Difference: WorkBookMuonAnalysis (89 vs. 90)

Revision 902017-08-14 - JhovannyMejia

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META TOPICPARENT name="WorkBook"

7.8 Muon Analysis

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  Muon reconstruction overview:
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muonreco.png
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muonreco.png
  The muon reconstruction chain starts with the "local reconstruction". First, hits in DTs, CSCs and RPCs are reconstructed from digitized electronics signals. Hits within each DT and CSC chamber are then matched to form "segments" (track stubs). The production of hits and segments in muon sub-systems is discussed in detail in the tutorial on local muon reconstruction. The reconstruction of the tracks inside the silicon tracker is described in the tutorial on track reconstruction.
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Standalone muons

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<!--STARTWORKBOOK-->
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In the offline reconstruction, the segments reconstructed in the muon chambers are used to generate "seeds" consisting of position and direction vectors and an estimate of the muon transverse momentum. These initial estimates are used as seeds for the track fits in the muon system, which are performed using segments and hits from DTs, CSCs and RPCs and are based on the Kalman filter technique. The result is a collection of reco::Track objects reconstructed in the muon spectrometer, which are referred to as "standalone muons". To improve the momentum resolution, a beam-spot constraint can be applied in the fit. Two collections of "standalone muons", with and without the beam-spot constraint, are available in the event record (in both RECO and AOD). A more detailed description of the reconstruction of tracks in the muon system alone can be found in Section 4 of CMS AN 2008/097.
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In the offline reconstruction, the segments reconstructed in the muon chambers are used to generate "seeds" consisting of position and direction vectors and an estimate of the muon transverse momentum. These initial estimates are used as seeds for the track fits in the muon system, which are performed using segments and hits from DTs, CSCs and RPCs and are based on the Kalman filter technique. The result is a collection of reco::Track objects reconstructed in the muon spectrometer, which are referred to as "standalone muons". To improve the momentum resolution, a beam-spot constraint can be applied in the fit. Two collections of "standalone muons", with and without the beam-spot constraint, are available in the event record (in both RECO and AOD). A more detailed description of the reconstruction of tracks in the muon system alone can be found in Section 4 of CMS AN 2008/097.
 
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A tutorial explaining how to run standalone muon reconstruction and to analyze its results is available at this link.

Global muons

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For each standalone muon track, a search for tracks matching it among those reconstructed in the inner tracking system (referred to as "tracker tracks", "inner tracks" or "silicon tracks") is performed, and the best-matching tracker track is selected. For each "tracker track" - "standalone muon" pair, the track fit using all hits in both tracks is performed, again based on the Kalman filter technique. The result is a collection of reco::Track objects referred to as "global muons". More details on the reconstruction of global muons can be found in Section 5 of CMS AN 2008/097.
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For each standalone muon track, a search for tracks matching it among those reconstructed in the inner tracking system (referred to as "tracker tracks", "inner tracks" or "silicon tracks") is performed, and the best-matching tracker track is selected. For each "tracker track" - "standalone muon" pair, the track fit using all hits in both tracks is performed, again based on the Kalman filter technique. The result is a collection of reco::Track objects referred to as "global muons". More details on the reconstruction of global muons can be found in Section 5 of CMS AN 2008/097.
 
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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.
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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

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 These refitted tracks are accessible through TrackToTrackMap's linking to the corresponding global muon track:
Handle <reco::TrackToTrackMap> tevMap;
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iEvent.getByLabel("tevMuons", "refit_name", tevMap);
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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.

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 const reco::MuonCollection muonC = *(MuCollection.product());

for(imuon = muonC.begin(); imuon = muonC.end(); ++imuon)

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reco::TrackRef pmcTrack = muon::tevOptimized(*imuon, tevMap1, tevMap2, tevMap3);
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reco::TrackRef pmcTrack = muon::tevOptimized(*imuon, tevMap1, tevMap2, tevMap3);
 
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An EDProducer, MuonsFromRefitTracksProducer, has been implemented to
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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
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 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
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track. The module's use from a python configuration file is illustrated here. After that, PAT modules can be used; e.g.
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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')
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Tracker muons

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Tracker muons

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<!--STARTWORKBOOK-->
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An approach complementary to the global-muon reconstruction consists in considering all tracker tracks to be potential muon candidates and in checking this hypothesis by looking for compatible signatures in the calorimeters and in the muon system. Tracker tracks identified as muons by this method are referred to as "tracker muons". A detailed description of the reconstruction of tracker muons can be found in Section 6 of CMS AN 2008/097.
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An approach complementary to the global-muon reconstruction consists in considering all tracker tracks to be potential muon candidates and in checking this hypothesis by looking for compatible signatures in the calorimeters and in the muon system. Tracker tracks identified as muons by this method are referred to as "tracker muons". A detailed description of the reconstruction of tracker muons can be found in Section 6 of CMS AN 2008/097.
 
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The tracker-muon algorithm is particularly useful for the identification of low-pT muons (with pT of the order of several GeV), which may not leave enough hits in the muon stations for a standalone muon to be reconstructed. The default criteria for tagging a tracker track as "tracker muon" are very loose (in CMSSW_3_X_Y series, every track with p > 2.5 GeV and pT > 0.5 GeV matched with at least one segment in the muon stations is labeled as "tracker muon"), so tracker muons should in general not be used without further requirements. Several sets of such requirements recommended by the muon POG are described in Sections 6-8 of CMS AN 2008/098; the corresponding flags can be retrieved from the reco::Muon object (see WorkBook section on muon ID).
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The tracker-muon algorithm is particularly useful for the identification of low-pT muons (with pT of the order of several GeV), which may not leave enough hits in the muon stations for a standalone muon to be reconstructed. The default criteria for tagging a tracker track as "tracker muon" are very loose (in CMSSW_3_X_Y series, every track with p > 2.5 GeV and pT > 0.5 GeV matched with at least one segment in the muon stations is labeled as "tracker muon"), so tracker muons should in general not be used without further requirements. Several sets of such requirements recommended by the muon POG are described in Sections 6-8 of CMS AN 2008/098; the corresponding flags can be retrieved from the reco::Muon object (see WorkBook section on muon ID).
 
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#RPCMu

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  #RPCMu

RPC muons

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An approach similar to Tracker Muons is followed to define the RPC Muons: in this case a match is sought between the extrapolated inner track and hits on the RPC muon detectors.
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A description of the algorithm and the performance measurements is contained in the CMS AN-2012/482 .
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A description of the algorithm and the performance measurements is contained in the CMS AN-2012/482 .
 The effects of including the RPC hits in the global muon reconstruction have also been studied and are described in M.S.Kim, JINST 8 (2013) T03001 .
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The main twiki page documenting the RPC Muon algorithm is linked HERE .
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The main twiki page documenting the RPC Muon algorithm is linked HERE .
 
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Calorimeter-based muons

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Calorimeter-based muons, or "calo muons" for short, represent a subset of all tracker tracks reconstructed in the event, which includes tracks with energy depositions in the calorimeters compatible with those of a minimum-ionizing particle. The fake rate of these muon candidates is high and they should not be used when muon purity is essential. A typical use case for "calo muons" is the reconstruction of the J/ψ decaying to low-momentum muons that have little or no information in the muon system, thus improving signal to background ratio compared with the inner tracks. In the event record, "calo muons" are stored in a collection of dedicated objects, reco::CaloMuon's.
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Calorimeter-based muons, or "calo muons" for short, represent a subset of all tracker tracks reconstructed in the event, which includes tracks with energy depositions in the calorimeters compatible with those of a minimum-ionizing particle. The fake rate of these muon candidates is high and they should not be used when muon purity is essential. A typical use case for "calo muons" is the reconstruction of the J/ψ decaying to low-momentum muons that have little or no information in the muon system, thus improving signal to background ratio compared with the inner tracks. In the event record, "calo muons" are stored in a collection of dedicated objects, reco::CaloMuon's.
 

Available information

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The information on various types of muons discussed above is assembled in one single collection of reco::Muon objects, "muons". This collection serves as the main entry point for accessing muon-related information in CMSSW: various quantities are either directly included in the reco::Muon objects, or can be accessed from there via provided references. One exception to this rule is "calo muons", which are stored in a separate collection of reco::CaloMuon objects. The full list of muon collections available in the event record is available in SWGuideDataFormatRecoMuon.
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The information on various types of muons discussed above is assembled in one single collection of reco::Muon objects, "muons". This collection serves as the main entry point for accessing muon-related information in CMSSW: various quantities are either directly included in the reco::Muon objects, or can be accessed from there via provided references. One exception to this rule is "calo muons", which are stored in a separate collection of reco::CaloMuon objects. The full list of muon collections available in the event record is available in SWGuideDataFormatRecoMuon.
 

reco::Muon

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Full description of the reco::Muon class can be found here: DataFormats/MuonReco/interface/Muon.h.
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Full description of the reco::Muon class can be found here: DataFormats/MuonReco/interface/Muon.h.
  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.
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reco::Muon timing information

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Timing information is calculated from information in the DT, CSC and ECAL (RPC timing information is not available for offline at all; efforts to implement HCAL timing are currently under way). Information is stored in reco::MuonTime format. It contains a summary of the combined fit of all subdetector data with two estimates of time at the interaction point and its error, assuming an outside-in or inside-out β=1 particle, plus an estimate of the muon's direction.
 
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More detailed information about muon timing (time, velocity with and without the assumption of the muon being produced in-time, etc.) separated into subsystems is available in a set external products of type reco::MuonTimeExtra linked to the muons via association maps. If necessary, these objects can be regenerated independent of the muon reconstruction chain using the MuonTimingProducer.
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Timing information is calculated from information in the DT, CSC and ECAL (RPC timing information is not available for offline at all; efforts to implement HCAL timing are currently under way). Information is stored in reco::MuonTime format. It contains a summary of the combined fit of all subdetector data with two estimates of time at the interaction point and its error, assuming an outside-in or inside-out β=1 particle, plus an estimate of the muon's direction.
 
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For a simple example of accessing all the information, please take a look at MuonTimingValidator.cc in here. For more details on the interpretation of the different quantities and performance in early data, see CMS IN-2010/003.
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More detailed information about muon timing (time, velocity with and without the assumption of the muon being produced in-time, etc.) separated into subsystems is available in a set external products of type reco::MuonTimeExtra linked to the muons via association maps. If necessary, these objects can be regenerated independent of the muon reconstruction chain using the MuonTimingProducer.

For a simple example of accessing all the information, please take a look at MuonTimingValidator.cc in here. For more details on the interpretation of the different quantities and performance in early data, see CMS IN-2010/003.

 

Muon identification

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Detailed information about muon identification and the various selection algorithms can be found in the CMS AN-2008/098 analysis note. There is a detailed accounting of changes made to the tracker-muon-based muon identification algorithms since AN-2008/098 (over 1.5 years ago as of this writing) at SWGuideTrackerMuons. Below is a brief summary.
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Detailed information about muon identification and the various selection algorithms can be found in the CMS AN-2008/098 analysis note. There is a detailed accounting of changes made to the tracker-muon-based muon identification algorithms since AN-2008/098 (over 1.5 years ago as of this writing) at SWGuideTrackerMuons. Below is a brief summary.
  There are several categories of muon identification algorithms that have been developed:
  • Cut-based ID for global muons, which consists of a set of track-quality requirements
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 TMLastStationOptimizedBarrelLowPtLoose = 22, // combination of TMLastStation and TMOneStation with low pT optimization in barrel only TMLastStationOptimizedBarrelLowPtTight = 23 // combination of TMLastStation and TMOneStation with low pT optimization in barrel only };
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}
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The isGoodMuon method and the available selection types are defined in DataFormats/MuonReco/interface/MuonSelectors.h, so one must include this header file to utilize the selectors.
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The isGoodMuon method and the available selection types are defined in DataFormats/MuonReco/interface/MuonSelectors.h, so one must include this header file to utilize the selectors.
 
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The muon-identification algorithms recommended by the muon POG can be found in SWGuideMuonId. Tracker-muon selectors (TMLastStationLoose, etc.) are described in SWGuideTrackerMuons. GlobalMuonPromptTight consists of the following requirements, designed to suppress hadronic punch-throughs and muons from decays in flight:
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The muon-identification algorithms recommended by the muon POG can be found in SWGuideMuonId. Tracker-muon selectors (TMLastStationLoose, etc.) are described in SWGuideTrackerMuons. GlobalMuonPromptTight consists of the following requirements, designed to suppress hadronic punch-throughs and muons from decays in flight:
 
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muon.isGlobalMuon() && muon.globalTrack()->normalizedChi2() < 10. && muon.globalTrack()->hitPattern().numberOfValidMuonHits() > 0
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muon.isGlobalMuon() && muon.globalTrack()->normalizedChi2() < 10. && muon.globalTrack()->hitPattern().numberOfValidMuonHits() > 0
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  • the track is identified as a global muon
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  • χ2/ndof of the global muon fit < 10
  • number of valid muon-detector hits used in the global fit > 0
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  • χ2/ndof of the global muon fit < 10
  • number of valid muon-detector hits used in the global fit > 0
  The following additional track-quality cuts using the tracker-track information are recommended to further suppress non-prompt muons, although they are not explicitly included in the selection types above:
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  • Transverse impact parameter of the tracker track (or global muon) relative to the beam spot position, |d0|, < 2 mm. This loose cut preserves efficiency for muons from decays of b and c hadrons.
  • Number of hits in the tracker track, Nhits, > 10.
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  • Transverse impact parameter of the tracker track (or global muon) relative to the beam spot position, |d0|, < 2 mm. This loose cut preserves efficiency for muons from decays of b and c hadrons.
  • Number of hits in the tracker track, Nhits, > 10.
  One can also impose cuts on the last point in the global fit to reject punch-throughs that terminate in the first station of the muon detector:
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  // endcap region if ( abs(z) > 600 && abs(z) < 650 && r < 300) keepMuon = false;
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if ( abs(z) > 680 && abs(z) < 730 && r < 480) keepMuon = false;
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if ( abs(z) > 680 && abs(z) < 730 && r < 480) keepMuon = false;
 This can reduce the fake rate by ~20% with minimal loss in efficiency.
<!--STOPWORKBOOK-->
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More information can be found in CMS AN 2008/098, in SWGuideTrackerMuons, and in several dedicated talks: "Muons in CMSSW_2_1_X", "Global Muon Selections" and "Summary of the Muon ID Note".
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More information can be found in CMS AN 2008/098, in SWGuideTrackerMuons, and in several dedicated talks: "Muons in CMSSW_2_1_X", "Global Muon Selections" and "Summary of the Muon ID Note".
  The muon-identification criteria used by the Vector-Boson Task Force of the Electroweak PAG follow closely muon-POG recommendations and can be found here.
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  %IF{"defined PRINT" then="
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> EdmFileUtil -d /store/mc/2007/10/20/RelVal-RelValTTbar-1192895175/0000
/00C41641-2A81-DC11-B6EA-0019DB29C620.root dcap://cmsdca3.fnal.gov:24142/pnfs/fnal.gov/usr/cms/WAX/11/store/mc/2007/10/20/
RelVal-RelValTTbar-1192895175/0000/00C41641-2A81-DC11-B6EA-0019DB29C620.root

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> EdmFileUtil -d /store/mc/2007/10/20/RelVal-RelValTTbar-1192895175/0000
/00C41641-2A81-DC11-B6EA-0019DB29C620.root dcap://cmsdca3.fnal.gov:24142/pnfs/fnal.gov/usr/cms/WAX/11/store/mc/2007/10/20/
RelVal-RelValTTbar-1192895175/0000/00C41641-2A81-DC11-B6EA-0019DB29C620.root

 

" else="

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 root [0] Cintex::Enable() root [1] gSystem->Load("libFWCoreFWLite"); root [2] AutoLibraryLoader::enable();
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root [3] f = TFile::Open("events.root")
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root [3] f = TFile::Open("events.root")
  For each muon candidate in the event, let's look at the global-muon pT, inner-track pT, and ECAL energy deposition associated with the candidate:
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 ********************************************************* * 0 * 0 * 26.570809 * 26.387926 * 0.1964492 * * 1 * 0 * 9.6703965 * 9.6947574 * 5.2962083 *
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* 1 * 1 * 25.099496 * 25.039489 * 0.3504051 *
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* 1 * 1 * 25.099496 * 25.039489 * 0.3504051 *
  Isolation variables for the muon candidates (e.g., sum of pT's of tracks and sum of ET's for ECAL in a cone of R = 0.5) can be obtained with the following command:
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 ********************************************* * 0 * 0 * 0 * 4.5468702 * * 1 * 0 * 16.282365 * 66.035476 *
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* 1 * 1 * 7.0197601 * 2.9328436 *
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* 1 * 1 * 7.0197601 * 2.9328436 *
 

Review status

 
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