BABAR Beam Background Simulation Steven Robertson

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1 BABAR Beam Background Simulation Steven Robertson 2nd Hawaii Super B Factory Workshop April 21, 2005

2 Beam background conditions result in detector occupancy, radiation damage and degradation of data quality Background characterization is based on dedicated beam background experiments single and two beam colliding/non-colliding, trickle injection etc. permit extrapolation to future running conditions (under assumptions about beam conditions) Simulation is needed in order to: # of crystals with significant energy Motivation EMC L = 3.3x1034 cm-2 s-1 DCH 1) Validate and aid interpretation of background data 2) Identify sources and underlying causes of background 3) Evaluate sensitivities to specific sources (e.g. details of IR geometry) 4) Evaluate effects of future upgrades on bg rates Extrapolate to Super-B? 2

3 Background simulation Recent effort to simulate two of the dominant contributions to observed subdetector background rates Lost-particle single beam backgrounds produced by bremmstrahlung or Coulumb scattering of primary beam particles from beam gas Luminosity backgrounds associated with small-angle radiative bhabha events in which electron/positron strikes machine elements outside of nominal BABAR fiducial acceptance observed neutron background attributed to this source Ingredients: machine lattice & apertures (TURTLE) - T. Feiguth, R. Barlow Geant 4 IR geometry, materials model - M. Bonioli, G.Calderini Geant 4 IR magnetic field model - G. Bower small angle bhabha generators - B. Lockman, D. Strom, N. Blount e/gamma nuclear physics modeling - D. Wright study and interpretation of results - subdetector groups 3

4 Interaction region 4

5 Simulation Tools Magbends Decay Turtle Transport of Coulomb and bremsstrahlung final state particles to vicinity of IR Modeling of beam phase space and beam tails Knowledge of apertures and assumptions regarding vaccuum pressure profile propagates charged particles through magnetic fields Geant4 Full modeling of materials and magnetic fields in vicinity of IR (+/- 8m) Contains physics of particle interactions, detector materials and response Can be used as stand-alone simulation of physics processes (e.g. Bhabha) or using Turtle rays as input Data Impact of background occupancy in data modeled in BABAR physics Monte Carlo from cyclic triggers in data 5

6 Lumi backgrounds at KEK? Why is Belle apparently not as sensitive to luminosity backgrounds? Magbends model suggests that most radiative Bhabha daughters hit machine elements further from the IP LER Radiative Bhabhas V Ge 10 cm 0 9 GeV 9 GeV V Ge m M. Sullivan Feb. 8, 2004 API88k3_R5_RADBHA_TOT_7_5M however, radiative Bhabha daughters still hit in the vicinity of Belle detector so still surprising that NO lumi term is observed, particularily if neutrons contribute... 6

7 Turtle ray simulation Coulomb scattering in Arcs (yplane) IP Vacuum pipe / mask apertures e- Bremsstrahlung Normalized to: - uniform pressure profile of 1 nt - 1 A beam current in last 26 m (x-plane) IP 7

8 GEANT4 IR Simulation Most subdetector background occupancies are due to flux of low energy secondary particles rather than primary electrons/positrons from bhabhas or beam particles EM shower fragments or neutrons from primary particle hits in various machine elements Need full geometry+magnetic fields+materials+interactions for IR outside of nominal detector acceptance previously only B1 & Q1 geometry were modeled, but no magnetic fields 8

9 Radiative Bhabha background Believed to be responsible for sizable luminosity background observed Studies for BABAR TDR and predicted to be a possible background source Observed in data in ~2000; currently a dominant background source Recently, significant interest in simulation proof of principle using MagBends with off-energy electrons/positrons: Mike Sullivan 9

10 Radiative Bhabha simulation In absence the of complete magnetic field model in GEANT4, TURTLE model has been used to simulate radiative bhabha backgrounds Use magnetic field and machine aperture definitions from TURTLE decks Yields information on primary particle impact point but no information on secondaries (i.e neutrons, EM showers) Achille Stochi & Patrick Roudeau Use small-angle bhabha generator (bbbrem) to obtain particle trajectories and energy flux through beamline elements Results recently updated using new HER/LER Turtle decks including BABAR solenoid electrons Golden orbit photons HER radiative bhabhas 10

11 Turtle-based bhabha simulation Simulation reproduces main features predicted from Magbends accelerator magnetic field model Obtain quantitative estimates of effective cross section and total energy flux through beamline elements: Q1 Q2 Z range (m) σ (mb) Exσ (mb GeV) < <Z< <Z< <Z< LER Radiative Bhabhas cm 0 9 GeV 9 GeV <Z ev G ev 1 G m M. Sullivan Feb. 8, 2004 API88k3_R5_RADBHA_TOT_7_5M 11

12 GEANT4 Bhabha Simulation Bbbrem generator adapted and tested in BABAR framework ( BrmBbbrem ) Working on including BHLUMI generator as well Initial ghit-level studies performed with single-particle generator (i.e. no crossection info) yield results consistent with magbends/turtle: Off energy electrons Off energy positrons Ben Campbell, McGill 12

13 A complication: neutrons Recently discovered neutron background source believed to be due to radiative Bhabhas striking in vicinity of Q2 septum Do neutrons interact in detector? rates, radiation damage Can (in principle) be simulated using full Geant4 with e/ - nuclear processes Neutron detectors added to Geant4 detector model J. Va'vra BF3 counter installed on forward Q4 magnet - high rate (>10 khz) during colliding beams, but not single beam running 13

14 Low-Energy Nuclear processes Dennis Wright Electro-nuclear and gamma-nuclear physics processes included for the first time in BABAR simulation Results in production of additional low-energy (0-20 MeV) particles Important for EMC (?) Neutron kinetic energy (MeV) 1 ms High precision neutron processes included in test releases for study of thermalized neutrons produced by beam backgrounds Neutron TOF (0.025 ev cutoff) Will be included in future production releases with enable/disable switch (off by default!) 14

15 Status and plans Still some problems with GEANT4 model and with TURTLE apertures Q2 septum geometry incomplete and Q5 magnets still missing Magnetic fields believed to be correct Detailed validation of full simulation chain in progress Comparison of TURTLE/GEANT particle trajectories and impact positions Large scale production of TURTLE lost-particle events Implement background (e.g. neutron) detectors in Geant model Validation of simulated neutron production and interactions Validation of small-angle bhabha generators (BHWIDE/BHLUMI/BBBREM) Study of sources and rates Study of luminosity backgrounds Absolute predictions of background rates in subdetectors and detailed studies of sources etc. 15

16 Background Sources Synchrotron Radiation Lost particle (beam gas bremsstrahlung & coulomb scattering) Well characterized in data, simulated with Turtle Also inelastic beam-gas / beam-wall contribution to L1 trigger rate Touschek Well measured, but not well understood (neutrons?) Beam-beam Contribution to background rates not well understood Luminosity (Bhabha/radiative Bhabha) Well understood and not significant (by design!) in current IR mechanism understood, measured in data Trickle injection related Characterized from data and not currently problematic 16

17 Background simulation studies Synchrotron radiation Beam-beam collimation affects orbit of outgoing beam particles; collimate downstream? Beam gas Coulomb scattering and bremsstrahlung Magbends studies for IR upgrades Turtle and Turtle/Geant4 similar to ~2000 era studies but with upgraded simulation tools Radiative Bhabha ( luminosity ) background Separate Magbends, Turtle and Geant4 studies in progress use fields & apertures in Turtle to study trajectories of charged particles or physics event can be generated by directly by G4 sim (but need extended beamline geomerty!) Questions: Where are primary particles produced and where do they go? Secondaries, neutrons and potential for shielding? Effect of 2005 IR upgrade, Super-B IR etc... 17

18 Synchrotron radiation Synchrotron radiation simulated using Magbends as intrinsic part of IR design No primary synchrotron radiation background seen in BABAR detector Relevent for heating etc. of machine elements in the vicinity of IR Ongoing simulation studies as part of PEP-II upgrade program 18

19 GEANT4 Simulation Proceeding concurrently with updating of HER & LER Turtle decks Recently, a substantial effort has been made to expand Geant4 simulation to include beamline geometry out beyond Q4: Include also various background sensors (pin diodes, diamond, quartz and CsI) Modeling of magnetic fields (incl. Solenoid) and validation against Magbends & Turtle Significant improvements in geometry and materials modeling compared to old Geant3 version 19

20 GEANT4 beamline model Challenging geometry! problems with G4 representation (M. Bondioli and G. Calderini) (Almost) complete model out to Q5 magnets Final checks of geometry in progress (overlapping Geant volumes etc) Magnetic field modeling (G. Bower) Validated against Magbends/Turtle Technical issues tracking particles through a varying field ( resolved ) Remaining issues: extended model runs ~10x slower than nominal detector model Sources outside of +/-10m? Do we need additional beam elements? 20

21 Pressure zones zones are empirically defined based on observation that lost particles from different regions have differing characteristics: Zone 1 X (mm) Bremmsstrahlung in field-free region April 21, 2005 Zone 3 Bremmsstrahlung BBBTF X (mm) X (mm) HER Zone Bremmsstrahlung Zone 2 Range (m) -4, 4-4, , , , LER Zone McGill University, Institute of Particle Physics Range (m) -4, 4 4, 10 10, 21 21, 36 36, 62 62,

22 Simulates production and propagation of Bremsstrahlung and Coulomb scattered primary beam particles through PEP-II magnet lattice gives rates and impact point of particles in vicinity of IR T.Feiguth, R.Barlow Zone 1 Bremmsstrahlung in field-free region requires knowledge of vacuum profile in the rings (particularily incoming HER and LER straights near the IR) Effort in progress to update TURTLE magnet and aperture descriptions not updated since 1998 (commissioning run!) LER optics done but apertures still to come Bremmsstrahlung Zone 3 X (mm) X (mm) TURTLE ray simulation HER optics (essentially) done, aperture description is available but not yet implemented April 21, 2005 BBBTF McGill University, Institute of Particle Physics 22

23 Comparison with data April 21, 2005 BBBTF McGill University, Institute of Particle Physics 23

24 Recent turtle results... Updated HER deck to 2004 configuration Aperture and orbit checks performed LER deck update still in progress Coulomb scattering in HER (2004 configuration): Scattered e- impact point April 21, 2005 BBBTF Scattered e- production zone McGill University, Institute of Particle Physics 24

25 Turtle level studies (~2000) bremsstrahlung and Coulomb scatter events generated uniformly around ring assuming a flat 1nTorr pressure profile Reweight to known profile to get absolute predictions Record location, energy etc of primary particles hitting in vicinity of IR Useful information about impact regions and background sensitivities to regions of the rings: April 21, 2005 BBBTF McGill University, Institute of Particle Physics 25

26 Geant3 simulation (<2001) Used during commissioning phase and first few years of running Modeled BABAR detector and beam line out to Q5 (+/- 8m from IR) Turtle ray input to allow lost particle background studies Some known issues with beamline geometry, fields and material model occasionally primary particles would vanish occasional discrepancies between Turtle z-hit position and Geant hit position Replaced by Geant4 detector simulation in ~2002 Beamline simulation only out to ~Q2! April 21, 2005 BBBTF McGill University, Institute of Particle Physics 26

27 Turtle-Geant3 studies (circa 1998) SVT pin-diode simulation studies Chih-Hsiang Cheng G-hit based study using Turtle rays as input to Geant-3 pin-diode detector model April 21, 2005 BBBTF Used during initial PEP-II commissioning and early data taking phases of BABAR Predictions for SVT background sensitivities to HER and LER zones Some diodes appeared to be better modeled than others, but overall agreement with data to within a factor ~2.5 McGill University, Institute of Particle Physics 27

28 More Turtle-Geant studies 4-vectors of Turtle rays which strike apertures near IP are recorded at a point ~8m upstream, then passed to Geant Geant propagates particle into IR and simulates interactions in beampipe/detector material permits identification of turtle rays which produce activity (e.g. from secondary particles in specific detector elements April 21, 2005 BBBTF McGill University, Institute of Particle Physics 28

29 EMC Occupancy predictions Extrapolate simulated lostparticle induced detector occupancies according to measured (or assumed) vacuum profile Once appropriate backgrounds data was available, this was done using data instead Full detector response to backgrounds can be simulated to obtain reconstructed information e.g. clusters, tracks and even triggers April 21, 2005 BBBTF McGill University, Institute of Particle Physics 29

30 EMC Radiation dose RadFET calorimeter radiation monitoring gives integrated dose in various regions of the EMC Total radiation dose estimated by integrating estimated flux rate from simulation Not necessarily representative of dose in individual crystals Assume pressure profile Reasonable agreement with RadFET data (~30%) RadFET Data Interesting features! simulation predicted region of reduced dose in forward barrel (naively expected to be high dose) April 21, 2005 BBBTF McGill University, Institute of Particle Physics 30