  Since the initial data taking of the CERN LHC, the CMS experiment has
undergone substantial upgrades and improvements. This paper discusses the CMS
detector as it is configured for the third data-taking period of the CERN LHC,
Run 3, which started in 2022. The entire silicon pixel tracking detector was
replaced. A new powering system for the superconducting solenoid was installed.
The electronics of the hadron calorimeter was upgraded. All the muon electronic
systems were upgraded, and new muon detector stations were added, including a
gas electron multiplier detector. The precision proton spectrometer was
upgraded. The dedicated luminosity detectors and the beam loss monitor were
refurbished. Substantial improvements to the trigger, data acquisition,
software, and computing systems were also implemented, including a new hybrid
CPU/GPU farm for the high-level trigger.
