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

Introduction Overview of the CMS Trigger System Trigger Commissioning in 2010 Trigger Rates and Performance HLT Processing Times At design L = cm -2 s -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 June 11, 2011TIPP Outline: Trigger is essential

The CMS Detector June 11, 20113TIPP 2011

CMS Trigger System June 11, 2011TIPP The CMS trigger system is structured in two levels: Level-1 (L1) and HLT Level-1 trigger 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

Electrons,Photons,Jets,ME T Muons 3<|  |<5 |  |<3 |  |<3 |  |< <|  |<2.4 |  |<1.2 The Level-1 Trigger Runs synchronously at the LHC clock rate of 40 MHz Selects muons, electrons, photons, jets –E T and location in detector –Also Missing E T, Total E T, H T, 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 June 11, 20115TIPP 2011 For details about the Level-1 system and it’s performance, see Thursday's talk by P. KlabbersThursday's talk

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 June 11, 2011TIPP 20116

June 11, 2011TIPP 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) HLT Algorithm Design L1 seeds L2 unpacking (MUON/ECAL/HCAL) Local Reco (RecHit) L2 Algorithm Filter L2.5 unpacking (Pixels) L2.5 Algorithm Local Reco (RecHit) “Local”: using one sub-detector only “Regional”: using small (η, φ) region

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/cm 2 trigger 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 June 11, 2011TIPP 20118

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 June 11, 2011TIPP 20119

CMS Web Based Monitoring Summary of all HLT paths Similar page available for L1 June 11, 2011TIPP Sync problem ECAL Problem (Muon trigger unaffected) A dedicated talk on the WBMdedicated talk system will be given on Monday

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?” June 11, 2011TIPP

HLT Performance June 11, 2011TIPP

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 p T threshold = 15, 30, and 50 GeV –Shown for Barrel and Endcap regions June 11, 2011TIPP 2011 Comparison with MC

Performance of MET Triggers Missing E T calculated from algebraic sum of transverse energies of calorimeter objects plus muons Good agreement between MC and data June 11, 2011TIPP L1 MET (thresh=20 GeV) efficiency vs. Offline MET HLT MET (thresh=45 GeV) efficiency vs. Offline MET

HLT Muon Reconstruction First Stage: –Confirm L1 “seeds”: refit hits in the muon chambers 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 –pT resolution much better compared to 1st stage Optional: Isolation in calorimeters (at 1st Stage) and tracker (at 2nd Stage) June 11, 2011TIPP

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 p T > 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 June 11, 2011TIPP Barrel Endcaps

HLT e/γ Reconstruction Common stage for Photons and Electrons: –Spatial matching of energy deposits (clusters) in the Electromagnetic Calorimeter (ECAL) with e/γ candidates at L1 –Form super-clusters (group of clusters; bremsstrahlung/conversions recovery) –E T 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 June 11, 2011TIPP

Performance of Photon and Electron Triggers June 11, 2011TIPP Trigger efficiency for selected offline superclusters matched to L1 objects to pass a photon trigger with a threshold of E T > 15 GeV, plotted vs. the supercluster E T. Efficiency for offline reconstructed electrons, which have passed a photon trigger with a threshold of E T > 15 GeV, to pass an electron trigger with similar E T threshold.

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) June 11, 2011TIPP Basic unpacking and book-keeping: common to all HLT paths “L2” section: regional unpacking, jet reco, pixel tracking “L3” section: full regional tracking, multiple objects L1 seeding and prescale: fast rejection of events

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 ~1.3x10 33 Hz/cm 2 (max in 2010 was ~2x10 32 Hz/cm 2 ) and could reach as high as 5x10 33 Hz/cm 2 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) June 11, 2011TIPP Upcoming challenges in 2011