During the early months of 2020, as the COVID-19 threat increased, CMS activity was adapted to gradually transfer responsibility to locally resident collaborators. LS2 work was accelerated in the experimental hall, so as to reach a critical LS2 milestone with a target configuration optimized in order to be ready for the required month-long warm-up of the CMS solenoid magnet. This warm-up was made necessary by the failure last year of a highly specialised insulation vacuum pump. As the pandemic approached, the technical teams made it feasible to perform the delicate warm-up task in lab closure conditions. The magnet yoke configuration was adapted to close both ends of the barrel part, since that configuration is optimum for magnetic field containment, while the first endcap disks were arranged to enable access in case there might be a need for detector cooling repairs. This configuration will also enable a simpler re-start of the critical path muon system activities. The compromised magnet insulation vacuum system was reinforced by an additional temporary pump.

After an exceptional, “down to the wire”, effort by many teams, the CMS closure configuration was reached late on 18 March, one day before the closure deadline. Pre-arranged daily safety tours started immediately and have been controlling the detector status since then. The CMS closure condition is similar to the annual end-of-year closure but with substantially more computer processors and racks in operation to assist tele-working as well as the CMS contribution to anti-COVID-19 computing. Detector cooling systems remain operational to prevent corrosion in the detector systems.

Planning for the re-start of CMS began immediately, with the P5 teams focussing on the procurement or local manufacture of the appropriate protective equipment (masks, visors, distancing aids) and associated disinfection and disposal facilities. Meanwhile, the CMS team is working in harmony with the CERN against COVID-19 programme and has made significant and innovative additional contributions to COVID-19 counter-measures.

On April 20, with CERN directorate authorisation, the CMS magnet warm-up started and is progressing well.  In addition, several pilot activities at various CMS technical zones around the laboratory are about to be restarted. Each planned activity will be subjected to a rigorous analysis of compliance with the recently issued CERN COVID-19 precautions, as well as with additional work-site and task specific precautions which the CMS Safety team will apply. New procedures will cover distribution, use and disposal of protective equipment (masks, visors, proximity warning aids etc), social distancing choreography, ventilation checks and disinfection protocols. Experience and confidence gained from these pilot activities will enable other carefully selected LS2 and upgrade related work-packages to be re-started.  Progress will inevitably be affected by the availability of supplies and of CMS Collaboration specialists, or specialist teams, currently in their home countries. Fortunately, many LS2 and upgrade teams remained in the local area and have been covering safety shifts, on-call duties and re-start preparations during the lab closure.  These teams of experts will help facilitate a progressive return to work, as conditions in the local area and around the world allow.