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ATLAS B-triggers

ATLAS B-triggers. John Baines. Constraints of the Online System. Constraints : B-Physics can take a maximum of about 5-10% of the bandwidth => ~100 Hz from LVL2 to the EF 10 Hz from EF to recording Highly selective trigger : Exclusive or semi-inclusive reconstruction of decays

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ATLAS B-triggers

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  1. ATLAS B-triggers John Baines

  2. Constraints of the Online System Constraints : • B-Physics can take a maximum of about 5-10% of the bandwidth • => ~100 Hz from LVL2 to the EF • 10 Hz from EF to recording • Highly selective trigger : Exclusive or semi-inclusive reconstruction of decays • Time per event: • ~10ms at LVL2 • ~1s at EF • Fast Algorithms – can’t reconstruct the whole event in the ID (except at very low luminosity). Instead use Regions of Interest at LVL2 and EF • RoI defined by LVL1: • Muon • EM • Jet

  3. Not normalized Bm+ m- B K* m+ m- Di-m Mass, (MeV) Di-muon trigger for rare decays & B->J/y(mm) LVL1:2mRoI pT(m) > 6GeV Rate ~500Hz @ L=1033cm-2s-1 Main background – singlemdoubly counted due to overlaps LVL2: • Confirm each LVL1m: • In precision muon chambers • Combine m with Inner Detector track (~5kHz @ L=1033cm-2s-1) • Vertex fit & vertex quality cuts • EF: • Refit ID tracks in Level-2 RoI • Decay vertex reconstruction, Vertex Quality cuts • Mass cuts for B->mm, J/y(mm) • Reject prompt J/y(mm)using Transverse Decay length cut: • e.g. Lxy > 500mm • Investigating other cuts to select B->mmK* etc. • Try to avoid Lower mass cut – but can bias assymetry measurements

  4. Other RoI based Triggers • Trigger with single muon at LVL1 • Use low threshold LVL1 EM and Jet RoI to identify Regions of Interest that might contain a B-Jet, electron or photon • Min. pT are determined by need to keep LVL1 RoI mutiplities under control: • ~ 5 GeV for electrons, ~15GeV for B in Jet roI (very soft threshold) • Strategy at LVL2 depends on signatures we are looking for: • For Hadronic decays such as Bs->Dsf(kk)p: • Reconstruct ID tracks in Jet RoI • For Final states including J/y(ee) either: • Look for1 electron in each of 2 RoI • look for 2 electrons in a large RoI about a single LVL1 EM cluster • For B->K*g and B->fg • Look for tracks in a very large RoI about the gamma • For J/y(mm) : Can use a single LVL1 muon and look for the second muon in the Inner Detector in a large RoI about the Trigger Muon

  5. 78 (%) 76 74 72 J/psi Efficiency, 70 68 66 0.2 0.25 0.3 0.35 0.4 0.45 Background rate, (kHz) Lowering 2ndm threshold for J/y->mm Efficiency vs. delta eta (RoI) 1.1 • Single muon at LVL1 (22kHz) • LVL2 • confirm muon (first in muon detector and then combined with ID) => rate ~ 5 kHz • Open region around muon and search for J/psi in inner detector in RoI ~1x1. • Mass cut (M(mumu)>2.8GeV) • Extrapolate tracks to muon system – require muon detector hits – more layers for higher purity • Apply mass cuts, vertex fit 1 0.9 0.8 Efficiency 0.7 0.6 0.5 0.4 0.3 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Delta eta J/y  m+ m- efficiency vs. background rate Efficiency 68% (1st m 6Gev,2nd m 3GeV) Background 260Hz Might be able to use a similar method to improve efficiency for other di-muon channels at low lumi – but with larger RoI

  6. Full ID Reconstruction • Full ID reconstruction for events with a LVL1 MU • Takes about ~4 times longer than reconstructing in a Jet RoI (1.5x1.5) • Possible to reconstruct tracks down to ~1 GeV => improve efficiency for low pT B • Could use when luminosity is very low e.g. startup, provided enough capacity in HLT CPU farms.

  7. Flexible Trigger Strategy • Startup (900GeV run) • HLT in “transparent” mode – no selection • Run LVL2 ID full-scan for events with min. bias trigger or LVL1 muon triggers • First Physics Running (2008): • Luminosity <~1032 cm-2s-1: • Di-Muon Triggers • Other Triggers based on ID full-scan • Lumi : 1033: • DiMuon Triggers • RoI based triggers for hadronic final states and final stated with e & g • High Lumi Running: 1034: • Dimuon Trigger for Rare Decays : B->mm(X)

  8. Future Work • First B-Trigger selection available in offline releases (12.0.3) • For some Triggers Bs(DsPi) and B(mm) TriggerDecision in the AOD • B-trigger not yet complete, several components missing especially at Event Filter • Very small group currently working on B-Triggers in ATLAS • Need more people needed to: • test and validate the existing selections • optimise cuts • start looking at performance: efficiencies, rates, execution time • help to develop the missing parts • develop strategies for measuring Trigger efficiencies & systematics • assess impact of trigger on physics channels => Now is an exciting time to join the B-trigger work!!

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