Machine Background Status & issues in BaBar/PEP-II

1. Machine Background Status & issues in BaBar/PEP-II • Background sources • Characterization experiments • Long-term projections & vulnerabilities W. Kozanecki, CEA-Saclay

2. Background sources in PEP-II • Synchrotron radiation (this bkg negligible in PEP-II) • Beam-gas (bremsstrahlung + Coulomb) • HEB only: BHbg ~ IH * (pH0 + PHDyn * IH) Note: p0 = f(T) ! • LEB only: BLbg ~ IL * (pL0 + PLDyn * IL) Note: p0 = f(T) ! • beam-gas x- term: BLHbg ~ cLH * IL * IH (LEB+HEB, out of collision) (?) • Luminosity (radiative-Bhabha debris) – major concern as L  • BP ~ dP * L (strictly linear with L) • Beam-beam tails • from LER tails: BL, bb ~ IL * fL(xL,H+/-) • from HER tails: BH, bb ~ IH * fH(xL,H+/-) • Trickle background: BLi ,BHi(injected-beam quality/orbit + beam-beam) • Touschek: BLT(signature somewhat similar to bremsstrahlung; so far small)

3. Data: Jan 04 (bef. therrmal outgassing crisis) Background characterization measurements Step 1: Beam-current scans  single-beam terms

4. Total occupancy • HER single beam • LER single beam • Beam-beam term • present in all subdetectors • fluctuations, short - & long-term •  parametrization optimistic ? Step 2: L & beam-beam terms EMC cluster multiplicity SVT occupancy (FL1 M01-f)

5. IDCH = DCH Step 3: Background Parametrizations • DCH example: total current & occupancies Step 4: Background Extrapolations 60 L Tracking efficiency drops by roughly 1% per 3% occupancy PEP-II parameter projections LER contribution very small

6. EMC Looked at number of crystals with any/significant energy & clusters Small quadratic term from single beam data # of crystals used in cluster finding Currently physics events have ~110 digis and 8 clusters Long term impact on physics analysis not clear yet

7. Luminosity background e+ e- e+e-g • elm shower debris • neutrons! • no contribution from coasting HEB or LEB • maydominate DCH, DIRC rate

8. Neutron Background Effort underway to measure neutron background in BaBar BF3 counter installed on fwd Q4 Sees large rate (>10 kHz) during colliding beams, not single beam Rate only seen with polyethylene moderator~1MeV neutrons Neutrons thought to be from radiative Bhabhas hitting Q2 septum mask and inside support tube - Shielding of BaBar is being investigated

9. E E Fwd q index Bkwd EMC default digi map: luminosity background (N. Barlow) f index W

10. DCH + TRG When combined with higher trigger rates, long read-out time leads to unacceptable deadtime. A major DCH elx upgrade is now in progress.

11. Backward: East Top West Bottom Background strongly - dependent By 2007 predict 80% occupancy right in MID-plane In layer 1, 10% will be above 20% occupancy NOW 2004 2005 2006 2007 Forward: East Top West Bottom Yearly dose will be more than 1 Mrad/year by 2007 SVT Background now is ~75% HEB [LEB negligible (!)] In 2007, it will be 50% HER, 50% L • It has recently been realized that • in the SVT (but not in other subdetectors), a large fraction of the “Luminosity”background is most likely due to a HER-LER beam-gas X-term (but: similar extrap’ltn). • the HER single-beam background in Jan 04 is about 2x what it was in 2002  improve?

12. Data: 27 Jan 04 HEB current scan

13. HEB scan: evidence for Touschek & beam-beam background

14. Outgassing storms (April 04) • New (?) major background source: thermally-enhanced beam-gas • in incoming LER straight (exacerbated by NEG activation; now limits Ib) • sensitive to LER current; several time constants in a time-dependent mix • suspect: NEGs, ion pumps, collimator jaws, misc. vac. pipe secs •  SVT dose + occupancy (E-MID); minor impact on dead time • in incoming HER straight (triggered the NEG activation; now limits Ib) • sensitive to HER current, very long time constants •  BaBar dead time + SVT occupancy (W-MID) • in (or very close to) the shared IR vacuum system (now limits Ib ) • sensitive to both beam currents; at least 2 time constants • suspect: NEG + complicated IR ‘cavity’ (Q2L  Q2R) + HOM interference •  BaBar dead time + SVT occupancy (W-MID + E-MID) • HOM dominant heating mechanism • mostly long to very long time constants (30’ - 3 h): suggests low power • sensitive to: bunch pattern, VRF, collimator settings, Z(IP), hidden var’s • Many “??”(minor, inocuous changes  large effects, good or bad) • detailed analysis by GW

15. 12 hours Thermal time constants VGCC3027  (incoming LEB) BE diamond  (LEB sensitive) LER current VGCC2187 (HER sensitive)  A potential limitation for run 5!  BW diamond [+ BBR dead time] (HEB sensitive)

16. HOM interference in IR Data: 13 Apr 04 VGCC2187 (HER sensitive) VGCC3027 (incoming LEB) Collision phase = [t(e-) - t(e+)]IP <ZIP> (BaBar) BE diamond (LEB sensitive) BW diamond (HEB sensitive)

17. Thermal outgasssing now limits the beam current Babar dead time (%) VGCC2187 (nT, HEB side) HER current BE diamond (mR/s) VGCC3027 (nT, incoming LEB) LER current

18. Summary (I) • Trickle injection • is a major success in terms of improving • machine stability + abort frequency  integrated L • overall injection quality • accumulated SVT dose • The associated detector backgrounds appear largely negligible (most – but not all – of the time) • Improved understanding of background & abort sources • genuine radiation aborts down to < 1 /day • clear & reproducible correlations between diamond dose rates, on-line SVT occupancies, dead time, and pressure measurements in incoming HER & LER straights • “lumi[background]is[really due to] lumi”! (except in the SVT – maybe) • Stored-beam backgrounds (dose rate, data quality, dead time) • OK most of the time (& better w/ trickle) until recently • thermal outgassing now limits beam currents (primarily in the HER)

19. Summary (II) • Background characterization experiments • were highly valuable in identifying the origin, magnitude & impact of single- & two-beam backgrounds. • On the long term, the dominant backgrounds are expected to be, in order of decreasing importance: • radiative-Bhabha debris (all subdetectors), incl. a significant neutron flux • HER beam-gas (SVT, TRG), especially that due to thermal outgassing • beam-beam tails & their fluctuations (DCH, EMC, TRG, IFR  wall!) • In the medium term (2005-07), the main vulnerabilities are • beam-gas backgrounds from HOM-related thermal outgassing as I+,- • high dead time associated with growing data volume & trigger rates [Mainly HER beam-gas (TRG, SVT) & radiative-Bhabha debris (DCH)] • high occupancy and radiation ageing in the mid-plane of the SVT •  local loss of tracking coverage (?) • closely monitor the HER single-beam background & keep it similar to 2002 levels • high n flux (~ 1 MeV) correlates with L, some spikes is it an issue?

20. Spare slides

21. B. Petersen L.Piemontese Run-4 radiation-abort history • ~ 60% of stable-beam radiation aborts “sympathetic” • 2/19 – 4/29: < 0.9 (genuine) rad. aborts/day (out of ~7 total avrg)

22. Stored-beam background history IDCH, msrd/pred DCH current normalized to Jan 04 background data 04 20% 20% SVT ocp’cy @ f = p (HEB-sensitive) SVT ocp’cy @ f = 0 (LEB-sensitive) 10%

23. B Petersen N. Barlow M. Cristinziani/T. Glanzman J. Malcles Jan 2004 Apr 2004 Feb 2002 SVT occupancy Jan 2004 DCH current (mA) Apr2004 Feb 2002 HER current (A) Evolution of HER single-beam background, 2002-04 Jan 2004 EMC clusters Apr 2004 DCH current

24. Implies replacement of mid-plane modules during 2005 shutdown SVT: projected integrated dose Dose projections assume negligible injection background

25. DCH current vs. Luminosity during a X scan (all currents constant) DCH current (microA) Luminosity (1E33)

26. DCH/TRG background extrapolations • HER single-beam & lumi (bkg + physics) terms dominate • Trickle: only average shown. Must be able to accomodate large fluctuations. • Beam-beam: only best case shown. Data taken since then show beam-beam can easily be 2 x larger – not counting short-term fluctuations. • LER single beam: small (mostly beam-gas), no fluctuations expected

28. Fill March 28, 12pm-3 pm Data points End of injection mRad/s Fit VP3044 VGCC3027 VP3147 0 1 2 3 h Time evolution of the thermal outgassing background • The different time dependencies of the pressure readings allow to fit the measured background level (BE diamond) as a linear combination of 4 LER Pumps, on a fill by fill basis • The 4 pumps are located in the incoming LER straight and all exhibit HOM-induced thermal outgassing (e.g. change of pressure associated with change of bunch length) • A very satisfactory description of the background was obtained in all cases • The sensitivity coefficients for each pump were then extracted. They represent the N2-equivalent pressure integral with the same time dependence as the pump reading.

29. Evolution of the sensitivity coefficients (Apr 04 outgassing storms) VP3044 VGCC3027 • The coefficients are normalized to their pre-NEG activation values , indicated by the red line (1 point per long fill) • The background problem was not related to an increase in local pressure reading (at the pump) but to a huge increase in background sensitivity • The problem was solved (serendipitously) by: • continued processing • opening collimator jaw(s) • changing in bunch pattern These changes had different actions on the various background drivers 200 10 Days in March (April 1 = day 32)

30. BE diamond (LEB sensitive) VGCC3027 (incoming LEB) NEG actvtd NEG actvtd NEG actvtd NEG actvtd BW diamond (HEB sensitive) VGCC2187 (HER sensitive) Mismatch (x 10-100) betw. time evolution of msrd p and of bkgd demonstrated by detailed analysis of local pressure contributions to background signals

31. NEG actvtd NEG actvtd NEG actvtd NEG actvtd Large variety of processing times, mechanisms, & bkg sensitivities

32. Coulomb scattering in Arcs (y-plane) IP e-Brems-strahlung in last 26 m (x-plane) Vacuum pipe / mask apertures Lost-particle backgrounds Normalized to: - uniform pressure profile of 1 nT - 1 A beam current IP

33. Zone 3 X (mm) Zone 2 X (mm) Zone 1 X (mm) Zone 4 IP The “Background Zones” reflect the combined effect of.... • beam-line geometry (e.g. bends) • optics at the source and at the detector • aperture restrictions, both distant(good!) & close-by (bad!) Bremmsstrahlung Bremmsstrahlung in field-free region Coulomb scattering in Arcs Bremmsstrahlung