The Development of the CMS Zero Degree Calorimeters to Derive the Centrality of AA Collisions

Abstract

The centrality of PbPb collisions is derived using correlations from the zero degree calorimeter (ZDC) signal and pixel multiplicity at the Compact Muon Solenoid (CMS) Experiment using data from the heavy ion run in 2010. The method to derive the centrality takes the two-dimensional correlation between the ZDC and pixels and linearizes it for sorting events. The initial method for deriving the centrality at CMS uses the energy deposit in the HF detector, and it is compared to the centrality derived by the correlations in ZDC and pixel multiplicity. This comparison highlights the similarities between the results of both methods in central collisions, as expected, and deviations in the results in peripheral collisions. The ZDC signals in peripheral collisions are selected by low pixel multiplicity to obtain a ZDC neutron spectrum, which is used to effectively gain match both sides of the ZDC.