1. Status of Beam Halo/Gas Simulation A.Stradling(Wisconsin) P. Steinberg(BNL) G. Usai (Chicago) Standard Model Group 6 December 2006 A.Stradling(Wisconsin) P. Steinberg(BNL) G. Usai (Chicago) Standard Model Group 6 December 2006 1

2. Intro Review of the practical issues surrounding beam halo and beam gas Simulation progress Initial plots of unweighted beam halo events 2

3. Beam Halo The LHC’s contribution to background Results from proton losses, including: Gases and impurities in the beampipe Collimator impacts (less important) Beam configuration and optics are taken into account 3

4. Beam Halo Details These effects are simulated in the IHEP MARS package I. Azhgirey, I. Baishev, K.M. Potter et al. Methodical Study of the Machine Induced Background in the IR8 of LHC. CERN LHC Project Note 258, Geneva, 2001; I. Azhgirey, I. Baishev, K.M. Potter, V. Talanov, Cascade simulations for the machine induced background study in the IR1 of the LHC. CERN LHC Project Note 324, Geneva, 2003. Particles were generated to represent one second of run, totaling 8.2 M particles Particles are produced all along the accelerator, but most of those we see are produced in the inner triplet magnets just before the cavern ~10x more hadrons than muons, concentrated at a smaller radius 4

5. Beam Halo Files The resulting file is specific to: ATLAS, Side A (Beam 1), Optics v6.4, and possibly outdated collimator configuration Design lumi (14 TeV, β* of 0.5 m and beam current of 0.54 A) It includes nothing after the “scoring plane”, where the experimental volume is defined as beginning. This plane lies at 23000 mm from the IP, between the concrete wall and the beam shields /afs/cern.ch/atlas/project/MachineBackgrounds/Sam 5

6. Beam Halo Scaling (1) Many of these previous assumptions have changed LHC Optics version = 6.5, β* of 10 m or worse for initial runs Collimators are still being finalized Beam gas distributions account only for adsorbed gases (no other junk or impurities) It should also be borne in mind that the LHC is not completely symmetric, and there will be differences in intensity between Beams 1 and 2 6

7. Beam Halo Scaling (2) Most of these are relatively simple to handle: Scaling for changes in beam intensity, β*, optics, beam gas density, beam asymmetries and other similar effects is linear - we just change the number of particles that come in at a time We have 1.8 M particles per second total for 40 M bunch crossings - most bunch crossings will have little or no beam halo 7

8. Beam Halo Scaling, 900 GeV The 900 GeV run is a problem, however - it requires a redo of the MARS simulation Simple scaling is impossible - the differences lie not in the particle multiplicity, but in the produced particle energies and species proportions The machine group is (obviously) fairly occupied now, and taking another crack at the simulation effort will take time 8

9. Beam Halo Work in Progress Giulio Usai is working on getting the events unweighted and translated into HepMC format for simulation Collaboration with the G4 community to get the odd detector volume issues (like having a vertex at 23 m) worked out MC group is excited to have this in their production system 9

10. Beam Halo Particles (1) Kinetic energy of beam halo particles MeV Ratio - Weighted/Unweighted Weighted Unweighted

11. Beam Halo Particles (2) Initial radius of halo particles on the scoring plane Ratio - Weighted/Unweighted Weighted Unweighted cm

12. Beam Halo Particles (3) PDG ID histogram of halo particles 1:1 Ratio - Weighted/Unweighted Weighted Unweighted

13. Beam Gas Generation HIJING (A+A, p+A generator) in Athena Simulation of various gas species in the beampipe of the detector Starting simple - will eventually make adjustments for gas density profiles and etc. Generated events are vertex-shifted from 0,0,0 to other locations during simulation 13

14. Beam Gas Questions The accelerator group did their work assuming that the only important gases in the beampipe were H 2, CO 2, CO, CH 4 Other things may be present (N, Ti, V, Zr from the getter, Be from the beampipe, etc) - what will they look like? Will there be any significant presence? 14

15. Timing Complications 40 MHz bunch crossings give us a τ of 25 ns 25 ns at c translates to 7.5 m Lots of bunches in flight Timing the beam halo and beam gas relative to the physics event is crucial 15

16. Beam Halo Timing The beam halo comes in with the same timing as the bunches themselves Offsets of 25 ns Use digitization to overlay beam halo events onto simulated events, as with pileup events for realistic TRT, LAr and Muon simulation Differences - we will need to open up the number of bunch crossings to 3 on each side 16

17. Beam Gas Timing More challenging - does not often happen in sync with the event (can be anywhere in the pipe) Time offsets in the digitization need to be tunable (but are easy to calculate from the vertex position) - I have not yet determined whether this has been implemented in digi yet. Suggestions and recipes welcome. 17

18. Next Get good jobOptions for generation/simulation of the beam gas and beam halo, validate some output Examine parameters (scaling of beam halo, proportions of beam gas components) Pass to ProdSys Start analyses of the effects of beam gas and beam halo on MinBias event triggering 18

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