CMS Triggering

1. Level 1 Trigger

1) Introduction

  • L1MuonParticle
    • Status of isolation and MIP bits (use the isIsolated() and isMip() member functions).
    • Corresponding hardware class: L1MuGMTCand (in DataFormats/L1GlobalMuonTrigger).

2) Level 1 Trigger : running software

L1Trigger?/GlobalTriggerAnalyzer/test
  • run emulator chain (L1 muon chain + L1 calorimeter chain + L1 Global Trigger) + L1 Global Trigger Analyzer on GEN + SIM + DIGI events
     cmsRun L1GtAnalyzerDigi.cfg
     
  • run L1 Global Trigger Analyzer on GMT + GCT + GT output file
      cmsRun L1GtAnalyzer.cfg
     

3) Level 1 Software Guide

Reference page - L1SWGuide? L1SWGuide

2. HLT (High Level Trigger)

1) Introduction

Why a trigger ?

  • Most of events/processes produced at a high-energy hadronic collider are not interesting : an early decision, i.e. online selection, has to be made to avoid running the offline code on millions of uninteresting events

High Level Trigger (HLT) :

  • Takes events accepted by the L1, first level of the CMS Trigger
  • Decides, based on more elaborated algorithms, whether the event should be kept
Therefore, it is a crucial part of the CMS data flow since it's the HLT algorithms and filters which will decide whether an event should be kept for an offline analysis : any offline analysis depends on the outcome of HLT, i.e. on the HLT efficiency.

Important benchmark ideas :

  • Rates (at 2E33) :
    • 13 MHz -> L1 -> 50kHz
    • -> HLT -> 150 Hz
  • Conceive and run approx. 200 triggers, as efficient as possible
  • Reconstruction :
    • Seeded by L1
    • As close as possible to offline reconstruction
    • Regional reconstruction : saving CPU
    • HLT level : typically :
      • L2 : Calorimeter and Muon information
      • L3 : Tracking information

Example of HLT reconstruction : Muons :

  • L2 :
    • Uses L1 seeds : Up to 4 muons provided by Global Muon Trigger (GMT), information : (pT, charge, phi, eta)
    • "StandAlone" muons : using muon segments, charge clusters, then Outside->In fitting
    • Filters : pT, invariant-mass
    • Filters : Calorimeter-based isolation
  • L3 :
    • Uses L2 seeds
    • "Global regional muon reconstruction" : using tracker information in muon window
    • Filters : pT, invariant-mass, Impact-Parameter, track-quality
    • Filters : Tracker-based isolation

2) HLT Software

SWGuide - HLT Software Guide HLTSoftware

Reference page - 4.6 HLT Tutorial on the WorkBook? (HLT Tutorial)

3. Muon Triggering

1) Muon HLT (description)

Every HLT muon path must be understood as a chain composed of two different types of pieces: pieces from the HLT muon reconstruction chain and pieces from the HLT filter logic.

The reconstruction chain is common to all trigger paths containing muons. This guarantees that only one instance of each reconstruction producer is used, independently of the number of muon trigger paths present in HLT. The savings in processing time are clear. Even if only a given subset of muon candidates may be relevant for a trigger path, the reconstruction chain will process all muon candidates (up to 4) selected by an initial filter common to all paths. This filter is applied on the Level 1 Global Muon Trigger information. Again, this is necessary to guarantee the use of the same collections (products) for all paths. It enormously simplifies the logic and facilitates the addition of new trigger paths, because a minimal additional time will be spent for them in what concerns muon reconstruction.

Some important details:

  • The HLT muon reconstruction chain starts only if the L1 trigger decision for at least one of the paths containing muons ( "SingleMuon", "DoubleMuon", "Muon+Jet", ...) is positive.
  • The reconstruction chain continues if at least one of the trigger paths with muons decides to do so. Conversely, it stops at a given point if all muon trigger paths decide that the subsequent reconstruction steps are not necessary. The usual case is a negative decision for all trigger paths if no muon pass some given thresholds in the initial steps.

In detail, the HLT muon reconstruction consists of the following steps:

  • The parameters from the L1 muon candidates are converted into seeds for the Standalone muon reconstruction (also known as Level 2 muon reconstruction): RecoMuon/L2MuonSeedGenerator.
  • The Level 2 muon reconstruction itself: RecoMuon/L2MuonProducer.
  • Calorimetric isolation for Level 2 candidates, for trigger paths involving isolation: RecoMuon/L2MuonIsolationProducer.
  • The global muon reconstruction (also known as Level 3 muon reconstruction), using as seeds the Level 2 muon candidates. Regional reconstruction in the CMS tracker is an important ingredient: RecoMuon/L3MuonProducer.
  • Tracker isolation for Level 3 candidates, for trigger paths involving isolation: RecoMuon/L3MuonIsolationProducer.

Several trigger filters can be interleaved between the previous pieces of reconstruction code. These filters are specific for each trigger path. Their role is to stop the trigger sequence if muon candidates do not satisfy some given conditions provided via configuration files. In addition, they provide as output a reduced list of candidates that should be considered in subsequent filter steps. For instance the single muon trigger path with isolation sequence proceeds in the following way:

  • Level 1 filter: it filters L1 candidates according to their quality, Pt, ... The generic filter is: HLTrigger/Muon/src/HLTMuonL1Filter.cc.
  • L1 seeds for L2, plus L2 reconstruction (common reconstruction sequence: only if not done yet by a previous path).
  • Filter on L2 output (mainly Pt thresholds). The generic filter is: HLTrigger/Muon/src/HLTMuonPreFilter.cc.
  • Calorimetric isolation (common reconstruction sequence: only if not done yet by a previous path).
  • L2 isolation filter, requiring at least one isolated muon from the list of candidates selected by the previous filter: The generic filter is: HLTrigger/Muon/src/HLTMuonIsoFilter.cc.
  • L3 reconstruction (common reconstruction sequence: only if not done yet by a previous path).
  • Filter on L3 output (mainly Pt thresholds and impact parameter cuts).
  • Track isolation (common reconstruction sequence: only if not done yet by a previous path).
  • L3 isolation filter, requiring at least one isolated muon from the list of candidates selected by the previous filter. The event is considered as accepted by this trigger path if it passes the criteria of this last filter.

The configuration files for the basic HLT muon paths at different luminosities $LUMI ($LUMI=1032, 2x1033, ...) are provided in the HLTrigger/Muon/data/ directory:

  • CommonModules_$LUMI.cfi: It contains all the sequences that are common to all trigger paths containing muons, and in particular all reconstruction pieces.
  • PathSingleMu_$LUMI_Iso.cff: Sequence for the single muon trigger path with isolation.
  • PathSingleMu_$LUMI_NoIso.cff: Sequence for the single muon trigger path without isolation (relaxed).
  • PathDiMuon_$LUMI_Iso.cff: Sequence for the dimuon trigger path with isolation.
  • PathDiMuon_$LUMI_NoIso.cff: Sequence for the dimuon trigger path without isolation (relaxed).
  • HLTMuon.cfg: Example of full configuration file, filtering events according to the "OR" of the previous triggers.

Additional muon filters for muon pairs (using invariant mass cuts, for instance) are included in the global HLT table. They are necessary for the implementation of specific dimuon triggers (J/psi, Upsilon, Z, same-sign dimuons, multi-muons, difractive triggers).

-- written by Main.jalcaraz - 25 Jul 2007 Reference page - Muon HLT (Muon HLT)


-- DongHoMoon - 07 Nov 2007

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Topic revision: r1 - 2007-12-03 - GaramHahn
 
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