Event data

Track parametrization

A trajectory in a magnetic field is generally parameterized by a set of at least five parameters (when being bound to a surface). Two different categories are used in Acts: so-called bound parameters, i.e. parameter bound to a surface and curvilinear parameters. Curvilinear parameters are defined in an implicit planar and normal (to the track) reference plane that follows the track. The center of the implicitly defined plane is at the current track position, while the normal vector points along the momentum direction. Per definition the local parameters of a curvilinear parametrization are thus fixed to (0,0).

In Acts the parametrization is can be changed, provided the according transformations into global coordinates are given, it can be changed by adapting the relevant coordinate_transformation definition. This shows an excerpt of the default implementation:

namespace Acts {
namespace detail {
   * @brief helper structure summarizing coordinate transformations
  struct coordinate_transformation
    typedef ActsVector<ActsScalar, Acts::NGlobalPars> ParVector_t;

    static Vector3
    parameters2globalPosition(const ParVector_t& pars, const Surface& s)
      return s.localToGlobal(Vector2(pars(Acts::eBoundLoc0), pars(Acts::eBoundLoc1)),

    static Vector3
    parameters2globalMomentum(const ParVector_t& pars)
      Vector3 momentum;
      double         p     = std::abs(1. / pars(Acts::eBoundQOverP));
      double         phi   = pars(Acts::eBoundPhi);
      double         theta = pars(Acts::eBoundTheta);
      momentum << p * sin(theta) * cos(phi), p * sin(theta) * sin(phi),
          p * cos(theta);

      return momentum;

    static ParVector_t
    global2curvilinear(const Vector3&,
                       const Vector3& mom,
                       double                charge)
      ParVector_t parameters;
      parameters << 0, 0, mom.phi(), mom.theta(),
          ((std::abs(charge) < 1e-4) ? 1. : charge) / mom.mag();

      return parameters;
  // ...
} // end of namespace detail
} // end of namespace Acts

Changing the default parameter definition and transformation needs recompilation of the Acts Software and redefinition of the relevant plugin:


Neutral and charged representations

Track parameters come in a charged and neutral flavor, the flavor is defined using a template parameter, which either is a ChargedPolicy class for charged track parameter representation:

namespace Acts {
    typedef SingleTrackParameters<ChargedPolicy>            TrackParameters;
    typedef SingleCurvilinearTrackParameters<ChargedPolicy> CurvilinearTrackParameters;
    typedef SingleBoundTrackParameters<ChargedPolicy>       BoundTrackParameters;
}  // end of namespace Acts

Or, respectively, a NeutralPolicy object

namespace Acts {
    typedef SingleTrackParameters<NeutralPolicy> NeutralParameters;
    typedef SingleCurvilinearTrackParameters<NeutralPolicy>
    typedef SingleBoundTrackParameters<NeutralPolicy> NeutralBoundTrackParameters;
} // end of namespace Acts  

Multi-variant representation

Multi-variant fitters, such as the Gaussian Sum filter rely on a multi-component description of the track, which requires the definition of multi-variant track parameters. For Acts, as Propagator and Extrapolator are written for type templates, a multi-variant definition of track parametrization is planned in order to integrate directly with the core software.


Measurements exist as uncalibrated and calibrated types. While uncalibrated measurements are the direct output of the data formation, during the track fit, when trajectory information (or information about the trajectory hypothesis) is available, certain calibration steps can help to improve the track fit quality.

Uncalibrated measurements

Measurements in a detector can be one to many-dimensional (covering the full range up to the full track parametrization).

template <typename Identifier, BoundIndices... params>
class Measurement
  // check type conditions
                "'Identifier' must be copy-constructible");
                "'Identifier' must be move-constructible");
                "'Identifier' must be copy-assignable");
                "'Identifier' must be move-assignable");

  // private typedef's
  typedef ParameterSet<params...>
      ParSet_t;  ///< type of the underlying ParameterSet object
// ...

In order to minimize the computational cost (and differently from the original ATLAS code base), the dimension of the measurement has to be fixed at compile time.

Calibrated measurements

Calibrated measurements are temporary objects needed for track fitting and belonging to a track. Still to be defined