CMS detector being built. The CMS (Compact Muon Solenoid) is a detector at the LHC (Large Hadron Collider), at CERN (the European particle physics laboratory) near Geneva, Switzerland. The central hole (above man's head) will surround part of the LHC, a 27-kilometre-long underground ring. Proton beams will be accelerated by the LHC to collide head-on. The collision energy will create new particles that will decay into other particles. These will be detected by the many detector layers that will fill the central space. Here, there are the layers (red) of the muon detector. A muon is a heavy version of an electron. The LHC is scheduled to start operating in 2007.