Track Propagator Algorithms
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Introduction
Propagators are used during track building and fitting to extrapolate track from one layer to the next, or to any other point in space, such as the track's point of closest approach to the beam-line. There are three commonly used types of propagator, which in increasing order of sophistication (and slowness) are:
- AnalyticalPropagator just solves analytically for the intercept of a helix with a plane or cylinder.
- MaterialPropagator uses (1) but then also uses MaterialEffectsUpdater, which uses EnergyLossUpdator to apply an mean energy loss correction to the helix based on Bethe-Block (or Bethe-Heitler for electrons), and also uses MultipleScatteringUpdator to inflate the track error matrix according to the expected rms scattering angle (from Moliere formula).
- RungeKuttaTrackPropagator is like (2) but determines intercept by iterating the track trajectory in many small steps (of automatically chosen size) using a 4th order Runge-Kutta iteration (which is a mathematical technique for solving first order differential equations, such as the ones describing the particle's motion in the B-field). It can take into account variations in the B-field etc.
In general, the AnalyticalPropagator is used only during track-finding, when determining which layers a track crosses, and so which layers should be checked for hits. The MaterialPropagator is used for all other purposes during track-finding, such as extrapolating the TrackCandidates from layer to layer, when searching for additional hits to add to them. The RungeKuttaTrackPropagator is only used for the final fit/smoothing of the found track.
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