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Trigger Strategy and Performance of the LHCb Detector

Trigger Strategy and Performance of the LHCb Detector. Mitesh Patel (CERN) (on behalf of the LHCb Collaboration) Thursday 7 th July 2005. Introduction. LHCb experimental goals : Precision measurements of CP Violation in B decays

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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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