The present CMS muon system consists of three different detector technologies: drift tubes (DT) and cathode strip chambers (CSC) are used in the barrel and endcap regions of the spectrometer as offline tracking and triggering devices, whereas resistive plate chambers (RPC) are installed both in barrel and endcaps and are exploited mostly in the trigger.
To cope with the challenging conditions of increasing luminosity expected at HL-LHC, several upgrades of the muon detectors and trigger system are planned. In the case of DT and CSC, the electronics will be upgraded to handle higher rates, but there is no plan to replace the existing DT, CSC and RPC chambers. Therefore, accelerated ageing tests are being performed to assess the performance stability of all muon detectors under conditions which exceed, by one order of magnitude, the design specifications. New micropattern gas detectors will be added to improve the performance in the forward region, which is characterized by high background rates and a smaller, nonuniform magnetic field. Large-area triple-foil gas electron multiplier (GEM) detectors (GE1/1) are presently being installed during the second LHC long shutdown covering the pseudo-rapidity ($\eta$) region 1.6 < $|\eta|$ < 2.4. They will limit the rate of background triggers, while preserving high trigger efficiency for low transverse momentum muons. For the HL-LHC operation, the muon forward region will also be enhanced with another large area GEM-based station (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 few $kHz/cm^2$.
In addition, an ME0 station of GEM chambers will be installed behind the new forward calorimeter to cover up to $|\eta|$ = 2.8 and take advantage of the pixel tracking coverage extension.
We present results about the expected performance stability of the existing muon detectors at HL-LHC. Moreover, we report on the outcome of simulation-based studies, which describe the impact of the muon upgrades to the trigger and the reconstruction of muon physics objects.