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Optimizing the CMS Trigger System for High-Energy Physics at the LHC

This paper presents an in-depth analysis of the CMS Trigger System used in particle physics experiments at the Large Hadron Collider (LHC). It discusses the challenges posed by high bunch crossing rates and the need for efficient data processing and storage. The system employs a High Level Trigger (HLT) with a processor farm that allows for flexibility and sophisticated algorithm applications while minimizing in-house elements to reduce costs. Key simulation and reconstruction techniques, including the use of GEANT3, as well as various trigger algorithms for electron, photon, jet, and muon detection, are detailed to highlight their contributions to successful physics data selection.

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Optimizing the CMS Trigger System for High-Energy Physics at the LHC

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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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