Performance of the ALICE Zero Degree Calorimeters and upgrade strategy

The Zero Degree Calorimeters (ZDCs) of the ALICE experiment were designed with the twofold purpose of both estimating the centrality in heavy ion collisions by measuring the energy carried away by the spectator nucleons and of measuring the luminosity delivered to the experiment exploiting the high cross sections for neutron emission from electromagnetic dissociation processes. The measurement of centrality has been successfully extended to p–A collisions with the detection of nucleons ejected from the nucleus by the collisions with the projectile proton (“gray” nucleons) and those resulting from de-excitation processes (“black” nucleons). The applications of the detector in triggering and analysis have expanded during the years of operation in the LHC RUN1 and RUN2. These now include both the reaction plane and the longitudinal asymmetry measurements in heavy ion collisions. Moreover the ZDCs are used to reject the parasitic interactions of main bunches with satellite bunches in A–A and p–A collisions and to tag diffractive events in pp collisions. The foreseen operation in RUN3 with the tenfold increase in the luminosity delivered by LHC in heavy ion collisions, together with the continuous acquisition strategy that is being adopted by ALICE, will be challenging for the ZDC readout system. The readout upgrade will be based on FMC digitizers with trigger, timing and charge integration functionality performed through FPGA.