Giant Particle Detector Repair Making Progress at CERN (Video)
Muons are charged particles 200 times heavier than electrons. Because muons can pass through several metres of matter without interacting, they are not stopped by any of the calorimeters on CMS. So chambers to detect muons are placed at the very edge of the detector, where muons are the only particles likely to register a signal.
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 12,500-tonne weight.
Drift tubes consist of a wire stretched along a tube filled with gas. When a muon or any charged particle passes through the tube it knocks electrons from the atoms in the gas. These follow the electric field ending up at the positively charged wire, registering a signal.
Resistive plate chambers consist of two parallel plates - an anode and a cathode - separated by a volume of gas. As in the drift tubes, muons passing through the chamber knock electrons out of the atoms. These electrons hit other atoms causing an avalanche of electrons that is picked up by external metallic strips after a small but precise time delay. The pattern of hit strips gives a quick measure of a muon's momentum, which is used to make immediate decisions about whether the data are worth keeping.
The package seen in the video was extracted from its original position and brought to the ground on 19 August 2013 to fix a high-voltage problem present in the drift tube chamber since 2009. The chamber was reinstalled on 21 August 2013 after performing repairs and quality checks. -- CERN