FLUKA beam-gas simulations extending to the arc R . Bruce,

1. FLUKA beam-gas simulations • extending to the arc • R. Bruce, • R.W. Assmann, V. Boccone, F. Cerutti, M. Huhtinen, A. Lechner, A. Mereghetti,

2. Outline • Simulation setup and FLUKA geometry • Results of inelastic beam-gas simulations • Conclusions

3. Introduction • LHC machine background in experiments from several sources: • Beam halo from upstream collimation impacting on TCTs, shower propagating to IP • Local inelastic beam-gas scattering close to detector • Large-angle elastic beam-gas scattering around the ring sending protons directly on TCTs without hitting primary collimators first, shower reaching IP • Cross-talk from other IPs. • Particles entering detector given as input to experiments - simulation of detector not treated here Today’s topic!

4. Inelastic beam-gas simulations • Shower originating from inelastic beam-gas interaction close to the IP simulated with FLUKA • Simulated already last year, but up to s=150m (at TCTs) • MARS results by N. Mokhovat 7 TeV for IR5 using MARS showed that muons coming from farther away could be important. Hinted lso by previous FLUKA studies. • Therefore, new extended geometry going to s=550m from IP1 (Q13) implemented by FLUKA team (same value as used in MARS for IR5)

5. IR1 FLUKA geometry, A. Lechner • Magnetic field maps in triplet, D1, MQTL, MQY, main dipoles • Analytic fields only in pipe in remaining magnets • Quadrupoles in matching section adapted for beta*=1m • All fields adapted for 3.5 TeV • No correctors powered (no crossing angle) Interface plane Inner triplet TCTs D1

6. FLUKA geometry (continued) ~10m horizontal offset of beam center at the end of the model

7. Check of magnetic fields Outgoing beam MADX Incoming beam MADX Incoming beam FLUKA D1 Interface plane triplet TCTs

8. Check of magnetic fields • Particles tracked in MAD-X and FLUKA • Overall accuracy is about 10μm

9. Simulation setup • Sampling forced inelastic interactions homogenously along ideal orbit, between s=22.6 m and s=546.6 m • Equivalent to assuming a homogenous pressure profile • Real inhomogeneous pressure profiles can be accounted for by weighting the particles according to the initial interaction point, or selecting events with a probability according to pressure profile. • This approach provides a maximum flexibility, as the CPU intense FLUKA simulation does not have to be rerun for each case considered • Right side of IR1 (B2) • “To first order” no difference expected w.r.t. IR5 • No biasing used, cutoff at 20 GeV(to keep CPU and file size down but still track important high-energy muons) • Particles at interface plane at 22.6 m saved in file

10. Results - energy spectra 3.5 TeV • 177 240 000 primary beam-gas events simulated • 323GeV/primary reaches interface plane on average • Compare N. Mokhov simulations 7 TeV: ~300 GeV/primary @ interface plane, but uses non-homogenous pressure profile • Showing 3 curves: new simulation (20 GeV cut in energy, no cut in z), new simulation (20 GeV cut in energy, 150m cut in z), old simulation (20 GeV cut in energy, 150m cut in z) old plot from last meeting New result 20 GeV

11. Energy spectra 3.5 TeV Excellent agreement with old simulation in region z<150m Less energy per primary interaction reaches interface plane when averaging over all events with z<550m as expected

12. Radial energy 3.5 TeV • Impinging energy on interface plane strongly peaked in the center • Difference in radial tails: • At large radii, more energy entering interface plane in old model • At r>200 cm, more energy per primary if counting the arc - muons!

13. Radial energy - muons • Difference in radial energy at 200<r<500cm caused by muons • When cutting at 20 GeV, we miss the tail at large radii (as expected) Old simulation New result

14. Radial energy distribution 3.5 TeV • When cutting at 20 GeV, we miss the tail at large radius (as expected) 20 GeV cut

15. Distribution in z • Energy at interface plane binned in z-coordinate of the initial beam-gas interaction • Largest contribution inside TCTs, but arc is not-negligible, even for all particles and protons • Note linear scale

16. Distribution in z • Arc not important for e+- and photons TCTs • Upstream of TCTs very important for muons(as expected from N. Mokhov) Beginning of arc

17. Slide from M. Huhtinen, ATLAS Non-Collision Backgrounds Meeting 2012.03.05 Comparison with N. Mokhov 3.5 TeV possible next step?

18. Summary • Simulations of particle distributions entering the interface plane between machine and detectors in ATLAS for beam-gas interactions sampled on N between s=22.6m and s=547m at 3.5 TeV • Only IR1 simulated, but to first order no difference expected w.r.t. IR5 • Simulation between 22.6m and TCTs agrees well with old simulation • Completely unbiased but cut-off at 20 GeV to keep down CPU usage and file size • Muons beyond old model (s~150m) important contribution • All data files and plots available on http://macbe13119 • See presentation M. Huhtinen in ATLAS background meeting for additional plots (muon phi distribution etc) • Next steps: • Normalization with measured pressure profile obtained from vacuum group • Beam-halo simulations: ongoing work to understand local discrepancies at TCTs between SixTrack simulations and measurements