The LHCb Trigger Niko Neufeld CERN, PH It is sometimes said that LHCb is a trigger and a RICH. Even if that’s an oversimplification it is a good pretence for this complementary talk at a RICH workshop

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 Visible cross section 100 p Niko Neufeld LHCb Trigger, RICH-2004

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

Trigger Rates Overview 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 HLT L1-confirmation HLT Full reconstruction Level-0 Level-1 Niko Neufeld LHCb Trigger, RICH-2004

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

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

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

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 standard Gigabit Ethernet Mezzanine card via up to 4 Gigabit Ethernet links (over copper) Niko Neufeld LHCb Trigger, RICH-2004

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

Using the RICH in the High Level Trigger

RICH in HLT: What does it get us & what does it cost us? HLT involves exclusive selection of (many) B decay modes by checking the invariant-mass of combinations of 2–8 tracks eg Bs  Ds+Ds-  K+K-p+ K+K-p- Long tracks (tracks having info from VeLo and T-stations) are used ~ 30 per event If 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 2007 20 ms/event for exclusive selections Niko Neufeld LHCb Trigger, RICH-2004

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: RICH-2 Hit pixels shown on HPD detector planes, for a typical event Crosses mark impact point of tracks (as if they were reflected) Niko Neufeld LHCb Trigger, RICH-2004

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

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

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

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

Then apply correction: qC = 30 r / (r - A cos 2f) mrad 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

Single track All tracks 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

Selecting the kaon band : 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

Local HLT algorithms: performance Kaon-ID efficiency (purple) / pion misid (blue) using gas info Cut pion band Select kaon band Good performance & fast Niko Neufeld LHCb Trigger, RICH-2004

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

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

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

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

Backup Slides

Data Flow 1MHz Level 0 trigger Readout Network Switch Switch Switch HLT Traffic Level-1 Traffic Front-end Electronics FE FE FE FE FE FE FE FE FE FE FE FE TRM 323 Links 4 kHz 1.6 GB/s 126 Links 44 kHz 5.5 GB/s Multiplexing Layer 62 Switches Switch Switch Switch Switch Switch 29 Switches 64 Links 88 kHz 32 Links Readout Network L1-Decision Sorter TFC System Storage System 94 Links 7.1 GB/s SFC Switch CPU SFC Switch CPU SFC Switch CPU 94 SFCs SFC Switch CPU SFC Switch CPU CPU Farm Gb Ethernet Level-1 Traffic Mixed Traffic HLT Traffic ~1800 CPUs Niko Neufeld LHCb Trigger, RICH-2004

L1 efficiencies overview Offline selected events: (1.55 +/- 0.18)% - Why so low? Reconstructible events: Hadrons trigger e and ! 4-prong give best generic efficiency! Niko Neufeld LHCb Trigger, RICH-2004

LHCb Software triggers Level-1 HLT Input rate 1 MHz Stage 1) 40 kHz Stage 2) 10 kHz Data used VeLo, TT, L0 summary Stage 1) like L1 Stage 2) all Output rate 40 kHz O(2 kHz) out of which 200 Hz fully reconstructed Mean time on 2007 CPU 1 ms 10 ms (total) 10 ms (Stage 2) Maximum allowed time 58 ms (data transport included) not applicable (limited only by available CPU power) Niko Neufeld LHCb Trigger, RICH-2004

Level-0: B-signatures m Global event variables: Trigger on B’s: reject “complicated” events Trigger on B’s: thresholds on highest ET: h-cluster e-cluster g-cluster p0-cluster HCAL clusters m Or highest pT m’s >90% p/K decay Algorithm: at least one cluster/m >threshold and “Global”<threshold or Di-muon>threshold Nominal threshold Nominal threshold Niko Neufeld LHCb Trigger, RICH-2004

Dedicated HLT RICH algorithms First step: parameterise ring distortions so that problem can be solved on HPD planes, without having to solve quartic equation for each pixel-track combination: RICH 2 Study distortion by plotting local radius - mean radius vs photon azimuth. Works well – but not so good for aerogel Niko Neufeld LHCb Trigger, RICH-2004

Local HLT algorithms: principle Plot Cherenkov angle for nearby hits to track of interest and look for peak – below shown integrated over many tracks Aerogel Gas, R2 Gas R1 Method OK for gas rings, but is not so good (yet) for aerogel Niko Neufeld LHCb Trigger, RICH-2004

Selecting the kaon band : Cutting the pion band : Selecting the kaon band : RICH 1 RICH 1 Cut on (rS/B max-rtrue pion) Cut on (rS/B max-rtrue kaon) Niko Neufeld LHCb Trigger, RICH-2004