SWGuideTransientTracks < CMSPublic

CMSPublic Web>SWGuide>SWGuideVertexReco>SWGuideTransientTracks (2013-08-19, IanTomalin) (raw view)

---+!! Transient Tracks
%COMPLETE3%

%TOC{title="Contents:"}%

---++ 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): [[https://cmssdt.cern.ch/SDT/doxygen/CMSSW_5_3_5/doc/html/d9/d6e/classreco_1_1TrackTransientTrack.html][TransientTracks]] , 
[[https://cmssdt.cern.ch/SDT/doxygen/CMSSW_5_3_5/doc/html/d0/db2/classTransientTrackBuilder.html][TransientTrackBuilder]] .

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

<verbatim>
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. 
</verbatim>
You may need to include header files into your analyzer, look for example here: [[http://cmslxr.fnal.gov/lxr/source/TrackingTools/TransientTrack/test/TransientTrackBuilderTest.cc][TransientTrackBuilderTest.cc]].

To get the !TransientTrackBuilder from the !EventSetup:

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

You can now build your !TransientTracks from =Track=, =TrackRef=, or directly from the =edm::handle= to a =TrackCollection=:
<verbatim>
    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;
</verbatim>

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

---++ Examples (including calculation of track impact parameters)

<verbatim>
    // 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);
</verbatim>

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. 

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

<verbatim>
// 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()
</verbatim>

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

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

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

#ReviewStatus
---++!! Review Status

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

%RESPONSIBLE% Main.speer (Thomas Speer) %BR%

Topic revision: r20 - 2013-08-19 - IanTomalin

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