Centrality and collision event-plane determination in ALICE at the LHC

Abstract

Investigation of physical phenomena in heavy-ion collisions requires knowledge about collision geometry, which is characterized by the energy distribution in the overlap region of the colliding nuclei and of the collision spectators. Both the event-by-event distributions of produced particles and the spectator nucleons can be used to estimate the collision geometry. Due to the pressure gradients, the spatial anisotropy of initial state geometry is converted during the system evolution to an anisotropy in momentum space. The event-planes of this anisotropy can be estimated via measured azimuthal distributions of particles produced in the collision. We report on the performance of the ALICE experiment at the LHC for the centrality and the event-plane determination for different harmonics using measured distribution of produced particles at central and forward rapidity, and the energy distribution of the spectator neutrons at beam rapidity.