Up to this point, the detectors have told us only about charged particles. But what about neutral particles? Instead of a track that shows you where the particle went and how it responded to the magnetic field, the calorimeter gives you a measurement of the total energy of the particles hitting it in any direction, regardless of whether they are neutral or charged.

The calorimeter is so large that it didn't make sense to try to make a seamless detector. So when we talk about the “calorimeter,” we are referring to a series of wedges that are arranged into circles along both sides of the detector, fitting snugly around the COT and magnet. Each wedge is about eight feet long and is actually two detectors in one. First is the electromagnetic calorimeter, which measures the energy of lighter particles such as electrons and photons. This detector is made of sheets of a plastic scintillator that absorb energy and emit light, sandwiched between 3/4-inch layers of lead. Behind it is the much larger hadronic calorimeter, which measures the energy of hadrons—more massive particles made of quarks. This piece of detector uses steel instead of lead in its scintillator sandwich.

Why is the hadronic calorimeter so much larger than the EM calorimeter? It isn't only direct collisions with atoms in the steel and lead that cause incoming particles to "shower." The electromagnetic attraction of protons and electrons in the metals can also cause charged incoming particles to shower merely by changing their direction. Since electrons are much lighter than hadrons, they yield to the electromagnetic force more easily, and shower more often. But hadrons have to wait until they have a head-on collision with the nucleus of an atom in the lead or steel. This means hadrons usually penetrate deeper into the machine before they are converted into lower-energy particles, so a hadronic calorimeter needs more space to contain the energy from its cascading particles.

Uses of the calorimeter

Calorimeters are especially good for telling you about the neutral particles that are coursing through your detector. Remember that neutral particles (such as neutrons and high-energy photons) were invisible up to this point, since the inner detectors rely on ionization to see particles and neutral particles do not ionize atoms. But charged particles also deposit energy in the calorimeters. So how do you tell the difference between the two? If you see a track leading straight into a hit in the calorimeter, you're seeing a charged particle. If you see nothing and then suddenly you see a hit in the calorimeter, chances are it was a neutral particle.

The ability to measure the energy of each particle helps physicists determine the masses of particles such as the top quark or W and Z bosons. But the energy that is "missing" can be just as important as the energy they "catch." Physicists know that according to the law of conservation of energy, what they put into the system should equal what they get out of it. Missing energy means the collision created some particles that cannot be seen by the detector. These could be particles such as neutrinos that don't interact with matter very often. Or they could be particles that haven't been discovered yet. Many searches for new physics postulate the existence of particles that the detector wouldn't be able to "see" directly. Looking for missing energy is the only way to validate the existence of these particles.