The muon is, compared to the electron, a heavy (106 MeV) negatively charged lepton (i.e. a matter particle). It is unstable and therefore it decays into other particles. Its mean lifetime is 2.2 µs.

Different particles have different capabilities to penetrate through matter, and the muon has a very high penetration capability. Only muons (and neutrinos) have the ability to penetrate first the track detectors and then the calorimeters. The muon detectors are therefore located furthest out in the layer of detectors.

As with hadron calorimeters, muon detectors consist of layers of iron interleaved with tack chambers. The muons interact with the iron in the detector and give rise to secondary particles, which are detected in the track chambers. The muon detectors register charged particles that pass through the hadron calorimeters without creating particle showers.