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 video shows the installation of a "muon barrel package" in the uppermost sector of the central barrel of CMS. During the construction of CMS from 2004 to 2007, 250 such packages were installed. Each package contains two main types of detector: drift tubes and resistive plate chambers.
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.