Determination of geometry of heavy ion collisions with forward hadron calorimeter (FHCal) at MPD/NICA

The main purpose of the FHCal is to provide an experimental measurement of a heavy-ion collision centrality (impact parameter) and orientation of its reaction plane. FHCal consists of two identical arms placed at the left/right sides from the beam collision point. Due to the fine modular structure and detection of spectators in both forward/backward regions, the angular resolution of the reaction plane reconstruction is below 30 degrees. Since the heavy fragments escape into beam holes, it is not possible to distinguish the central and peripheral collisions using only the deposited energies in FHCal. The subdivision of the calorimeter into two, inner and outer parts, and the calculation of the energy depositions separately in these calorimeter parts allow one to construct a new observable, the energy asymmetry. Taking the two-dimensional correlation between the energy asymmetry and full energy deposition in the calorimeter, it would be possible to resolve the ambiguity in the centrality determination.