Operation and performance of the ALICE Muon IDentifier RPCs during LHC Run3

Abstract

ALICE, which stands for A Large Ion Collider Experiment, is designed to study hadronic collisions at ultrarelativistic energies at the LHC. The primary objective of ALICE is to investigate the properties of quark-gluon plasma (QGP), a state of matter where quarks and gluons are deconfined under extreme conditions of temperature and energy density. One of the key observables for studying QGP is the production of hadrons carrying heavy quarks in Pb-Pb collisions. To detect heavy quarks via their muonic decays, ALICE is equipped with a forward muon spectrometer (MS). During LHC Run 1 and Run 2, a dedicated muon trigger system based on Resistive Plate Chambers (RPCs) was used for muon physics events selection. In preparation for Run 3, ALICE underwent a major upgrade, transitioning to a continuous readout (triggerless) mode to cope with higher interaction rates. As a result, the muon trigger system was replaced by the Muon Identifier (MID). In order to prevent ageing effects and to improve the RPC rate capability, the RPCs are now operated with lower gain by reducing the operating voltage while maintaining the same gas mixture. The front-end and readout electronics were also upgraded to support low-gain operation and triggerless readout. This paper presents an assessment of the stability and performance of the MID RPCs during the first two and a half years of Run 3 at unprecedented collision energies.