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The CMS Trigger System

The CMS Trigger System. Chris Seez, Imperial College, London IV International Symposium on LHC Physics and Detectors Fermilab, 1 st -3 rd May 2003. Physics Selection at LHC. The CMS Trigger. Formidable task: Bunch crossing rate  permanent storage rate for events with size ~1MB

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The CMS Trigger System

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  1. The CMS Trigger System Chris Seez, Imperial College, London IV International Symposium on LHC Physics and Detectors Fermilab, 1st-3rd May 2003

  2. Physics Selection at LHC IV International Symposium on LHC Physics and Detectors

  3. The CMS Trigger • Formidable task: • Bunch crossing rate  permanent storage rate for events with size ~1MB • 40MHz  O(102)Hz • CMS design: • Beyond Level-1 there isa High Level Trigger running on a single processor farm IV International Symposium on LHC Physics and Detectors

  4. High Level Trigger • Advantages of using processor farm for all selection beyond Level-1: • Benefit maximally from evolution of computing technology • Flexibility: no built-in design or architectural limitations — maximum freedom in what data to access and in sophistication of algorithms • Evolution, including response to unforeseen backgrounds • Minimize in-house elements • cost • maintainability IV International Symposium on LHC Physics and Detectors

  5. CMS DAQ and Trigger System • Event size: 1MB from ~700 front-end electronics modules • Level-1 decision time: ~3s — ~1s actual processing(the rest in transmission delays) • DAQ designed to accept Level-1 rate of 100kHz • Modular DAQ: 8 x 12.5kHz units • HLT designed to output O(102)Hz – rejection of 1000 • ~1000 processor units IV International Symposium on LHC Physics and Detectors

  6. Software for simulation and reconstruction • Full GEANT3 simulation of CMS detector • Digitization and reconstruction in C++ code • Many samples digitized at both 2x1033 and 1034 • Digitization includes both in- and out-of-time pileup (i.e. min-bias type events in the same and neighbouring bunch-crossings) • Results presented in full in CMS DAQ TDR (Dec 2002) • Results presented here for rates, efficiencies of the complete CMS trigger system (Level-1 + HLT) use event samples comprising ~7M events produced in 2002 IV International Symposium on LHC Physics and Detectors

  7. Level-1 Trigger

  8. Level-1 Trigger • Information from Calorimeters and muon detectors • Electron/photon triggers • Jet and missing ET triggers • Muon triggers • Synchronous, pipelined • Time needed for decision (+its propagation) ≈ 3 s • Bunch crossing time = 25 ns • Algorithms run on coarse local data • Only calorimeter and muon information • Special-purpose hardware (ASICs), but also wide use of FPGAs • Backgrounds are huge • Large rejection factor: is 40MHz (x20 ev/crossing)  100kHz (≈ 8,000) • Rates: steep functions of thresholds IV International Symposium on LHC Physics and Detectors

  9. EISO SORT ASICs EISO Calorimeter Trigger • 18 Calorimeter trigger crates • ≈ 4000 Gb/s serial links • 224 inputs/crate • 18 bits/(trigger tower) • 32 towers/card • ASICs: process 8 or 16 towers IV International Symposium on LHC Physics and Detectors

  10. Electron/photon Trigger • Electromagnetic trigger based on 3x3 trigger towers • Each tower is 5x5 crystals in ECAL (barrel; varies in end-cap) • Each tower is single readout tower in HCAL Cuts put on: - e/h fraction - Fine shape in ECAL (acts as local isolation) - Isolation in both ECAL and HCAL sections Trigger threshold on sum of two towers IV International Symposium on LHC Physics and Detectors

  11. Electron/photon Trigger Response to electrons: Rate (jet background): Top 4 candidates in each category passed to global trigger IV International Symposium on LHC Physics and Detectors

  12. Jet and  Triggers • Single, double, triple and quad thresholds possible • Possible also to cut on jet multiplicities • Also ETmiss, SET and SET(jets) triggers • Sliding window: • granularity is 4x4 towers • = trigger region • jet ET summed in 3x3 regions • , = 1.04 “-like” shapes identified for  trigger IV International Symposium on LHC Physics and Detectors

  13. L = 1034 cm-2s-1 |h| < 2.1 Muons • Issue is pT measurement of real muons IV International Symposium on LHC Physics and Detectors

  14. Muon trigger • Level-1 m-trigger info from: • Dedicated trigger detector: RPCs (Resistive plate chambers) • Excellent time resolution • Muon chambers with accurate position resolution • Drift Tubes (DT) in barrel • Cathode Strip Chambers (CSC) in end-caps • Bending in magnetic field  determine pT IV International Symposium on LHC Physics and Detectors

  15. Drift tube and CSC trigger Drift Tubes CSC • Extrapolation: using look-up tables • Track Assembler: link track segment-pairs to tracks, cancel fakes Implementation: ASICs for Trigger Primitive Generators FPGAs for Track Finder processors IV International Symposium on LHC Physics and Detectors

  16. Level-1 muon global trigger • Information from different detectors combined(RPC, CSC and DT) • Match muon candidates from different systems • Different sub-systems complement one another • Maximize efficiency, minimize rate • Identify 4 “best” muons and pass them on to the Global Trigger 1034 cm-2s-1 IV International Symposium on LHC Physics and Detectors

  17. Modular DAQ Level-1 Settings and Rates… • Current CMS plan is for phased installation of DAQ • Startup (L=2x1033 cm-2s-1): can handle 50kHz • High luminosity (L=1034 cm-2s-1): can handle 100kHz • Model assumes safety factor of three • To account for simulation uncertainties, and beam conditions… • Startup (L=2x1033 cm-2s-1): set thresholds for 16kHz • High luminosity (L=1034 cm-2s-1): set thresholds for 33kHz • Start iteration by allocating the rate equally between:  Electrons/photons;  Muons;  Tau-jets;  Jets and combined triggers • Priority: guarantee discovery physics • Then choose allocation between single and double objects, etc IV International Symposium on LHC Physics and Detectors

  18. Choice of operating point • Example of electrons • Look at efficiency to trigger on Zee versus efficiency to trigger on We IV International Symposium on LHC Physics and Detectors

  19. Level-1 Trigger table (2x1033) IV International Symposium on LHC Physics and Detectors

  20. Level-1 Trigger table (1034) IV International Symposium on LHC Physics and Detectors

  21. High-Level Trigger

  22. High-Level Trigger • Runs on CPU farm • Code as close as possible to offline reconstruction code • Ease of maintenance • Able to include major improvements in offline reconstruction • Selection must meet CMS physics goals • Output rate to permanent storage limited to O(102)Hz • Reconstruction on demand • Reject as soon as possible • Hence trigger “Levels”: • Level-2: use calorimeter and muon detectors • Level-2.5: also use tracker pixel detectors • Level-3: includes use of full information, including tracker • And “regional reconstruction”: e.g. tracks in a given road or region IV International Symposium on LHC Physics and Detectors

  23. Pixel L_1 D Pixel L_2 e t Si L_1 e c t o ECAL r HCAL Pixel L_1 D Pixel L_2 e t Si L_1 e c t o ECAL r HCAL 14 HLT regional reconstruction Regional rather than Global reconstruction • Slices must be of appropriate size • Need to know where to start reconstruction (seed) • Seeds from Level-1: • e/g triggers • m triggers • Jet triggers • Seeds ≈ absent: • Other side of lepton • Global tracking • Global objects (ET, ETmiss) IV International Symposium on LHC Physics and Detectors

  24. HLT requirements and operation • Boundary conditions: • Code runs in a single processor, which analyzes one event at a time • HLT has access to full event data (full granularity and resolution) • Only limitations: • CPU time • Output selection rate (~102 Hz) • Precision of calibration constants • Main requirements: • Satisfy physics program: high efficiency • Selection must be inclusive (to discover the unpredicted as well) • Must not require precise knowledge of calibration/run conditions • All algorithms/processors must be monitored closely IV International Symposium on LHC Physics and Detectors

  25. HLT selection: , , jets and ETmiss • Muons • Successive refinement of momentum measurement; + isolation • Level-2: reconstructed in muon system; must have valid extrapolation to collision vertex; + calorimeter isolation • Level-3: reconstructed in inner tracker; + tracker isolation • -leptons • Level-2: calorimetric reconstruction and isolation • Very narrow jet surrounded by isolation cone • Level-3: tracker isolation • Jets and Etmiss • Jet reconstruction with iterative cone algorithm • ETmiss reconstruction (vector sum of towers above threshold) IV International Symposium on LHC Physics and Detectors

  26. Level-1 ECAL reconstruction Threshold cut Level-2 Level-2.5 Pixel matching Level-3 Electrons Track reconstruction E/p, matching (Dh) cut Photons Threshold cut Isolation HLT selection: electrons and photons • Issue is electron reconstruction and rejection • Higher ET threshold on photons • Electron reconstruction • key is recovery of radiated energy • Electron rejection • key tool is pixel detector IV International Symposium on LHC Physics and Detectors

  27. super-cluster basic cluster Electron selection: Level-2 • “Level-2” electron: • Search for match to Level-1 trigger • Use 1-tower margin around 4x4-tower trigger region • Bremsstrahlung recovery “super-clustering” • Select highest ET cluster • Brem recovery: • Road along f— in narrow -window around seed • Collect all sub-clusters in road  “super-cluster” IV International Symposium on LHC Physics and Detectors

  28. Full pixel system Staged option Electron selection: Level-2.5 • “Level-2.5” selection: use pixel information • Very fast, large rejection with high efficiency (>15 for e=95%) • Before most material  before most bremsstrahlung, and before most conversions • Number of potential hits is 3: demanding  2 hits quite efficient IV International Symposium on LHC Physics and Detectors

  29. Electron selection: Level-3 • “Level-3” selection • Full tracking, loose track-finding (to maintain high efficiency) • Cut on E/p everywhere, plus • Matching in h (barrel) • h/e (endcap) • Optional handle (used for photons): isolation 2x1033 cm-2s-1 IV International Symposium on LHC Physics and Detectors

  30. HLT table • Issues: • Purity of streams is not the same (e.g. electrons vs muons) • Kinematic overlap provides redundancy • To answer the sort of question, when a problem is under investigation in Wen: do we see this in the muons? • Comparisons of unlike things: • How much more bandwidth should go to lower-pT muons than to electrons? • How should one share the bandwidth between jet*ETmiss and di-electrons? • Only final guidance is efficiency to all the known channels • While keeping the selection inclusive • This is online: events rejected are lost forever. IV International Symposium on LHC Physics and Detectors

  31. HLT Summary: 2x1033 cm-2s-1 IV International Symposium on LHC Physics and Detectors

  32. HLT performance — signal efficiency • With previous selection cuts IV International Symposium on LHC Physics and Detectors

  33. CPU time usage • All numbers for a 1 GHz, Intel Pentium-III CPU • Total: 4092 s for 15.1 kHz  271 ms/event • Therefore, a 100 kHz system requires 1.2x106 SI95 • Expect improvements, additions. Time completely dominated by muons (GEANE extrapolation) – this will improve • This is “current best estimate”, with ~50% uncertainty. IV International Symposium on LHC Physics and Detectors

  34. HLT summary • Today: need ~300 ms on a 1GHz Pentium-III CPU • For 50 kHz, need 15,000 CPUs • Moore’s Law: 2x2x2 times less time (fewer CPUs) in 2007 • Central estimate: 40 ms in 2007, i.e. 2,000 CPUs • Thus, basic estimate of 1,000 dual-CPU boxes in TDR • (Note: not an excess of CPU, e.g. no raw-data handling) • Start-up system of 50kHz (Level-1) and 105 Hz (HLT) can satisfy basic “discovery menu” • Some Standard Model physics left out; intend to do it, at lower luminosity and pre-scales as luminosity drops through fill • Examples: inclusion of B physics (can be done with high efficiency and low CPU cost; limitation is Level-1 bandwidth); details in TDR [see talk by Vitalliano Ciulli]. Also low-mass di-jet resonances. • Single-farm design works IV International Symposium on LHC Physics and Detectors

  35. Overall Summary • Using a full and detailed simulation of the CMS trigger (Level-1 + HLT) a model trigger table has been developed which: • Meets target rates for Level-1 • and for final output to permanent storage • While maintaining high efficiency for signal events • and wide inclusive selection (open to the unexpected) • The system outlined has huge flexibility • This is only the beginning — there are many challenges ahead • Final tuning will clearly be done with the final event generator: LHC collisions IV International Symposium on LHC Physics and Detectors

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