Difference: WorkBookMuonAnalysis (90 vs. 91)

Revision 912017-08-30 - JhovannyMejia

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7.8 Muon Analysis

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  This document explains how to get access to muon tracks and associated information available in the event record, which can be used for muon identification purposes as well as analyses. Brief summary of muon reconstruction algorithms and of muon selectors is provided. (See WorkBookSetComputerNode for instructions on how to set up a proper CMSSW computing environment.)
Nota bene: We will access muon information in miniAOD files. Please, check the muon information stored in the miniAOD. This link here could be very useful to get familiar with muons in the miniAOD too.

Introduction to muon reconstruction

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  References to the matching tracker tracks, standalone muon tracks, and global muon tracks are assembled into one single collection of reco::Muon objects described below.

High-pT muons

As the muon traverses the iron yoke, multiple scattering and/or showers can corrupt the measurement of the muon trajectory, so using all available hits in the track fit is not always the best choice. In order to provide improved performance for energetic muons, the global muon track is refitted with different hit content. The results of two alternative global track refits are available in RECO and AOD collections:

  • "Tracker plus the first muon station" (TPFMS) algorithm uses all tracker hits and hits in the innermost muon station containing hits.
  • "Picky muon reconstructor" (PMR) uses all tracker hits and hits in those muon stations which do not appear to contain electromagnetic showers.
These refitted tracks are accessible through TrackToTrackMap's linking to the corresponding global muon track:
Handle <reco::TrackToTrackMap> tevMap;
iEvent.getByLabel("tevMuons", "refit_name", tevMap);

where refit_name can be default (a refit of the global trajectory with all hits), firstHit (TPFMS refit) or picky (PMR refit).

A method for selecting the best refit on a track-by-track basis is also provided, and is illustrated below. This "cocktail" of fits is intended to provide the best performance for high-pT muons in terms of resolution and controlling the non-gaussian tails.

#include "DataFormats/MuonReco/interface/MuonCocktails.h"
iEvent.getByLabel("tevMuons", "default", tevMapH1);
const reco::TrackToTrackMap tevMap1 = *(tevMapH1.product());
iEvent.getByLabel("tevMuons", "firstHit", tevMapH2);
const reco::TrackToTrackMap tevMap2 = *(tevMapH2.product());
iEvent.getByLabel("tevMuons", "picky", tevMapH3);
const reco::TrackToTrackMap tevMap3 = *(tevMapH3.product());
iEvent.getByLabel(MuonTags_, MuCollection);
const reco::MuonCollection muonC = *(MuCollection.product());

for(imuon = muonC.begin(); imuon != muonC.end(); ++imuon) 
  reco::TrackRef pmcTrack = muon::tevOptimized(*imuon, tevMap1, tevMap2, tevMap3);

An EDProducer, MuonsFromRefitTracksProducer, has been implemented to create reco::MuonCollection out of any of the above tracks, so that the refit muons may be used anywhere a reco::Muon can be (e.g. using View and modules in the PAT, such as CandViewCombiner). For each reco::Muon object in the input collection, the corresponding track is found in the selected refit collection. A clone of the reco::Muon is then stored in the output collection, with its kinematic variables (momentum, charge, vertex) taken from the refit track. The module's use from a python configuration file is illustrated here. After that, PAT modules can be used; e.g.

 # Make dimuons out of the cocktail muons we just made.
 process.cocktailDimuons = cms.EDProducer('CandViewCombiner',
   decay = cms.string('cocktailMuons@+ cocktailMuons@-'),
   cut = cms.string('mass > 110.0')

Tracker muons

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  Brief summary of available information:
  • Methods to check to which category of muon candidates a given reco::Muon object belongs: isStandAloneMuon(), isGlobalMuon(), isTrackerMuon() (isCaloMuon() is currently not used). Note that a single object can belong to more than one category.
  • References to the corresponding reco::Track objects for tracks reconstructed in the inner tracker (tracker track), muon system alone (standalone muon track with a beam-spot constraint), and to a combined tracker-muon fit (global muon track). Refits available for high-pT muons can be accessed through TrackToTrackMap's linking to the global muon track as described above.
  • References to the corresponding reco::Track objects for tracks reconstructed in the inner tracker (tracker track), muon system alone (standalone muon track with a beam-spot constraint), and to a combined tracker-muon fit (global muon track). High-pT muons can be accessed through isHighPtMuon.
  • For tracker muons:
    • Energy depositions in ECAL, HCAL and HO in crossed elements as well as 3x3 shapes around them.
    • Compatibility with muon hypothesis based on energy depositions in calorimeters.
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