Long-term aging studies on Resistive Plate Chambers (RPC) of the CMS muon system for HL-LHC

During the phase II of the LHC physics program, called High Luminosity LHC (HL-LHC), the accelerator will increase the instantaneous luminosity up to 5$\times$10$^{34}$cm$^{-2}$s$^{-1}$, providing to experiments an additional integrated luminosity of about 3000 fb$^{-1}$ over 10 years of operation, starting in 2026. The present Compact Muon Solenoid (CMS) Resistive Plate Chambers (RPC) system has been certified for 10 LHC years operation, at maximum background rate of 300 Hz/cm$^{2}$ and 50 mC/cm$^{2}$ as integrated charge. Based on Run2 data and assuming a liner dependence of the background rates as a function of the instantaneous luminosity, the expected rates and integrated charge at HL-LHC are about 600 Hz/cm$^{2}$ and 840 mC/cm$^{2}$, respectively (including a safety factor of three). HL-LHC will be threfore a challenge for the RPC system since the expected operating conditions are much higher with respect those for which the detectors have been designed, and could induce non-recoverable aging effects which can alter the detector properties. A new longevity test is then needed to estimate the impact of HL-LHC conditions up to an integrated charge equivalent to the integrated luminosity of 3000 fb$^{-1}$, to confirm that the RPC system will survive to the harsher background conditions expected at HL-LHC. A dedicated consolidation program is ongoing at CERN Gamma Irradiation Facility (GIF++), where few RPC detectors are exposed to intense gamma radiation. The main detectors parameters (currents, rate, resistivity) are under monitoring as a function of the integrated charge and the performance studied with muon beam. After having collected a significant amount of the total irradiation preliminary results will be presented.