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Trigger Strategy and Performance of the LHCb Detector

This presentation by Mitesh Patel on behalf of the LHCb Collaboration at HCP2005 discusses the trigger system's critical role in meeting the experimental goals of the LHCb experiment. Emphasizing precision measurements of CP Violation in B decays, the talk elaborates on the detectors integrated into the trigger environment and the strategic decisions involved in selecting trigger channels. The trigger overview outlines the three levels of trigger system operation and their respective functions in selecting exclusive and inclusive triggers for physics channels. The Level 0 Trigger's functionality, including the muon and calorimeter triggers, is detailed with insights into efficiency and performance metrics. The talk rounds off with Level 0 and Level 1 trigger performance data illustrations, showcasing the trigger system's efficiency improvements.

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Trigger Strategy and Performance of the LHCb Detector

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  1. Trigger Strategy and Performance of the LHCb Detector Mitesh Patel (CERN) (on behalf of the LHCb Collaboration) Thursday 7th July 2005

  2. Introduction • LHCb experimental goals : • Precision measurements of CP Violation in B decays • Aim to (over-)constrain the unitarity triangle by making measurements in multiple channels • b production predominately at small polar angles → LHCb optimised as single forward arm spectrometer • To meet the physics goals require a trigger which can select : • Multitude of signal channels in the LHCb experimental environment • Channels required for calibration, alignment and systematic studies • Channels that allow the efficiency of the tagging of B flavour to be evaluated • Unbiased control channels • System must be simple, robust and flexible HCP2005 - Mitesh Patel

  3. The LHCb Trigger Environment • LHC Bunch crossing frequency: 40 MHz Non empty bunches → 30 MHz • LHCb Luminosity : 2×1032 cm-2s-1 • 10-50 times lower than ATLAS, CMS • B decays → displaced secondary vertex, need ~1 interaction/event • ‘Visible’ interactions at 10 MHz • 100 kHz bb events (800 kHz cc) • 15% of bb events: all decay products of at least one B in detector • Branching ratio of interest : 10-3 to 10-7 • Use information from a variety of LHCb’s detectors to reduce 10 MHz … HCP2005 - Mitesh Patel

  4. Detectors in the LHCb Trigger VErtex LOcator primary vertex impact parameter displaced vertex Scintillator Pad Detector Charged multiplicity Trigger Tracker p, pT Calorimeters PID: e,, 0 Trigger on hadr. Muon System Pile-up system multiple interactions, charged multiplicity HCP2005 - Mitesh Patel

  5. Trigger Strategy • The physics goals of LHCb motivate : • Exclusive triggers : ‘hot’ physics eg. BsDsh, Bsff, B0J/y KS, B0D*p, B(s)h+h-, B0K*m+m- ,B0D0 K*, Bsm+m-,BsJ/y f, Bsfg • Inclusive triggers : • Inclusive single-muon sample[independent of signal type ] • Sample triggered independent of signal type – unbiased on the signal side • Signal trigger efficiencies • Inclusive di-muon sample [selected without lifetime information] • Clean mass peaks for alignment, momentum (B field) calibration • Proper time resolution using prompt J/ events • Inclusive D* sample [selected without RICH information] • Clean signal of D*D(K) • Measure PID performance as a function of momentum → Data mining HCP2005 - Mitesh Patel

  6. Trigger Overview • LHCb will use three levels of trigger : • Level 0Trigger [4 ms] [hardware] • ‘high’ pT particles in calorimeters and m detector • Pile-up System throws away busy events • Level 1Trigger [~1 ms] [software] • Partial read-out: Vertex Detector (VeLo), Trigger Tracker (TT) and L0 summary • Find high IP tracks, estimate pT of tracks, link to L0 objects • pT of the two highest pT tracks • High Level Trigger[~10 ms][software] • Confirmation of L1 decision then full reconstruction of event • Exclusive selections for most important physics channels • Inclusive selections 10 MHz 1 MHz Already well developed, relatively fixed 40 kHz Now being developed 200 Hz 1800 Hz + • L1 and HLT will be run on a single ~1600 CPU PC farm HCP2005 - Mitesh Patel

  7. The Level 0 Trigger: Overview 2 highest pTmuons m CHAMBERS • Fast search for ‘high’ pT particles • Cut on global variables • L0 has 4 ms latency “L0 OBJECTS” Highest ETg, electron, p0, hadron candidates CALORIMETERS L0 decision unit L0DU SET CALORIMETERS GLOBAL VARIABLES z and # trks in 1st, 2nd vtx PILE-UP SYSTEM Charged particle multiplicity SPD, PILE-UP SYSTEM HCP2005 - Mitesh Patel

  8. Level 0: Muon Trigger • Search for high pT muons • Five muon stations M1-5 • Variable granularity • Projective geometry • 2 highest pT candidates per quadrant sent to L0 decision unit • p/p ~ 20% for b-decays • Typical Performance: ~88% efficiency on B→J/(µµ)X HCP2005 - Mitesh Patel

  9. Level 0: Calorimeter Trigger ECAL HCAL SPD-PreShower FE ECAL: 6000 cells, 8x8 to 24x24 cm2 HCAL: 1500 cells, 26x26, 52x52 cm2 • Look for high ET candidates in the calorimeters : • 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 of each type • Global variables • Total calorimeter energy • SPD multiplicity • Typical Performance: 30-50% efficiency on hadronic channels for about 700 kHz bandwidth ECAL HCAL Pre-Shower Detector (PS) Scintillator Pad Detector (SPD) Validation cards Selection crates g e± p0 p0 ETtot hadr SPD mult. HCP2005 - Mitesh Patel

  10. Level 0 : Performance • L0 Decision unit : • OR of high ET candidates • Applies cuts on global variables • Performance : • Efficiency ~50% for hadronic channels, 90% for m channels, 70% for radiative channels • 1% bb →3% after L0 • 8% cc →10% after L0 HCP2005 - Mitesh Patel

  11. The Level 1 Trigger: Overview Find high IP tracks (VErtex LOcator) Confirm track / Estimate pT from Trigger Tracker Link VELO tracks to L0-objects L1 algorithm run on PC Farm Average latency: 1 ms (max 50 ms) 2D (rz) tracking in VELO Primary Vertex search Allow up to 3 PV 2D track selection 0.15mm < IP < 3mm L0 m match • Multiple routes through L1 : • Generic Line : • L1-Variable: log(pt1)+log(pt2) • pt1,2 two highest pT tracks • Muon lines : • Single muon: pT>2.3 GeV, IP >0.15 mm • Dimuons: J/ ± 500 MeV window • OR : mµµ>500MeV and IP>0.05mm • OR : mµµ>2.5GeV • Photon, electron lines : • L1-Variable (relaxed) + ECAL>3.1 GeV 3D (rfz) VELO tracking Confirm IP L0 m match p, pT estimation Use VELO-TT track + fringe B field OR : VELO track + L0 muon L1 decision HCP2005 - Mitesh Patel

  12. Level 1: Event Reconstruction • L1 relies on the LHCbVErtexLOcator (VELO) : • Silicon tracker before the LHCb magnet • Angular coverage of full angular range of downstream detector • Sensors ~7mm away from beam, retractable (injection), in secondary vacuum • Foils protect against RF pickup from the LHC beam • 21 sensor stations : 2 R- and 2 f-measuring sensors per station • Gradual increase of pitch (40 mm to 103 mm) Si Sensors RF foils Interaction region fsensor R sensor 100 cm HCP2005 - Mitesh Patel

  13. Fast tracking strategy : ~70 tracks/event after L0 : perform tracking in R-z view (using only R sensors) Primary vertex σZ ~ 60 mm, σX,Y ~ 20 mm Select 2D tracks with 0.15 < IP < 3 mm → 8.5 tracks/event 3D tracking for selected tracks pT measurement using Trigger Tracker TT : two layers of Si detectors with 200 mm pitch Only 0.15Tm of B field between VELO and TT → DpT / pT ~ 20-40% allows rejection of low p tracks which can fake high IP Matched L0-m : →DpT / pT ~ 5% Level 1: Event Reconstruction Example: 2D tracks in 45o z-vertex histogram sz~60mm xy-vertex sx,y~20mm The Trigger Tracker HCP2005 - Mitesh Patel

  14. Level 1 : Performance • L1 decision : • Take OR- of the multiple routes through L1 • Tuned for retention of 4% of minimum bias L0 triggers (40 kHz L1 output rate) • Performance : eg. Generic L1-Variable: log(pt1)+log(pt2) log(pt1)+log(pt2) L0 efficiency L1 efficiency L0L1 eff • 3% bb after L0 → 16% after L1 • 10% cc after L0 → 18% after L1 HCP2005 - Mitesh Patel

  15. The High Level Trigger: Overview Inclusive D* Generic HLT Photons, electrons Loose D0→hh Bd → D*p D0→Kp, KK Bd → D0K* f →KK Bs → fg p, K Bs → ff Ds→KKp Exclusive HLT Bs → Dsp K*→Kp Bd → mmK* muons Loose dimuons B → J/yX Muon Highway Inclusive B→m Inclusive di-m • High Level Trigger (HLT) : • Generic Algorithm: repeat L1 then full readout of the detector • Muon Highway feeds inclusive muon modes • Form basic particles, composites, search for signatures of hot physics channels (exclusive), D* inclusive • 10ms to run (in 2007) – algorithm run on same CPU farm as L1 HCP2005 - Mitesh Patel

  16. HLT Generic • Redo “L1” with improved: • momentum resolution • muon matching • HLT generic reconstructs ~1/3 of tracks in the event and redoes L1 in ~4 ms • Reduces rate from 40 kHz input from L1 to 10 kHz: • 16% bb after L1 →38% after HLT Generic • 18% cc after L1→27% after HLT Generic • Then have ~24 ms for further HLT selections HCP2005 - Mitesh Patel

  17. HLT Exclusive • HLT Exclusive being tuned for ~10 core physics channels • In these channels cuts tuned to take ~15Hz MB / channel • B mass resolutions ~30 MeV • Mass windows >±500 MeV Bs→ DsK Bs→Dsp Bd→D*p Bd→pp Bs→KK Ds→KKp Bs→mm Bd→Kp Bs→pK B → hh reconstructed as B→pp HCP2005 - Mitesh Patel

  18. HLT : Performance • HLT still under study, efficiencies on L0L1 and offline selected events 60-90% • Philosophy to try and trigger channels in many ways • Further inclusive triggers will make us more robust to the unknown • Limited time available for online tracking → different to offline : inefficiency in high multiplicity channels – strategies being explored to resolve this … HCP2005 - Mitesh Patel

  19. Bs→ff selection on L0L1 and offline selected events : • Online tracking etracking = 0.73 • HLT selection cuts eselection = 0.97 → HLT exclusive selection efficiency e = 0.71 • Select only 3 of the 4 tracks (fK), use online RICH to control the background : • Online tracking etracking = 0.93 • HLT selection cutseselection = 0.94 → HLT exclusive selection efficiency e= 0.87 1200MeV 1000MeV Before RICH cut 4 track 3 track After RICH cut fK mass ff mass HCP2005 - Mitesh Patel

  20. Overall Trigger Performance Bs J/Y f~70% Bdp+p- ~37% Bs DsK~23% Level-1 Level-0 HLT Total 2 KHz bb 900 Hz cc 600 Hz HCP2005 - Mitesh Patel

  21. Trigger Robustness • Several scenarios considered : • Event multiplicity • Noise, misalignment, resolution • Increased material • LHC beam position • LHC background • Size of the CPU farm • The performance of L0 is stable within 10%, L1 is stable within 20% • The execution time and L1 event size is stable to within 30% HCP2005 - Mitesh Patel

  22. Real Time Trigger Challenge • But will it work … ?! • The Real Time Trigger Challenge : • Operate one (few) subfarms of the DAQ under realistic conditions • 44 double CPU boxes • Full-speed Data Input • Long-term operation (hours) • Exercise realistic Level-1/HLT code • Exercise/evaluate realistic overheads • Establish performance of ‘modern’ CPUs compared to (today’s) standard CERN • Happening now ! HCP2005 - Mitesh Patel

  23. Conclusions • LHCb will use three level of trigger to deliver inclusive and exclusive samples of B decays suitable for it’s physics goals : • Exclusive selection of core physics channels • Samples to allow calibration/alignment studies • Data that allows the trigger efficiencies to be determined • The L0 and L1 triggers are well developed and performance is good • The High Level Trigger continues to evolve as our understanding of the LHCb physics potential evolves • The trigger system is robust and flexible HCP2005 - Mitesh Patel

  24. Level 0: Pileup System B A Silicon r-sensors k’ k RB RA ZB ZA ZPV’ ZPV RA ZPV - ZA RB ZPV - ZB k = • Two planes of Silicon sensors upstream of the interaction point • Used to identify and reject multi-Primary-Vertex events : • Measure R coordinate (-4.2<<-2.9) • From hits on two planes  produce a histogram of z on beam axis • Remove hits contributing to largest peak, look for 2nd peak above threshold • L0 Decision Unit cuts on # of tracks in the second peak + hit multiplicity • Performance : • Vetoes 60% of double interactions keeping 95% of single interactions HCP2005 - Mitesh Patel

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