momentum smearing

[Ben Kilminster]

Hi,

I am looking at the momentum smearing function
and confused as to why the Eta and Phi are calculated
from the position 4-vector rather than from the momentum 4-vector.
I understand the position 4-vector should be based on the vertex of the particle,
not on the direction it is traveling.

If the smearing code is doing the right thing, then I am wondering
if the vertex position is being correctly filled since it shouldn't be
that the position 4-vector and momentum 4-vector are identical.

[Pavel Demin]

Normally, the MomentumSmearing module is called after the ParticlePropagator module.

In this case, the position of the particle corresponds to the detector region where this particle exits the tracking system.

Then this position is used to select the resolution function that corresponds to this detector region in terms of eta.

We suppose that tracking system resolution (that the MomentumSmearing module tries to simulate) depends mostly on the position of the particle in the detector and not on the direction of the particle momentum.