After the Large Hadron Collider (LHC) was shut down in December 2018 following the successful completion of RUN 2 operation (2015-2018), CMS is undergoing an intensive upgrade and maintenance program during the current 3-years break (“Long Shutdown 2” or LS2) in order to ensure an excellent performance of the detector in the subsequent physics program.
As the name “Compact Muon Solenoid” suggests, detecting muons is one of CMS’s most important tasks. They can be produced as decay products of a number of interesting particles; for instance, one of the clearest signatures of the Higgs boson is its decay into four muons. Resistive Plate Chambers (RPC) are fast gaseous muon detectors that provide a muon trigger system. RPCs complement other muon detection technologies namely Drift Tubes (DT), Cathode Strip Chambers (CSC) and Gas Electron Multipliers (GEM). These four systems comprise the CMS Muon detector, a robust, redundant and efficient muon spectrometer. There are 1020 RPC detectors installed over almost the whole CMS coverage with more than 130,000 separate readout channels.
The CMS RPC group has planned an intensive campaign of maintenance of existing detectors and electronics and to install the services for the future improved technology RPC detectors, which are to be installed in the coming years. One of the key interventions during LS2 was to dismount 72 RPC detectors called as Super Modules (SM) from the two RE4 stations (the external disks of CMS or “endcaps”) for maintenance and to create the space for partner muon detectors (CSC) extraction for their electronics upgrade. In 2019, a team composed of Pakistani, Colombian, Brazilian, Bulgarian, Georgian, Mexican, Belgian and CERN CMS members successfully dismounted the 72 Super Modules. The extraction of very delicate RE4 Super Modules of about 4 meters long and weight of 230 kg each was challenging. This was the very first extraction of this type of module as none of the SM has ever been extracted. The revalidation of all 72 detectors was performed on surface in a newly built lab applying specific Quality Control tests. All half detectors were reinstalled at the end of 2019 and the remaining half end of 2020. Last intervention consisting of reinstallation of 36 SM was performed in the month of November 2020 following all the COVID19 precautions.
At present, detector experts are working hard in order to prepare the RPC system for excellent data delivery once the LHC resumes operation in 2021 and start taking data for significant new physics searches.
Work on CMS Muon Detector (RPC) during Long Shutdown 1 (LS1)