Jet Tagging

AddTaskEmcalJetTagger.C contains two methods:

• one takes as input already existing jet arrays
• the other takes care of running the jet algorithm according to the settings.

The actual tagging is performed in the method AliAnalysisTaskEmcalJetTagger::MatchJetsGeo. The distance between pair of jets in the two inputs array is compared and those who are reciprocally closest are matched.

There are two tagging types:

enum JetTaggingType {
  kTag      = 0,
  kClosest  = 1
};

• kTag is used for "hard core" tagging and switches the TagStatus of the AliEmcalJet to 1 and set the TaggedJet to the companion jet.
• kClosest is used for the matching of the reconstructed and particle level jets in PYTHIA. It sets the ClosestJet (AliEmcalJet::fClosestJets[0]) and the distance.

Hard core tagging

Used to reject combinatorial jets without applying an explicit high-pT cut on the constituents. Two jet finders are needed:

• normal jet finder used in the analysis (e.g. AKT, p_T,const > 0.15 GeV/c, E_min > 0.3 GeV) –> jet
• jet finder with harder constituent cut (e.g. AKT, p_T,const > 4 GeV /c, E_min > 0.3 GeV) –> tag

The jets are matched geometrically to the tags and flagged as "tagged".

JetFinderAKTCharged_R04_Escheme

gROOT->LoadMacro("$ALICE_PHYSICS/PWGJE/EMCALJetTasks/macros/AddTaskEmcalJet.C");
AliEmcalJetTask *jet = AddTaskEmcalJet(kUsedTracks, "", 1, 0.4, 1, kTrackPtCut, kClusPtCut, 0.005, 0, "Jet", 0., kFALSE, kFALSE, kFALSE);

//optional: background subtraction utils
AliEmcalJetUtilityGenSubtractor* genUtil = jet->AddUtility(new AliEmcalJetUtilityGenSubtractor("GenSubtractor"));
genUtil->SetUseExternalBkg(kTRUE);
genUtil->SetRhoName("RhoSparseR040");
genUtil->SetRhomName("RhoMassSparseR040");
genUtil->SetGenericSubtractionJetMass(kTRUE);

JetFinderAKTCharged_R04_Escheme_4GeV

AliEmcalJetTask *jettag = AddTaskEmcalJet(kUsedTracks, "", 1, 0.4, 1, 4., 4., 0.005, 0, "Jet", 0., kFALSE, kFALSE, kFALSE);

JetTaggerChargedR040_v2

gROOT->LoadMacro("$ALICE_PHYSICS/PWGJE/EMCALJetTasks/macros/AddTaskEmcalJetTagger.C");
AliAnalysisTaskEmcalJetTagger *jetTagger = AddTaskEmcalJetTagger("Jet_AKTChargedR040_PicoTracks_pT0150_E_scheme", "Jet_AKTChargedR040_PicoTracks_pT4000_E_scheme", 0.4, "", "", kUsedTracks, "", "TPC", "", kPhysSel);
jetTagger->SetNCentBins(1);
jetTagger->SetIsPythia(kIsPythia);
jetTagger->SetJetTaggingType(AliAnalysisTaskEmcalJetTagger::kTag);
jetTagger->SetJetTaggingMethod(AliAnalysisTaskEmcalJetTagger::kGeo);
AliJetContainer *contBase = jetTagger->GetJetContainer(0);
contBase->SetJetPtCut(0.15);
AliJetContainer *contTag = jetTagger->GetJetContainer(1);
contTag->SetJetPtCut(4.);

Matching PYTHIA detector and PYTHIA particle level jets

It is done geometrically, as explained before. It is possible to request that the detector level jet has a given fraction of p_T of the particle level jet. This doesn't affect the tagging but the result is shown in one of the output histograms (fh2PtJet2VsFraction) and the cut is applied to the histograms related to the "tagged" jets.

AliAnalysisTaskEmcalJetTagger *taskJetTagger = AddTaskEmcalJetTagger(nJetsThrm.Data(),nJetsPyt.Data(),0.4,rhoName.Data(),rhoMassName,"ThrmTracksEmb","","TPC",kCentEst,pSel,"","");
     taskJetTagger->SelectCollisionCandidates(AliVEvent::kAny);
     taskJetTagger->SetNCentBins(1);
     taskJetTagger->SetJetTaggingType(AliAnalysisTaskEmcalJetTagger::kClosest);
     taskJetTagger->SetMinFractionShared(fracShared);

Minimum p_T fraction shared

Note that the method:

AliJetContainer::GetFractionSharedPt(const AliEmcalJet *jet1, AliParticleContainer *cont2)

can be used without specifying cont2 if the track containers of the two jets is the same. If cont2 is specified, a geometrical matching of the constituents of jet1 and jet2 = jet1->ClosestJet() is performed. The tracks of jet2 contained in cont2 that have very similar (equal) #eta, #phi, and p_T as the tracks of jet1 in cont1 are accounted for the shared p_T fraction.