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The LHCb Trigger

The LHCb Trigger. Niko Neufeld CERN, PH. Outline. The 3 levels of the LHCb Trigger Level-0 hardware trigger Fully synchronous and pipe-lined (deadtime < 0.5%) Pile-up System Calorimeter and Muon Flexible L0 Decision unit Level-1 software trigger

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The LHCb Trigger

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  1. The LHCb Trigger Niko Neufeld CERN, PH

  2. Outline • The 3 levels of the LHCb Trigger • Level-0 hardware trigger • Fully synchronous and pipe-lined (deadtime < 0.5%) • Pile-up System • Calorimeter and Muon • Flexible L0 Decision unit • Level-1 software trigger • Partial read-out: Vertex Detector (VeLo), Trigger Tracker (TT) and L0 summary • High Level software trigger (HLT) • Full read-out: all detector data • Using the RICH in the High Level Trigger Common hardware Niko Neufeld LHCb Trigger, RICH-2004

  3. The LHCb trigger at a single glance Niko Neufeld LHCb Trigger, RICH-2004 ~ O(1) kHz

  4. Trigger Rates Overview HLT L1-confirmation HLT Full reconstruction Level-0 Level-1 Level-0 • 30 MHz pp x-ings • 10 MHz “visible” @2.1032 • Multiplicity/Pile-Up: 7 MHz • ET(m1,m2,h,e,g,p0): 1 MHz Level-1 • VELO: impact parameter • VELO+TT: momentum • VELO+L0-m: Mmm HLT • L1(VELO+TT)+T: 10 kHz • VELO+TT+T: dp/p<1%exclusive channels, full reconstruction for 200 Hz Niko Neufeld LHCb Trigger, RICH-2004

  5. Bandwidth (kHz) Adjusted for overlap Generic 30.0 (75.2%) 30.0 (75.2%) Single-muon 8.8 (22.1%) 3.0 ( 7.4%) Dimuon, general 1.7 ( 4.1%) 1.2 ( 3.0%) Dimuon, J/Psi 1.8 ( 4.6%) 1.1 ( 2.7%) Electron 3.9 ( 9.9%) 2.3 ( 5.9%) Photon 4.0 (10.0%) 2.3 ( 5.8%) Level-1 Decision Algorithm PT1,PT2 µµ µ γ e Parallel (overlapping) trigger lines PT1,PT2? PT1,PT2? Bandwidth division: L1J/ψ L1µµ L1µ L1γ L1PT L1e OR(L1) ⇒ L1 yes/no Overlaps are absorbed in this direction Niko Neufeld LHCb Trigger, RICH-2004

  6. Efficiency for generic L1 trigger Niko Neufeld LHCb Trigger, RICH-2004

  7. Key features of DAQ hardware • All detectors use standardized read-out boards (two variants  next slide) • Use commercial (mostly even commodity) components wherever possible: PCs, (copper) Gigabit Ethernet, Ethernet routers • Use the same infrastructure (network and computer farm) for L1 and HLT • Accommodate a soft real-time requirement: Level 1 latency no larger than 58 ms • Large system: 3000 Gigabit Ethernet links, 1800 PCs, several 100 Ethernet switches Niko Neufeld LHCb Trigger, RICH-2004

  8. The RICH readout board • Standardised read-out boards: 9U x 400 mmm • Gets data from detectors, from up to 48 optical links de-serialises, zero-suppresses, etc… • All boards are controlled by commercial Microcontroller (Creditcard-PC) • Data sent out by standardGigabit Ethernet Mezzanine card via up to 4 Gigabit Ethernet links (over copper) Niko Neufeld LHCb Trigger, RICH-2004

  9. Switch Switch Switch Switch Switch Readout Network SFC SFC SFC SFC SFC Switch Switch Switch Switch Switch CPU CPU CPU CPU CPU CPU CPU CPU CPU CPU CPU CPU CPU CPU CPU Software trigger Hardware (DAQ) Front-end Electronics Level-1Traffic HLTTraffic FE FE FE FE FE FE FE FE FE FE FE FE TRM 40 kHz 1.6 GB/s 1000 kHz 5.5 GB/s Multiplexing Layer TIER0 L1-Decision Sorter TFCSystem StorageSystem ~ 250 MB/s total 7.1 GB/s 94 SFCs Scalable in depth:more CPUs Scalable in width: more detectors in Level-1 CPUFarm ~1800 CPUs Niko Neufeld LHCb Trigger, RICH-2004

  10. Using the RICH in the High Level Trigger

  11. RICH in HLT: What does it get us & what does it cost us? • HLT involves exclusive selection of (many) B decay modesby checking the invariant-mass of combinations of 2–8 trackseg Bs  Ds+Ds- K+K-p+ K+K-p- • Long tracks (tracks having info from VeLo and T-stations) are used ~ 30 per eventIf only selection is on charge, then for Bs  Ds+Ds- ~ 156 combinations  107 • If K/p identification were available, the number of combinations to be checked would be substantially reduced ~ 24×102 103 combinations for Bs  Ds+Ds- example HLT could take less time per channel, if K/p ID is fast • 10 ms/event to run HLT on a CPU in 200720 ms/event for exclusive selections Niko Neufeld LHCb Trigger, RICH-2004

  12. First step: parameterise ring distortions so that problem can be solved on HPD planes avoiding the need to determine the Cherenkov angle for each pixel-track combination: Hit pixels shown on HPD detector planes, for a typical eventCrosses mark impact point of tracks (as if they were reflected) RICH-2 Niko Neufeld LHCb Trigger, RICH-2004

  13. Same event: now ray trace “fake” photons to the detector plane, emitted from each track at fixed qC = 30 mrad,uniformly distributed around azimuthal angle f: Niko Neufeld LHCb Trigger, RICH-2004

  14. Plot radius r vs f to calibrate the distortion of the rings • If plotted relative to the average radius r, all rings show  the same distortion: Dr = A cos 2f (A 2.5 mm) Niko Neufeld LHCb Trigger, RICH-2004

  15. Reconstructed Cherenkov angle for all long tracks passing through RICH-2, vs momentum (on log scale) e p p K Niko Neufeld LHCb Trigger, RICH-2004

  16. Same plot selecting out the true kaons onlyNote that below threshold, peak search finds ~ random qC Niko Neufeld LHCb Trigger, RICH-2004

  17. Determine the average ring radius for each track by ray tracing a few photons (currently 6) — fast • Then apply correction: qC = 30 r / (r-A cos 2f) mrad Ray traced photons Pixel hits from full simulation s= 0.1 mrad s= 0.7 mrad  ~ as good as offline resolution on qC Niko Neufeld LHCb Trigger, RICH-2004

  18. Plot reconstructed qC for all photons relative to a track • “Local” algorithm can be made by searching for peak, treating hits from other tracks as background • Scan over qC to find value that maximizes significance Single track All tracks Niko Neufeld LHCb Trigger, RICH-2004

  19. Cutting the pion band : Selecting the kaon band : RICH 2 RICH 2 Cut on (rS/B max-rtrue pion) Cut on (rS/B max-rtrue kaon) Niko Neufeld LHCb Trigger, RICH-2004

  20. Local HLT algorithms: performance Kaon-ID efficiency (purple)/ pion misid (blue)using gas info Cutpionband Selectkaonband Good performance & fast Niko Neufeld LHCb Trigger, RICH-2004

  21. Global HLT algorithm: principle Implement global LogLikelihood with some simplifications with respect to full glory of “offline” reconstruction: • Cherenkov angle calculated on HPD plane (discussed) • Do not use aerogel (for the moment) • Do not calculate contribution of every pixel to every track; • rather assign pixels to ring image closest to track • Only consider pion vs kaon hypotheses in LL Main advantages of this w.r.t. local approach: • Simultaneous treatment of signal and background • Method much better suited to looking for below threshold • kaons, which is a v. challenging problem for local method Niko Neufeld LHCb Trigger, RICH-2004

  22. Global HLT algorithm: performance Online Offline e(KK) = 88% e(pK) = 15% e(KK) = 92% e(pK) = 11% Good (but not exact) correlation with offline. Speed 11 ms (1 GHz PIII) per event when only “long” tracks are used. Promising! Niko Neufeld LHCb Trigger, RICH-2004

  23. Summary • LHCb uses a 3 level trigger system • Two levels of software trigger provide maximum flexibility at high rate • RICH information is available in the trigger and potentially very useful • Fast algorithms are being developed and look promising • We are currently implementing, deploying and eagerly awaiting 2007 Niko Neufeld LHCb Trigger, RICH-2004

  24. Acknowledgements • The work of many people has been presented in this talk • I would like thank in particular the LHCb RICH, Online and Electronics groups Niko Neufeld LHCb Trigger, RICH-2004

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