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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: &kappa = -q • B_z/p_t.
θ: polar angle of the momentum vector.
φ₀: azimuthal angle of the momentum vector at the PCA to the beam line.
d₀: 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