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Cavern background

Cavern background

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Cavern background

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  1. Cavern background Charlie Young (SLAC)

  2. Outline • Reminder of what is cavern background • Simulation of cavern background • Physics event generation • Tracks at scoring volume surface • Detector hits for pile-up digitization • Estimated hit rates without detector hits • Status and outlook

  3. What Is Cavern Background?

  4. Cavern Background Cavern Background refers to the • typically low energy (MeV and below) • predominantly neutral (g and n) • long lived (compared with LHC bunch spacing of 25 nsec and revolution time of ~0.1 msec) background in the ATLAS cavern coming from p-p collisions.

  5. Typical Energy Distribution GeV meV MeV KeV eV

  6. Typical Energy Distribution 70 / 4x106 above 1 GeV

  7. Particle Type Distribution Photon Neutron Electron Proton Muon - Positron Muon + Pion - Pion + See for all FLUKA particle codes

  8. Typical Time Distribution ~ LHC revolution 25nsec BC

  9. Typical Time Distribution ~1.5x106 / 4x106 after 1 turn ~ LHC revolution

  10. Uncertainty in Hit Rate • Estimated by Radiation Background Task Force • Three major factors inmuon detector hit rate uncertainty: • 1.3x for p-p cross section • 2.5x for calculation of background particle flux • 1.5x for detector response • Overall uncertainty ~5x

  11. Simulation of Cavern Background

  12. Overlay vs Pile-Up • Overlay using zero-bias data should be closest to reality but not predictive • Different beam energies • Shielding plans • LHC and/or ATLAS upgrades • Complementary tool based on simulation • Cavern background similar to other sources such as min-bias, beam gas and beam halo • Min-bias and cavern background both come from p-p collisions – more later • Save background event as Geant4 hits • Add to signal event during pile-up digitization

  13. Steps

  14. p-p Event • Using PHOJET • Same as RBTF for comparison • Other event generators a very small differences • Not well constrained by data Physics events studied mostly in |h| < 5 From Energy is significantly more forward peaked at |h| ~ 7

  15. FLUGG: Background Calculation • FLUGG combines Geant4 and FLUKA • Geant4 description of detector • Detector simplified from Athena description for speed • E.g. no internal structure in calorimeters • More realistic compared with RBTF • Shielding and cavern description more important than in physics simulation, e.g. curved end walls • Release-to-release changes unimportant • FLUKA physics • Low energy n and g exiting shielding and calorimeter just like shielding calculations where FLUKA is the standard

  16. Some Geometry Updates Access shafts Curved ceiling Flat barrel chambers Curved end wall

  17. RBTF vs Current Geometry Curved end wall Energy deposition Chamber location Barrel torid Missing shielding added

  18. FLUGG Output • Fluence maps in (r,z) space – see examples later • Track every particle, including decay and interaction products, until it falls below (very low) energy cut. • Define scoring volumes • Imaginary surfaces surrounding each region of interest, e.g. muon station, MBTS and MPX • Implemented as cylinders • Record “4-vector” when entering any scoring volume • (x,y,z,t) and (px,py,pz,E) • Note: a particle may have more than one record because of multiple crossings into scoring volumes In practice, direction cosines and kinetic energy

  19. Athena: Simulated Hits • Treat “4-vector” output from FLUGG (after some massaging… ) as event generator • Low energy particles • Starting points on scoring volume surfaces near muon chambersa reasonable program speed • Standard Athena/G4 with full detector description to produce simulated hits • Knows detailed detector structure such as number of detection planes • Detector response as coded in Athena • Kill when exiting muon volume to avoid double counting • Different double counting from the one on next page • Existing cavern background sample affected and patched • Usually re-run in each production cycle with latest Athena updates

  20. Massaging • Kill prompt charged particles • Logically identical particles also present in minimum-bias events – both come from p-p collision • Existing cavern background sample did not do this, leading to double counting of prompt muons and artificially high trigger rates in pile-up samples • Modify hit times to be within 25 nsec of a particle traveling at speed of light • Athena/G4 has time cut that would otherwise remove most hits • Effect identical to bunch train with 25-nsec separation

  21. Shortcut for Hit Rate • Convolute 4-vectors produced by FLUGG with detector response curves • Different responses for CSC, MDT, RPC and TGC • Different responses for n and g • Taken from RBTF Report – examples on next pages • Account for path length due to incident angle • Benefits • Much faster • Higher statistics from artificially boosted response • Global boost factor a no change in relative numbers • Drawbacks • No knowledge of detector structure or granularity • Overlap between sectors • Number of detection planes • Response curve in general different from Athena implementation • Comparisons to date have used this result

  22. MDT Efficiency for Neutrons 1% 0.1% KeV MeV

  23. TGC Efficiency for Photons 1% 0.1% KeV MeV

  24. Changes from RBTF • Geometry • No longer cylindrically symmetric • Improved detector and cavern description • Many cross-checks with Mike Shupe! • Flux incident on muon chambers ~75% of before • Beam energy • 7 TeV ~75% of 14 TeV • Overall change roughly factor of two • Depends on location • No double counting of prompt muons in pile-up • Correlated hits directly affecting trigger rates • No need to patch double counting of low-energy particles • Uncorrelated hits

  25. RBTF vs Current Geometry Neutron Fluence Now generally lower in barrel and endcap chamber areas

  26. 7 TeVvs 14 TeV Flux of neutron (energy > 100 KeV) in kHz/cm2

  27. Neutron Flux Ratio: 7 to 14 TeV r z Bin-by-bin flux ratio of neutron (energy > 100 KeV)

  28. Ratio of 7 to 14 TeV Average over histogram bins is not very sensible procedure but indicative of change.

  29. Status and Outlook

  30. FLUGG Step • Geometry stable for some time • Starting production at 7 TeV • Initial plan of 106 events • Approximately one month • Need to improve production setup and scripts to utilize grid resources for higher production rate • Presently running in batch at SLAC only • What energy in future production? • More 7 TeV events to support analysis of 2010 data • 8(?) TeV for 2011 run • 14 TeV for upgrade studies

  31. Comparison with Data • Started in summer 2010 • Identifying “cavern background” in data • Bunch structure • See next talk for • CSC photon hit rate • MDT hit rate in data and simulation • RPC HV current vs simulated hit rate • MIP rate in MPX • Lots more to be done!

  32. Other Items • Earlier study found ~2x improvement in background with Beryllium beam pipe (already Be for |z| < 3.5 m) • Confirmed in our study • Beam pipe made of vacuum is even better, but would be an engineering challenge  • Simulation predicted background coming out of barrel endcap gap region • Improving description with an eye to proposing changes • High background for small wheel • Check simulation geometry some more! • Possible shielding improvements for the future

  33. Effect of Beam Pipe Material Change in nfluence From Change in gfluence muon chamber locations

  34. Gap Region Information extracted from CATIA by Georgian Team.

  35. Upgrade • Detector coverage and shielding are correlated • Need to break deadlock to move ahead • Iterations • Detector technology not important to estimating incident rate because of use of scoring volumes • Manpower limited 