sPHENIX tracking discussion

[Michael Joseph Peters <mjpeters AT mit.edu>]

Hi,

The material corrections used in the Kalman filter and propagator depend on several parameters that are likely material specific (​see https://github.com/sPHENIX-Collaboration/coresoftware/blob/master/offline/packages/trackreco/GPUTPCTrackParam.cxx#L340). Namely:

- Density of TPC gas (current value: 2.27925e-3 g/cm^3)

- "Density effect first junction point" (current value: 2)

- "Density effect second junction point" (current value: 4)

- Mean excitation energy of TPC gas (current value: 14e-9 GeV)

- Mean Z/A of TPC gas (current value: 0.47999)

- Radiation length of TPC gas (current value: 14.403 g/cm^2)

The value for density is calculated from the CF4 density given at https://github.com/sPHENIX-Collaboration/coresoftware/blob/master/simulation/g4simulation/g4main/PHG4Reco.cc and the Geant4 Materials Database for G4_Ne. I'm confident this one is correct.

The two "density effect junction points" are copied from ALICE code; I don't know how to find the correct values for these.

The mean excitation energy comes from an unknown source, and is likely incorrect; the ALICE value is 140e-9 GeV, which is similar to the excitation energy of pure neon (listed as 137 eV in the Geant4 Materials Database).

The current value for the mean Z/A is actually slightly incorrect; calculation just now gives a value of 0.48640, and I'm not sure where the current value comes from. 

Likewise, I'm not sure where the current value of radiation length comes from.

Does anyone have any idea where to find the correct values for these parameters?

Thanks,

Michael Peters