Upgrade and commissioning of the ALICE muon spectrometer

ALICE (A Large Ion Collider Experiment) at the CERN Large Hadron Collider (LHC) isdesigned to study proton–proton and heavy–ion collisions at ultra-relativistic energies. The maingoal of the experiment is to assess the properties of quark–gluon plasma, a state of matter wherequarks and gluons are deconfined, which is reached in extreme conditions of temperature and energydensity. The production of quarkonia (cc and bb bound states) is among the main observablesto study the QGP. During the ongoing Long Shutdown 2 (LS2) of the LHC (2019-2021), ALICEunderwent a major upgrade of its apparatus, in view of the LHC Run 3, which started in 2022.The upgrade will allow a new ambitious program of high-precision measurements. However, thedetectors will have to cope with an increased collision rate, which will go up to 50 kHz in Pb–Pbcollisions. For the Muon Spectrometer (MS) ALICE is implementing new hardware and softwaresolutions. The installation of a new vertex tracker, the Muon Forward Tracker (MFT) upstreamfrom the absorber in the acceptance of the MS will improve the current measurements and enablenew ones. It will allow one to separate, for the first time in ALICE in the forward-rapidity region,the prompt (i.e. the ones directly produced in the interaction point) and non-prompt (i.e. the onescoming from beauty-hadron decays) contributions to the charmonium yield. The matching of themuon tracks reconstructed in the MFT with those in the Muon Spectrometer will provide a precisedetermination of the track parameters in the vicinity of the interaction point allowing one to resolvethe decay vertices of non-prompt charmonia in a broad interval of transverse momenta down to 𝑝 T= 0. It will also improve significantly the invariant mass resolution, allowing for a better separationof the J/𝜓 and 𝜓(2S) states. In addition, the front-end and readout electronics of the Muon Trackingsystem (Cathode Pad - Cathode Strip Chambers) and of the Muon Identification system (ResistivePlate Chambers) has been upgraded, in order to optimize the detector performance in the newrunning conditions. A detailed description of the MS upgrades, together with the results from thecommissioning with cosmic rays and the first LHC beams, will be presented in this talk.