4.6 HLT Tutorial

Complete: 4
Detailed Review status

This page has been updated for CMSSW 52X and the latest 2012 HLT studies

Goals of this page:

This page is intended to familiarize you with HLT ideas, software and utilities. In particular, you will learn:

  • how to run various trigger paths,
  • how to analyze HLT related information.



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

Please do visit Trigger, HLT pages :

Subscribe to hypernews :

Producing event with Triggers

Triggers defined as path blocks : sequence of modules and operands :

path MyTrigger = {doL1Reco & doL1Seeding & ApplyPrescale & doHltReco, HltCondition}
Modules :
  • Reconstruction : doL1Reco, doHltReco, etc...
  • Prescale : ApplyPrescale
  • Filter : HltCondition
Operands :
  • the "," or dependency operator, the operand to the right is dependent on the operand on the left (i.e. the right accesses data produced by the left)
  • the "&" or sequencing operator, the operand on the left is executed first, followed by the operand on the right, but they are not dependent on each other

Consequences :

  1. The result of each operand is a boolean : the final outcome of MyTrigger is "reject" or "accept"
  2. For a given path, the overall answer is the "AND" of all operands : If ever one of the operands on the left fails, reject AND stop processing : Saving CPU time !

Recommended reading : PathTriggerBits

The HLT configuration in CMS is stored in a dedicated database system, ConfDB. Use the dedicated GUI to create, manipulate and store trigger path configurations. Use the ConfDB web browser to browse the content of the configuration database and inspect available HLT menus. The command-line tool edmConfigFromDB allows you to retrieve (complete or partial) configurations as either ascii or python configuration files to be fed to cmsRun.

A quick look at the code

Please visit :

HLT code in HLTrigger

Running Trigger paths

Follow instructions from HLTtable

For input files : go on the Data Aggregation System page DAS

Analyzing Trigger/Offline information

Motivation : Get in one place, on event-by-event basis :

  • L1-, HLT-related information
  • Offline reconstructed information
  • HLT information without filters: "Open HLT" mode
Enables to :
  1. Study Trigger efficiencies, as function of offline reconstructed quantities...
  2. Get Trigger rejections, overlaps, rates...
  3. ...for L1, HLT conditions

Code :

  • Package : HLTrigger/HLTanalyzers : HLTanalyzers
  • Driving code : HLTAnalyzer .h .cc : an EDAnalyzer
  • Configuration file to run : HLTrigger/HLTanalyzers/test/HLTAnalysis.cfg

More details in HLTAnaManual

Getting the L1 information

Motivation : Have the information of HLT seeds at disposal

Using L1Extra objects, from MCTruth or from the L1 Emulator : In the HLTAnalysis.cfg file :

module HLTAnalyzer = {
   string l1extramc = l1extraParticles

Physics objects Variables stored L1Extra Class Instances
Muons E, pT, phi, eta, isolation, mip L1MuonParticle  
EM particles E, ET, phi, eta L1EmParticle “Isolated” and “NonIsolated”
Jets E, ET, phi, eta L1JetParticle "Forward" and "Central"
Taus E, ET, phi, eta L1JetParticle "Tau"
MET ET, ET(tot), ET(had), phi L1EtMissParticle  

Branches of variables created per instance.

Code in : HLTInfo .h .cc

Getting the HLT information

How do I get information about Trigger Results ?

  • From the TriggerResults class : Associate Trigger Path to Decision. HLTAnalyzer dynamically creates as many branches as Triggers present, and fill them with corresponding Trigger decision. Trigger branches pop up in form of "TRIGG_".

Code in : HLTInfo .h .cc

If you want to get the names of the paths (e.g. for accessing them by path name rather than by bit number), see the example code in HLTrigger/HLTanalyzers/src/HLTrigReport.cc.

See also the hypernews discussion here.

Getting the Offline-reconstructed and other information

Specify in HLTAnalysis.cfg the instances of reconstructed objects that you want :

module HLTAnalyzer = 
    string muon = "muons"
    string Electron = "pixelMatchGsfElectrons"
    string Photon = "correctedPhotons"
    string recjets = "iterativeCone5CMS.CaloJets"
    string genjets = "iterativeCone5GenJets"
    string recmet = met
    string genmet = genMet
    string calotowers = towerMaker

Physics objects Variables stored Instances
Muon E, ET, pT, eta, phi muons
Electron E, ET, pT, eta, phi pixelMatchGsfElectrons
Gamma E, ET, pT, eta, phi correctedPhotons
Jet E, ET, pT, eta, phi iterativeCone5CMS.CaloJets, iterativeCone5GenJets
MET pT, phi, ET(sum) met, genMet
Calo Towers E, ET, E(em), E(had), E(out), eta, phi towerMaker
MC truth Id, Vtx(X,Y,Z), pT  

Code in : HLTEgamma, HLTMuon, HLTJets (deals with MET as well)

  • Getting also information about the MC truth information about generated particles : pT, Identity, Vertex... Code in HLTMCtruth

Offline collections can be accessed when running simultaneously and RAW+RECO samples. This can be achieved using crab with use_parent=1 option. Running on a RECO dataset will then allow to access its parent dataset, namely the corresponding RAW dataset.

Getting the MC truth information

Motivation : Have the generator level information at disposal

Object Variables stored Instances
Generated particle PDG-Identity, particle status, [x,y,z]-component of primary vertex genParticles
Generated particle pT, phi, eta genParticles
  Event pT-hat, i.e. scale genEventScale

Calculate the rate of a new trigger:

Please have a look at: How to run OpenHLT

You may also want to take a look at the recipes from the STEAM group

Recipes for producing ntuples

These versions of the analyzer have the OpenHLT capability included.

For additional tags needed to run on real data consistent with online releases, consult this page.

For the correct GlobalTag to use in a given release consult this page.

Review status

Reviewer/Editor and Date Comments
Pedrame Bargassa - 1 Dec 2007 Author
XuanChen - 31 Jul 2014 changed links from cvs to github, replaced dbs by das

Responsible: Pedrame Bargassa
Last reviewed by: ChristosLeonidopoulos - 22 Feb 2008

Topic attachments
I Attachment History Action Size Date Who Comment
Unknown file formatcfg HLTtable.cfg r3 r2 r1 manage 5.6 K 2007-04-18 - 17:18 UnknownUser File to run CMSSW_1_3_0 trigger table on 1_2_0 MC
JPEGjpg hltana_root.jpg r2 r1 manage 272.5 K 2007-10-26 - 18:30 PedrameBargassa  
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Topic revision: r201 - 2014-10-29 - MurielVanderDonckt



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