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Download High Level Triggering Fred Wickens. High Level Triggering (HLT) Introduction to triggering and HLT systems –What is Triggering –What is High Level Triggering

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  • High Level TriggeringFred Wickens

  • High Level Triggering (HLT)Introduction to triggering and HLT systemsWhat is TriggeringWhat is High Level Triggering Why do we need itCase study of ATLAS HLT (+ some comparisons with other experiments)Summary

  • Simple trigger for spark chamber set-up

    EMBED Word.Picture.8


  • Dead timeExperiments frozen from trigger to end of readoutTrigger rate with no deadtime = R per sec.Dead time / trigger = t sec.For 1 second of live time = 1 + Rt secondsLive time fraction = 1/(1 + Rt)Real trigger rate = R/(1 + Rt) per sec.

    Rate in Hz

    Dead time ms.

    Live time %

    Trigger rate Hz









  • Trigger systems 1980s and 90sbigger experiments more data per eventhigher luminosities more triggers per secondboth led to increased fractional deadtime

    use multi-level triggers to reduce dead-timefirst level - fast detectors, fast algorithmshigher levels can use data from slower detectors and more complex algorithms to obtain better event selection/background rejection

  • Trigger systems 1990s and 2000sDead-time was not the only problemExperiments focussed on rarer processesNeed large statistics of these rare eventsBut increasingly difficult to select the interesting eventsDAQ system (and off-line analysis capability) under increasing strain - limiting useful event statisticsThis is a major issue at hadron colliders, but is also significant at ILCUse the High Level Trigger to reduce the requirements forThe DAQ systemOff-line data storage and off-line analysis

  • Summary of ATLAS Data Flow RatesFrom detectors> 1014 Bytes/sec

    After Level-1 accept~ 1011 Bytes/sec

    Into event builder~ 109 Bytes/sec

    Onto permanent storage~ 108 Bytes/sec ~ 1015 Bytes/year

  • TDAQ Comparisons

  • The evolution of DAQ systems

  • Typical architecture 2000+

  • Level 1 (Sometimes called Level-0 - LHCb)Time:one very few microsecondsStandard electronics modules for small systemsDedicated logic for larger systems ASIC - Application Specific Integrated CircuitsFPGA - Field Programmable Gate ArraysReduced granularity and precisioncalorimeter energy sumstracking by masksEvent data stored in front-end electronics (at LHC use pipeline as collision rate shorter than Level-1 decision time)

  • Level 21) few microseconds (10-100) hardwired, fixed algorithm, adjustable parameters

    2) few milliseconds (1-100)Dedicated microprocessors, adjustable algorithm3-D, fine grain calorimetrytracking, matchingTopologyDifferent sub-detectors handled in parallelPrimitives from each detector may be combined in a global trigger processor or passed to next level

  • Level 2 - contd3) few milliseconds (10-100) - 2006Processor farm with Linux PCsPartial events received with high-speed networkSpecialised algorithmsEach event allocated to a single processor, large farm of processors to handle rate

    If separate Level 2 data from each event stored in many parallel buffers (each dedicated to a small part of the detector)

  • Level 3millisecs to secondsprocessor farmmicroprocessors/emulators/workstationsNow standard server PCsfull or partial event reconstructionafter event building (collection of all data from all detectors)Each event allocated to a single processor, large farm of processors to handle rate

  • Summary of IntroductionFor many physics analyses, aim is to obtain as high statistics as possible for a given processWe cannot afford to handle or store all of the data a detector can produce!What does the trigger doselect the most interesting events from the myriad of events seenI.e. Obtain better use of limited output band-widthThrow away less interesting eventsKeep all of the good events(or as many as possible)But note must get it right - any good events thrown away are lost for ever!High level trigger allows much more complex selection algorithms

  • Case study of the ATLAS HLT systemConcentrate on issues relevant for ATLAS (CMS very similar issues), but try to address some more general points

  • Starting points for any HLT systemphysics programme for the experimentwhat are you trying to measureaccelerator parameterswhat rates and structuresdetector and trigger performancewhat data is availablewhat trigger resources do we have to use it

  • Physics at the LHCInteresting events are buried in a sea of soft interactionsHiggs productionHigh energy QCD jet productionB physics top physics

  • The LHC and ATLAS/CMSLHC has design luminosity 1034 cm-2s-1 (In 2008 from 1031 - 1033 ?)bunch separation 25 ns (bunch length ~1 ns)This results in ~ 23 interactions / bunch crossing~ 80 charged particles (mainly soft pions) / interaction ~2000 charged particles / bunch crossingTotal interaction rate109 sec-1b-physicsfraction ~ 10-3106 sec-1t-physicsfraction ~ 10-810 sec-1Higgsfraction ~ 10-1110-2 sec-1

  • Physics programmeHiggs signal extraction important but very difficult Also there is lots of other interesting physics B physics and CP violationquarks, gluons and QCDtop quarksSUSYnew physicsProgramme will evolve with: luminosity, HLT capacity and understanding of the detectorlow luminosity (first ~2 years)high PT programme (Higgs etc.)b-physics programme (CP measurements)high luminosityhigh PT programme (Higgs etc.)searches for new physics

  • Trigger strategy at LHCTo avoid being overwhelmed use signatures with small backgroundsLeptonsHigh mass resonancesHeavy quarks

    The trigger selection looks for events with: Isolated leptons and photons, -, central- and forward-jets Events with high ETEvents with missing ET

  • Example Physics signatures

    ObjectsPhysics signaturesElectron 1e>25, 2e>15 GeVHiggs (SM, MSSM), new gauge bosons, extra dimensions, SUSY, W, topPhoton 1>60, 2>20 GeVHiggs (SM, MSSM), extra dimensions, SUSYMuon 1>20, 2>10 GeVHiggs (SM, MSSM), new gauge bosons, extra dimensions, SUSY, W, topJet 1j>360, 3j>150, 4j>100 GeVSUSY, compositeness, resonancesJet >60 + ETmiss >60 GeVSUSY, leptoquarksTau >30 + ETmiss >40 GeVExtended Higgs models, SUSY

  • ARCHITECTURE40 MHzTriggerDAQ~1 PB/s (equivalent)Three logical levelsLVL1 - Fastest: Only Calo and Mu HardwiredLVL2 - Local: LVL1 refinement + track associationLVL3 - Full event: Offline analysis~2 ms~10 ms~1 sec.Hierarchical data-flowOn-detector electronics: PipelinesEvent fragments buffered in parallelFull event in processor farm

  • Selected (inclusive) signatures





    (2 em, ET>20 GeV

    2 , ET>20 GeV

    H0Z Z* + +

    (2 em, ET>20 GeV(2 , pT>6 GeV(1 em, ET>30 GeV(1 , pT>20 GeV

    2 e, ET>20 GeV2 , ET>6 GeV, I 1 e, ET>30 GeV1 , ET>20 GeV, I


    (2 em, ET>20 GeV(2 , pT>6 GeV(1 em, ET>30 GeV(1 , pT>20 GeV

    2 e, ET>20 GeV2 , ET>6 GeV, I 1 e, ET>30 GeV1 , ET>20 GeV, I

    t EQ \x\to(t) leptons+jets

    (1 em, ET>30 GeV(1 , pT>20 GeV

    1 e, ET>30 GeV1 , ET>20 GeV, I

    W', Z'jets

    (1 jet, ET>150GeV

    1 jet, ET>300GeV


    (1 jet, ET>150GeVETmiss

    3 jet, ET>150GeVETmiss

  • Trigger design - Level-1Level-1 sets the context for the HLTreduces triggers to ~75 kHzhas a very short time budget few micro-sec (ATLAS/CMS ~2.5 - much used up in cable delays!)Detectors used must provide data very promptly, must be simple to analyseCoarse grain data from calorimetersFast parts of muon spectrometer (I.e. not precision chambers)NOT precision trackers - too slow, too complex(LHCb does use some simple tracking data from their VELO detector to veto events with more than 1 primary vertex)Proposed FP420 detectors provide data too late

  • ATLAS Level-1 trigger systemCalorimeter and muontrigger on inclusive signaturesmuons; em/tau/jet calo clusters; missing and sum ETHardware triggerProgrammable thresholdsSelection based on multiplicities and thresholds

    EMBED Word.Picture.8


  • ATLAS em cluster trigger algorithmSliding window algorithm repeated for each of ~4000 cells


  • ATLAS Level 1 Muon trigger RPC: Restive Plate ChambersTGC: Thin Gap ChambersMDT: Monitored Drift TubesRPC - Trigger Chambers - TGCMeasure muon momentum with very simple tracking in a few planes of trigger chambers

  • Level-1 SelectionThe Level-1 trigger - an or of a large number of inclusive signals - set to match the current physics priorities and beam conditionsPrecision of cuts at Level-1 is generally limitedAdjust the overall Level-1 accept rate (and the relative frequency of different triggers) byAdjusting thresholds Pre-scaling (e.g. only accept every 10th trigger of a particular type) higher rate triggersCan be used to include a low rate of calibration eventsMenu can be changed at the start of run Pre-scale factors may change during the course of a run

  • Example Level-1 Menu for 2x10^33

    Level-1 signatureOutput Rate (Hz)EM25i120002EM15i4000MU208002MU6200J2002003J902004J65200J60 + XE60400TAU25i + XE302000MU10 + EM15i100Others (pre-scaled, exclusive, monitor, calibration)5000Total~25000

  • Trigger design - Level-2Level-2 reduce triggers to ~2 kHzNote CMS does not have a physically separate Level-2 trigger, but the HLT processors include a first stage of Level-2 algorithmsLevel-2 trigger has a short time budget ATLAS ~10 milli-sec average Note for Level-1 the time budget is a hard limit for every event, for the High Level Trigger it is the average that matters, so a some events can take several times the average, provid


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