Class Acts::ImpactPointEstimator

template<typename input_track_t, typename propagator_t, typename propagator_options_t = PropagatorOptions<>>
class ImpactPointEstimator

Estimator for impact point calculations A description of the underlying mathematics can be found here: https://github.com/acts-project/acts/pull/2506 TODO: Upload reference at a better place.

Public Functions

inline ImpactPointEstimator(const Config &cfg)

Constructor.

Parameters

cfg – Configuration object

Result<double> calculateDistance(const GeometryContext &gctx, const BoundTrackParameters &trkParams, const Vector3 &vtxPos, State &state) const

Calculates 3D distance between a track and a vertex.

Parameters

  • gctx – The geometry context

  • trkParams – Track parameters

  • vtxPos – 3D position to calculate the distance to

  • state – The state object

Returns

Distance

Result<BoundTrackParameters> estimate3DImpactParameters(const GeometryContext &gctx, const Acts::MagneticFieldContext &mctx, const BoundTrackParameters &trkParams, const Vector3 &vtxPos, State &state) const

Estimates the track parameters at the 3D PCA (i.e., a point of minimal 3D distance) to a vertex.

The track parameters are defined wrt a reference plane that has its origin at the vertex position and whose z-axis points in the direction of the track momentum. The plane’s x-axis points approximately from the vertex to the 3D PCA (it is only approximate because we force it to be orthogonal to the z-axis). The y-axis is calculated as a cross product between x- and z-axis.

Parameters

  • gctx – The geometry context

  • mctx – The magnetic field context

  • trkParams – Track parameters

  • vtxPos – Reference position (vertex)

  • state – The state object

Returns

Track parameters at the 3D PCA

Result<double> get3DLifetimeSignOfTrack(const BoundTrackParameters &track, const Vertex<input_track_t> &vtx, const Acts::Vector3 &direction, const GeometryContext &gctx, const MagneticFieldContext &mctx) const

Estimates the sign of the 3D lifetime of a given track w.r.t.

a vertex and a direction (e.g. a jet direction)

Parameters

  • track – Track to estimate the IP from

  • vtx – Vertex the track belongs to

  • direction – The direction

  • gctx – The geometry context

  • mctx – The magnetic field context

Returns

The value of the 3D lifetime

template<unsigned int nDim>
Result<std::pair<Acts::ActsVector<nDim>, Acts::Vector3>> getDistanceAndMomentum(const GeometryContext &gctx, const BoundTrackParameters &trkParams, const ActsVector<nDim> &vtxPos, State &state) const

Calculate the distance between a track and a vertex by finding the corresponding 3D PCA.

Returns also the momentum direction at the 3D PCA. The template parameter nDim determines whether we calculate the 3D distance (nDim = 3) or the 4D distance (nDim = 4) to the 3D PCA.

Note

For straight tracks we use an analytical solution; for helical tracks we use the Newton method.

Note

If nDim = 3 we only consider spatial dimensions; if nDim = 4, we also consider time. Other values are not allowed.

Template Parameters

nDim – Number of dimensions used to compute compatibility

Parameters

  • gctx – Geometry context

  • trkParams – Track parameters

  • vtxPos – Vertex position

  • state – The state object

Result<ImpactParametersAndSigma> getImpactParameters(const BoundTrackParameters &track, const Vertex<input_track_t> &vtx, const GeometryContext &gctx, const MagneticFieldContext &mctx, bool calculateTimeIP = false) const

Calculates the impact parameters of a track w.r.t.

a vertex. The corresponding errors are approximated by summing the variances of the track and the vertex.

Parameters

  • track – Track whose impact parameters are calculated

  • vtx – Vertex corresponding to the track

  • gctx – The geometry context

  • mctx – The magnetic field context

  • calculateTimeIP – If true, the difference in time is computed

Result<std::pair<double, double>> getLifetimeSignOfTrack(const BoundTrackParameters &track, const Vertex<input_track_t> &vtx, const Acts::Vector3 &direction, const GeometryContext &gctx, const MagneticFieldContext &mctx) const

Estimates the sign of the 2D and Z lifetime of a given track w.r.t.

a vertex and a direction (e.g. a jet direction) by propagating the trajectory state towards the vertex position and computing the scalar product with the direction vector

Parameters

  • track – Track to estimate the IP from

  • vtx – Vertex the track belongs to

  • direction – The direction

  • gctx – The geometry context

  • mctx – The magnetic field context

Returns

A pair holding the sign for the 2D and Z lifetimes

template<unsigned int nDim>
Result<double> getVertexCompatibility(const GeometryContext &gctx, const BoundTrackParameters *trkParams, const ActsVector<nDim> &vertexPos) const

Estimates the compatibility of a track to a vertex based on their 3D (if nDim = 3) or 4D (if nDim = 4) distance and the track covariance.

Note

Confusingly, a smaller compatibility means that a track is more compatible.

Note

If nDim = 3 we only consider spatial dimensions; if nDim = 4, we also consider time. Other values are not allowed.

Template Parameters

nDim – Number of dimensions used to compute compatibility

Parameters

  • gctx – The Geometry context

  • trkParams – Track parameters at point of closest approach in 3D as retrieved by estimate3DImpactParameters

  • vertexPos – The vertex position

Returns

The compatibility value

struct Config

Public Functions

inline Config(std::shared_ptr<const MagneticFieldProvider> bIn, std::shared_ptr<const propagator_t> prop)

Config constructor if magnetic field is present.

Parameters

  • bIn – The magnetic field

  • prop – The propagator

inline Config(std::shared_ptr<propagator_t> prop)

Config constructor without B field -> uses NullBField provided)

Parameters

prop – The propagator

Public Members

std::shared_ptr<const MagneticFieldProvider> bField

Magnetic field.

int maxIterations = 20

Max. number of iterations in Newton method.

double precision = 1.e-10

Desired precision of deltaPhi in Newton method.

std::shared_ptr<const propagator_t> propagator

Propagator.

struct State

State struct.

Public Functions

inline State(MagneticFieldProvider::Cache fieldCacheIn)

The state constructor.

Parameters

fieldCacheIn – The magnetic field cache

Public Members

MagneticFieldProvider::Cache fieldCache

Magnetic field cache.