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The CMS High-Level Trigger

The CMS High-Level Trigger. Leonard Apanasevich University of Illinois at Chicago On behalf of the CMS collaboration. Outline. Introduction to the CMS Detector Trigger Challenges at the LHC Trigger Architecture Level-1 Trigger High Level Trigger HLT Processing Times

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The CMS High-Level Trigger

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  1. The CMS High-Level Trigger Leonard Apanasevich University of Illinois at Chicago On behalf of the CMS collaboration

  2. Outline • Introduction to the CMS Detector • Trigger Challenges at the LHC • Trigger Architecture • Level-1 Trigger • High Level Trigger • HLT Processing Times • Trigger Commissioning with Cosmic Data • Trigger Rates and Menus for Startup ICCMSE 2009

  3. The Compact Muon Solenoid (CMS) General purpose pp collider experiment: Searches for Higgs bosons, other new particles (SUSY,...) and new phenomena; Precision measurement of SM parameters like top and W masses, ...; Heavy ion program. For details see e.g.: The CMS experiment at theCERN LHC, JINST 0803:S08004,2008; ICCMSE 2009

  4. The Large Hadron Collider ICCMSE 2009

  5. Trigger Challenges at the LHC • At design L = 1034 cm-2s-1 • Interaction rate: ~1 GHz • ~20 pp events per 25 ns crossing • 1 kHz W events • 10 Hz top events • < 104 detectable Higgs decays/year • Event size: ~1 MB (≈75M channels) → 1000 TB/sec for 1 GHz input rate • ~300 MB/sec affordable • Enormous rate reduction necessary! • but without losing interesting physics • Select in stages: • Level-1 Triggers • 1 GHz to 100 kHz (1/10000 reduction) • High Level Triggers • 100 kHz to 100 Hz (1/1000 reduction) ICCMSE 2009

  6. Trigger Architecture Detectors Detectors Lvl-1 Front end pipelines Lvl-1 Front end pipelines Readout buffers Readout buffers Lvl-2 Switching network Switching network Lvl-3 HLT Processor farms Processor farms CMS: 2 physical levels “Traditional”: 3 physical levels • CMS has a two-tiered system to handle the LHC challenge • Level-1 trigger reduces rate from 40 MHz (xing freq) to 100 kHz (max) • High-Level triggers reduce rate from 100 kHz to O(100 Hz) • Progress in networking/switching has justified CMS choice ICCMSE 2009

  7. Level 1: Only Calorimeter & Muon ComplexAlgorithms Hugeamounts ofdata Simple Algorithms Small amounts of data ICCMSE 2009

  8. The Level-1 Trigger Electrons, Photons, Jets, MET Muons • Reduce data rate from 40 MHz to 100 kHz while keeping the interesting physics events • Custom electronic boards and chips • Selects muons, electrons, photons, jets • ET and location in detector • Also Missing ET, Total ET, HT, and jet counts • Total decision latency: 3.2 s • 128 Level-1 trigger bits. Bits can be set using logical combinations of Level-1 objects. 3<||<5 ||<3 ||<3 ||<2.1 0.9<||<2.4 ||<1.2 ICCMSE 2009

  9. Calorimeter Trigger Geometry Trigger towers: =  = 0.087 HCAL ECAL HF EB, EE, HB, HE map to 18 RCT crates Provide e/g and jet, t, ET triggers ICCMSE 2009

  10. Muon Trigger Geometry • DT: drift-tube system • CSC: cathode strip chamber system • RPC: resistive plate chamber system Initial coverage of RPC is staged to <1.6 Initial coverage of CSC 1st station is staged to <2.1 ICCMSE 2009

  11. Level-1 Trigger Menu • Determined by the physics priorities of the experiment • L1 Menu optimized to fit within the L1 bandwidth • Designed with a safety factor of 3 in mind • underestimation of input cross sections, poor beam conditions, detector performance, etc. • Realistic menu including double and mixed triggers for specific physics channels • Two L1 menus currently available • L = 8e29 cm-2s-1 (day-1 menu) • L = 1e31 cm-2s-1 (MC studies) ICCMSE 2009

  12. High Level Trigger Strategy 40 MHz 100 kHz ~150 Hz ICCMSE 2009

  13. High Level Trigger (HLT) 1 PB/s • High-Level triggers reduce rate from 100 kHz to O(150 Hz) • HLT does event reconstruction “on demand” seeded by the L1 objects found, using full detector resolution • Algorithms are essentially offline quality but optimized for fast performance • Farm of Processors • ONE event, ONE processor • High Latency • Simpler I/O • Sequential programming 300 MB/s ICCMSE 2009

  14. HLT Algorithm Design L1 seeds L2 unpacking (MUON/ECAL/HCAL) Local Reco (RecHit) L2 Algorithm Filter L2.5 unpacking (Pixels) Local Reco (RecHit) L2.5 Algorithm • Each HLT trigger path is a sequence of modules • Processing of the trigger path stops once a module returns false • Reconstruction time is significantly improved by doing regional data-unpacking and local reconstruction across HLT • All algorithms regional • Seeded by previous levels (L1, L2, L2.5) “Local”: using one sub-detector only “Regional”: using small (η, φ) region ICCMSE 2009

  15. CMS Trigger/DAQ Evolution • CMS DAQ is a number of functionally identical, parallel, small DAQ systems • Build up 512512 switch from 8 6464 switches (DAQ slices) • HLT filter farm is currently comprised of 720 dual-quad core, 2.6 GHz, 16 GB memory Dell PE 1950 PC’s • 7 out of 8 cores (~5000) dedicated to HLT processing • Remaining processor does event building, acts a resource broker, and other services. ICCMSE 2009

  16. m triggers: performance • Signal efficiency (eHLT/[eL1*A]) • Background rates atL = 1032 cm-2 s-1 Threshold: double m non-isolated Threshold: single m isolated Threshold: single m non-isolated Di-muon Single muon ICCMSE 2009

  17. e/g triggers: performance Background rates at L= 1032 cm-2 s-1 • Efficiency on benchmark channels (estimated from MC simulation) • Electrons: • Photons: Electrons Photons ICCMSE 2009

  18. Jets and Missing ET Rates for L= 1032 cm-2 s-1 • Jets: • At HLT level, using an iterative cone algorithm with cone radius: out of calorimeter “towers” (HCAL + projection on ECAL - deposits above certain thresholds) • ETmiss: • Algebraic sum of “transverse energies” (ET = E sinq) of calorimeter objects • Objects can be individual calorimeter cells or jets • Can be used in combination with 1 or more jets ICCMSE 2009

  19. t- and b-tagging at HLT • Hadronict decays: • Level 1: seed from calorimeter towers in a narrow region • Level 2: HLT jet reconstructions and ECAL isolation • Level 3: Track isolation using pixel tracks in a smaller rectangular region or full tracks in a larger region • b-tagging triggers: • b-lifetime: • Jet requirements + global pixel track reconstruction to determine primary vertex at HLT level  requirement of at least 2 tracks with transverse impact parameter significance above a certain threshold • b → μ: • Jet requirements + L2/L3 muon reconstruction  requirements on DR(m-jet) and pT(m) relative to the jet direction ICCMSE 2009

  20. HLT TimingConsiderations Allowable CPU performance: • L1 input rate / number of processors • @ 50 kHz: 100 ms/evt • @ 100 kHz: 50 ms/ evt HLT Timing is influenced by: • Trigger menu (L1T & HLT) • Determined by physics priorities • Input L1Trigger rate • Limited by bandwidth • Parameters of various L1T algorithms, e.g., H/E • HLT algorithms and configuration • Standard trigger paths at HLT seeded by L1 trigger bits • Order of modules and filters in a path • Parameters of the modules and filters ICCMSE 2009

  21. HLT Processing Times • Average time needed to run full Trigger Menu on L1-accepted events: 43 ms/event • Core 2 5160 Xeon processor running at 3.0 GHz • CPU times strongly dependent on HLT input • “Tails” have a significant impact on the average time • Will eliminate with time-out mechanism ICCMSE 2009

  22. Trigger Commissioning • CMS has collected over 300 million cosmic ray events • Can use these data for: • compare data and L1 emulation • check synchronization of signals from the various sub-detectors • Measure trigger rates from cosmic rays and from noise • Evaluate trigger efficiencies • Compare resolutions between trigger and reconstructed objects ICCMSE 2009

  23. Trigger Commissioning (II) Synchronization of signals from different detector sub-systems demonstrated • L1 emulator: bit by bit software emulation of the L1 trigger • Excellent agreement between emulation and data L1 trigger efficiency as a function of offline reconstructed pT ICCMSE 2009

  24. Trigger Menu for Startup Trigger Rates by Object • Muons: • L1/L2/L3 muons run unprescaled at pT=20/9/3 GeV • Electrons: • No isolation requirements at L1 • Unprescaled at pT=10 GeV with large pixel-matching window (LW) • Photons: • No isolation requirements at L1 • Unprescaled at pT=15 GeV with no isolation requirement • Jets: • No L1/HLT jet corrections applied at startup • Unprescaled at pT=30 GeV (~60 GeV corr.) • MinBias: • Several algorithms installed based upon HF-tower ET over threshold, HF ET ring sums, Ecal ET, and pixel triplets • Muon: 33 Hz • Electron: 30 Hz • Photon: 9 Hz • Jet: 36 Hz • MET & HT: 5 Hz • B-Tau: 4 Hz • MinBias: 14 Hz • Cosmics/Halo: 7 Hz • Total: 138 Hz Rates for L=8e29 cm-2s-1 ICCMSE 2009

  25. Summary LHC Trigger is Challenging • A new crossing every 25 ns with ~ 20 events at design luminosity  1 GHz of events input • All data stored 3 s then all but 50-100 kHz rejected • Rejection of 99.99% of data without losing discovery physics! • Remaining event filtering done on a farm of CPUs running offline quality reconstruction algorithms • Rate of storage to archive is ~ 150 Hz • Rejection of 99.99997% of data without losing discovery physics!! • Fast selection and high efficiency is obtained for the physics objects and processes of interest • Commissioning work already going on using Cosmic data • HLT and L1 have been developed for early luminosities • The overall CPU requirement is within the system capabilities CMS trigger system is ready for collisions!!! ICCMSE 2009

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