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The CMS Trigger

The CMS Trigger. Cross sections for different processes vary by many orders of magnitude inelastic: 10 9 Hz W -> l n : 100 Hz tt: 10 Hz Higgs (100 GeV): 0.1 Hz Higgs (600 GeV): 0.01 Hz Required selectivity 1 : 10 10 - 11

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The CMS Trigger

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  1. The CMS Trigger

  2. Cross sections for different processes vary by many orders • of magnitude • inelastic: 109 Hz • W ->ln: 100 Hz • tt: 10 Hz • Higgs (100 GeV): 0.1 Hz • Higgs (600 GeV): 0.01 Hz • Required selectivity • 1 : 10 10- 11 • Trigger - Cross Sections and Rates C.-E. Wulz

  3. Principle of Triggering T( ) Event accepted? YES NO Successive steps Depends on Event type Properties of the measured trigger objects Trigger objects (candidates): e/g, m, hadronic jets,t-Jets, missing energy, total energy Trigger conditions: according to physics and technical priorities

  4. CMS Detector (Compact Muon Solenoid)

  5. Trigger Levels in CMS Level-1 Trigger Macrogranular information from calorimters and muon system (e, m, Jets, ETmissing) Threshold and topology conditions possible Latency: 3.2 ms Input rate: 40 MHz Output rate: up to 100 kHz Custom designed electronics system High Level Trigger (several steps) More precise information from calorimeters, muon system, pixel detector and tracker Threshold, topology, mass, … criteria possible as well as matching with other detectors Latency: between 10 ms and 1 s Input rate: up to 100 kHz Ouput (data acquisition) rate: approx. 100 Hz Industral processors and switching network

  6. Conventional Concept with 3 Steps Investment in specialized processors, control

  7. Advantages: Fewer components, scalable Investment in band width and commercial components CMS Concept with 2 Steps

  8. Evolution of Trigger Requirements ATLAS/CMS: Rather high rates and large event sizes Interaction rates: ~ Factor 1000 larger than at LEP, ~ Factor 10 larger than at Tevatron 8 Split, Oct. 2002

  9. Level-1 Trigger Only calorimeters and muon system involved Reason: no complex pattern recognition as in tracker required (appr. 1000 tracks at 1034 cm-2s-1 luminosity), lower data volume Trigger is based on: Cluster search in the calorimeters Track search in muon system

  10. RPC Hits Architecture of the Level-1 Trigger GLOBAL TRIGGER Global Calorimeter Trigger Global Muon Trigger Regional DT Trigger Regional CSC Trigger RPC Trigger Regional Calorimeter Trigger Local DT Trigger Local CSC Trigger Local Calorimeter Trigger Calorimeter cell energies DT Hits CSC Hits

  11. Strategy of the Level-1 Trigger • Local • Energy measurement in single calorimeter cells or groups of cells (towers) • Determination of hits or track segments in muon detectors Regional • Identification of particle signature • Measurement of pT/ET (e/g, m, hadron jets, t-jets) • Determination of location coordinates (h,f) and quality • Global • Sorting of candidates by pT/ET, quality and retaining of the • best 4 of each type together with location and quality information • Determination of SET, HT, ETmissing, Njets for different thresholds and h ranges • Algorithm logic • thresholds (pT/ET, NJets) • geometric correlations • - e.g. back-to-back events, forward tagging jets • - more detailed topological requirements optional • - location information for HLT • - diagnostics

  12. Level-1 Calorimeter Trigger Goals Identify electron / photon candidates Identify jet / t-jet candidates Measure transverse energies (objects, sums, missing ET) Measure location Provide MIP/isolation information to muon trigger

  13. e/g f Jet h Local / Regional Electron/Photon Trigger Trigger primitive generator (local) Flag max of 4 combinations (“Fine Grain Bit”) Regional calorimeter trigger ET cut Hit +max of > ET threshold Longitundinal cut hadr./electromagn. ET / < 0.05 Hadronic and electromagnetic isolation < 2 GeV One of < 1 GeV Electron / photon

  14. Typical e/g Level-1 Rates and Efficiencies • Single isolated e/g rate at 25 GeV threshold: 1.9 kHz • 95% efficiency at 31 GeV

  15. { Jet / t Trigger • Jet ET is obtained from energy sum of 3 x 3 regions - • sliding window technique, seamless coverage up to |h| < 5 • Up to |h| < 3 (HCAL barrel and endcap) the regions are 4 x 4 trigger towers Narrow jets are tagged as t-jets in tracker acceptance (|h| < 2.5) if ET deposit matches any of these patterns

  16. Typical Level-1 Jet Rates and Efficiencies • Single jet rate at 120 GeV threshold: 2.2 kHz, 95% efficiency at 143 GeV • Dijet rate at 90 GeV: 2.1 kHz 95% efficiency at 113 GeV • Single t-jet rate at 80 GeV threshold: 6.1 kHz C.-E. Wulz 16 Split, Oct. 2002

  17. HT Trigger HT, which is the sum of scalar ET of all high ET objects in the event is more useful than total scalar ET for the discovery or study of heavy particles (SUSY sparticles, top): • Total scalar ET integrates too much noise and is not easily calibrated • At Level-1 tower-by-tower ET calibration is not available • However, jet calibration is available as f(ET, h, f) • Implemented in Global Calorimeter Trigger • Rate with cutoff at 400 GeV: 0.7 kHz, 95% efficiency at 470 GeV

  18. Muon Trigger Detectors Drift Tube Chambers and Cathode Strip Chambers are used for precision measurements and for triggering. Resistive Plate Chambers (RPC’s) are dedicated trigger chambers.

  19. Drift Tube Chambers Cathode Strip Chambers Local Muon Trigger Superlayer Vector of 4 hit cells Comparators allow resolution of 1/2 strip width Station Correlator combines vectors to track segment 6 hit strips form track segment

  20. Regional DT/CSC Muon Trigger (Track Finder) • Trigger relies on track segments pointing to the vertex and correlation of several detector planes • Tracks with small pT often do not point to the vertex • (multiple scattering, magnetic deflection) • Tracks from decays and punchthrough do not point to vertex in general • Punchthrough and sailthrough particles seldom transverse all muon detector • planes

  21. Regional DT/CSC Muon Trigger (Track Finder) Longitudinal plane Track Segment and direction of extrapolation Bending plane Drift Tube Trigger (CSC Trigger similar)

  22. 4/4 High pT 3/4 3/4 Low pT 4/4 Regional RPC Muon Trigger RPC-Trigger is based on strip hits matched to precalculated patterns according to pT and charge. For improved noise reduction algorithm requiring conincidence of at least 4/6 hit planes has been designed. Number of patterns is high. FPGA solution (previously ASICs) seems feasible, but currently expensive. Solutions to reduce number of patterns under study.

  23. Global Muon Trigger DR/CSC/RPC: combined in Global Muon Trigger • Optimized algorithm (no simple AND/OR) with respect to • efficiency • rates • ghost suppression -> Make use of geometry + quality

  24. L1 Single & Di-Muon Trigger Rates Trigger rates in kHz |h| < 2.1 18, 8;8eW =84.9 %eZ =99.7 %eBsmm = 7.2 % 20, 5;5eW =82.3 %eZ =99.7 %eBsmm =15.1 % 14, -;-eW =89.6 %eZ =99.8 %eBsmm =27.1 % 25, 4;4eW =74.2 %eZ =99.5 %eBsmm =18.4 % working points compatible with current L1 pT binning L = 1034cm-2s-1 L = 2x1033cm-2s-1

  25. Global/Central Trigger within ATLAS and CMS Algorithm bits Algorithm bits Thresholds already set in calorimeters and muon system. The Central Trigger Processor receives object multiplicities. It does not receive location information, therefore topological conditions are impossible. Separate RoI electronics for the Level-2 Trigger is necessary. Thresolds set in Global Trigger. The processor receives objects with location information, therefore topological conditions are possible. Special HLT algorithms or lower thresholds can be used for selected event categories. Sorting needs some time, however.

  26. Global Trigger • The trigger decision is taken according to similar criteria as in data analysis: • Logic combinations of trigger objects sent by the Global Calorimeter Trigger and the • Global Muon Trigger • Best 4 isolated electrons/photons ET, h, f • Best 4 non-isolated electrons/photonsET, h, f • Best 4 jets in forward regions ET, h, f • Best 4 jets in central region ET, h, f • Best 4 t-Jets ET, h, f • Total ETSET • Total ET of all good jets HT • Missing ETETmissing, f(ETmissing) • 12 jet multiplicities Njets (different ET thresholds and h-regions) • Best 4 muons pT, charge, f, h, quality, MIP, isolation • Thresholds (pT, ET, NJets) • Optional topological and other conditions (geometry, isolation, charge, quality)

  27. Algorithm Logic in Global Trigger Object Conditions Logical Combinations 128 flexible parallel running algorithms implemented in FPGA’s. Trigger decision (Level-1-Accept) is a function of the 128 trigger algorithm bits (for physics). 64 more technical algorithms possible.

  28. Bs -> mm: Example of Topological Trigger D- distributions Dh similar. Can ask for opposite charges in addition.

  29. Bs -> mm: Example of Topological Trigger Topological conditions: Dh < 1.5, D < 2 rad, opposite muon charges Gain by topological conditions and requiring no threshold on second muon with respect to working point 20; 5,5: 1 kHz rate, 7.8%efficiency (15.1% without, 22.9% with cuts).

  30. High Level Trigger Goals • Validate Level-1 decision • Refine ET/pT thresholds • Refine measurement of position and other parameters • Reject backgrounds • Perform first physics selection Detailed algorithms: see talks of A. Nikitenko (e/g,jets), N. Neumeister (m) and other talks related to CMS physics and wait for DAQ/HLT TDR due Dec. 2002.

  31. High Level Trigger Challenges • Rate reduction • Design input rate: 100 kHz (50 kHz at startup with luminosity 2x1033 cm-2s-1), i.e. 1 event every 10 ms. Safety factor of 3: 33 kHz (16.5 kHz). • Output rate to tape: order of 100 Hz • Reduction factor: 1:1000 • Example of allocation of input bandwidth to four categories of physics objects plus service triggers (1 or 0.5 kHz): • electrons/photons (8 or 4 kHz) • muons (8 or 4 kHz) • t-jets (8 or 4 kHz) • jets + combined channels (8 or 4 kHz) • Algorithms • The entire HLT is implemented in a processor farm. Algorithms can almost be as sophisticated as in the off-line analysis. In principle continuum of steps possible. Current practice: level-2 (calo + muons), level-2.5 (pixels), level-3 (tracker), full reconstruction.

  32. High Level Trigger and DAQ Challenges Processing time Estimated processing time: up to 1 s for certain events, average 50 ms About 1000 processors needed. Interconnection of processors and frontend Frontend has O(1000) modules -> necessity for large switching network. Bandwidth Average event size 1 MB -> For maximum L1 rate need 100 GByte/s capacity.

  33. Conclusions • CMS has designed a Trigger capable of fulfilling physics and technical requirements at LHC. • The Trigger consists of 2 distinct levels. Level-1 is custom designed, the HLT is implemented with industrial components. • Prototypes of many Level-1 electronics boards exist. • Integration and synchronization tests are scheduled for 2003/2004. • The HLT/DAQ Technical Design Report is finalized and due to be ready at the end of 2002. 33

  34. Acknowledgments • my colleagues in CMS and at HEPHY Vienna, especially P. Chumney, • J. Erö, S. Dasu, N. Neumeister, H. Sakulin, W. Smith, G. Wrochna, • A. Taurok. • to the Organizing Committee of LHC Days in Split for the invitation and • the enjoyable conference.

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