Muon Isolation

Contacts

Slava Krutelyov (slava77@fnalNOSPAMPLEASE.gov)

Goals

• develop algorithms to define muon isolation quantities using calorimeters, tracker tracks, etc. for different types of muon candidates (standalone muons, global muons, etc.)
• define resulting object(s) to be stored
• provide algorithms which can be used for offline and HLT
• validate and optimize the performance of these algorithms

Status and introduction

Isolation of a muon is done relative to measurements in the detector (tracks, calorimeter towers and hits) that can be viewed as a swarm of other objects. Isolation of a muon relative to a list of other objects is not a trivial task in general. Highly flexible methods are provided in the CMSSW (>=1_5_0) together with the configurations which are run by the standard reconstruction with outputs stored in the event record. The outputs of these methods are intended to be flexible enough to suite most analysis purposes.

The following data can be obtained from the event record

• Sum-pt and n-track, sum-et in hcal, ecal and outer-cal (ho) in cones of dR = √(dφ^2 + dη^2) = 0.3 and 0.5 around the muon trajectory can be obtained in reco::Muon.
• A list of track-pt or tower et separately in hcal, ho, ecal (called deposits below) with their directions relative to the muon are available in the objects of IsoDeposit type.
  • These objects are stored in the event record as associations (MuIsoDepositAssociationMap in CMSSW_1_6_X, IsoDepositMap in >=2_0_X).
  • Easy access to the IsoDeposits is provided by PAT: use pat::Muon::isoDeposit(key). It is a recommended way to access IsoDeposits associated to muons.
  • Based on the content stored in IsoDeposit it is possible to recalculate nearly any quantity related to (pt,eta,phi) of the deposits around the muon: sum in a different cone (or even a rectangle, solid angle, or a combination of them), count above threshold, all the above relative to a different direction, remove (veto) any of the deposits based on their direction.
  • You can get a sumPt by simply calling deposit.depositWithin(0.3);//sum of deposits in cone 0.3.

SHORT DESCRIPTION OF CUTS AND SELECTION USED IN THE CODE IS GIVEN in this not well-formatted text. Documentation note is currently in development: it is available in cvs.

Frequently Asked Questions (FAQs)

CMSSW packages (doxygen documentation)

• DataFormats/MuonReco
• RecoMuon/MuonIsolationProducers
  • NB: IsolationExample.cc in releases <=CMSSW_2_0_0 is out of date. For studies in CMSSW_1_6_X use the examples available below.
• RecoMuon/CMS.MuonIsolation
• RecoMuon/L2MuonIsolationProducer
• RecoMuon/L3MuonIsolationProducer

Suggested cuts

• For tracker
  • trackIso = sumPt < 3 GeV in a cone of radius 0.3 around the muon will give a 96.7 \pm 0.7 % efficiency for isolated muons from W or Z decays with about a factor of 10 rejection of muon candidates reconstructed in QCD events.
    • You might want to require chi2/ndof < 10, nTrackerHits > 10 as a quality cut on reconstructed muon (applied to a global muon, this cut allows to suppress fake muons from K/pi). See more details in CMS-AN2008/098.
• For the calorimeters
  • caloIso = sumEtHcal+sumEtEcal < 5 GeV in the cone of 0.3.

The cuts above are given only as a guideline for a fairly loose isolation requirement for muons from electroweak (W/Z and alike) kind of sources. If desired, further background reduction can be achieved by tightening the above cuts: either reduce the value of the cut or apply a cut on the relative isolation relIso = (trackIso+caloIso)/muonPt < x, which allows to effectively suppress backgrounds with softer spectrum than the signal muons. The optimal isolation selection is specific to each particular analysis.

Examples of access to isolation

For many analyses sums in cones 0.3 and 0.5 available as reco::Muon::getIsolationR03,05().{sumPt, emEt, hadEt, hoEt, nTracks} are enough to perform muon isolation relative to tracks or calorimeter measurements.

Access to Isolation using IsoDeposit maps

gSystem->Load("libFWCoreFWLite");
AutoLibraryLoader::enable();
f = new TFile("isoTest.root");
e = (TTree*)f->Get("Events");
e->SetAlias("hcaldep","recoIsoDepositedmValueMap_muIsoDepositCalByAssociatorTowers_hcal_RECO.obj.values_");
// e.g., muonEnergy() == sumEt close to a muon (r<0.1 for hcal).
e->Draw("hcaldep.muonEnergy()", "abs(hcaldep.direction().eta())<1.", "");
// e.g., depositWithin(0.4) == sumEt in dR<0.4, excluding deposit from a muon.
e->Draw("hcaldep.depositWithin(0.4)", "abs(hcaldep.direction().eta())<1.", "");