Roberto Crupi INFN Lecce & Physics Department, University of Salento - Italy on behalf of the

1. Muon reconstruction and selection at the last trigger level of the ATLAS experiment Roberto Crupi INFN Lecce & Physics Department, University of Salento - Italy on behalf of the ATLAS Collaboration 11th ICATPP Conference on Astroparticle, Particle, Space Physics, Detectors and Medical Physics Applications Villa Olmo, Como 5-9 October 2009 R.Crupi

2. Outline • Overview on the ATLAS Trigger System • The Muon Event Filter • Performance Studies R.Crupi

3. ATLAS (A Toroidal LHC ApparatuS) R.Crupi

4. MDT RPC TGC CSC The ATLAS Muon Spectrometer The Muon Spectrometer has the standalone capability of measuring muon momenta pT/pT < 10% up to 1 TeV ΔpT/pT of few % up to 100 GeV Technologies deployed in the muon system: • Tracking Chambers: • MDT (Monitored Drift Tubes) • ||<2 • CSC (Cathode Strip Chambers) • 2<||<2.7 • Trigger Chambers: • RPC (Resistive Plate Chambers) • ||<1.05 • TGC (Thin Gap Chambers ) • 1.05<||<2.4 A magnetic field allows muon momentum measurements R.Crupi

5. BUNCH CROSSING RATE 40 MHz ~75 kHz 2.5 ms ~3 kHz 40 ms ~200 Hz 4 s The ATLAS Trigger To obtain the required event rate reduction, the trigger is structured in three levels: • Level-1 : hardware based, with coarse granularity from calorimeters and muon systems. It reduces the ~40MHz initial rate to 75 kHz in a maximum latency of 2.5 s. It looks for regions of activity in the detector (Regions of Interest,RoIs) • Level-2: softwarebased, algorithms optimized for fast rejection. It reduces Level-1 output rate to ~3 kHz with a 40 ms mean processing time. It accesses the full granularity data inside a Level-1 RoI. • Event Filter: software based. It uses • the same algorithms as in the offline • and runs after event building, then • it can access the complete event data. • It reduces Level-2 output rate to • ~200 Hzwith a 4s mean processing • time.Itaccesses the full granularity • data inside a Level-2 RoI. • Level-2 & EF are together referred as High Level Trigger (HLT) R.Crupi

6. Reconstruction and identificationin the muon system • TheMuon Spectrometer is a powerful tool for precise measurements of high pT muons and identification of low pT muons • The longer lever armimprovescharge determination for high energy muons • Extrapolation to the Interaction Point (IP) can better discriminate from secondary muons and the ones coming from the in-flight decay of π/K mesons. • The Inner Detector (ID) performs precise track reconstruction with high efficiency for low pT muons • Muon Identification is achieved by combining ID measurements with the ones provided by MS • The final momentum resolution takes advantage of both ID and MS • Isolation criteria applied to muons before reaching the MS allows a goodmuons discrimination from the ones deriving from jets R.Crupi

7. Region of Interest selected By Level-1 Level-2: • muFast: standalone MS muon reconstruction, fast muon pT estimation via LUT • muComb: combined ID/MS muon reconstruction; improvement of pT resolution wrt μFast • muIso: Calorimeter and ID isolation • muTile: Muon Tagging in Tile Calorimeter Level-1 result • rejection of low pT muons • rejection of secondary muons deriving from the decay in flight of π/k • rejection of fake muon tracks generated by hits due to cavern background Level-2 tasks: The Muon Trigger • Event Filter (EF)performs muon • Identification and Reconstruction • thanks to dedicated offline packages • developed in the ATHENA framework • Refines Level-2 estimates • Confirms/Discards Level-2 Hypothesis • For debug purposes EF could directly • receive a seed from Level-1 RoIs R.Crupi

8. Muon Event Filter Two algorithms have been implemented in muon EF: TrigMuonEFandTrigMuGirl. Both are wrappers of muon offline reconstruction tools. They have an opposite strategy in muon reconstruction/identification: TrigMuonEFstarts from the reconstruction in the MS and performs a backward extrapolation to the IP and track combination in the inner detector. TrigMuGirlstarts muon reconstruction from the ID and extrapolates tracks to MS. R.Crupi

9. FEX HYPO Updated TE Muon Event Filter • Trigger selection strategy is based on the definition of: • Trigger Elements (TEs): abstract objects representing the state of the reconstructions. • Chains: sequences of several TEs • For each chain, the trigger steering runs the sequence of algorithms configured to produce all the output TEs at each step of the chain. TE • TrigMuonEF andTrigMuGirl are structured in : • Feature EXtraction (FEX) algorithms: normally RoI seeded, retrieve detector data inside RoI and look for feature in this data. • HYPOthesis (HYPO) algorithms: compare features provided by the FEXs against some hypothesis and validate or reject the TE output according to the success or failure of the hypothesis R.Crupi

10. TrigMuonEF Implementation • TrigMuonEFis a wrapper of offline packages developed for the purpose of muon reconstruction and identification: • TrigMuonEF is composed of: • A sequence offourFEXalgorithms which compute physics quantities. • FourHYPOalgorithmsafter each FEX. • Possible configurations: • Seed from Level 2 (standard) • Seed from Level 1 (for debug) • Unseeded (full event reconstruction) R.Crupi

11. TrigMuonEF Implementation • Segment Finder: selects MDT precision hits to found a segment. • Track Builder: Reconstructs muon tracks inside the MS taking advantage of its high precision tracking system and provides a precise measurement of the track parameter outside the calorimeter. • TrackExtrapolator: performs a backward extrapolation to the interaction region through the calorimeters. • It takes into account the magnetic fieldandcorrectionsforenergy loss and multiple scattering effectsfrom all crossed material • TrackCombiner: Extrapolated tracks are combined with the corresponding matching tracks, if they exist, in the ID. • MS and ID tracks are matched by forming a c2 built with the parameters of both tracks • use a global fit of all the hits collected in both ID and MS R.Crupi

12. EF ID Tracks In Muon RoI CandidateTool TrigMuGirl ANNSelectionTool GlobalFitTool HypoAlg TrigMuGirl Implementation • Muon candidate starts from an ID track inside a Level-2 RoI, provided by EF ID algorithms. • Tracks are extrapolated to the MS chambers. • A road in eta and phi is created around the extrapolated coordinates. • The algorithm looks for hits around the extrapolated track to fit segments. • The best fittedsegment is used to refine the estimated track coordinates and road in the current station. • Hits and segments collected in the various stations of the MS chambers allow TrigMuGirl to improve extrapolation and the to identify muon-like candidate. • Muon-like candidates are selected using precalibrated ANN functions. • A global fit, icluding ID and MS hits is applied to tracks belonging to identified muons for a further improvement of the momentum estimate. R.Crupi

13. Source 1034 rate (Hz) 1031 rate(Hz) Mtruth=110 GeV M = 113.4 ± 9.7 GeV L2_mu40 Slow Slow M=110 GeV c-cbar 125 2.5 0.0024 b-bbar 123 1.65 0.0016 Wmunu 19 0.4 0.0004 t-tbar 0.2 small 0.0000 zmumu small 0.0000 TrigMuGirl: Slow Particle Mode Background Rates • TrigMuGirl includes a dedicated toolto trigger Stable Massive Particles (SMP, e.g. in Susy R-parity violating scenario). • Reconstruction efficiency takes advantage of hits collected from both the previous and the followingBunch Crossing. • b and mass reconstruction using RPC and MDT technologies. • Specific R-Hadrons selection for candidates that don’t have an ID track. • Seed search in the MS from a Level-2 trigger element. SMP trigger runs in a separate chain (Slow) in the trigger Menu for 1031 L1: mu10 L2: beta/mass reco in the barrel EF: TrigMuGirl beta/mass reco Estimated beta resolution and reconstructed mass for single stau R.Crupi

14. EF efficiencies wrt Level-2 ATLAS Preliminary pT resolution vs pT Muon EF Performance • Simulatedt-tbar sample • Association to true muon in a cone of DR<0.002 for muon EF combined algorithms. • No pT thresholds have been applied for TrigMuonEF while the requirement of a 10GeV pT muonin the event isappliedfor TrigMuGirl. • The pT resolution as a function of transverse momentum is shown for • Extrapolator • Combiner • TrigMuGirl • with respect toLevel-2muComb R.Crupi

15. ZμμMC sample Efficiency wrt MC Truth TrigMuGirl TrigMuonEF Level-2: muFast muComb Muon EF Performance eta resolution vs pT • TrigMuonEF and TrigMuGirl: • improve significantly trigger rejection power due to better pT resolution wrt Level-2 • have similar performance in efficiency and resolutions phi resolution vs pT R.Crupi

16. Results on 2008 cosmic data • TrigMuonEFTrackBuilder algorithm: comparison of eta ()and phi (Φ) wrt to offline algorithm =0.007, =17mrad • Solenoidaland toroidal field on R.Crupi

17. Conclusions • The Muon Event Filter has been designed and implemented to cope with the demanding requirements of the ATLAS trigger system in the high luminosity and background enviroment at LHC • TrigMuonEF and TrigMuGirl have been successfully integrated in the Muon EF and constantly tested and validated • TrigMuonEF and TrigMuGirl show very good and similar performance • At the startup muon selection will take advantage of both algorithms running R.Crupi

18. BACK UP SLIDES R.Crupi

19. LHC • Center of Mass Energy (p-p) • 14 TeV (design) • 10 TeV (start-up) • Instantaneous Luminosity: • L = 1031cm-2s-1 • L = 1034cm-2s-1 • Integrated Luminosity / year (@ 1034) •  Ldt  100 fb-1 • The cross sections of interesting physics processes are highly suppressed w.r.t. stot • Calibrations and precision physics • Leptonic W decay • New Physics • Higgs boson production 10-6 10-9 • Event rate at LHC (@ design lumi) ~1 GHz • 40 MHz (BC frequency) x 23 interactions per BC • Max allowed acquisition rate ~200 Hz • Bandwidth ~ 300 MByte/s • Event Size ~ 1.5 MByte R.Crupi

20. 1031 Muon menu and rates Total of muons: 1.7k -220 ~40 R.Crupi

21. Slow Particle Trigger R.Crupi

22. TrigMuGirl for Slow Particles At Level-2; StauHypo: pT> 40GeV, b < 0.097 and m>40 GeV • Enables to select candidates when segment reconstruction is imperfect • Efficiency for low b is improved using TGC hits form next BC • b estimation using RPC- as in Level-2 • b estimation using MDT • Loop over possible b • Change TOF according to b –MDT radii change accordingly • Create MDT segments from the re-time radii • Minimize segment c2 wrt b → estimate • Determine b and Mass R.Crupi

23. New mu20_slow is in trigger menu for 1031 • L1 mu10  • L2 stau (B) | mu20 (EC)  • TrigMuGirlSlow • Estimated background rates R.Crupi

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