Commissioning and first performances of the ALICE MID RPCs

ALICE (A Large Ion Collider Experiment) at the CERN Large Hadron Collider (LHC) is designed to study pp and Pb–Pb collisions at ultra-relativistic energies. ALICE is equipped with a Muon Spectrometer (MS) to study the heavy charmonia in pp and heavy ion collisions via their muonic decay. At first, in the LHC Run 1 and 2 the selection of interesting events for muon physics in the MS was performed with a dedicated Muon Trigger system based on Resistive Plate Chambers (RPCs) operated in maxi-avalanche mode. During the LHC Long Shutdown 2 (LS2), ALICE underwent a major upgrade of its apparatus in order to fully profit from the increased luminosity of Pb–Pb collisions (from 20 kHz in Run 2 to 50 kHz in Run 3). Many sub-systems of the ALICE experiment are now running in continuous readout (triggerless). In this context the Muon Trigger system became the Muon IDentifier (MID). In order to reduce the RPC ageing and to increase the rate capability, it was decided to use a new front-end electronics FEERIC with a pre-amplification stage to minimize the charge released per hit inside the gas gap. A description of the MID upgrades, together with the results and performances of the RPCs from the commissioning, is presented in this paper.