SWGuideTrackerMuons

Contacts

Claudio Campagnari (claudio@hepNOSPAMPLEASE.ucsb.edu), Dmytro Kovalskyi (dmytro@slacNOSPAMPLEASE.stanford.edu)

General idea and links to the documentation

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.

The tracker-muon algorithm is particularly useful for the identification of low-p_T muons (with p_T 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 p_T > 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).

Changes with respect to the above notes

As of May 2010.

In what follows a "required station" is a station in which a tracker muon traverses well into the active fiducial volume of a muon chamber in that station. A tracker muon traverses well into the active fiducial volume of a muon chamber if it's expected position is inside the chamber and at least 3cm AND 3 multiple-scattering sigma from the chamber edge.

Changes to existing algorithms

• The default reco::Muon::ArbitrationType for all reco::Muon methods has changed from reco::Muon::SegmentArbitration to reco::Muon::SegmentAndTrackArbitration. At the time we were paranoid about inefficiencies introduced by the track arbitration, but as we have never recommended that users use anything but reco::Muon::SegmentAndTrackArbitration we changed the default to that.

• All tracker-muon-based muon identification algorithms now explicitly require that the given reco::Muon be a tracker muon. It may of course also be other types of muons, but it must be a tracker muon.

• TMLastStationLoose, TMLastStationTight:
  • An inefficiency was discovered for tracker muons in the barrel region whereby high pT tracker muons that have only one required station have only one segment match -- this can happen for prompt muons traversing the gaps between the MB0 and MB+-1 chambers. However, the TMLastStation algorithms required that the tracker muon have >=2 segment matches. The algorithms were updated so that if the number of required stations is less than the minimum number of segment matches, the minimum number of segment matches is reduced to the number of required stations. This is done only in the barrel, for |eta| < 1.2.
  • If there are zero required stations, then rather than requiring that the outermost required station have a good segment match, we look at the outermost station for which we have a segment match and require that it be good.

• TMLastStationOptimizedLowPtLoose, TMLastStationOptimizedLowPtTight
  • When early 900GeV data began to arrive, those searching for low mass resonances used these algorithms blindly because of the "LowPt" in the names. Little did they know that "LowPt" considerations were made only for muons in the barrel region. These, in fact, are the same considerations addressed above for the TMLastStationLoose and TMLastStationTight algorithms. The algorithms were modified to extend the same number of required stations vs. minimum number of segment matches considerations to all eta which increases greatly the efficiency for low pT endcap muons where it is easy to have only one segment match.

• TM2DCompatibilityLoose, TM2DCompatibilityTight:
  • The default reco::Muon::ArbitrationType used in the segmentCompatibility, which is utilized heavily in the compatibility algorithms, is now reco::Muon::SegmentAndTrackArbitration. It used to be reco::Muon::SegmentArbitration as that was the default for the various reco::Muon methods, however, that it was not always reco::Muon::SegmentAndTrackArbitration for the segmentCompatibility is considered a bug.

New algorithms

• TMLastStationAngLoose, TMLastStationAngTight
  • These algorithms are nearly identical to the TMLastStationLoose and TMLastStationTight algorithms. There are two differences:
    1. The number of required stations vs. minimum number of segment matches considerations are extended to all eta -- recall they are limited to the barrel region in the TMLastStationLoose and TMLastStationTight algorithms.
    2. In addition to applying position and pull cuts to the segment match in the deepest required station, angular pull cuts are applied as well. In TMLastStationAngLoose the dXdZ pull, defined as delta(dXdZ)/sigma(delta(dXdZ)) between the muon segment and the extrapolated track positions, must satisfy |dXdZ pull| < 3. In TMLastStationAngTight the match must satisfy |dXdZ pull| < 3 and |dYdZ pull| < 3. These algorithms have been used widely in low pT analyses as the angular cuts do well to fight background.

• TMOneStationAngLoose, TMOneStationAngTight
  • These algorithms are identical to the TMOneStationLoose and TMOneStationTight algorithms, except that in addition to applying position and pull cuts to segment matches, angular pull cuts are applied as well. In TMOneStationAngLoose the dXdZ pull must satisfy |dXdZ pull| < 3. In TMOneStationAngTight the match must satisfy |dXdZ pull| < 3 and |dYdZ pull| < 3.

• TMLastStationOptimizedBarrelLowPtLoose, TMLastStationOptimizedBarrelLowPtTight
  • These algorithms are identical to what were previously known as TMLastStationOptimizedLowPtLoose and TMLastStationOptimizedLowPtTight. The names were changed because indeed the low pT optimization applies only to muons in the barrel region. We hope this will be less confusing for future generations of CMS members.

Dimuon selectors

In order to suppress dimuon fakes originating from the same physical muon users are advised to use the following method available in DataFormats/MuonReco/interface/MuonSelectors.h

bool overlap( const reco::Muon& muon1, const reco::Muon& muon2, double pullX = 1.0, double pullY = 1.0, bool checkAdjacentChambers = false)

The method checks if two muon trajectories overlap. The overlap check is performed by comparing distance between the two trajectories if they cross the same muon chamber. Trajectories overlap if distance divided by uncertainty is smaller than allowed pullX and pullY. The optional argument checkAdjacentChambers allows to get rid of fake dimuon pairs if two segments represent one physical muon crossing two overlapping chambers. The default values give a reasonable starting point that slightly improves signal/background ratio for J/psi->mumu.

Code is available in CMSSW_3_6_1 and later releases.

Configuration parameters

The tracker-muon algorithm has many parameters that are configurable. For a full list of parameters and their default values, see RecoMuon/CMS.MuonIdentification/python/muons_cfi.py

Responsible: DmytroKovalskyi
Last reviewed by: SlavaValuev - 04 Mar 2009