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- The Track parameters are always evaluated at the origin
- the reference point of the Track is therefore (0, 0, 0), but getReferencePoint() returns the same point as getOrigin of the particle that the track was instantiated with.
- The Parameters of the Track are never swum to the reference point and therefore the track parameters are therefore wrong by design for all particles that don't come from the origin
Issues
This has the result that you cannot reliably swim the Track to a given point. Because the reference point for the track is 0, the Swimmer claims that the tracklength to the origin is 0. However, since the Track has been instantiated from a particle that did not originate at the IP, the track parameters do not actually correspond to the origin.
In other words: The POCA on the track to the IP results in the wrong point.
The result of this can be seen in 
, 
What is more: The following is an example of how to instantiate Tracks from Particles.
The approach is simple: Get a Particle, make a Track from it and compare Track momentum and particle momentum. This is kind of the whole idea of a track.
Naive approach 1: Swim the Track to the Origin of the Particle and compare momenta.
OK, that looks pretty bad. Now, remember that the old Track has the wrong reference point, and that it is instantiated at the origin of the MCParticle. Let's compare this then to a new fast mc track that is properly smeared and swum to the origin of the particle.
Aha ! So as long as we use Tracks from the IP (which admittedly is the majority of the tracks) or know exactly where in the detector the Track was created from the particle (...) this seems to do OK.
Just for kicks: The new interface can of course turn off smearing. So let's make some unsmeared tracks and swim them.
A new Track
Interface Design goals:
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NewFastMCTrackFactory factory = new NewFastMCTrackFactory(event, false); // beamConstraint when smearing ?
for (MCParticle daughter : part.getDaughters()) {
Track unsmearedTrack = tf.getTrack(
new SpaceVector(daughter.getMomentum()), // momentum
new SpacePoint(daughter.getOrigin()), // position
new SpacePoint(part.getEndPoint()), // reference point
(int)daughter.getCharge(), // charge
rand, // random generator for smearing
false); // smearing ? default is true
Track smearedTrack = tf.getTrack(
new SpaceVector(daughter.getMomentum()),
new SpacePoint(daughter.getOrigin()),
new SpacePoint(part.getEndPoint()),
(int)daughter.getCharge(),
rand,
true);
}
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