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

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)

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

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

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

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

Luminosity background e+ e-  e+ e- g elm shower debris neutrons! no contribution from coasting HEB or LEB may dominate DCH, DIRC rate

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 ~1 MeV neutrons Neutrons thought to be from radiative Bhabhas hitting Q2 septum mask and inside support tube - Shielding of BaBar is being investigated

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

A major DCH elx upgrade is now in progress. 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.

Yearly dose will be more than 1 Mrad/year by 2007 SVT Yearly dose will be more than 1 Mrad/year by 2007 Backward: Forward: Top East West Bottom NOW 2004 2005 2006 2007 Background now is ~75% HEB [LEB negligible (!)] In 2007, it will be 50% HER, 50% L Background strongly  - dependent By 2007 predict 80% occupancy right in MID-plane In layer 1, 10% will be above 20% occupancy 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?

Data: 27 Jan 04 HEB current scan

HEB scan: evidence for Touschek & beam-beam background

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

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

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)

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

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)

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?

Spare slides

Run-4 radiation-abort history 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)

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%

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

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

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

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

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. Fill March 28, 12pm-3 pm 3 h 1 2 Data points VP3044 VGCC3027 Fit VP3147 End of injection mRad/s

Evolution of the sensitivity coefficients (Apr 04 outgassing storms) 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 VP3044 VGCC3027 200 10 Days in March (April 1 = day 32)

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

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

Lost-particle backgrounds IP Coulomb scattering in Arcs (y-plane) e- Brems-strahlung in last 26 m (x-plane) Vacuum pipe / mask apertures Normalized to: - uniform pressure profile of 1 nT - 1 A beam current 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!) X (mm) Zone 1 X (mm) Zone 2 Bremmsstrahlung Bremmsstrahlung in field-free region Zone 3 X (mm) IP Zone 4 Coulomb scattering in Arcs Bremmsstrahlung