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Particle background in ZEUS

Particle background in ZEUS. Hera background workshop 21-24 Oct 2002 R.Carlin. Particle background. Lot of work from lots of people

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Particle background in ZEUS

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  1. Particle background in ZEUS Hera background workshop 21-24 Oct 2002 R.Carlin

  2. 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)

  3. Sources of particle background for ZEUS: e-gas p-gas (e scraping, p scraping) All possible positions and tilt scans were performed for both beams. We always get “bathtub” plots • No evidence of particle scraping • No chance to reduce backgrounds with beam tuning • Concentrate on e-gas, p-gas • Vacuum becomes an important issue

  4. 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

  5. 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 e-gas: origin of the problem C5A lower momentum e

  6. Early runs Late runs After GG,GO warm-up Study on the data: trigger rates sensitive to e-gas • 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

  7. 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

  8. Distributions from e-gas are well reproduced by the simulation • Azimutal ditribution of condensates in RCAL • Energy around beam pipe in RCAL

  9. Blu:  > 100 keV Red: e > 100 keV e-gas simulation The electrons interact in C5A but also in the beam pipe and in the calorimeter C5A

  10. Can we improve the CTD currents by 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

  11. A lead shield around C5A will not make thing much worse

  12. 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)

  13. Extrapolation to nominal beam current From the long e fill CTD ~ 7*Ie+1.33*Ie+0.093*Ie 2 (worse case) if linearCTD ~ 7*Ie+3*Ie • ICTD=180 @ 18mA Ie • 30% e-gas @ 18 mA Ie • e-gas  aIe+bIe2 • Dotted: e-gas  Ie synch+e-gas synch e-gas Ie • Synchrotron radiation reduced by a factor 6 • e-gas reduced by a factor 2 CTD ~ 1.17*Ie+0.66*Ie+0.046*Ie 2 if linear CTD ~ 1.17*Ie+1.5*Ie e-gas synch+e-gas synch Ie

  14. e- e+ C5A IP to arc p(e-) e+ - e- Switching magnet 8mm e- p(e+) e+ Separator magnet to arc Differences between e+ and e- • soft separation for e+ less synchrotron radiation (factor 2) • beam more centered for e-  less scattering from low momentum •  Probably the e-gas background will be better with e-

  15. 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

  16. p-gas Trigger rates studies C5 veto • Rates for p-gas • upstream • 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?), Before venting Last days, high GG Temp Recent runs 1998 1999 2000 2000

  17. Rates for inside p-gas improved but not as much as outside • Intercepts still much higher than in 1999 • Vacuum inside still high? Before venting Last days, high GG Temp Recent runs 1998 1999 2000

  18. GG+GO inner wall at higher T C5V/Ip Improvement in trigger rates with p-only runs SRTDv/Ip

  19. How to extract the p-gas contribution to CTD currents ? • CTD currents on • p-only runs: • Intercept = 1.52 • With warm GG, intercept = 1 • p-gas contribution to CTD • currents on ep runs: • Slope (recent runs) = 0.11 • With warm GG 110K ep runs Subtract the contribution from e-beam: CTD ~ 7*Ie+1.3*Ie+0.093*Ie 2

  20. CTD p-gas currents compared with 2000 2002 early runs 2002 late runs 2000

  21. P-gassimulation and comparisons • Monte Carlo: • p-gas (p-p) interactionstuned 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) • 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

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

  23. 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

  24. Good agreement Can be made much better by reweighting the vacuum profile C5C? Peak from C5A Its contribution to the CTD current is limited, but prominent in event selections with tracking triggers Vacuum profile

  25. Z vertex EMC fraction RCAL E

  26. Absolute vacuum profiles normalize to events/ proton current, active time, cross section • Reconstructed vacuum • profile confirms our • present understanding: • Dynamic vacuum is dominant upstream • Recent improvements happened mostly upstream • Need to improve vacuum around IP Equivalent effective vacuum ep bad vac ep high GG T p only bad vac ep good vac p only good vac p only high GG T

  27. 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

  28. Other results from p-gas MC • Removal of C5 will only give marginal (15%) improvement • May be quite useful for the tracking trigger Energy in CTD (Mev/evt) Thick C5A Energy in CTD (Mev/evt) Thin C5A CTD SL Extra collimator at –3.6m will make a very slight improvement CTD SL

  29. Other results from p-gas MC • Also a thinner C5C has a marginal effect Energy in CTD (Mev/evt) CTD SL

  30. @ nominal Ip=130 mA @ nominal Ie=58 mA total total p-gas p-gas e-gas e-gas synch synch Finally, extrapolation of total CTD current • Total currents CTD SL1 after the improvement on the collimators • Synch. radiation reduced to 1/6 • e-gas reduced to ½ • 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) ICTD=20+1.5(1+0.11*Ie)*Ip+1.17*Ie+0.66*Ie0.046*Ie 2

  31. 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

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