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Commissioning and Performance of the CMS High Level Trigger

Commissioning and Performance of the CMS High Level Trigger. Leonard Apanasevich University of Illinois at Chicago f or the CMS collaboration. Introduction. Outline:. Overview of the CMS Trigger System Trigger Commissioning in 2010 Trigger Rates and Performance HLT Processing Times.

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Commissioning and Performance of the CMS High Level Trigger

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  1. Commissioning and Performance of the CMS High Level Trigger Leonard Apanasevich University of Illinois at Chicago for the CMS collaboration

  2. Introduction Outline: • Overview of the CMS Trigger System • Trigger Commissioning in 2010 • Trigger Rates and Performance • HLT Processing Times • At design L = 1034cm-2s-1 • ~ 1 GHz input rate • 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 Trigger is essential TIPP 2011

  3. The CMS Detector TIPP 2011

  4. CMS Trigger System • The CMS trigger system is structured in two levels: Level-1 (L1) and HLT • Level-1trigger reduces the rate from 40 MHz to 100 kHz (max) • Custom-designed electronic boards and chips • High-Level trigger further reduces the rate to O(200 Hz) • Software-based using the full granularity of the CMS detector • Runs offline-quality algorithms running on a ~5K processor filter farm TIPP 2011

  5. The Level-1 Trigger Electrons,Photons,Jets,METMuons • Runs synchronously at the LHC clock rate of 40 MHz • Selects muons, electrons, photons, jets • ET and location in detector • Also Missing ET, Total ET, HT, and jet counts • 128 Level-1 trigger bits. Bits can be set using (up to 128) logical combinations of Level-1 objects • e.g: • Total decision latency: 3.2 s 3<||<5 ||<3 ||<3 ||<2.1 0.9<||<2.4 ||<1.2 For details about the Level-1 system and it’s performance, see Thursday's talk by P. Klabbers TIPP 2011

  6. High Level Trigger • The HLT runs as a dedicated configuration of the CMS reconstruction software on a 672 node DAQ filter farm • Dual 4-core machines • 2.66 GHz, 16 GB Ram • 1 Master / 7 Event Processors • With a nominal input rate of 100 kHz, running on 4704 processes in parallel, each process has available an average of 47 ms to • read the input data • run all the trigger algorithms (~150 at the end of 2010; currently, ~300) • take the final accept/reject decision • stream the data to the Storage Managers • Nominal output rate ~200 Hz • For comparison, offline reconstruction takes ~5 sec per event TIPP 2011

  7. 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 runs independently from the others • Processing of a 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 TIPP 2011

  8. Trigger Commissioning in 2010 • CMS trigger menu was continuously adopting to the LHC conditions • Developed ~12 trigger menus covering a large range of luminosity scenarios over the course of the 2010 run • Low luminosity regime: • e.g.: L=1E28,1E29,4E29 Hz/cm2trigger menus • Trigger decision based on simple threshold cuts • Large fraction of the bandwidth (~30%) reserved for calibration and minimum bias triggers to ensure complete understanding of the detector performance • High(er) luminosity regime: • e.g.:2E31, 6E31, 2E32 menus • More elaborate paths: require isolation and identification besides thresholds TIPP 2011

  9. Trigger Menu Development • Using data to develop trigger menus for higher instantaneous luminosities • Most paths exhibit fairly linear behavior vs luminosity • Extrapolation errors minimized by using most recent data to keep the rate non-linearities under control • Large discrepancies understood to be due to paths with significant cosmic/noise components or depending on the beam conditions (beam gas and beam halo) • Menus were prepared for each twofold increase in luminosity • Target rate between 200 and 400Hz TIPP 2011

  10. CMS Web Based Monitoring • Summary of all HLT paths • Similar page available for L1 A dedicated talk on the WBM system will be given on Monday ECAL Problem (Muon trigger unaffected) Sync problem TIPP 2011

  11. Lessons Learned… • Overplan • never trust the machine schedule ! • always be ready for an extra factor 2-3 in luminosity • Involve • HLT menus are increasing in complexity with luminosity • need a tighter involvement/collaboration with the physics users • Document • the most common questions from users are: • “when was [insert favorite trigger here] first deployed?” • “why is it prescaled?” TIPP 2011

  12. HLT Performance TIPP 2011

  13. Performance of Jet Triggers • Jets at HLT were reconstructed using an iterative cone algorithm with cone size R = 0.5 • Switched to Anti-kT (R=0.5) in 2011 • Jet algorithm is identical to the one used in the offline analysis • Efficiency for offline reconstructed jets to pass HLT jet triggers with pT threshold = 15, 30, and 50 GeV • Shown for Barrel and Endcap regions Comparison with MC TIPP 2011

  14. Performance of MET Triggers • Missing ET calculated from algebraic sum of transverse energies of calorimeter objects plus muons • Good agreement between MC and data L1 MET (thresh=20 GeV) efficiency vs. Offline MET HLT MET (thresh=45 GeV) efficiency vs. Offline MET TIPP 2011

  15. HLT Muon Reconstruction • First Stage: • Confirm L1 “seeds”: refit hits in the muonchambers with full granularity of the detector • Kalman filter iterative technique • Reconstruction in L1 regions of interest • Second Stage: • Inclusion of tracker hits • Regional tracker reconstruction • Combine 1st-stage objects to charged particle tracks in the tracker • pTresolution much better compared to 1st stage • Optional: Isolation in calorimeters (at 1st Stage) and tracker (at 2nd Stage) TIPP 2011

  16. Performance of Muon Triggers • Efficiency for a high-quality offline reconstructed muon matched to L1 object to pass the HLT single muon trigger with a threshold of pT > 3 GeV, plotted as a function of pT • Events collected with the minimum bias trigger • Lower than expected efficiency due to time calibration at start-up Endcaps Barrel TIPP 2011

  17. HLT e/γ Reconstruction • Common stage for Photons and Electrons: • Spatial matching of energy deposits (clusters) in the Electromagnetic Calorimeter (ECAL) with e/γcandidates at L1 • Form super-clusters (group of clusters; bremsstrahlung/conversions recovery) • ET cut applied • ECAL super-cluster shape consistent with an electromagnetic object • Calorimetric (ECAL+HCAL) isolation • Photons • Tight track isolation in a solid cone • Electrons: • Matching with hit pairs in pixel detectors • Electron track reconstruction • Angular matching of ECAL cluster and full track • Loose track isolation in a hollow cone TIPP 2011

  18. Performance of Photon and Electron Triggers • Trigger efficiency for selected offline superclusters matched to L1 objects to pass a photon trigger with a threshold of ET > 15 GeV, plotted vs. the supercluster ET. • Efficiency for offline reconstructed electrons, which have passed a photon trigger with a threshold of ET > 15 GeV, to pass an electron trigger with similar ET threshold. TIPP 2011

  19. HLT CPU Performance • Study performed on a Minimum Bias sample of collected data (average pile up ~ 1.5 event/xing) • Filter farm machine specs: • Processors: 2 Quad Core Intel ® Xeon ® 5430 • 2.66 GHz nominal frequency • 16 GB of memory • Average CPU time budget at 100kHz (50kHz) of input rate is 50 ms (100 ms) “L3” section: full regional tracking, multiple objects “L2” section: regional unpacking, jet reco, pixel tracking Basic unpacking and book-keeping: common to all HLT paths L1 seeding and prescale: fast rejection of events TIPP 2011

  20. Conclusions • The CMS trigger performed amazingly well in 2010 • Successfully captured physics over 5 orders of magnitude in luminosity • Less then 30 mins downtime due to HLT during entire 2010 run • Good understanding of the evolution of trigger rates and CPU timing with instantaneous luminosity allowed optimal operation of the HLT • In general, the physics triggers exhibit sharp turn-on curves and are in good agreement with the simulation • LHC has already achieved instantaneous luminosities of ~1.3x1033 Hz/cm2 (max in 2010 was ~2x1032 Hz/cm2) and could reach as high as 5x1033 Hz/cm2 • The trigger paths are becoming more and more complex and specialized as we try to maintain low thresholds to capture all the physics signals of interest • Over 350 paths in the current trigger menu (max in 2010 was 174) • Bandwidth issues are becoming important • Need to continue to develop strategies for controlling trigger rates in a high pileup environment • Not a significant issue so far for the trigger, but we may have up to ~18 evts/crossing before the end of the year (currently ~5 evts/crossing) Upcoming challenges in 2011 TIPP 2011

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