LHCb trigger: algorithms and performance

1. LHCb trigger: algorithms and performance Introduction The LHCb trigger: Level 0 Level 1 HLT Performance Hugo Ruiz on behalf of the LHCb collaboration Hugo Ruiz

2. LHCb environment • LHC: • 40 MHz crossing rate • 30 MHz with bunches from both directions • Luminosity: 2·1032 cm-2 s-1 • 10 to 50 times lower than @ ATLAS, CMS (well under machine design!) • Reason: single interaction preferred to identify secondary vertices from B mesons • Relevant rates: (for visible events  at least 2 tracks in acceptance) • Total rate (minimum bias): 10 MHz • bb:100KHz • Whole decay of one B in acceptance: 15KHz • cc: 600KHz HERA-LHC workshop. Hugo Ruiz

3. Detector overview Muon System RICHES: PID: K, separation VELO: primary vertex impact parameter displaced vertex PileUp System Interaction point Calorimeters: PID: e,, 0 Trigger Tracker: p for trigger Tracking Stations: p of charged particles HERA-LHC workshop. Hugo Ruiz

4. MC generation • PYTHIA 6.2 used • Minimum bias: hard QCD, single / double diffraction, elastic scattering • Signal: forcing B-mesons in a minimum bias event to decay into specific final state • Charged particle distributions tuned to data fors < 1.8 TeV • Predicted cross-sections: sinel = 79.2 mb, sbb=633 mb • Pileup (multiple interactions in single bunch crossing) simulated • GEANT3 for full simulation of all events (minimum bias, signal) • Moving to GEANT4 in this year’s Data Challenge • Additional backgrounds: off-beam muons, low-energy background at muon chambers • Spilloversimulated in detector response (from two preceding and one following bunch crossings). HERA-LHC workshop. Hugo Ruiz

5. Trigger overview Pileup system VELO + Trigger tracker Calorimeters + Muon system 10 MHz L0: hight pT + not too busy • Fully synchr. (40 MHz), 4ms latency • On custom boards 1 MHz L1: IP + high pT • Ave. latency: 1 ms (max 50 ms) • Buffer: 58254 events 40 KHz HLT + reconstruction • Full detector: ~ 40 kb / evt ~ 200 Hz Single PC farm ~1800 CPUs HERA-LHC workshop. Hugo Ruiz

6. Level 0 • Fast search for ‘high’ pT particles(calorimeters, muon syst) • Charged hadrons: HCAL (~ 3 GeV) • Electrons, photons, p0: ECAL (~ 3 GeV) • Muons: muon system (~ 1 GeV) • Cut on global variables: • Require minimum total ET in HCAL (calorimeters) • Reduces background from halo-muons • Rejection of multi-PV and busy events (Pileup system, SPD) : • fake B signatures (IP) • Busy events spend trigger resources without being more signal-like • Better throw them early and use bandwidth to relax other cuts HERA-LHC workshop. Hugo Ruiz

7. Level 0: calorimeter trigger ECAL HCAL SPD-PreShower FE Scintillator Pad Detector (SPD) • The LHCb calorimeter: • ECAL: 6000 cells, 8x8 to 24x24 cm2 • HCAL: 1500 cells, 26x26, 52x52 cm2 • Trigger strategy: look for high ET candidates: • In regions of 2x2 cells • Particle identification from • ECAL / HCAL energy • PS and SPD information • ET threshold ~ 3 GeV • Sent to L0 decision unit: • Highest ET candidate each type • Global variables: • Total calorimeter energy • SPD multiplicity ECAL HCAL Pre-Shower Detector Validation cards Selection crates g ETtot e± p0 hadr SPD mult HERA-LHC workshop. Hugo Ruiz

8. Level 0: muon trigger • The LHCb muon system: • 5 stations • Variable segmentation • Projective geometry • Trigger strategy: • Straight line search in M2-M5 • Look for compatible hits in M1 • Momentum measurement • Sent to L0 decision unit: 2 highest pT candidates per quadrant threshold HERA-LHC workshop. Hugo Ruiz

9. Level 0: Pile-up system Interaction region • Pileup system: • 2 silicon planes • Measure R coordinate • backwards from interaction point  no tracks from signal B • Trigger strategy: veto multi-PV evts • From hits on two planes  produce a histogram of z on beam axis • Sent to L0 Decision Unit: height of two highest peaks + multiplicity HERA-LHC workshop. Hugo Ruiz

10. Level 0: Decision • L0 decision unit: • OR of high ET candidates • Applies cuts on global properties • Thresholds and partial rates: (Trigger TDR, Sept 2003) • Composition: HERA-LHC workshop. Hugo Ruiz

11. L1-HLT infrastructure Front-end Electronics FE FE FE FE FE FE FE FE FE FE FE FE TRM 126 links 44 kHz 5.5 GB/s Multiplexing Layer Switch Switch Switch Switch Switch 64 Links L1-Decision Readout Network Sorter 94 Links 7.1 GB/s 94 SFCs SFC SFC SFC SFC CPUFarm … ~1800 CPUs Switch Switch Switch Switch CPU CPU CPU CPU CPU CPU CPU CPU CPU CPU CPU CPU Gb Ethernet Level-1 Traffic HLT Traffic Mixed Traffic • L1 & HLT share infrastructure: • Ethernet network • Sub-farm controllers • Computing nodes • Provides flexibility, scalability • HLT & reconstruction run in background • L1 task has top priority • CPU share: ~ 55% L1, 25% HLT, 20% reconstruction HERA-LHC workshop. Hugo Ruiz

12. Level 1  sensor R sensor 100 cm • Trigger strategy: • find high IP tracks (tracking in VELO) • Confirm track / Estimate pT from TT • Special treatment for clear L0 signatures • The LHCb VELO: • 21 stations (~ 100 cm) • Alternated R-f sensors • 40 μm to 100 μm pitch • Busy environment: • ~ 70 tracks/event after L0 • but low occupancy in VELO (~0.5%) Interaction region HERA-LHC workshop. Hugo Ruiz

13. Level 1: IP at VELO • Fast-tracking strategy: • First in R-Z view (only R sensors) • Primary vertex σZ ~ 60 mm • Select tracks with IP in (0.15, 3) mm • about 8.5 / event • 3D tracking for those tracks • pT measurement using TT • Silicon, 2 layers, 200 mm pitch • Only 0.15 T.m between VELO and TT DpT / pT ~ 20-40% • Rejects most low momentum tracks, which can fake high IP HERA-LHC workshop. Hugo Ruiz

14. Level 1 decision • Based on IP and pt of the two highest pt tracks • Decision modified on clear L0 signatures matching L1 tracks • High pT electrons & photons • High mass di-muons: • If mass lies within ±500MeV of J/y or B, accept automatically • Else, ‘bonus’ proportional to di-mass Minimum bias Signal events • Composition: HERA-LHC workshop. Hugo Ruiz

15. Possible HLT flow diagram 40 KHz (9.7% bb, 14.2% cc) Lepton-like evts Re-reconstruct L1 tracks (now using all tracking stations) “Lepton highway” Rest Confirm L1 decision (p)/p ~ 0.6 %! Reject uds, e > 95% HLT no 20 KHz (14.0% bb, 14.7% cc) Reconstruct whole event Specific: Exclusive (ex: BDsh) Inclusive (ex: DX) J/y-like + Tagging-lepton- enriched HLT no Generic algorithm Long-lived b sample (systs, backgrounds) CP channels with e ~100% Open charm Full reconstruction / Storage HERA-LHC workshop. Hugo Ruiz

16. Performance: L0 x L1 • Results from Trigger TDR (Sept 2003) • Efficiencies computed on offline selected events • Overall L0xL1 efficiency: • 30% for • hadronic channels • e/γ/π0 channels • 60-70% for di-muons • Software and hardware prototyped and working, within time budget • see Trigger TDR, Sept 2003 L0 efficiency L1 efficiency L0L1 efficiency HERA-LHC workshop. Hugo Ruiz

17. Performance: HLT • A complete implementation of HLT is being prepared for the Computing TDR (due summer 2005) • The reconstruction part has already been implemented and tested in terms of performance and time consumption, in particular: • Tracking • L1 confirmation • First results on exclusive selections show that individual b physics channels give rates of ~ 10Hz with • 500 MeV side bands for b mass • Affordable time consumptions • > 95% efficiencies HERA-LHC workshop. Hugo Ruiz

18. Expected event yield • Taking into account efficiency from: • L0xL1 • Offline selection HERA-LHC workshop. Hugo Ruiz