Speaker
Description
The Compact Muon Solenoid (CMS) is one of the two general-purpose experiments at the Large Hadron Collider (LHC), performing precision measurements of Standard Model processes and searches for new physics. The CMS Muon System includes more than 1,000 Resistive Plate Chambers (RPCs), gaseous detectors that provide nanosecond-level time resolution, essential for muon triggering and reconstruction. These capabilities will be even more critical during the High-Luminosity LHC (HL-LHC), the LHC upgrade phase with significantly increased luminosity and higher background rates. CMS RPCs currently operate with a gas mixture of 95.2% C$_{2}$H$_2$F$_4$ (high primary ionization yield), 4.5% iC$_4$H$_{10}$ (suppression of photon-feedback effects), and 0.3% SF$_6$ (electron quencher). Because C$_2$H$_2$F$_4$ and SF$_6$ have large global warming potential, CMS has implemented mitigation measures to reduce greenhouse-gas (GHG) emissions in line with evolving CERN and European Union policies, while preserving detector performance for high-rate data taking. Meanwhile, degradation of gas pipes in RPC barrel chambers have led to leaks. More than 100 chambers were repaired with an initial protocol that replaced only the damaged pipe segment; however, additional breaks sometimes appeared elsewhere along the line. A new leak repair procedure has recently been developed, restoring 10 chambers and permanently recovering detectors previously taken out of operation. In addition, a recuperation system has been deployed to recover and reinject C$_2$H$_2$F$_4$ with an efficiency of about 80%. This contribution presents these operational strategies and the latest R&D on alternative gas mixtures aiming to replace GHG components. Recent results show good detector performance, with efficiencies above 95% at high background rates and stable cluster size, while longevity studies are ongoing.