Upgrades of the CMS muon system in preparation of HL-LHC

The present CMS muon system operates three different detector types: barrel drift tubes (DT) and resistive plate chambers (RPC), along with cathode strip chambers (CSC) and another set of RPCs in the forward regions. In order to cope with increasingly challenging conditions, various upgrades are planned to the trigger and muon systems. In view of the operating conditions at HL-LHC, it is vital to assess the detector performance under high luminosity. New irradiation tests had to be performed to ensure that the muon detectors will survive the harsher conditions and operate reliably. The new CERN GIF++ (Gamma Irradiation Facility) allowed aging tests to be performed on these large muon detectors. We present results in terms of system performance under high backgrounds and after accumulating charge through an accelerated test to simulate the expected dose. New detectors will be added to improve the performance in the critical forward region: large-area triple-foil gas electron multiplier (GEM) detectors will have already been installed in LS2 in the pseudo-rapidity region $1.6 < \lvert\eta\rvert < 2.4$, aiming at suppressing the rate of background triggers while maintaining high trigger efficiency for low transverse momentum muons. For the HL-LHC operation, the muon forward region will be enhanced with another large area GEM-based station called GE2/1, and with two new generation RPC stations called RE3/1 and RE4/1, having low resistivity electrodes. These detectors will combine tracking and triggering capabilities and can stand particle rates up to a few kHz/cm$^2$. In addition to taking advantage of the pixel tracking coverage extension, a new detector, the ME0 station, will be installed behind the new forward calorimeter, with coverage up to $\lvert\eta\rvert = 2.8$.