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CMS High Level Trigger Selection

EPS-HEP 2003 Aachen, Germany. CMS High Level Trigger Selection. Giuseppe Bagliesi INFN-Pisa On behalf of the CMS collaboration. Outline. LHC Environment High Level Trigger strategy Object selection e/ g, m, Jet /n, t , b HLT rates and efficiencies Conclusions. p-p collisions at LHC.

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CMS High Level Trigger Selection

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  1. EPS-HEP 2003 Aachen, Germany CMS High Level Trigger Selection Giuseppe Bagliesi INFN-Pisa On behalf of the CMS collaboration

  2. Outline • LHC Environment • High Level Trigger strategy • Object selection • e/g, m, Jet/n,t, b • HLT rates and efficiencies • Conclusions G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  3. p-p collisions at LHC Event rate Crossing rate 40 MHz Event Rates: ~109 Hz Max LV1 Trigger 100 kHz Event size ~1 Mbyte Readout network 1 Terabit/s Filter Farm ~106 Si95 Trigger levels 2 Online rejection 99.9997% (100 Hz from 50 MHz) System dead time ~ % Event Selection: ~1/1013 Luminosity Low 2x1033 cm-2 s-1 High 1034 cm-2 s-1 “Discovery” rate G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  4. Operating conditions: one “good” event (e.g Higgs in 4 muons ) + ~20 minimum bias events) All charged tracks with pt > 2 GeV Reconstructed tracks with pt > 25 GeV Trigger environment 40 MHz Clock driven Custom processors 100 kHz Event driven PC network Totally software 100 Hz To mass storage two trigger levels Level-1 (~µs) 40 MHz High-Level ( ms-sec) 100 kHz Event Size ~ 106 Bytes G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  5. HLT: Reconstruction and selection of electrons, photons, muons, jets, missing ET, and b and t tagging. HLT has access to full event data (full granularity and resolution) maximum flexibility Main requirements: Satisfy CMS physics program with high efficiency Inclusive selection (we like to see also unexpected physics!) Must not require precise knowledge of calibration/run conditions Efficiency must be measurable from data alone The HLT code/algorithms must be as close as possible to the offline reconstruction Limitations: CPU time Output selection rate (~102 Hz) Precision of calibration constants High Level Trigger requirements and operation G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  6. Regional Reconstruction Global • process (e.g. DIGI to RHITs) each detector fully • then link detectors • then make physics objects Regional • process (e.g. DIGI to RHITs) each detector on a "need" basis • link detectors as one goes along • physics objects: same G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  7. e/g selection Level-1 ECAL reconstruction Threshold cut Level-2 Pixel matching Level-2.5 • In addition: • Isolation cuts (ECAL, pixel, track) • Had/EM isolation • p0 rejection Level-3 Photons Threshold cut Electrons Track reconstruction E/p, matching (Dh) cut G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  8. Level-2 electron: 1-tower margin around 4x4 area found by Lvl-1 trigger Apply “clustering” Accept clusters if EHCAL /EECAL <0.05 Select highest ET cluster HLT Electron selection (I) • Brem recovery: • Seed cluster with ET>ETmin • Road in f around seed • Collect all clusters in road •  “supercluster”and • add all energy in road Supercluster G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  9. Level-2.5 selection: add pixel information Very fast, high rejection high efficiency (e=95%), high background rejection (14) Pre-bremsstrahlung: Matching hits given by most electrons and by few photons Require at least 2 hits (3 pixel hits available almost always) HLT Electron selection (II) G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  10. HLT Electron selection (III) • Level-3electron: • Build tracks from pixel seeds found in pixel-matching step • Very loose track requirements • for high efficiency for radiating tracks: • 3-hit layers • Allow 2 consecutive missing layers • Track selection: • Barrel: E/p and Dh(track-cluster) • Endcap: E/p • Also (non-track): H/E • With tight cuts is always possible to select almost no-radiating electron with very high purity G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  11. Level-3 Algorithmic efficiency  Single muons 10<Pt<100 GeV/c HLT muon track reconstruction • Standalone Muon Reconstruction: “Level-2” • Seeded by Level-1 muons • Kalman filtering technique applied to DT/CSC/RPC track segments • GEANE used for propagation through iron • Trajectory building works from inside out • Track fitting works from outside in • Fit track with beam constraint • Inclusion of Tracker Hits: “Level-3” • Define a region of interest through tracker based on L2 track with parameters at vertex • Find pixel seeds, and propagate from innermost layers out, including muon G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  12. L2 & L3 muon pTresolution and efficiency PT resolution barrel Efficiency vs PT threshold L2 s=0.11 10 GeV threshold L3 L3 L1 s=0.013 30 GeV L2 10. 30. 50. (1/pTrec-1/pTgen) /(1/pTgen)

  13. Before isolation After isolation Isolation and physics content after muon Level-3 Isolation is based on transverse energy (ET ) or momentum (PT ) measurements in cones around the muon Calorimeter isolation - ET from calorimeter towers in a cone around the muon Pixel isolation - PT of 3-hit tracks in the pixel detector in cone around the muon - Requires that contributing tracks come from same primary vertex as the Level-3 muon (to reduce pile-up contamination) Tracker isolation - PT of tracks in the Tracker (regional reconstruction around L3 muon) Muons from b,c,K,p decays are greatly suppressed by isolation G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  14. HLT efficiencies on H WW  2m2n L3 muon thresholds at low luminosity: Single m: 19 GeV Doublem: 7 GeV L3 threshold Efficiency @ low lumi: MH=120 GeV: single mu 74% , di-mu exclusive 14% , combined: 87 % MH=160 GeV: single mu 87% , di-mu exclusive 5% , combined: 92 % G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  15. Jet rates and thresholds • Low luminosity: • 1 kHz at Level-1: 177 GeV (1 jet), 85 GeV (3 jet), 70 GeV (4 jet) • 1 Hz at HLT: 657 GeV (1 jet), 247 GeV (3 jet), 149 GeV (4 jet) • High luminosity: • 1 kHz at Level-1: 248 GeV (1 jet), 112 GeV (3 jet), 95 GeV (4 jet) • 1 Hz at HLT: 860 GeV (1 jet), 326 GeV (3 jet), 199 GeV (4 jet) • Very high rates and thresholds! • HLT triggers need some other condition • to have acceptably low threshold • MET, leptons, isolation, vertices… G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  16. MET Rates • Calorimeter coverage: |h|<5 • Generator level: • real neutrinos -> ETmiss>60 GeV • ETmiss<60 GeV mostly due to limited coverage • Much higher ETmiss at HLT than • at generator level • “ETmiss” objects selection is done in association with other requirements, like a energetic jet G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  17. Momentum resolution Full tracker Impact parameter resolution Full tracker Partial track reconstruction strategy at HLT Reconstruct only a ROI (Region Of Interest) from LVL1 candidate objects (regional tracking) Use a reduced number of hits (conditional tracking) • At HLT ultimate resolution is not needed • Good track parameter resolution is obtained already with 4 or more hits • The time for track reconstruction increases linearly with the number of hits

  18. Tracker @ HLT: tau tagging • TEST CHANNELS • A0/H0 (200, 500 GeV) -> t-jet t-jet, t-jet lepton • H+(200, 400 GeV) -> t-jet n • Efficiencies ~ 40-50%, • Background rej. after LVL1 ~103 • Regional Tracking: • Look only in Jet-track matching cone • Loose Primary Vertex association Conditional Tracking: Stop track as soon as Pixel seed found (PXL) / 6 hits found (Trk) If Pt<1 GeV with high C.L. Reject event if no “leading track” found Regional seeding: look for seeds in a specific region • Regional Tracking: • Look only inside isolation cone • Loose Primary Vertex association Conditional Tracking: Stop track as soon as Pixel seed found (PXL) / 6 hits found (Trk) If Pt<1 GeV with high C.L. Essential at High Luminosity activity well advanced Reject event as soon as additional track found

  19. Inclusive b tagging at HLT Regional Tracking: Look only in Jet-track matching cone Loose Primary Vertex association Conditional Tracking: Stop track as soon as Pixel seed found (PXL) / 6 hits found (Trk) If Pt<1 GeV with high C.L. ~300 ms low lumi ~1 s high lumi Performance of simple signed IP “track counting” tags ~ same as after full track reconstruction Use tracks to define Jet axis (if rely on L1 Calo Jet ~ randomize signed IP) Inclusive b tag at HLT possible, provided alignment under control G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  20. Level-1 rate “DAQ staging”: 50 KHz Total Rate: 105 Hz Average HLT CPU: 300ms*1GHz Improvements are possible HLT table: LHC start… • HLT performances: • Priority to discovery channels G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  21. All numbers for a 1 GHz, Intel Pentium-III CPU HLT: CPU usage Total: 4092 s for 15.1 kHz  271 ms/event Time completely dominated byslow GEANE extrapolationin muons– will improve! Consider ~50% uncertainty! • Today: ~300 ms/event • on a 1GHz Pentium-III CPU • Physics start-up (50 kHz LVL1 output): • need 15,000 CPUs • Moore’s Law: 2x2x2 faster CPUs in 2007 • ~ 40 ms in 2007, ~2,000 CPUs • ~1,000 dual-CPU boxes in Filter Farm G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

  22. The regional/conditional reconstruction is very useful to reduce CPU time and very effective in the HLT selection Tracker at HLT: Essential for muons, electron and tau selection inclusive/esclusive b-trigger is possible Standard Model physics: “just do it” at lower initial luminosity (“dedicated” triggers could be implemented) Pre-scale or lower thresholds when luminosity drops through fill Conclusions Start-up system 50kHz (Level-1) and 105 Hz (HLT) satisfy basic “discovery menu” The HLT design based on a purely software selection will work: Maximum flexibility and scalability Possibility to use “off-line” reconstruction/algorithms Summary G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003

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