] T. Yue, Q.-L. Xiu, Y.-W. Wang, Zh. Duan, and Q. Qin. “Physics design of collimators to reduce lost particle background at the CEPC”. In: Chinese Physics
C 40 (2016), p. 117001. DOI: 10 . 1088 / 1674 - 1137 / 40 / 11 / 117001. URL:
https://doi.org/10.1088/1674-1137/40/11/117001.
[37] K. Ohmi and T. Ishibashi. “Private communication”. In: KEK-SuperKEKB accelerator group (Feb. 2022), p. 1.

25

30

35

40

[38] K. Shibata et al. “Commissioning Status of SuperKEKB Vacuum System”. In:
Proc. 62nd ICFA ABDW on High Luminosity Circular e+ e− Colliders (eeFACT’18),
24-27 Sep. 2018 (Hong Kong, China). 62. JACoW Publishing, Apr. 2019, pp. 226–
233. ISBN: 978-3-95450-216-5. DOI: doi : 10 . 18429 / JACoW - eeFACT2018 WEOAB01. URL: http://jacow.org/eefact2018/papers/weoab01.pdf.
[39] A. Beaulieu. “The Study and Shielding of Electromagnetic Radiation from SuperKEKB Electron and Positron Beam Interactions”. In: University of Victoria
BELLE2-PTHESIS-2019-001 (2019), p. 256. URL: %7Bhttps://docs.belle2.
org/record/1474%7D.
[40] Yukiyoshi Ohnishi et al. “Accelerator design at SuperKEKB”. In: Progress ofTheoretical and Experimental Physics 2013.3 (Mar. 2013). 03A011. ISSN: 20503911. DOI: 10 . 1093 / ptep / pts083. eprint: https : / / academic . oup . com /ptep / article - pdf / 2013 / 3 / 03A011 / 4439973 / pts083 . pdf. URL: https ://doi.org/10.1093/ptep/pts083.
[41] H. Ikeda et al. “Beam Loss Monitor at SuperKEKB”. In: Proc. International
Beam Instrumentation Conference (IBIC’14). TUPD22. 2014, pp. 459–462. URL:
https://accelconf.web.cern.ch/ibic2014/papers/tupd22.pdf.

158

[42] S. Bacher et al. “Performance of the diamond-based beam-loss monitor system
of Belle II”. In: Nucl. Instrum. Meth. A 997 (2021), p. 165157. ISSN: 01689002. DOI: https://doi.org/10.1016/j.nima.2021.165157. URL: https://www.sciencedirect.com/science/article/pii/S0168900221001418.
5

10

15

[43] M. Gabriel et al. “A time resolved study of injection backgrounds during thefirst commissioning phase of SuperKEKB”. In: The European Physical Journal
C 81 (2021), p. 972. DOI: https://doi.org/10.1140/epjc/s10052- 02109769 - 3. URL: https : / / link . springer . com / article / 10 . 1140 / epjc /s10052-021-09769-3.
[44] A. Natochii et al. Snowmass White Paper: Beam background expectations for
Belle II at SuperKEKB. 2022. DOI: 10.48550/ARXIV.2203.05731. URL: https://arxiv.org/abs/2203.05731.
[45] M. Tobiyama, J. Flanagan, and A. Drago. “Bunch by bunch feedback systems
for SuperKEKB rings”. In: Proc. 13th Annual Meeting of Particle Accelerator Society of Japan (PASJ2016). TUOM06. 2016, p. 144. URL: https://www.pasj.
jp/web_publish/pasj2016/proceedings/PDF/TUOM/TUOM06.pdf.
[46] Shinji Terui et al. “The Report of Machine Studies Related to the Vertical Beam
Size Blow-Up in SuperKEKB LER”. In: JACoW IPAC2022 (2022), WEPOTK050.
DOI : 10.18429/JACoW-IPAC2022-WEPOTK050.

2025[47] A. Natochii et al. “Improved simulation of beam backgrounds and collimation
at SuperKEKB”. In: Phys. Rev. Accel. Beams 24 (8 Aug. 2021), p. 081001. DOI:
10.1103/PhysRevAccelBeams.24.081001. URL: https://link.aps.org/doi/
10.1103/PhysRevAccelBeams.24.081001.
[48] S. Agostinelli et al. “Geant4—a simulation toolkit”. In: Nucl. Instrum. Meth. A
506.3 (2003), pp. 250–303. ISSN: 0168-9002. DOI: https : / / doi . org / 10 .
1016 / S0168 - 9002(03 ) 01368 - 8. URL: https : / / www . sciencedirect . com /science/article/pii/S0168900203013688.
[49] J. Allison et al. “Geant4 developments and applications”. In: IEEE Transactions
on Nuclear Science 53.1 (2006), pp. 270–278. DOI: 10.1109/TNS.2006.869826.

30

35

40

[50] J. Allison et al. “Recent developments in Geant4”. In: Nucl. Instrum. Meth. A
835 (2016), pp. 186–225. ISSN: 0168-9002. DOI: https://doi.org/10.1016/
j . nima . 2016 . 06 . 125. URL: https : / / www . sciencedirect . com / science /article/pii/S0168900216306957.
[51] P.M. Lewis et al. “First measurements of beam backgrounds at SuperKEKB”.
In: Nucl. Instrum. Meth. A 914 (2019), pp. 69–144. ISSN: 0168-9002. DOI:
https : / / doi . org / 10 . 1016 / j . nima . 2018 . 05 . 071. URL: https : / / www .
sciencedirect.com/science/article/pii/S0168900218306909.
[52] Z. Liptak et al. “Measurements of beam backgrounds in SuperKEKB Phase 2”.
In: Nucl. Instrum. Meth. A 1040 (2022), p. 167168. ISSN: 0168-9002. DOI:
https : / / doi . org / 10 . 1016 / j . nima . 2022 . 167168. URL: https : / / www .
sciencedirect.com/science/article/pii/S0168900222005149.

159

5

[53] J. Schueler et al. “Application of recoil-imaging time projection chambers to
directional neutron background measurements in the SuperKEKB accelerator
tunnel”. In: Nucl. Instrum. Meth. A 1040 (2022), p. 167291. ISSN: 0168-9002.
DOI : https://doi.org/10.1016/j.nima.2022.167291. URL : https://www.
sciencedirect.com/science/article/pii/S0168900222006179.
[54] Belle II Collaboration. Belle2 Physics List. https : / / github . com / belle2 /basf2/blob/release-06-00-09/simulation/doc/Belle2PhysicsList.rst.
2021 (accessed June 2023).

10[55] Geant4 Collaboration. Guide for Physics Lists. https://geant4-userdoc.web.
cern.ch/UsersGuides/PhysicsListGuide/html/index.html. 2022 (accessed
June 2023).
[56] L.Bosisio et al. “SVD Radiation and Environmental Monitoring: General Requirements”. In: BELLE2 - NOTE - 43 (2015).

15[57] S. Bacher et al. “Performance of the diamond-based beam-loss monitor system
of Belle II”. In: Nucl. Instrum. Meth. A 997 (2021), p. 165157. DOI: 10.1016/
j.nima.2021.165157.
[58] R. S. Sussmann. CVD diamond for electronic devices and sensors. 1st. Wiley,2009. DOI: 10.1002/9780470740392.

20[59] E. Berdermann. “Diamond for particle and photon detection in extreme conditions”. In: Comprehensive Hard Materials / Sarin, Vinod. Elsevier Science,
2014, pp. 407–467. DOI: 10.1016/B978-0-08-096527-7.00054-4.
[60] L. Vitale. “EoI: RMBA Upgrade”. In: Internal Document (2021). URL: https://confluence.desy.de/display/BI/Upgrade+EOI+Submission.

25[61] P.M. Lewis et al. “First Measurements of Beam Backgrounds at SuperKEKB”.
In: Nucl. Instrum. Meth. A 914 (2019), pp. 69–144. DOI: 10.1016/j.nima.
2018.05.071. arXiv: 1802.01366 [physics.ins-det].
[62] Latika Aggarwal et al. “Snowmass White Paper: Belle II physics reach andplans for the next decade and beyond”. In: (July 2022). arXiv: 2207 .06307[hep-ex].

3035[63] Valerio Bertacchi et al. “Track finding at Belle II”. In: Comput. Phys. Commun.
259 (2021), p. 107610. DOI: 10.1016/j.cpc.2020.107610. arXiv: 2003.12466[physics.ins-det].
[64] F. Abudinén et al. “B-flavor tagging at Belle II”. In: Eur. Phys. J. C 82.4 (2022),
p. 283. DOI: 10 . 1140 / epjc / s10052 - 022 - 10180 - 9. arXiv: 2110 . 00790[hep-ex].
[65] H. Kakuno et al. “Measurement of —V(ub)— using inclusive B —> X(u) lnu decays with a novel X(u) reconstruction method”. In: Phys. Rev. Lett. 92(2004), p. 101801. DOI: 10.1103/PhysRevLett.92.101801. arXiv: hep- ex/
0311048.

160

5[66] Bernard Aubert et al. “Determination of |Vub | from measurements of the electron and neutrino momenta in inclusive semileptonic B decays”. In: Phys.
Rev. Lett. 95 (2005). Ed. by R. Brenner, C. P. de los Heros, and J. Rathsman. [Erratum: Phys.Rev.Lett. 97, 019903 (2006)], p. 111801. DOI: 10.1103/
PhysRevLett.95.111801. arXiv: hep-ex/0506036.
[67] I. Adachi et al. “Evidence for B − → τ − ν̄τ with a Hadronic Tagging Method Using the Full Data Sample of Belle”. In: Phys. Rev. Lett. 110.13 (2013), p. 131801.
DOI : 10.1103/PhysRevLett.110.131801. arXiv: 1208.4678 [hep-ex].

10[68] I. Adachi et al. “Evidence for B + → K + ν ν̄ Decays”. In: (Nov. 2023). arXiv:
2311.14647 [hep-ex].
[69] Jan Kieseler. “Object condensation: one-stage grid-free multi-object reconstruction in physics detectors, graph and image data”. In: Eur. Phys. J. C 80.9(2020), p. 886. DOI: 10.1140/epjc/s10052-020-08461-2. arXiv: 2002.03605[physics.data-an].

15

20

25

[70] Marc Neu et al. “Real-time Graph Building on FPGAs for Machine Learning
Trigger Applications in Particle Physics”. In: (July 2023). arXiv: 2307.07289[hep-ex].
[71] Lea Reuter et al. “Graph Neural Network based Track Finding in the Central
Drift Chamber at Belle II”. CHEP23. 2023. URL: https://indico.jlab.org/
event/459/contributions/11761/attachments/9441/13689/GNN_based_
TrackFinding_Belle2_LReuter.pdf.
[72] Isabel Haide et al. “Improved Clustering in the Belle II Electromagnetic Calorimeter with Graph Neural Networks”. CHEP23. 2023. URL: https : / / indico .
jlab . org / event / 459 / contributions / 11756 / attachments / 9700 / 14154 /IsabelHaide_CHEP_Talk_110523.pdf.
[73] F. Wemmer et al. “Photon Reconstruction in the Belle II Calorimeter Using
Graph Neural Networks”. In: (June 2023). arXiv: 2306.04179 [hep-ex].

30[74] T. Keck et al. “The Full Event Interpretation: An Exclusive Tagging Algorithm
for the Belle II Experiment”. In: Comput. Softw. Big Sci. 3.1 (2019), p. 6. DOI:
10.1007/s41781-019-0021-8. arXiv: 1807.08680 [hep-ex].
[75] Gianluca Aglieri Rinella. “The ALPIDE pixel sensor chip for the upgrade of the
ALICE Inner Tracking System”. In: Nucl. Instrum. Meth. A. Proceedings of theVienna Conference on Instrumentation 2016 845 (2017), pp. 583–587. DOI:
10.1016/j.nima.2016.05.016.

35

40[76] W. Snoeys et al. “A process modification for CMOS monolithic active pixel
sensors for enhanced depletion, timing performance and radiation tolerance”.
en. In: Nucl. Instrum. Meth. A 871 (Nov. 2017), pp. 90–96. ISSN: 0168-9002.
DOI : 10.1016/j.nima.2017.07.046. URL : http://www.sciencedirect.com/
science/article/pii/S016890021730791X (visited on 04/29/2020).
[77] C. Bespin et al. “DMAPS Monopix developments in large and small electrodedesigns”. In: Nucl. Instrum. Meth. A 978 (Oct. 2020), p. 164460. DOI: 10.1016/
j.nima.2020.164460.

161

5

10

[78] Ivan Perić et al. “The FEI3 readout chip for the ATLAS pixel detector”. en. In:
Nucl. Instrum. Meth. A. Proceedings of the International Workshop on Semiconductor Pixel Detectors for Particles and Imaging 565.1 (Sept. 2006), pp. 178–
187. ISSN: 0168-9002. DOI: 10 . 1016 / j . nima . 2006 . 05 . 032. URL: http ://www.sciencedirect.com/science/article/pii/S0168900206007649 (visited on 11/19/2020).
[79] Maurice Garcia-Sciveres, Jorgen Christiansen, and Flavio Loddo. “RD53B Manual”. fr. In: 1) Detailed ATLAS Manual pdf from gitlab (Mar. 2019). Number:
CERN-RD53-PUB-19-002. URL: https://cds.cern.ch/record/2665301 (visited on 04/04/2022).
[80] C. Bespin. Performance of unirradiated TJ-Monopix 2. Valencia, Spain, Apr.
2023. URL: https : / / indico . cern . ch / event / 1191719 / contributions /5368096/ (visited on 08/23/2023).

1520[81] L. Massaccesi. A CMOS pixels upgrade for the Belle II Vertex Detector. Trento,
Italy, Feb. 2023. URL: https://indico.cern.ch/event/1223972/contributions/
5262030/ (visited on 08/23/2023).
[82] M. Babeluk et al. “CMOS MAPS upgrade for the Belle II Vertex Detector”. en.
In: Nucl. Instrum. Meth. A 1048 (Mar. 2023), p. 168015. ISSN: 0168-9002.
DOI : 10.1016/j.nima.2023.168015. URL : https://www.sciencedirect.com/
science/article/pii/S0168900223000050 (visited on 01/14/2023).
[83] M. Schwickardi. Upgrade of Belle II Vertex Detector with CMOS Pixel Technology· Indico. Oslo, Norway, June 2023. URL: https://indico.cern.ch/event/1247911/contributions/5394507/ (visited on 08/23/2023).

25

30

35

40

[84] Hendrik Jansen et al. “Performance of the EUDET-type beam telescopes”. en.
In: EPJ Techniques and Instrumentation 3.1 (Dec. 2016). Number: 1 Publisher:
SpringerOpen, pp. 1–20. ISSN: 2195-7045. DOI: 10.1140/epjti/s40485-0160033- 2. URL: https://epjtechniquesandinstrumentation.springeropen.
com/articles/10.1140/epjti/s40485-016-0033-2 (visited on 11/14/2022).
[85] T. Fillinger. “Detector Simulation for a Potential Upgrade of the Vertex Detectorof the Belle II Experiment”. en. In: Acta Physica Polonica B 52.8 (2021), p. 909.
ISSN : 0587-4254, 1509-5770. DOI : 10.5506/APhysPolB.52.909. URL : http://www.actaphys.uj.edu.pl/findarticle?series=Reg&vol=52&page=909(visited on 10/14/2021).
[86] Konstantinos Moustakas. “Design and Development of Depleted Monolithic
Active Pixel Sensors with Small Collection Electrode for High-Radiation Applications”. PhD Thesis. U. Bonn (main), Bonn U., 2021. DOI: CERN-THESIS2021-146.
[87] Alessandra Fantoni. “Upgrade of the ALICE inner tracking system: Construction and commissioning”. In: 95.8 (2020), p. 084011. DOI: 10.1088/14024896/aba0f7.

162

5

[88] K. Adamczyk et al. “The design, construction, operation and performance of
the Belle II silicon vertex detector”. en. In: Journal of Instrumentation 17.11(Nov. 2022). Publisher: IOP Publishing, P11042. ISSN: 1748-0221. DOI: 10.
1088/1748-0221/17/11/P11042. URL: https://dx.doi.org/10.1088/17480221/17/11/P11042 (visited on 11/30/2022).
[89] J. P. Cachemiche. “The PCIe-based readout system for the LHCb experiment”.
In: Topical Workshop on Elect. for Part. Phys. 2015. URL: https : / / indico .
cern.ch/event/357738/sessions/71607/.

10[90] Akimasa Ishikawa et al. “New pixel detector concept DuTiP for Belle II upgrade
and the ILC with an SOI technology”. In: Nucl. Instrum. Meth. A 978 (2020),
p. 164404. DOI: 10.1016/j.nima.2020.164404.
[91] Y. Ono et al. “Development of the Pixel OR SOI detector for high energy physics
experiments”. In: Nucl. Instrum. Meth. A 731 (2013). Ed. by Yoshinobu Unno
et al., pp. 266–269. DOI: 10.1016/j.nima.2013.06.044.

1520[92] Shun Ono et al. “Development of a monolithic pixel sensor based on SOI technology for the ILC vertex detector”. In: Nucl. Instrum. Meth. A924 (2019),
pp. 431–435. DOI: 10.1016/j.nima.2018.06.075.
[93] Zhigang Wu et al. “A prototype SOI pixel sensor for CEPC vertex”. In: Nucl.
Instrum. Meth. A 924 (2019), pp. 409–416. DOI: 10.1016/j.nima.2018.06.
059.
[94] Roma Bugiel et al. “Test-beam results of a SOI pixel-detector prototype”. In:
Nucl. Instrum. Meth. A 901 (2018), pp. 173–179. DOI: 10.1016/j.nima.2018.
06.017.

25[95] R. Bugiel et al. “High spatial resolution monolithic pixel detector in SOI technology”. In: Nucl. Instrum. Meth. A 988 (2021), p. 164897. DOI: 10.1016/j.
nima.2020.164897.
[96] Y Arai, T Miyoshi, and I Kurachi. “SOI Monolithic Pixel Technology for Radiation Image Sensor”. In: Tech Digst. IEDM (2019).

30[97] Kazuhiko Hara et al. “Radiation hardness of silicon-on-insulator pixel devices”.
In: Nucl. Instrum. Meth. A 924 (2019), pp. 426–430. DOI: 10.1016/j.nima.
2018.05.077.
[98] M. Šuljić. “ALPIDE: the Monolithic Active Pixel Sensor for the ALICE ITS upgrade”. In: JINST 11.11 (2016). Ed. by Claudia Gemme and Leonardo Rossi,
p. C11025. DOI: 10.1088/1748-0221/11/11/C11025.

35

40[99] Andreas Löschcke Centeno et al. A TPC-based tracking system for a future BelleII upgrade. 2022. arXiv: 2203.07287 [physics.ins-det].
[100] Y. Bilevych et al. “TwinGrid: A wafer post-processed multistage Micro Patterned Gaseous Detector”. In: Nucl. Instrum. Meth. A 610.3 (2009), pp. 644–
648. ISSN: 0168-9002. DOI: https : / / doi . org / 10 . 1016 / j . nima . 2009 .
09 . 054. URL: https : / / www . sciencedirect . com / science / article / pii /S0168900209017963.

163

[101] P. M. Kluit et al. “Pixel TPC R&D”. In: The 2020 International Workshop on theHigh Energy Circular Electron Positron Collider. 2020. URL: https://indico.
ihep.ac.cn/event/11444.
5

10

15

[102] Peter Križan (for the Belle II Collaboration). “The Belle II Upgrade Program”.
In: 41st International Conference on High Energy physics. 2022. URL: https ://arxiv.org/pdf/2211.13634.pdf (visited on 11/24/2022).
[103] I. Mostafanezhad et al. “Measurement results for the ASoC V3: A High Performance Waveform Digitizer System-on-Chip”. In: 2020 IEEE Nuclear Science
Symposium and Medical Imaging Conference (NSS/MIC). 2020, pp. 1–3. DOI:
10.1109/NSS/MIC42677.2020.9507893.
[104] L. Macchiarulo et al. “Design and initial Measurement Results for the second
revision of the High Density Digitizer System on Chip (HDSoCv2): A 64 Channel 1 GSa/s Waveform Digitizer for High Density Detectors”. In: 2023 IEEE
Nuclear Science Symposium, Medical Imaging Conference and International Symposium on Room-Temperature Semiconductor Detectors (NSS MIC RTSD). 2023,
pp. 1–1. DOI: 10.1109/NSSMICRTSD49126.2023.10338148.
[105] Xiyang Wang and Hongyu Zhang et al. “Design of a high-speed and low-noise
preamplifier for SiPM”. In: (Nov. 2023). ChinaXiv: 202305.00148.

20[106] G. Aielli et al. “Improving the RPC rate capability”. In: Journal of Instrumentation 11.07 (July 2016), P07014. DOI: 10.1088/1748-0221/11/07/P07014.
URL : https://dx.doi.org/10.1088/1748-0221/11/07/P07014.

25[107] K Abe et al. “Performance of glass RPC operated in streamer mode with SF6 gas
mixture”. In: Nucl. Instrum. Meth. A 455.2 (2000), pp. 397–404. ISSN: 01689002. DOI: https://doi.org/10.1016/S0168-9002(00)00518-0. URL: https://www.sciencedirect.com/science/article/pii/S0168900200005180.

30[108] M Yamaga et al. “RPC systems for BELLE detector at KEKB”. In: Nucl. Instrum. Meth. A 456.1 (2000). Proceedings of the 5th Int. Workshop on Resistive Plate Chambers and Related Detectors, pp. 109–112. ISSN: 0168-9002.
DOI : https : / / doi . org / 10 . 1016 / S0168 - 9002(00 ) 00973 - 6. URL : https ://www.sciencedirect.com/science/article/pii/S0168900200009736.
[109] K. Abe et al. “Detection of Muons, and K0L Mesons from e+e- collisions using
the endcap RPC module in the Belle detector”. In: IEEE Transactions on Nuclear
Science 47.6 (2000), pp. 1748–1752. DOI: 10.1109/TNS.2000.914440.

3540[110] A. Abashian et al. “The KL/µ detector subsystem for the BELLE experimentat the KEK B-factory”. In: Nucl. Instrum. Meth. A 449.1 (2000), pp. 112–124. ISSN: 0168-9002. DOI: https : / / doi . org / 10 . 1016 / S0168 - 9002(99 )01383- 2. URL: https://www.sciencedirect.com/science/article/pii/
S0168900299013832.
[111] H Sakaue et al. “Test of freonless operation of resistive plate chambers withglass electrodes—1mm gas gap vs 2mm gas gap”. In: Nucl. Instrum. Meth. A
482.1 (2002), pp. 216–225. ISSN: 0168-9002. DOI: https : / / doi . org / 10 .
1016 / S0168 - 9002(01 ) 01624 - 2. URL: https : / / www . sciencedirect . com /science/article/pii/S0168900201016242.
164[112] P Camarri et al. “Streamer suppression with SF6 in RPCs operated in avalanchemode”. In: Nucl. Instrum. Meth. A 414.2 (1998), pp. 317–324. ISSN: 01689002. DOI: https://doi.org/10.1016/S0168-9002(98)00576-2. URL: https://www.sciencedirect.com/science/article/pii/S0168900298005762.
5

10

15

20

25

[113] Gianluca Rigoletti, Roberto Guida, and Beatrice Mandelli. “Performance studies of RPC detectors operated with C2H2F4 and CO2 gas mixtures”. In: Nucl.
Instrum. Meth. A 1049 (2023), p. 168088. ISSN: 0168-9002. DOI: https://doi.org/10.1016/j.nima.2023.168088. URL: https://www.sciencedirect.
com/science/article/pii/S0168900223000785.
[114] Gianluca Rigoletti, Roberto Guida, and Beatrice Mandelli. “Studies on RPC detectors operated with environmentally friendly gas mixtures in LHC-like conditions”. In: The European Physical Journal Plus 138.9 (Sept. 2023), p. 841.
ISSN : 2190-5444. DOI : 10 . 1140 / epjp / s13360 - 023 - 04434 - y. URL : https ://doi.org/10.1140/epjp/s13360-023-04434-y.
[115] G. Myhre et al. “Anthropogenic and Natural Radiative Forcing”. In: Climate
Change 2013: The Physical Science Basis. Contribution of Working Group I to
the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.
Ed. by T.F. Stocker et al. Cambridge, United Kingdom and New York, NY, USA:
Cambridge University Press, 2013. Chap. 8, pp. 659–740. ISBN: ISBN 978-1107-66182-0. DOI: 10.1017/CBO9781107415324.018. URL: www.climatechange2013.
org.
[116] R Cardarelli et al. “Performance of RPCs and diamond detectors using a newvery fast low noise preamplifier”. In: Journal of Instrumentation 8.01 (Jan.
2013), P01003. DOI: 10.1088/1748-0221/8/01/P01003. URL: https://dx.
doi.org/10.1088/1748-0221/8/01/P01003.
[117] Hosein Hashemi et al. “Ultra-High-Resolution Detector Simulation with IntraEvent Aware GAN and Self-Supervised Relational Reasoning”. In: (Mar. 2023).
arXiv: 2303.08046 [physics.ins-det].

30[118] B. Schwenker et al. “A neural network for beam background decomposition
in Belle II at SuperKEKB”. In: Nucl. Instrum. Meth. A 1049 (2023), p. 168112.
DOI : 10.1016/j.nima.2023.168112. arXiv: 2301.06170 [hep-ex].
[119] LIU Yuxin. “Study of Sudden Beam Losses of SuperKEKB and Development of
3D Track Hardware Trigger using Machine Learning at Belle II”. MA thesis.
The graduate university for advanced studies, SOKENDAI, 2023.

35

40[120] Tomoyuki Shimasaki. “Study on first-level trigger of the Belle II experimentusing upraded silicon strip detector”. MA thesis. The university of Tokyo, 2023.
URL : https : / / docs . belle2 . org / record / 3400 / files / BELLE2 - MTHESIS 2023-004.pdf.
[121] M. Nakao. “Timing distribution for the Belle II data acquisition system”. In:
Journal of Instrumentation 7 (2012), p. C01028. DOI: 10.1088/1748-0221/7/
01/C01028.
[122] M. Nakao et al. “Minimizing dead time of the Belle II data acquisition systemwith pipelined trigger flow control”. In: Proceedings of 18th Real-Time Conference (2012). DOI: 10.1109/RTC.2012.6418145.
165

[123] S. Yamada et al. “Common Readout Subsystem for the Belle II Experiment and
Its Performance Measurement”. In: IEEE Trans. Nucl. Sci. 64 (2017), pp. 1415–
1419. DOI: 10.1109/TNS.2005.856913.
5[124] T. Higuchi et al. “Modular pipeline readout electronics for the SuperBelle drift
chamber”. In: IEEE Trans. Nucl. Sci. 52 (2005), pp. 1912–1917. DOI: 10.1109/
TNS.2005.856913.
[125] Julien Langouët. “Future DAQ boards”. In: 6th Workshop on LHCb upgrade II.
2023. URL: https://indico.icc.ub.edu/event/163.

10[126] R. Itoh et al. “Data Flow and High Level Trigger of Belle II DAQ System”. In:
IEEE Trans. Nucl. Sci. 60 (2013), pp. 3720–3724. DOI: 10.1109/TNS.2013.
2273091.
[127] D. M. Asner et al. “Snowmass 2021 White Paper on Upgrading SuperKEKB
with a Polarized Electron Beam: Discovery Potential and Proposed Implementation”. In: Snowmass 2021. May 2022. arXiv: 2205.12847 [physics.acc-ph].

1520[128] D. Charlet et al. “Conceptual study of a Compton polarimeter for the upgradeof the SuperKEKB collider with a polarized electron beam”. In: JINST 18.10(2023), P10014. DOI: 10.1088/1748-0221/18/10/P10014.
[129] J. P. Lees et al. “Precision e− Beam Polarimetry at an e+ e− B Factory using
Tau-Pair Events”. In: Phys. Rev. D 108.9 (2023), p. 092001. DOI: 10 . 1103 /PhysRevD.108.092001. arXiv: 2308.00774 [hep-ex].
[130] Shohei Nishida et al. “Development of an HAPD with 144 channels for the
aerogel RICH of the Belle upgrade”. In: Nucl. Instrum. Meth. A 595 (2008). Ed.
by Andrea Bressan et al., pp. 150–153. DOI: 10.1016/j.nima.2008.07.032.

25

30

35

[131] S.K. Sahu et al. “Measurement of radiation damage on a silica aerogel Cherenkov
radiator”. In: Nucl. Instrum. Meth. A 382.3 (1996), pp. 441–446. ISSN: 01689002. DOI: https://doi.org/10.1016/S0168-9002(96)00768-1.
[132] S. Iwata et al. “Particle identification performance of the prototype aerogel
RICH counter for the Belle II experiment”. In: Progress of Theoretical and Experimental Physics 2016.3 (Mar. 2016), 033H01. DOI: 10.1093/ptep/ptw005.
eprint: https://academic.oup.com/ptep/article- pdf/2016/3/033H01/
19301325/ptw005.pdf. URL: https://doi.org/10.1093/ptep/ptw005.
[133] R. Giordano et al. “Frame-Level Intermodular Configuration Scrubbing of OnDetector FPGAs for the ARICH at Belle II”. In: IEEE Trans. Nucl. Sci. 68.12(2021), pp. 2810–2817. DOI: 10.1109/TNS.2021.3127446. arXiv: 2010.16194[physics.ins-det].
[134] S. Korpar et al. “Silicon photomultiplier as a detector of Cherenkov photons”.
In: Nucl. Instrum. Meth. A 595 (2008). Ed. by Andrea Bressan et al., pp. 161–
164. DOI: 10.1016/j.nima.2008.07.013.

40[135] S. Korpar et al. “Timing and cross-talk properties of BURLE multi-channel MCP
PMTs”. In: Nucl. Instrum. Meth. A 595.1 (2008). RICH 2007, pp. 169–172.
ISSN : 0168-9002. DOI : https://doi.org/10.1016/j.nima.2008.07.022.

166

[136] “AIDAInnova”. In: Horizon 2020 H2020-INFRAINNOV-2020-2 project. 2021.
URL : https : / / cordis . europa . eu / project / id / 101004761 (visited on
02/10/2022).
5[137] S. Nishida et al. “Readout ASICs and Electronics for the 144-channel HAPDs
for the Aerogel RICH at Belle II”. In: Phys. Procedia 37 (2012). Ed. by Ted Liu,
pp. 1730–1735. DOI: 10.1016/j.phpro.2012.02.499.
[138] S. Gomez et al. “FastIC developments for LHCb and Belle RICH upgrades”. In:
Workshop on LHC Technologies (Red-LHC). 2021. URL: https://indico.cern.
ch/event/1064182/ (visited on 02/10/2022).

10[139] Hamamatsu. URL: https://www.hamamatsu.com/us/en/product/opticalsensors/apd/si-apd/S8664-55.html.
[140] Hamamatsu. URL: https://www.hamamatsu.com/us/en/product/opticalsensors/apd/si-apd/S8664-1010.html.

1520[141] Y. Jin et al. “Study of a pure CsI crystal readout by APD for Belle II end cap
ECL upgrade”. In: Nucl. Instrum. Meth. A 824 (2016). Ed. by Maria Giuseppina
Bisogni et al., pp. 691–692. DOI: 10.1016/j.nima.2015.07.034.
[142] S. Longo et al. “CsI(Tl) pulse shape discrimination with the Belle II electromagnetic calorimeter as a novel method to improve particle identification atelectron–positron colliders”. In: Nucl. Instrum. Meth. A 982 (2020), p. 164562.
DOI : 10.1016/j.nima.2020.164562. arXiv: 2007.09642 [physics.ins-det].
[143] FBK. URL: https://www.fbk.eu/en/.
[144] AdvanSid. URL: https://www.advansid.com/.

167