Belle II status(ARC review March 2024)
K.Trabelsi

KEK , 3- gokan , Room 202
karim.trabelsi@in2p3.fr

is...
Staf : 305, Post - docs: 131, PhD students : 264 =700 collaborators
and undergraduate/master students, technical members

SuperKEKB + Belle II , a flavour - factory ,(Belle ∼ 1 ab−1 )a rich physics program ...

'' on resonance' ' productione+ e− → Υ (4 S) → B0d B0d , B+ B−0+

Bd , Bb

Υ(4 S)

bbu,dbu,d0−∘ 2 B 's and nothing else !
Bd , B∘ 2 B mesons are created simultaneously
in a L=1 coherent state(e+e-→hadrons) [nb]∘ We plan to collect ( at least ) 50 ab−1 of e+ e− collisions at (or close to)the Υ (4 S) resonance , so that we have:− a (Super) B-factory ( ∼1.1 × 109 BB pairs per ab−1 )energy thresholdfor BB production− a (Super) charm factory (∼1.3 × 109 cc pairs per ab−1 )(but also charmonium , X , Y , Z , pentaquarks , tetraquarks , bottomonium...)9+ −−1− a (Super) τ factory ( ∼ 0.9 × 10 τ τ pairs per ab )− exploit the clean e+ e− environment to probe the existence of exotic
hadrons , dark photons/Higgs , light Dark Matter particles, ALPs , LLPs ...
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2− 2 −1⇒ to reach 6×10 cm s−1⇒ cumulate 50 ab by ∼2035

Belle II run 1 (2019 - 2022)
data taking from March 2019 to June 2022
→ despite dificult conditions since March 2020 (Covid , war in Ukraine , energy cost ...)
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2

−1

luminosity : 4.7×10 / cm / s ! > 2 fb

per day !
June, 2022

β *y = 1 mm , ILER /HER = 1.4/ 1.2 A
record of KEKB /Belle
2×1034 /cm2 /s currents >1 A
record of PEPII / BaBar
1×1034 /cm2 /s currents >2 A

L int = 428 fb −1
L int = 213 fb−1

L int = 74 fb−1
L int = 6.4 fb−1

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2

5 × 10 / cm / s∼ 1 BaBar / year3Long-shutdown (LS1) activity and plans
Belle II stopped taking data in Summer 2022 for a long shutdown- accelerator improvements: injection, non-linear collimators, monitoring...
- additional shielding (e.g. neutron) and increased resilience against beam bckg- installation of additional loss monitors and speed-up of abort signal- replacement of beam-pipe- installation of 2-layered pixel vertex detector (PXD2)- replacement of photomultipliers (MCP-PMT) of the central PID
detector (TOP)- improvement in CDC gas circulation and monitoring- completed transition to new DAQ boards (PCIe40)- work on other detectors as KLM, ECL...
- improved data-quality monitoring and alarm system

PXD2 at KEK since MarchVXD extraction in MayTOP MCP -PMT replacement work

CDC FE reinstallation work`4

PXD2Issue in gliding mechanism and damage of two L1 ladders in 1st half-shell: August 2022
Improve ladder gliding functionality
PXD2 was delivered to KEK: in March 2023
PXD2 mount completed on April 5th. After basic functionality checks (thermomechanicaland electrical) evaluated from internal review committee→ EB+TB gave green light for installation April 21st.
VXD uninstallation work during May
SVD attachment during June
Installation VXD in Belle II end of July
Commissioned with cosmic data in Fall (stand alone with SVD)
⇒ All 40 modules can be powered and deliver data
⇒ temperature, basic module functions

5

CDC∘ Resistance replacement of HV divider to decrease voltage drop due
to high injection background∘ Hardware repair to fix HV trip in a few sectors∘ Improved CDC gas system and monitoring- Origin of H2 0 increase is now understoodand under control with silica-gel dryer tubes- Redundant H2 0 and O2 sensors :to detect drift of calibration and improvereliability of monitor system

6TOP PMT replacement work
Performed PMT/electronics replacement
Allocated 2.5 months for TOP work (completed end of April 2023)
Replace degraded PMTs with newlife-extended ALD PMTs (upper half)
Replace degraded PMTs with best
PMTs available (lower half)
Replace problematic electronic boards⇒ > 99.5% channels operational

7

DAQ∘ upgrade of the readout system (COPPER to PCIe40), performance improvements∘ Reduction of time for SALS (stop-abort-load-start)∘ HLT reinforcement (+3 units) and new framework∘ Direct ROOT file recording

8

2 DAQ core members
rehired with M&O

Other consolidation activities∘ ECLImplementation new pedestal correction algorithm∘ KLM
Development and integration of bubbler flow monitor readout
Investigation NH 3 flushing with RPC test chambers∘ Environmental monitor and alarm system
EPIC - based framework alarm system∘ More shielding
PE shield for QCS neutron shield
PE+SWX 238 on QCS
against charged background (i . e. bellows shield)

9In Oct 2023, LS1 schedule delay: incident in FWD QCSR insertion
After QCS insertion , beam pipe vacuum sealing between pipes in QCS andis established by RVC (Remote Vacuum Connection)

RVC

QCS could reach final position but RVC hit the cable cage and consequently
couldn ' t get locked at this final position
QCS RVC
large displacements of the cable cage → large interference
possible that RVC gets stuck during its operationVXD∘ modifications of RVC / cable cage→ allowed to get back the needed clearance∘ LS 1 schedule revised and start of run 2 at the end of Jan 2024
thank the machine group for modifying cryostat and make VXD installation possible !

10LS1 chapter is closed

Run 1 is with us, run 2 has started
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first collisions of run 2 in February 2024

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Belle II operation shifts in run 2

Fraction of physics run time: 32.4%
Fraction of DAQ running time inphysics run time: 92.0%
Fraction of dead time in DAQ runningtime: 2.0%
Data taking efficiency: 90.2%

13

Among the first results of run 2, as an illustration
CDC performance on cosmic 2024 (better gas/water control and monitoring)⇒ performance much improved compared to end of run 1

exp 26 (2022)
exp 30 (2024)

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Among the first results of run 2, as an illustration
Eficiency map for PXD2

PXD2 eficiency
above 97 % for active modules
reaching 99.7% in forward regions:the maximum limited by Readout and Clear

Resolution from overlaps→ Run 2

Belle(II) , LHCb side by sideLHCb

Belle ( II)e+ e− →Υ (4S)→b b0+at Υ ( 4 S) : 2 B' s (B or B ) andnothing else ⇒ clean events

p p→b b X+0
production of B , B , Bs , Bc , Λ b ...
but also a lot of other particles in the event(flavour tagging , B tagging , missing energy ) ⇒ lower reconstruction eficiencies⇒ initial conditions are precisely known
σ b b much higher than at the Υ (4 S)

σ b b ∼ 1 nb ⇒ 1 fb−1 produces 106 B B
σ b b /σ total ∼ 1/ 4b b production cross - section at LHCb ∼ 500,000 × BaBar / Belle ! !
σ b b /σ total much lower than at the Υ (4 S)higher luminosity⇒ lower trigger and acceptance eficiencies
B mesons live relatively longmean decay length β γ c τ∼ 200 μ m

mean decay length β γ c τ ∼ 7 mm
(displaced vertices)
data taking period( s)−1[run I : 2010- 2012] = 3 fb
−1[run II: 2015 - 2018] = 6 fb−1[1999 -2010] = 1 ab[2019 -...] = ...

(near ) future−1[Belle II from 2019] → 50 ab−1
[LHCb upgrade from 2022 ] → 50 fb

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Rare/ Forbidden B decays
energy frontierintensity frontier→ NP beyond the directreach of the LHC
limited by beam energy

limited by statistics

New particles can for example contribute to loop or tree level diagrams
by enhancing/suppressing decay rates, introducing new sources of CP
violation or modifying the angular distribution of the final-state particles

Many interesting B - physics studies involve missing energyD( *) τ ν , K (* ) τ l , K (* ) τ τ , K ( * ) ν ν , π l ν , τ l , τ τ , τ ν , μ ν ....
Btag
hadronic tag
B→D( *) π , D(* ) ρ ...
ϵ < 1%

K

semileptonic tag( *)B→D l ν X
ϵ ∼ 2%

Bsig → K τ μτ →e ν ν , μ ν ν ,τ→ π ν , π π0 ν
(70 % of all τ decays)17

B→ K ν ν

[arXiv : 2311.14647]
accepted by PRD∘ B→K ν ν is known with high accuracy−6B(B→K ν ν) = (5.6 ± 0.4)×10 [arXiv : 2207.13371]∘ Extensions beyond SM may lead to significant rate increase∘ Very challenging experimentally , not yet observed− Low branching fraction , high background contributions− 3- body kinematics , no good kinematics∘ Unique for Belle II∘ Two analyses :more sensitive inclusive (ef = 8 %) , conventional hadronic tagging (ef = 0.4%)∘ Use event properties to suppress background with multiple variables combined∘ Use classifier output as (one of ) the fit variables , use simulation for signal andbackground templates∘ Use multiple control channels to validate simulation with data18

Evidence of B→ K ν ν[arXiv : 2311.14647]
accepted by PRD

inclusive tag
hadronic tag∘ Maximum likelihood fit to data using signal and background templates−5B incl = (2.7 ± 0.5 (stat) ± 0.5 (syst)) × 10+ 0.9+ 0.8−5

B had = (1.1 −0.8 (stat) −0.5 (syst)) × 10

∘ For inclusive analysis , evidence for B→K ν ν at 3.5 σbranching fraction within 3 σ of SM∘ For hadronic tag , the result is consistent with nullhypothesis and SM at 1.1 σ and 0.6 σ⇒ Combination of two analyses provides first evidenceof the decay at 2.7 σ from SM19

'' τ factory ''∘ SuperKEKB + Belle II is also a τ - factory !
∘ lepton flavour violating decays of the τ as NP probe⇒ LFV accidental symmetry of SM , many NP
models can naturally break this symmetry

expected Belle IIsensitivity

10−92040

cLFV : beyond the Standard Modelτ LFV searches at Belle II will be extremely clean with very little background (if any ),thanks to pair production and double - tag analysis technique .
[ Belle , PLB 687 (2010) 139]--+

τ →e e e---+τ →μ μ μ

how to improve further ?

-+−8⇒ 2.1× 10

at 90 % CL... considering τ →μ / e h h
in function of one prong
tag categories... for τ →3 muons ,improve μ - ID at low mom(ECL info)

In contrast , hadron collider experiments must contend
with larger combinatorial and specific backgrounds−1[ LHCb , JHEP02 (2015) 121, 2 fb ]−8

⇒ 4.6×10
CMS , full Run 2 dataset : 2.9×10−8at 90 % CL

at 90 % CL

Most improvement in coming decade is expected from Belle II , which can reach−9
1×10 [arXiv :1011.0352] and will do 21
even better if can achieve ∼ zero bckgd-τ → 3μ at Belle II
Analysis selection and results : inclusive approach
BDT trained on 32 variables :inputs from signal τ - , event tag side ,event shape and kinematics
ϵsig = (20.42 ± 0.06)% (3 × larger than Belle)+1.4

Expected BKG: 0.5−0.5 evts
No significant excess in 424 fb−1 of data

Obtained most stringent limit221.9 × 10−8

Lepton universality tests at Belle II
precise test of μ -e universality by measuring

ratio of leptonic branching fractions

R μ = 0.9675 ± 0.0037

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What are our goals for 2024 ?
(please a clear and sound message)

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2

Run stably at 10 / cm / s−1reach 150 fb

per month−1

exceed 1 ab

(setting the pace for run 2)(while doing good physics and working for the upgrade)...' This was their finest hour .'24

Upgrade: Conceptual Design Report(in the perspective of a LS2)

upgrade session at BPAC Feb 5th, 2024

CDR presents several possible detector
improvements on different time scales.
Main goal of the CDR is to have a document describing
the diferent proposals discussed in Belle II collaborationand make it public in a timely manner (spring 2024)
to allow the organization of the next steps on the path:R&D and engineering activities, preparation of TDRs
for the most promising proposals, submittal of funding
requests to funding agencies etc...
VTX - DMAPS(5 straight layers barrel,
using CMOS pixel sensors)25

Upgrade: Conceptual Design Report
Belle II's request to BPAC (Feb 2024)

BPAC short report (March 2024)
The CDR for the LS2 upgrade plans, presented during this meeting, described various
ideas for how to increase the machine luminosities, enhance the detector performanceand the tolerance against beam backgrounds and radiation damage. Some of them arewell advanced, in particular the new vertex detector system (VTX), which is envisioned
to replace the current vertex detector, for which the committee recommends that thegroup should advance towards the engineering level of studies, including the aspectsof system integration and operation.
The machine group presented an upgrade path with a new set of final focusing magnets.
Since they have just started machine operations after the Long Shutdown 1 with a lotof changes and improvements having been made, they need first to understand thebehaviour of the machine in order to identify the remaining causes of luminosity
limitation and to provide a definite scheme for the upgrade.

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Upgrade: Conceptual Design Report
Belle II's request to BPAC (Feb 2024)

BPAC short report (March 2024)

While the CDR provides an excellent initial framework to continue
the development of the plans for the upgrade, the BPAC is not yet
in a position to judge the validity of the plan described in the CDR.
While the committee recommends further R&D work to continue, italso thinks that a new document providing a coherent and integrated
description of the accelerator and detector upgrade plan witha quantitative demonstration of expected physics performance
should be submitted for making a decision on the project.

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Summary∘ Run 1 sample is providing competitive results...
- in B physics with missing energy (e.g. B→K ν ν )- in τ sector , dark sector (5- 6 PRLs) , measurements thatimprove hadronic vacuum polarization estimate muon g-2 ...

∘ A lot of improvements in Belle II during LS1...
- for better detector performance- for more eficient data taking∘ Run 2 is crucial to ...
- understand the behaviour of the machine in order to identify
the remaining causes of luminosity limitation- cumulate o(5 ab−1 ) in 4-5 years and impact the flavor/DS sector∘ Upgrade:- understanding the IR envelope and design as soon as possible isof primary importance- more details about the linac and RF upgrades- scope and timeline to flesh out and write upBackground mitigation expert (M&O)

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2

Reaching and running stably at 10 / cm / ssummershutdown

firstcollisions(or earlier)−1Reaching 150 fb

per month
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−2 −1

In end of April 2022, 15/fb per week could be reached when instantaneous luminosity was ∼ 3.5 × 10 cm s

−1

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−2 −1

⇒ 15×4×2.5 = 150 fb / month at L = 10 cm s