Muons & Taus

Last updated on 2024-07-09 | Edit this page

Estimated time: 10 minutes

Overview

Questions

  • How are muons reconstructed in CMS?
  • How are muons treated in CMS OpenData?

Objectives

  • Understand how muons are reconstructed in CMS
  • Learn variables for muon track-based quantities
  • Learn variables for identification and isolation of muons

Overview of muon reconstruction


Muons are the M in CMS (Compact Muon Solenoid). This is in part because they are reconstructed basically using all the CMS sub-detectors. As it nicely summarized here:

[A muon] is measured by fitting a curve to the hits registered in the four muon stations, which are located outside of the magnet coil, interleaved with iron “return yoke” plates. The particle path is measured by tracking its position through the multiple active layers of each station; for improved precision, this information is combined with the CMS silicon tracker measurements. Measuring the trajectory provides a measurement of particle momentum. Indeed, the strong magnetic field generated by the CMS solenoid bends the particle’s trajectory, with a bending radius that depends on its momentum: the more straight the track, the higher the momentum.

Depending essentially on the kind of sub-detectors were used to reconstruct muons, they are usually classified accroding to the summary image below.

Muons have many features in common with electrons, but their own unique identification algorithms. We will use the same NanoAOD file as in the previous episode to look at the Muon NanoAOD branches.

NanoAOD muon collection variables
Object property Type Description
Muon_charge Int_t electric charge
Muon_cleanmask UChar_t simple cleaning mask with priority to leptons
Muon_dxy Float_t dxy (with sign) wrt first PV, in cm
Muon_dxyErr Float_t dxy uncertainty, in cm
Muon_dxybs Float_t dxy (with sign) wrt the beam spot, in cm
Muon_dz Float_t dz (with sign) wrt first PV, in cm
Muon_dzErr Float_t dz uncertainty, in cm
Muon_eta Float_t eta
Muon_fsrPhotonIdx Int_t (index to Fsrphoton) Index of the associated FSR photon
Muon_highPtId UChar_t high-pT cut-based ID (1 = tracker high pT, 2 = global high pT, which includes tracker high pT)
Muon_highPurity Bool_t inner track is high purity
Muon_inTimeMuon Bool_t inTimeMuon ID
Muon_ip3d Float_t 3D impact parameter wrt first PV, in cm
Muon_isGlobal Bool_t muon is global muon
Muon_isPFcand Bool_t muon is PF candidate
Muon_isStandalone Bool_t muon is a standalone muon
Muon_isTracker Bool_t muon is tracker muon
Muon_jetIdx Int_t (index to Jet) index of the associated jet (-1 if none)
Muon_jetNDauCharged UChar_t number of charged daughters of the closest jet
Muon_jetPtRelv2 Float_t Relative momentum of the lepton with respect to the closest jet after subtracting the lepton
Muon_jetRelIso Float_t Relative isolation in matched jet (1/ptRatio-1, pfRelIso04_all if no matched jet)
Muon_looseId Bool_t muon is loose muon
Muon_mass Float_t mass
Muon_mediumId Bool_t cut-based ID, medium WP
Muon_mediumPromptId Bool_t cut-based ID, medium prompt WP
Muon_miniIsoId UChar_t MiniIso ID from miniAOD selector (1=MiniIsoLoose, 2=MiniIsoMedium, 3=MiniIsoTight, 4=MiniIsoVeryTight)
Muon_miniPFRelIso_all Float_t mini PF relative isolation, total (with scaled rho*EA PU corrections)
Muon_miniPFRelIso_chg Float_t mini PF relative isolation, charged component
Muon_multiIsoId UChar_t MultiIsoId from miniAOD selector (1=MultiIsoLoose, 2=MultiIsoMedium)
Muon_mvaId UChar_t Mva ID from miniAOD selector (1=MvaLoose, 2=MvaMedium, 3=MvaTight, 4=MvaVTight, 5=MvaVVTight)
Muon_mvaLowPt Float_t Low pt muon ID score
Muon_mvaLowPtId UChar_t Low Pt Mva ID from miniAOD selector (1=LowPtMvaLoose, 2=LowPtMvaMedium)
Muon_mvaTTH Float_t TTH MVA lepton ID score
Muon_nStations Int_t number of matched stations with default arbitration (segment & track)
Muon_nTrackerLayers Int_t number of layers in the tracker
Muon_pdgId Int_t PDG code assigned by the event reconstruction (not by MC truth)
Muon_pfIsoId UChar_t PFIso ID from miniAOD selector (1=PFIsoVeryLoose, 2=PFIsoLoose, 3=PFIsoMedium, 4=PFIsoTight, 5=PFIsoVeryTight, 6=PFIsoVeryVeryTight)
Muon_pfRelIso03_all Float_t PF relative isolation dR=0.3, total (deltaBeta corrections)
Muon_pfRelIso03_chg Float_t PF relative isolation dR=0.3, charged component
Muon_pfRelIso04_all Float_t PF relative isolation dR=0.4, total (deltaBeta corrections)
Muon_phi Float_t phi
Muon_pt Float_t pt
Muon_ptErr Float_t ptError of the muon track
Muon_puppiIsoId UChar_t PuppiIsoId from miniAOD selector (1=Loose, 2=Medium, 3=Tight)
Muon_segmentComp Float_t muon segment compatibility
Muon_sip3d Float_t 3D impact parameter significance wrt first PV
Muon_softId Bool_t soft cut-based ID
Muon_softMva Float_t soft MVA ID score
Muon_softMvaId Bool_t soft MVA ID
Muon_tightCharge Int_t Tight charge criterion using pterr/pt of muonBestTrack (0:fail, 2:pass)
Muon_tightId Bool_t cut-based ID, tight WP
Muon_tkIsoId UChar_t TkIso ID (1=TkIsoLoose, 2=TkIsoTight)
Muon_tkRelIso Float_t Tracker-based relative isolation dR=0.3 for highPt, trkIso/tunePpt
Muon_triggerIdLoose Bool_t TriggerIdLoose ID
Muon_tunepRelPt Float_t TuneP relative pt, tunePpt/pt
nMuon UInt_t slimmedMuons after basic selection (pt > 15

These branches for muons are very similar to those we saw earlier for electrons:

  • pt, eta, phi, and mass form the 4-vector
  • charge and tightCharge give electric charge information
  • dxy, dz, ip3d, and their uncertainties or significances give impact parameter information
muon 4-vector and track-related branches
Object property Type Description
Muon_charge Int_t electric charge
Muon_dxy Float_t dxy (with sign) wrt first PV, in cm
Muon_dxyErr Float_t dxy uncertainty, in cm
Muon_dxybs Float_t dxy (with sign) wrt the beam spot, in cm
Muon_dz Float_t dz (with sign) wrt first PV, in cm
Muon_dzErr Float_t dz uncertainty, in cm
Muon_eta Float_t eta
Muon_ip3d Float_t 3D impact parameter wrt first PV, in cm
Muon_mass Float_t mass
Muon_phi Float_t phi
Muon_pt Float_t pt
Muon_sip3d Float_t 3D impact parameter significance wrt first PV
Muon_tightCharge Int_t Tight charge criterion using pterr/pt of muonBestTrack (0:fail, 2:pass)

Muon identification and isolation

The CMS Muon object group has created member functions for the identification algorithms that store pass/fail decisions about the quality of each muon. A set of cut-based identification working points are available: looseId, mediumId, mediumPromptId, softId, highPtId. Another set of identification algorithms are based on MVA discriminants: mvaId, mvaLowPtId, mvaTTH, softMvaId.

Muon identification variables
Object property Type Description
Muon_highPtId UChar_t high-pT cut-based ID (1 = tracker high pT, 2 = global high pT, which includes tracker high pT)
Muon_looseId Bool_t muon is loose muon
Muon_mediumId Bool_t cut-based ID, medium WP
Muon_mediumPromptId Bool_t cut-based ID, medium prompt WP
Muon_mvaId UChar_t Mva ID from miniAOD selector (1=MvaLoose, 2=MvaMedium, 3=MvaTight, 4=MvaVTight, 5=MvaVVTight)
Muon_mvaLowPtId UChar_t Low Pt Mva ID from miniAOD selector (1=LowPtMvaLoose, 2=LowPtMvaMedium)
Muon_mvaTTH Float_t TTH MVA lepton ID score
Muon_softId Bool_t soft cut-based ID
Muon_softMvaId Bool_t soft MVA ID
Muon_tightId Bool_t cut-based ID, tight WP

Hard processes produce large angles between the final state partons. The final object of interest will be separated from the other objects in the event or be isolated. For instance, an isolated muon might be produced in the decay of a W boson. In contrast, a non-isolated muon can come from a weak decay inside a jet.

Muon isolation is calculated from a combination of factors: energy from charged hadrons, energy from neutral hadrons, and energy from photons, all in a cone of radius \(R = \sqrt{\eta^2 + \phi^2} < 0.3\) or \(<0.4\) around the muon. Many algorithms also feature a correction factor that subtracts average energy expected from pileup contributions to this con. The sum of the \(p_{T}\) of the charged hadrons associated to vertices other than the primary vertex, is used to correct for pileup contamination in the total flux of neutrals found in the muon isolation cone. A factor of \(\beta = 0.5\) is used to scale this contribution as:

\(I_{\mu} = \frac{1}{p_{T}} \sum_{R<0.4} \left( p_{T}^{\mathrm{charged\,hadrons}} + \max(p_{T}^{\mathrm{photons}} + p_{T}^{\mathrm{neutral\,hadrons}} - \beta p_{T}^{\mathrm{charged\,pileup}} , 0) \right)\)

Many forms of muon isolation are stored in NanoAOD, as shown in the table. The primary particle-flow isolation variable is Muon_pfIsoId. Another type of isolation in common us is “mini”-isolation, Muon_miniIsoId, which adapts the size of the cone to improve efficiency for leptons that might exist near jets because they were decay products of a high-momentum particle, such as a top quark.

Muon isolation variables
Object property Type Description
Muon_jetRelIso Float_t Relative isolation in matched jet (1/ptRatio-1, pfRelIso04_all if no matched jet)
Muon_miniIsoId UChar_t MiniIso ID from miniAOD selector (1=MiniIsoLoose, 2=MiniIsoMedium, 3=MiniIsoTight, 4=MiniIsoVeryTight)
Muon_miniPFRelIso_all Float_t mini PF relative isolation, total (with scaled rho*EA PU corrections)
Muon_miniPFRelIso_chg Float_t mini PF relative isolation, charged component
Muon_multiIsoId UChar_t MultiIsoId from miniAOD selector (1=MultiIsoLoose, 2=MultiIsoMedium)
Muon_pfIsoId UChar_t PFIso ID from miniAOD selector (1=PFIsoVeryLoose, 2=PFIsoLoose, 3=PFIsoMedium, 4=PFIsoTight, 5=PFIsoVeryTight, 6=PFIsoVeryVeryTight)
Muon_pfRelIso03_all Float_t PF relative isolation dR=0.3, total (deltaBeta corrections)
Muon_pfRelIso03_chg Float_t PF relative isolation dR=0.3, charged component
Muon_pfRelIso04_all Float_t PF relative isolation dR=0.4, total (deltaBeta corrections)
Muon_puppiIsoId UChar_t PuppiIsoId from miniAOD selector (1=Loose, 2=Medium, 3=Tight)
Muon_tkIsoId UChar_t TkIso ID (1=TkIsoLoose, 2=TkIsoTight)
Muon_tkRelIso Float_t Tracker-based relative isolation dR=0.3 for highPt, trkIso/tunePpt

Muon cross-reference indices

Like electrons, muons can be cross-referenced to other arrays in the NanoAOD file:

Muon cross-reference indices
Object property Type Description
Muon_fsrPhotonIdx Int_t (index to Fsrphoton) Index of the associated FSR photon
Muon_jetIdx Int_t (index to Jet) index of the associated jet (-1 if none)

Tau leptons


The CMS Tau object group relies almost entirely on pre-computed algorithms to determine the quality of the tau reconstruction and the decay type. Since this object is not stable and has several decay modes, different combinations of identification and isolation algorithms are used across different analyses. The Run 1 Tau ID page and Nutshell Recipe provide a large table of algorithms that remains a valuable reference.

Taus that decay to leptons are typically identified as electrons or muons in CMS. But taus that decay to hadrons can be identified in the calorimeters based on the characteristic size and shape of their clusters.

Tau collection branches
Object property Type Description
Tau_charge Int_t electric charge
Tau_chargedIso Float_t charged isolation
Tau_cleanmask UChar_t simple cleaning mask with priority to leptons
Tau_decayMode Int_t decayMode()
Tau_dxy Float_t d_{xy} of lead track with respect to PV, in cm (with sign)
Tau_dz Float_t d_{z} of lead track with respect to PV, in cm (with sign)
Tau_eta Float_t eta
Tau_idAntiEleDeadECal Bool_t Anti-electron dead-ECal discriminator
Tau_idAntiMu UChar_t Anti-muon discriminator V3: : bitmask 1 = Loose, 2 = Tight
Tau_idDecayModeOldDMs Bool_t tauID(‘decayModeFinding’)
Tau_idDeepTau2017v2p1VSe UChar_t byDeepTau2017v2p1VSe ID working points (deepTau2017v2p1): bitmask 1 = VVVLoose, 2 = VVLoose, 4 = VLoose, 8 = Loose, 16 = Medium, 32 = Tight, 64 = VTight, 128 = VVTight
Tau_idDeepTau2017v2p1VSjet UChar_t byDeepTau2017v2p1VSjet ID working points (deepTau2017v2p1): bitmask 1 = VVVLoose, 2 = VVLoose, 4 = VLoose, 8 = Loose, 16 = Medium, 32 = Tight, 64 = VTight, 128 = VVTight
Tau_idDeepTau2017v2p1VSmu UChar_t byDeepTau2017v2p1VSmu ID working points (deepTau2017v2p1): bitmask 1 = VLoose, 2 = Loose, 4 = Medium, 8 = Tight
Tau_jetIdx Int_t (index to Jet) index of the associated jet (-1 if none)
Tau_leadTkDeltaEta Float_t eta of the leading track, minus tau eta
Tau_leadTkDeltaPhi Float_t phi of the leading track, minus tau phi
Tau_leadTkPtOverTauPt Float_t pt of the leading track divided by tau pt
Tau_mass Float_t mass
Tau_neutralIso Float_t neutral (photon) isolation
Tau_phi Float_t phi
Tau_photonsOutsideSignalCone Float_t sum of photons outside signal cone
Tau_pt Float_t pt
Tau_puCorr Float_t pileup correction
Tau_rawDeepTau2017v2p1VSe Float_t byDeepTau2017v2p1VSe raw output discriminator (deepTau2017v2p1)
Tau_rawDeepTau2017v2p1VSjet Float_t byDeepTau2017v2p1VSjet raw output discriminator (deepTau2017v2p1)
Tau_rawDeepTau2017v2p1VSmu Float_t byDeepTau2017v2p1VSmu raw output discriminator (deepTau2017v2p1)
Tau_rawIso Float_t combined isolation (deltaBeta corrections)
Tau_rawIsodR03 Float_t combined isolation (deltaBeta corrections, dR=0.3)
nTau UInt_t slimmedTaus after basic selection (pt > 18 && tauID(‘decayModeFindingNewDMs’) && (tauID(‘byLooseCombinedIsolationDeltaBetaCorr3Hits’)

Tau identification variables

The following variables in the tau collection represent the identification and isolation variables.

Tau identification variables
Object property Type Description
Tau_decayMode Int_t decayMode()
Tau_idAntiEleDeadECal Bool_t Anti-electron dead-ECal discriminator
Tau_idAntiMu UChar_t Anti-muon discriminator V3: : bitmask 1 = Loose, 2 = Tight
Tau_idDecayModeOldDMs Bool_t tauID(‘decayModeFinding’)
Tau_idDeepTau2017v2p1VSe UChar_t byDeepTau2017v2p1VSe ID working points (deepTau2017v2p1): bitmask 1 = VVVLoose, 2 = VVLoose, 4 = VLoose, 8 = Loose, 16 = Medium, 32 = Tight, 64 = VTight, 128 = VVTight
Tau_idDeepTau2017v2p1VSjet UChar_t byDeepTau2017v2p1VSjet ID working points (deepTau2017v2p1): bitmask 1 = VVVLoose, 2 = VVLoose, 4 = VLoose, 8 = Loose, 16 = Medium, 32 = Tight, 64 = VTight, 128 = VVTight
Tau_idDeepTau2017v2p1VSmu UChar_t byDeepTau2017v2p1VSmu ID working points (deepTau2017v2p1): bitmask 1 = VLoose, 2 = Loose, 4 = Medium, 8 = Tight
Tau_rawIso Float_t combined isolation (deltaBeta corrections)
Tau_rawIsodR03 Float_t combined isolation (deltaBeta corrections, dR=0.3)

Other tau information

Information about the tau lepton 4-vectors, cross-reference indices, impact parameters, etc, are analogous to the variables for electrons and muons.

Key Points

  • Track access may differ, but track-related member functions are common across objects.
  • Physics objects in CMS are reconstructed from detector signals and are never 100% certain!
  • Muons typically use pre-configured identification and isolation variables”