Transient Tracks

Usage of TransientTracks objects

For most high-level algorithms, the reco::Track is unsuitable, since it does not have access to the magnetic field, which prevents one estimating the track parameters at arbitrary points along its trajectory. A TransientTracks has thus to be used. It has a pointer to the magnetic field, and can be given a pointer to the geometry as well. To easiest and safest way is to build the TransientTracks using the TransientTrackBuilder.

Link to Reference Manual (doxygen): TransientTracks , TransientTrackBuilder .

To use the TransientTrackBuilder, you have to add the following into your cfg:

process.load("TrackingTools/TransientTrack/TransientTrackBuilder_cfi")
process.load("Configuration.Geometry.GeometryIdeal_cff")
process.load("Configuration.StandardSequences.MagneticField_cff")
process.load('Configuration.StandardSequences.FrontierConditions_GlobalTag_cff')
process.GlobalTag.globaltag = 'GR_P_V42_AN3::All'  # or some other global tag depending on your CMSSW release and sample. 

TransientTrackBuilderTest.cc

To get the TransientTrackBuilder from the EventSetup:

    edm::ESHandle<TransientTrackBuilder> theB;
    setup.get<TransientTrackRecord>().get("TransientTrackBuilder",theB);

You can now build your TransientTracks from Track, TrackRef, or directly from the edm::handle to a TrackCollection:

    reco::TransientTrack * build ( const reco::Track * p)  const;
    reco::TransientTrack * build ( const reco::TrackRef * p)  const;
    std::vector<reco::TransientTrack> build ( const edm::Handle<reco::TrackCollection> & trkColl)  const;

You also have to include these packages into your BuildFile.xml: TrackingTools/TransientTrack and TrackingTools/Records.

Examples (including calculation of track impact parameters)

    // get RECO tracks from the event
    edm::Handle<reco::TrackCollection> tks;
    iEvent.getByLabel(trackLabel(), tks);

    //get the builder:
    edm::ESHandle<TransientTrackBuilder> theB;
    iSetup.get<TransientTrackRecord>().get("TransientTrackBuilder",theB);
    //do the conversion:
    vector<TransientTrack> t_tks = (*theB).build(tks);

One can use TransientTracks to estimate track impact parameters with respect to the beam line or primary vertex, taking into account the curvature of the track.

// with respect to beam-line
TransientTrack tk = ...;
TrajectoryStateClosestToBeamLine traj = tk.stateAtBeamLine();
Measurement1D meas = traj.transverseImpactParameter();
double d0 = meas.value();
double d0_error = meas.error();

// with respect to any specified vertex, such as primary vertex
GlobalPoint vert(pv.x(), pv.y(), pv.z());
TrajectoryStateClosestToPoint  traj = tk.trajectoryStateClosestToPoint(vert );
double d0 = traj.perigeeParameters().transverseImpactParameter();
double d0_error = traj.perigeeError().transverseImpactParameterError();
double z0 = traj.perigeeParameters().longitudinalImpactParameter()
double z0_error = traj.perigeeParameters().longitudinalImpactParameterError()

You can get more sophisticated impact parameters (e.g., 3D, signed ...) using the IPTools software, although it won't tell you z0. For example:

#include "TrackingTools/IPTools/interface/IPTools.h"
 double d03D = IPTools::absoluteImpactParameter3D(tk, pv).second;

where tk is a TransientTrack and pv is the PrimaryVertex.

A few more details

• The TransientTrack is reference counted.
• Different concrete implementation of the basic TransientTrack exist. This allows their use with different types of basic track objest (reco::GsfTrack, reco::Track, FreeTrajectoryState without persistent track, e.g. for refitted tracks). For a correct usage, the TransientTrack should be built using the TransientTrackBuilder, or, if you have a FreeTrajectoryState, using the TransientTrackFromFTSFactory.
• In case the TransientTrack has been created from a persistent track, the method track() allows to access the reco::Track. If not, a new reco::Track is created. This track can also be saved in the event.
• In addition, several reco::Track methods (e.g. the hit pattern) are implemented, and forward the call to the persistent track. An exception is thrown in case no persistent track is available.
• Since the TT is reference counted, if you request the builder a single TT, you do not get a pointer, and you do not need to delete at the and of your block. So use it like that:

const reco::TransientTrack transientTrack = theBuilder->build(&lepton);

Review Status

Editor/Reviewer and date	Comments
JennyWilliams - 28 Mar 2007	moved page into SWGuide
Main.speer - 17 Jul 2006	Page Author
ThomasSpeer - 27 Feb 2009	Review and update

Responsible: Main.speer (Thomas Speer)