Background Estimation
H.Nakayama (Belle(Belle‐II)II)
SuperKEKB international review
2011.02.08

Index• Introduction• Background estimation– 1. Touschek/gas scattering– 2.
2 Synchrotron
S hradiation
di i– 3. GEANT4‐based simulation framework

2Introduction• At SuperKEKB with 40 times higher luminosity of KEKB,detector background will also increase drastically.
– Nano‐beam(Nano beam(0.94um0.048/0.062nm
0 94um0 048/0 062nm),) higher current(x~2)
current(x~2),
smaller radius of inner‐most detector (rmin=18mm14mm)• To protect detector from background,background we carefully design– collimators in arc sections– IR beam pipe optimized to stop SR light– heavy‐metal shield on IR beam pipe to stop showers– Polyethylene shield to stop neutrons– etc…

3

Background sources at SuperKEKB
1. Touschek effect– Intra‐bunch scattering, Rate∝(beam size)‐1, ∝E‐3  mainly LER– Dominant background at SuperKEKB (nano(nano‐beam)
beam)– Scattered beam particles hit beam pipe create shower reach detector– Countermeasures：• Horizontal
H i t l collimator
lli t on bothb th (inner/outer)(i/ t ) sideid tot stopt off‐momentum
ffte+/e‐/• Heavy‐metal on IR beam pipe to protect inner detector from showerCollimator

2. Beam‐gas scattering– Scattering by remaining gas, Rate ∝IxP
– Vacuum level at SuperKEKB will be similar to KEKB
except for IP region.
– Particles scattered in IP region will be lost at downstream,
th f
therefore
willill nott beb detectord t t backgroundb kdhheavy‐metalt lIP4Background sources at SuperKEKB (cntd(cntd.))
3 Synchrotron
3.
S hradiation
di i– Rate ∝ E2B2: mainly from HER– Photons are emitted on final focusing magnet
hit IP beam pipe (Be) and penetrate  reach PXD– Countermeasures:• collimate beam pipe radius just before IP• “ridge (saw‐tooth)” on beam‐pipe inner wall
to prevent reflected photons to hit Be pipe

4. Q
QED pprocess– Physics process, Rate∝Luminosity• Rad. Bhabha: Gammas emitted in forward direction
 hit magnet iron create neutron: to be simulated• 2‐photon: e+e‐ pair will hit PXD: confirmed OK

5. Beam‐beam interaction– Scattered at IP,
IP by field of the other beam
– To be simulated

0.1%(<<1%)
occupancy
on PXD
5

Background tasks• Simulation– Prepare background flux (generate scattering, propagate to IR)– Implement IR geometries into GEANT4 models• Beam‐pipe, QC1/2 magnet, cryostat, non‐uniform magnetic field, etc.

– Full‐detector GEANT4 simulation• KEKB data analysis– Compare with KEKB simulation, validation of simulation scheme– Extrapolation toward SuperKEKB• Feedback to structural design– ShapeShoff beam‐pipe,bihheavy‐metall shield
hi ld on beam‐pipebi– Collimator– Cooling system of IP beam pipe
– etc…
6

1 Touschek/Beam‐gas
1.
dE/E: + or ‐bunch☑ Extrapolation of KEKB data
☑ KEKB: simulation validity check with data
☑ SuperKEKB: Flux simulationp
GEANT4 simulation□ SuperKEKB:

1 1 Extrapolation from KEKB data
1‐1.
• KEKB machine study (Jun. 2010)– Single beam (no collision), measure detector background– Separate Touschek
h k andd beam‐gas,busing beam‐sizebddependenced• kbeam‐gas, beam‐gas, kTou, Tou are measured“kTou, kbeambeam‐gas
gas are the probability of scattered particle to reachdetector and becomes background. It should depend on collimatorsettings, heavy metal shield amount, and detector performance.

– Extrapolation strategy for SuperKEKB：• The KEK value is assumed for kbeam‐gas, kTou,and beam‐gas.

• For Tou , SuperKEKB design Value (10min) is used.
• Other background (SR,QED,beam‐beam,..) are not considered.
8How to measure Touschek, beam‐gas

Beam size(m)smallersize
1/life (min‐1)(hour)

Nominalbeam size(~2um)

LER

Beam life(min)shorterlife

Touschek
beam‐gas(hour)

CDC current(A)
t( A)

1/y (m‐1)

y∞, Touschek 0moreBG
H.Nakayama(KEK)(hour)

9

How to measure kTouschek,kbeam‐gas
Nominalbeam size(~2um)CDC BG/i (A/mA)
LER
slope:
slope:
pkbeam‐gaskTouschek
BGTouschek
BGbeam‐gas
1/life (min‐1)

1/Touschek
Th k ~150min
150 i
H.Nakayama(KEK)1/beam‐gas ~800min

10min@SuperKEKB

10Sugihara(Tokyo)

Extrapolated background at SuperKEKB
Extrapolated BG (Belle‐II+SuperKEKB)

ECL~9 GeV/event, OK(introduce 5us time window)

TOP ~2.9MHz
2 9MH (=~3MHZ)( 3MHZ) TOF ratet eqiv.,i (replaced from current TOF)
CDC ~84kHz/wire (<200kHz/wire), OK(larger radius, more cell number)
SVD ~1.2% occupancy (<10%), OK((shorter integrationgtime))
PXD ~2.0% occupancy (>1%), (not including low pt particle & few keV gamma)

CDC

Note that only Touschek and beam‐gas BG
are considered. Other BGs are not included(SR QED,(SR,QED beam
beam‐beambeam, etc…).
etc )
11

Nakano(Tohoku)

1‐2. Simulation vs. Data (KEKB)
Nakano, B2GM7 parallel

Could be x1/10

Could be x1/10

Assumption for simulation:• 1nTorr CO (0.1~1nTorr?)(?)• Uniform Touschek in the ring
• 107 seconds/year (dose)• etc..
t

TURTLE+GEANT simulation isconsistent with experimental data.
12

Ohnishi(KEK)

1 3 SuperKEKB simulation
1‐3.
•
•

LER Lattice:
L tti ller1354f.sad1354f dScattering probability depends on:– Scattering position: ∝x(s0)‐1y(s0)‐1– |dE/E|: 14~100sigma(1.1 % ~ 8 %)•I=3.6A, Touschek life=600sec– Total loss: ‐6mA/sec, 240W, 380GHz e+(cf KEKB: 1.7A, life=7000sec, Total loss=8W, 12GHz e+)•Collimator setting (preliminary)– 4 collimators in arc section• s=‐2060m, ‐1300m, ‐550m, ‐200m– horizontal: both side (inner/outer)((KEKB: inner only)y)– +‐13.6mm from beam center(we can further close collimators to ~12mm
without losingg Touschek lifetime))•

Beam‐pipe aperture in IR (preliminary)– Updated after this simulation

13Loss distribution (ring)H4(27.8%)
H1(25.6%)
H2(9.3%)

H3(13.4%)

IR(16.2%)

Collimators

76.2%

Arc

7 6%
7.6%

IR(|s|<6m)

16.2%

s[m]
Collimatorsllstop ~76% off Touschek
h k lloss.
Outer collimator seems effective.
~16% reaches to IR region(|s|<6m).
More loss at masks can be achieved bynarrowingg collimators(+‐~12mm),(),
without losing Touschek life time.

Horizontal hit position @ H3

Inner+‐13.6mm

Outer14

Loss distribution (IR)7.0%(27GHz,16W)
2.6%(9GH 6 2W)(9GHz,6.2W)
2.4%(9GHz,5.8W)

multi‐turn
0 4%
0.4%
(1GHz, 1W)

Total loss in IR (|s|<6m):
16%,

Multi‐turn loss
in IR is small

s[m]
[ ]
4GeV e+Beam pipe material of this partshould be heavy, such as Tacf. KEKB
Main source of Touschek background islloss
at QC2(s=‐6~‐10m),() 60mW.

x60mW(0.8%) KEKB: W shield inside QCS.
15(6~10m)

Scattered position of IR loss
H2H3H4IPH1s [m]Before H1

0.5%

Before H2

2.9%

Before H3

6.0%

Before H4

4.3%

After H4

2.2%

Total

12.0%

These can be reducedwith narrower
collimators.
To reduce this,additional collimator
closer to IP is necessary.

Additionall collimator
Additilli t
at s=‐150m or closer to I
is necessary.

s [m]Beam pipe physical aperture
Beam‐pipe
horizontal hit
position [m]27GHz
4GeV e+Radius at s=‐1.6m
will be widened[m]particles
ti l llostt att s=‐1.6m
16will be lost closer to IP, or go through (not lost) to be simulated17

27GHz,16W
9GHz 6 2W
9GHz,6.2W

9GHz 5 8W
9GHz,5.8W

LER e+

18

Touschek: KEKB vs.
vs SuperKEKB
ler1354f.sad
KEKB(LER)( )S
SuperKEKB(LER)( )Beam energy

3.5 GeV

4.0 GeV

Beam current

1.7A

3.6A

Touschek lifetime~7000 sec

600 sec

Loss rate(Total)

8W 12GHz e+8W,

240W 380GHz e+
240W,

IR/Total
Loss rate(IR)IR loss position

I=eNf0f0=ve+/3000m =105[Hz]R= I/(ef0)/ [Hz]
W= REW
RE=IE/(efIE/(ef0)/ [W]

x30: almost fixed
0.7%
16%
x20: to be improved
60mW, 0.1GHz e+ 40W, 60GHz e+• Need narrower collimators(13.6mm~12mm)• Need additional collimator
ollimator
at s=~‐150m or closer to IP

s=‐6m~10m• Loss position should be
moved to upstream forshielding showers

s= ‐1.6m‐4m

Toy GEANT simulation
4GeV positron
Beam pipe (Tube)
4mm thickness
Beam pipe material: Cu, Ta+
40mm
4GeV eBeam pipe shape: simple tube
Incident angle: =1,2,4,6,10,20 mrad
Ennergy depposit [GeVV]••••Ta is preferable forbeam‐pipe material

Back‐scattering

X0

Energy deposit
on 4mm thickCu

14.4mm

~60% deposit

Ta4 3mm
4.3mm~70% deposit

Incident angle [mrad]

20

2. Synchrotron Radiation☑ Flux simulation□ GEANT4 simulation

Latest beam pipe design
Proposed at last B2GM, by K. KanazawaRidge

HERCollimation(20mm9mm)• Collimation part of incoming
pipe stops most of SR.
• The minimum distance of theduct wall from the beam stayy
clear is 2 mm.
• HOM can escape through thepipes for the outgoing beam.
• “Ridge” structure on innersurface of collimation part tohide IP from reflected SR.

Ridge structure

See Kanazawa’s talk22

M. Iwasaki

Synchrotron radiation

HER(5252 )HER(5252a)#photon vs.
vs beam pipe geometry

HERslice
l in thishplane
Forward straight part: 10cmY(cm)
10forward length(cm)010
Those photons hit
beam pipe‐1‐21 8x102/bunch (>5keV) photons hit
1.8x10straight part of beam pipe.

0.9

1.0

1.1
radius(cm)Gives requirements for fabrication
accuracy and alignment

Tanaka(KEK)

GEANT4 toy study
Simple beam‐pipe model
Quick GEANT4 study• 5 degree(worst case), 1‐30keV photon• After passing through the beam‐pipe,
3x10^(‐6) will survive for 20keV and less forlower energy(worst case)E ((keV))

Survive rate

1<1x10^‐65<1x10^‐610<1x10^‐620~3x10‐630~30 10^ 6~30x10^‐6PhotonbeamAu platingTaPXD requirement:<1% occupancy
 <6.4 hits/bunch on PXD• SR hits
hi on bbeam pipe:iO(10( 3)/bunch)/b h
 <<1hits/bunch on PXDOK!
• Back‐scattering effect (for high energy SR) should becheckedh k d withi h realisticli i beambpipe
i geometry.
• SR depends on leak field distribution.
• Reflection/tip‐scattering at “ridge” structure on
collimation
lli ti partt is
i nott easy tot simulate
i l t
 Beam‐test is planned.
24

3.GEANT4 simulation

Clement(Tokyo)

IR beam pipe
p p in GEANT4Heavy‐metal
Ht lshieldBe
B partIP +‐ 0.1m

Need to add
QC1/2, cryostat

Ta partIP +‐ 4.0m
IP +‐ 0.5m

Beam‐pipe,
p p , heavy‐metal
yshield is implemented.
pQC1/2 magnets, cryostat, non‐uniform magnetic field
will be implemented soon.
26

Further IR ggeometryy
Provided by Masuzawa
IP +‐+ 10m

Implementation will be step‐by step,from IP to far from IP+‐10m

27

28

Detector response of innerdetectors are implemented

29

SummaryTouschek•

Extrapolation of KEKB data toward SuperKEKB is performed.
– Expected background meets requirement for SVD,CDC,TOP,ECL.

•Simulation validity is confirmed.
confirmed– KEKB simulation results are consistent with KEKB data.

•Preliminary SuperKEKB simulation is performed.
– ~40W
40W loss in IR region (cf(cf.60mW@KEKB),
60mW@KEKB) but we have a room to improve:ex. narrower/additional collimators, wider IR beam‐pipe aperture

Synchrotron radiation•••

Collimation part on incoming beam‐pipebeam pipe stops most of SR.
SRSimple simulation shows expected PXD occupancy meets requirement.
Back‐scattering effect for high energy SR, tip‐scattering on collimation partshould be checked
checked. Leak field dependency should be also checked.
checkedGEANT4 simulation••

Full detector simulator is released recently.
IR magnets, cryostat geometry is missing: should be implemented a.s.a.p.
30

backup31

Final focusing magnets
 =22mrad

IP
SharedShdbby bbothh ring.
iStrong bend just after IP.
 =83mrad

IP
No shared final Q (no bend near IP)

Final Q for each ring.
Much less BG for:‐ Backscatter SR‐ e+e‐ from rad. Bhabha
32

Latest beam
beam‐pipe
pipe design

Paraffin flowSUSBeSUSHeavy metal shield (SuperKEKB)
Heavy‐metal
Belle‐IIBelle
II IR design(Preliminary )Heavy‐metal shield to protect PXD/SVD
from showers coming from upstream.

cf. Heavy‐metal shield @ Belle

35

Heavy metal shield(KEKB)
Heavy‐metal

36

Lead shield in QCS cryostat (KEKB)
QCS‐L cryostat

37

Neutron shield (KEKB)
Concrete wall

Polyethylene shield38

SuperB estimations

Touschek

Paoloni, XIV SuperB Meeting @ INFN‐LNF

Touschek is not a bigg p
problem for SuperB.
p
Large dispersion at some position movable mask is effective there40

SRPaoloni,, XIV SuperBp
Meetingg @ INFN‐LNF

hits/crossing on
beam pipe

SR is similar for SuperB.
p41

Paoloni, XIV SuperB Meeting @ INFN‐LNF

42

Christian(MPI)

2 photon processChristian(MPI)

Which is correct?

44

Christian(MPI)

Machine study might give answer

45

Christian(MPI)

Luminosity correction by CDC BG

We expected no luminosity
dependence for CDC BG, since2 h2‐photon
QED cannot reachhCDC.
However, we ffoundHddependence Use corrected
luminosity with CDC hits rate.

46

Christian(MPI)

SVD hits

SVD multiplicity
p y comingg fromQED‐2photon process, scaled bycorrected luminosity with CDC hit rate47

Christian(MPI)

QED summary• We have excluded SuperB value with machine study.
• Another analysis using reconstructed tracks areplanned.

Andreas(MPI)• We will start GEANT4 simulation soon.
48