The goal of the CMS experiment at CERN is to unravel the most fundamental properties of matter by studying collisions between protons and/or heavy ions at the highest energies and collision rates ever achieved. The LHC is situated in an underground (at an 100 m depth) tunnel of 27 km circumference near Geneva, and uses superconducting magnets to keep the accelerated particles on their orbit. Particles collide at four interaction points equipped with particle detectors. The CMS is one of these general purpose detectors, and measures 21 m in length, 15 m in diameter, and weighs a total of 14 000 tonnes. The CMS detector consists of many components based on a variety of particle detection technologies, allowing to identify and measure the energy and momentum of almost all particles created in the collisions. The CMS detector is built around a huge solenoid magnet. This takes the form of a cylindrical coil of superconducting cable that generates a field of 4 tesla, about 100,000 times the magnetic field of the Earth. The field is confined by a steel “yoke” that forms the bulk of the detector’s weight.

The CMS detector acts as a giant, high-speed camera, taking 3D “photographs” of particle collisions from all directions up to 40 million times each second. Although most of the particles produced in the collisions are “unstable”, they transform rapidly into stable particles that can be detected by CMS. By identifying these stable particles produced in each collision, measuring their momenta and energies, and then piecing together the information of all these particles like putting together the pieces of a puzzle, the detector can recreate an “image” of the collision for further analysis. While the CMS detector and all of its readout systems are located at CERN, the analysis of data is performed by a distributed computing network, GRID.

Belgian particle physics community contributes to the design, construction, integration and commissioning of various sub-detectors. For the Phase II upgrade of the CMS detector for example, we are involved in particle tracking detectors. While UAntwerp, VUB, UCL, ULB are mainly involved in silicon tracking detector upgrade for the EndCap region of the detector, UGent plays a leading role in the construction of muon detection systems using Resistive Plate Chambers (RPC) and Gas Electron Multipliers (GEM) technologies. The UGent was selected as an official CMS production site for both the GEM&RPC subsystems, where we are currently the only CMS site for RPCs. In 2017-2018 50 new chambers to equip a first new CMS GEM station were produced and certified at UGent. These chambers were moved to CERN for installation in CMS in 2019-2020, and are now taking data. The team is currently preparing a new chamber production, foreseen to start at UGent in January 2023, for a 2nd CMS GEM station.