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Understanding hits, space points, and trajectory points in Pandora

Note: this information is specific to the Pandora reconstruction workflow. It may also be true for other reconstructions, but no guarantees.

Hits are 2D objects: they exist for a specific plane and have co-ordinates (wire, time)

Space points are 3D best-fit representations of hits generated by the Pandora reconstruction. Because space points are a product of the reconstruction, they may not be a perfect 3D representation of the hits themselves. There also is no guarantee that a space point will be created for each hit (sometimes the reconstruction might not be able to find a good 3D position to match the given hits, for example). If you wanted to check how well the space points represent the hits, the easiest way would be to project each space point into 2D (wire,time) co-ordinates for each of the three planes, and compare those directly to the hits.

Trajectory points are 3D representations of the track fit. In Pandora, these are a result of a sliding fit to the space points, which are then used to generate a track.