As the trajectory of a track passes through the LHCb detector, it intersects many DetectorSurfaces (eg. detector elements, dead material, ... ). At such intersections, there may be measurements, and the parameterization of the track can be extrapolated to these DetectorSurfaces. As a result, the precise 'state' of a trajectory can be defined on such a DetectorSurface, by specifying the intersection of the trajectory with the DetectorSurface, and the ThreeMomentum at this intersection point. This is always done in the ReferenceFrame.

(MM:) The discussion is whether a trajectory actually passes through many surfaces or whether the validity of the trajectory is long enough to do so. This is part of the TrackTrajectory vs TrackTrack discussion.

(MM:) After re-considering I think the best option is to merge the TrackState and TrackTrajectory into a single object. In practice it means that we add a geometrical representation to a TrackState. This avoids creating another layer and gives all functionality for e.g. vertexing. In fact the current TrackState is basically a linear trajectory. It would also aid Panoramix to display TrackStates.

Given that the intersection+momentum have a redundant degree of freedom (as the z-coordinate of the intersection is used to parameterize the states), the TrackTrajectory is (locally) represented by the (x,y) coordinate of the intersection point, the slopes (tx,ty) of the TrackTrajectory with respect to x- and y-axis, and Q/P, where P is the magnitude of the momentum. (this is also in the ReferenceFrame?)

(MM:) There is the possibility that we include the time in the state vector. Do we exclude this in the current model or do we leave it open?

A state can be extrapolated to provide a prediction of the intersection of the trajectory with other DetectorSurfaces.

For the concrete implementation, see for example here

-- GerhardRaven - 03 Apr 2005

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Topic revision: r6 - 2005-04-21 - GerhardRaven
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