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Description

Persistent classes for Track Reco data format.

Public interface

reco::Track : reconstructed Track. Objects of this type are ment to be stored in the AOD.
Contains a reference to an object of type reco::TrackExtra .
reco::TrackExtra : Track extension. Objects of this type are ment to be stored in the RECO.

Auxiliary base classes

reco::TrackBase : common base class to reco::Track and reco::Muon
reco::TrackExtraBase : common base class to reco::TrackExtra and reco::MuonExtra. Contains references to objects of type TrackingRecHit .

typedefs

reco::TrackCollection : collection of reco::Track objects
reco::TrackRef : persistent reference to a reco::Track object
reco::TrackRefProd : reference to a reco::Track collection
reco::TrackRefVector : vector of references to reco::Track objects in the same collection
reco::track_iterator : iterator over a vector of references to reco::Track objects in the same collection
reco::TrackExtraCollection : collection of reco::TrackExtra objects
reco::TrackExtraRef : persistent reference to a reco::TrackExtra object
reco::TrackExtraRefProd : reference to a reco::TrackExtra collection
reco::TrackExtraRefVector : vector of references to reco::TrackExtra objects in the same collection
reco::trackExtra_iterator : iterator over a vector of references to reco::TrackExtra objects in the same collection

Track object "Tiers"

The complete track information is stored in two different object: reco::Track and reco::TrackExtra , that are stored in two different collection, physically written to fine into two different ROOT branches. This modularity allows the freedom to "drop" the reco::TrackExtra component, if not needed, and leave only reco::Track , in order to save disk space. This could be done, for instance while writing only the AOD component of a RECO collection.

A reco::Track has a reference ( edm::Ref ) to a reco::TrackExtra object that allows to access the information in the "extra" tier. Users don't necessarily have to dereference this persistent reference, since reco::Track has two sets of methods:

inherited from the base class reco::TrackBase , that only access data stored in the reco::Track object
specific to reco::Track , that return quantities derived from reco::TrackExtra information, dereferencing the corresponding reference.

For instance, the following methods will only access reco::TrackBase data members:

      reco::Track trk;
  double pt = trk.pt();
  double eta = trk.eta();

The following instead access data members contained in reco::TrackExtra :

      reco::Track trk;
  double outerPt = trk.outerPt();
  math::XYZPoint p = trk.outerPosition();

and are equivalent to:

      reco::Track trk;
  const reco::TrackExtraRef & trkExtra = trk.extra();
  double outerPt = trkExtra.outerPt();
  math::XYZPoint p = trkExtra.outerPosition();

Warning: collections of those two data types are potentially stored in different data tiers. So, if a reco::TrackExtra is not stored on disk dereferencing the reference contained in reco::Track may result in an exception.

Track Parametrization

Track helix parametrization follows the perigee model described in the internal CMS note CMS IN-2003/008 . The five parameters are:

κ : signed transverse curvature: κ = - q • B _z /p _t .
θ : polar angle of the momentum vector.
φ ₀ : azimuthal angle of the momentum vector at the PCA to the beam line.
d ₀ : signed transverse distance of the PCA. By convention, the sign is positive if the reference point is at the left of the PCA.
d _z : longitudinal distance of the PCA.

The five parameters are stored internally in a vector whose index varies from 0 to 4, in the order listed above. The indices are also defined as the enumerator index defined in the namespace reco::helix:

      enum index { i_transverseCurvature = 0 , i_theta, i_phi0, i_d0, i_dz };

So, for instance, in order to access specific track parameters a user can write the following code that requires no explicit numeric index:

     using namespace reco::perigee;
  reco::TrackRef track = ... ; 
  double d0 = track->parameter( i_d0 ); 
  double covThetaPhi = track->covariance( i_theta, i_phi0 );

Modules

None.

Unit tests and examples

None.

Status and planned development

To be reviewed.

Last updated: @ L. Lista, Th. Speer