The issues:

1. The invariant mass distribution: when primary tracks (mContamPairs branch in StMiniMcEvent class) are used to reconstruct Minv (Fig. 1) it is very different from Minv reconstructed using global tracks (mMatGlobPairs) - Fig 2.

   Update:The difference between width of J/psi are most probably due to momentum resolution: see Fig. 3 and Fig 4.

2. J/psi decay length: Update: I misunderstood the meaning of mStopR variable. It describes the location in the x-y plane and not the distance from primary vertex. I checked the difference between the J/psi stop radius and Monte Carlo Vertex radius (R = sqrt (x²+y²) and there are the same (see Fig 5).

List of cuts:

•   -30 < Vz < 30 cm
•   |η| < 1
•   pT > 1.1 GeV/c
•   nHistPts > 25 

Fig 1. J/psi invariant mass reconstrucerd using primary tracks (mContamPairs branch in StMiniMcEvent)

 

Fig 2. J/psi invariant mass reconstrucerd using global tracks (mMatGlobPairs branch in StMiniMcEvent)

 

Fig. 3 Single track momentum resolution

Fig. 4 Single track p_T resolution

Fig. 5 Difference between the J/psi stop radius and Monte Carlo Vertex radius (R = sqrt (x2+y2)

 

Fig. 6 Stop Radius for J/psi embedded in Cu+Cu 200GeV (run 5)