An upgraded muon identification system for ALICE at the LHC: from detector construction to testing and re-commissioning

ALICE (A Large Ion Collider Experiment) at the CERN Large Hadron Collider (LHC) is designed to study proton-proton (p-p) and Heavy-ion collisions at ultra-relativistic energies. The main goal of the experiment is to assess the properties of Quark Gluon Plasma (QGP), a state of matter where quarks and gluons are de-confined, reached in extreme conditions of temperature and energy density such as those obtained in Heavy-ion collisions at the LHC. The Universe is thought to have been in this state during the first few µs after the Big Bang. One of the main observables used to study the QGP is the production of Heavy quarkonia in Pb-Pb collisions. Quarkonia are bound states of a Heavy quark (c or b) and the corresponding anti-quark. The presence of QGP modifies the quarkonium production rates in Heavy-ion collisions via the competing processes of suppression by colour screening and regeneration by quark recombination. The study of p–p collisions is also crucial to the ALICE physics program since, while the production of the Heavy quark pairs in p–p collisions is relatively well understood, the description of the (non-perturbative) hadronisation process into quarkonium remains unsatisfactory. No model is able to simultaneously describe different aspects of quarkonium production, such as polarization, transverse momentum and energy dependence of the cross sections. For all the above reasons, ALICE is equipped with a forward muon spectrometer, detecting quarkonia via their di-muon decays. The muon spectrometer covers the pseudorapidity range 2.5 < η < 4; it is composed of a silicon forward tracker, a hadron absorber, a dipole magnet, a five-station tracking system and a muon identification system. The Muon IDentifier (MID) is the topic of my PhD project. The system is composed of 4 planes of 18 Resistive Plate Chamber (RPC) detectors each, located downstream of an iron wall. RPCs are gaseous detectors with resistive (bakelite) electrodes. During the Long Shutdown 2 (LS2) of LHC, ALICE underwent a major upgrade of its apparatus, in view of the LHC Run 3 started in 2022, in which the experiment is operated at a larger-than-ever luminosity. For the MID system, in order to cope with the increased collision rate (from the current ∼8 kHz to ∼50-100 kHz for Pb–Pb collisions), it is necessary to reduce the charge released per hit in the detector. For this reason, the detectors will be operated at a lower gain and the new front-end electronics will include an amplification stage. Moreover a new production of RPC detectors was launched. This was needed because, at the end of the LHC Run 2, some of the RPCs have integrated a non-negligible charge with respect to their expected life-time, and ageing effects might appear at some point. Therefore, about 25% of the detectors currently installed in ALICE will be replaced during Run 3. The first step of my PhD project was the testing of the new RPCs. These are made with a different type of bakelite, because the one used for the first generation of RPCs is no longer commercially available. The new bakelite has different surface and bulk properties, therefore it was necessary to verify if and how these new features affect the detector performance. The tests were performed in Torino, using cosmic rays and a dedicated set-up. Before the starting of my PhD, the RPCs production with the new bakelite was completely unsatisfactory in terms of performances. After several interactions with the RPC firm during the beginning of my PhD, a pre-production batch of 3 RPCs was launched. After the validation of this batch, the mass production of new modules was launched, completed and tested over my PhD. Starting from summer 2021, I took an active role in the commissioning of the detectors with the new front-end electronics. This included the integration of the upgraded on-line systems (read-out, Detector Control System, Data Quality Control) in the ALICE environment, and in parallel a cosmic-ray data-taking campaign aimed to a first tuning of the new working parameters. Finally, the commissioning of the system was completed with the very first p–p LHC beams, and finally validated with Pb–Pb collisions. The first collected data were used to parameterise in detail the detector response (efficiency, cluster size and charge released in the gas gap per hit) in the new operation mode, and feeded it into the ALICE software.