Reaction plane determination by means of the ALICE zero degree calorimeters

Abstract

A possible way to estimate the reaction plane of the ion-ion collision is to measure the sideward deflection of the spectator neutrons. In the ALICE experiment this kind of measurement can be performed by means of the two neutron zero degree calorimeters (ZN), which are located at opposite sides with respect to the beam intersection point (IP). In fact the ZN calorimeters, thanks to their segmentation in four towers, are position sensitive devices. Concerning their localizing capability, a spatial resolution of ∼3 mm has been measured for a 100 GeV/c hadron beam. This performance will be used to reconstruct, event by event, the centroid coordinate of the spectator neutron spot on the ZN front-face, which is sensitive to the directed flow (“bounce off”) of spectator neutrons. The measurement of the centroid will therefore allow to reconstruct the 1st-order event plane azimuth. A simulation is performed in order to estimate the dependence of the event plane resolution on the magnitude of the directed flow v₁ of the spectator neutrons and on the neutron multiplicity (event centrality). In particular, it will be shown that the event plane resolution is not dominated by the smearing on the centroid measurement, but by the smearing due to the transverse lead beam divergence at the IP. Finally a possible tool to select events with small lead beam divergence at IP is discussed, using the information coming from both the ZN calorimeters.