There are already several files which help you to analyse root files from primasoft:

1. PrimaSimAnalysis (can be found here: /geant4/src/PRIMASimAnalysis.cc, /geant4/include/PRIMASimAnalysis.hh)
2. Rootmacros (examples can be found here: /geant4/rootmacros)
3. Config file for Rootmacros (examples can be found here: /geant4/sim_ana_utilities)