Warm Iron Calorimeter

The outermost layer of the detector is the warm iron calorimeter, consisting of hundreds of long, thin gas-filled boxes stuffed between the 14 iron plates that make up the outer body of the detector. These boxes are strung with high-voltage wires and act, much like the drift chamber, as a collection of Geiger-counter-like detectors that record passage of any charged particle. Metal strips along the plastic boxes pick up signals that show directly which wire a particle passes, and other strips show where along the wire it went by. The result is a point on the path of the particle.

Most  particles produced in the collisions are stopped in the liquid argon calorimeter. Only the most energetic hadron showers and the muons penetrate into this last set of layers.

For hadrons, the interweaved layers of iron and tracking detectors function as an extension of the liquid argon calorimeter and help measure the hadron energy. This is why these layers are called the warm iron calorimeter. Unlike the liquid argon calorimeter, the these layers are relatively "warm" at room temperature.

High energy muons are recognizable because they are the only charged particle that penetrate the layers and leave the detector. They do not lose energy by forming a shower of particles because they have no strong interactions and, unlike electrons, they are not significantly deflected by inter-atomic electric fields. This is because the mass of a muon is so much larger than that of an electron.

The only type of particles that gets through all layers leaving no signal are neutrinos. We only know these have been produced when they carry off enough energy or momentum that we notice some is missing.