Particle background in ZEUS

Hera background workshop 21-24 Oct 2002

R.Carlin

Particle background– Lot of work from lots of people

– N.Brummer, R.Carlin, C.Catterall, S.Chekanov, J.Cole, T.Haas, R.Hall-Wilton, A. Geiser, U.Koetz, G.Levman, S.Limentani, W.Schmidke, U.Schneekloth, M.Sutton, E.Tassi, K.Tokushuku, T.Tsurugai, Y.Yamazaki, + Run Coordinators +…

– Show only a small fraction of the results

– Problems from particle backgrounds:

– Trigger rates and event sizes

– Radiation in MVD

– Currents on chamber wires

– Will concentrate on the last (CTD), as it is presently the limiting factor, but the others are not to be neglected

– All beam currents in mA– All CTD currents are for 1 quadrant of SuperLayer 1, in

“CTD units” (10 units=1uA)

• No evidence of particle scraping

• No chance to reduce backgrounds with beam tuning

All possible positions and tilt scans were performed for both beams. We always get “bathtub” plots

Concentrate on e-gas, p-gas Vacuum becomes an

important issue

Sources of particle background for ZEUS:e-gas p-gas (e scraping, p scraping)

e-gas, p-gas: analyze the vacuum

evolution:• Around day 240: venting to repair

a beam element on SL• Around day 260: improvement by

warming up GG and GO and firing pumps

• Now inner wall of GG, GO warmer

• SR: vacuum in many places much better than before day 240

•It is going up again close to the IP !• SL: vacuum went ~ back to

values before day 240

e-gas: origin of the

problem

• Low energy electrons generated by e-gas bremsstrahlung

• Bent into the beam pipe by the last bending magnet, now inside ZEUS

Not possible to insert a collimator

• Interact with the beam pipe or with C5A

C5A

lower momentum e

• e-gas rates should increase with a power of Ie

• Rate Ie*Vacuum• Vacuum a+b*Ie

• Power law not evident, apart from C5

• SRTD, R_ISOE quadratic term (b) less than 7% of the linear (a)

• Late runs compatible with power 1

• No difference on last week end runs (high T on GG,GO)

•Expected

• Rate on late runs not much better than before

• Consistent with vacuum measurements

Study on the data: trigger rates sensitive

to e-gas

Early runsLate runsAfter GG,GO warm-up

e-gas simulation

• Bremsstrahlung process from e-gas simulated from 132.8 m upstream

• Positrons tracked through the machine lattice and collimators up to ZEUS

• Particles fed into ZEUS MC with a geometry description around IP modifiable to study the different options for the collimators and shields

Distributions from e-gas are well reproduced by the

simulation

• Azimutal ditribution of condensates in RCAL

• Energy around beam pipe in RCAL

C5A

Blu: > 100 keVRed:e > 100 keV

e-gas simulation

The electrons interact in C5A but also in the beam pipe and in the calorimeter

Can we improve the CTD currentsby making C5A thinner?

Yes, but only a factor 2

Reason: currents in CTD dominated by the charged particles, not by

interactions downstream the CTD (C5A) are less important

A lead shield around C5A will not make thing much worse

e-gas fraction in CTD

Reflected light arrives later (reflection at 11m SR)

Get e-gas fraction from the drift times in the CTD (runs with isolated bunch)

synch

+e-gas

synch

e-gas

CTD ~ 7*Ie+1.33*Ie+0.093*Ie 2 (worse case)if linear CTD ~ 7*Ie+3*Ie

CTD ~ 1.17*Ie+0.66*Ie+0.046*Ie 2

if linear CTD ~ 1.17*Ie+1.5*Ie

e-gas

synch

•ICTD=180 @ 18mA Ie

•30% e-gas @ 18 mA Ie

•e-gas aIe+bIe2

• Dotted: e-gas Ie

•Synchrotron radiation reduced by a factor 6

•e-gas reduced by a factor 2

From the long e fill

Extrapolation to nominal beam current

Ie

Ie

synch

+e-gas

Differences between eDifferences between e++ and e and e--

IP

p(e-)

p(e+)e+

e-e+ - e- Switching magnet

8mm

Separator magnet

to arc

to arc

• soft separation for esoft separation for e+ less less synchrotron radiation (factor 2)synchrotron radiation (factor 2)

• beam more centered for ebeam more centered for e- less less scattering from low momentumscattering from low momentum

Probably the e-gas background will Probably the e-gas background will be better with ebe better with e--

ee-ee+

C5A

e-gas conclusions

• e-gas will be a serious contributor to the CTD current after we have solved the synchrotron radiation

• We cannot improve much by changing geometries around the IP

• Need to improve the vacuum in SL

• Running with electrons still to be properly simulated– Will be better for what regards e-gas

Before venting

Recent runs

Rates for p-gasupstream

• Big improvement after day 260, 1998 values ~ reached

• No effect seen at high GG T

•Vacuum may be getting worse again

• Intercepts small (1999 values?),

C5 veto

1998

1999

20002000

Trigger rates studies

Last days, high GG Temp

p-gas

• Rates for inside p-gas

improved but not as

much as outside

• Intercepts still much

higher than in 1999 Vacuum inside still

high?1998

19992000

Before venting

Recent runs

Last days, high GG Temp

C5V/Ip

SRTDv/Ip

Improvement in trigger rates with p-only runs

GG+GO inner wall at higher T

p-gas contribution to CTDcurrents on ep runs:•Slope (recent runs) = 0.11•With warm GG 110K

ep runs

CTD currents onp-only runs:• Intercept = 1.52•With warm GG,

intercept = 1

Subtract the contribution from e-beam: CTD ~ 7*Ie+1.3*Ie+0.093*Ie 2

How to extract the p-gas contribution to CTD currents ?

CTD p-gas currents compared with 2000

2000

2002 early runs

2002 late runs

Monte Carlo:

• p-gas (p-p) interactions tuned on UA5 (SPS) data

• flat vertex distribution along nominal p beam, -800 cm < z < 0 cm

• full simulation of ZEUS material, including GG, C5, veto wall

• partial simulation of upstream beam elements (material, no field)

P-gas simulation and comparisons

Data:• FCAL_BP_NOVETO triggers (FCAL Energy > 4 GeV)

-> small trigger bias, dominated by p-gas

• p only and ep runs

-> some e-gas (+ e-p) contamination

CTD acceptance for p-gas

Number of CTD hits ( CTD current) per event, as a function of the p-gas vertex position in z

To understand the vacuum profile near the IP:compare the distribution of quantities sensitive

to the p-gas vertex

• CTD reconstructed z vertex• RCAL energy• RCAL EMC fraction

— Flat vertex distribution between –8 m and 0— Flat vertex distribution between –3 m and 0

Peak from C5A

Its contribution to the CTD current is limited, but prominent in event selections with tracking triggers

Good agreement

Can be made much better by reweighting the vacuum profile

Vacuum profile

C5C?

Z vertex

EMC fraction

RCAL E

Equiv

ale

nt

eff

ect

ive v

acu

um

ep bad vac

p only bad vac

ep good vac

p only good vac

normalize toevents/proton current,active time,cross section

Absolute vacuum profiles

Reconstructed vacuum

profile confirms ourpresent

understanding:

• Dynamic vacuum is dominant upstream

• Recent improvements happened mostly upstream

• Need to improve vacuum around IP

p only high GG T

ep high GG T

Ratio of vacuum between ep and p-only runs at high T GG, as reconstructed from MC

Ratio of vacuum between two p-only runs, before and after high T GG, as reconstructed from MC

Other results from p-gas MC

Extra collimator at –3.6m will

make a very slight improvement

• Removal of C5 will only give marginal (15%) improvement

– May be quite useful for the tracking trigger

Thick C5A

Thin C5A

En

erg

y in

CTD

(M

ev/e

vt)

En

erg

y in

CTD

(M

ev/e

vt)

CTD SL

CTD SL

Other results from p-gas MC

• Also a thinner C5C has a marginal effect

En

erg

y in

CTD

(M

ev/e

vt)

CTD SL

• Total currents CTD SL1 after the improvement on the collimators

• Synch. radiation reduced to 1/6

• e-gas reduced to ½

ICTD=20+1.5(1+0.11*Ie)*Ip+1.17*Ie+0.66*Ie0.046*Ie 2

Running at higher T on

the

inner surface of GG, GO• Seen a factor 2 improvement

with p-only

• What to expect with ep?

• Factor 2 may be

optimistic

• First tests on ep rather

disappointing

(preliminary)

@ nominal Ie=58 mA @ nominal Ip=130 mA

tota

lto

tal

p-gas

e-gas

p-gas

e-gassynch synch

Finally, extrapolation of total CTD current

Conclusions

• Previous extrapolations have to be taken carefully

– Power law of e-gas still unclear

– Effect of warm GG, GO? Evolution of vacuum?

• We do need an improvement in the vacuum both for e-

gas and p-gas– The p-gas seems to be eventually the biggest problem

• Improvement from running GG and GO at higher inner

wall T promising but sofar unclear for e-p running– Need to study more

• Still simulations and test runs needed to get a reliable

quantitave model