calorimeter

How much energy is a particle carrying? One way to find out is to stop it completely and measure how much energy it dumps in the process — much as you could gauge a thrown ball's energy by how deep a dent it leaves in soft clay. A calorimeter does exactly this for particles: it is a thick block of material designed to absorb a particle entirely and turn the energy it deposits into a measurable signal. Unlike a tracker, which tries not to disturb the particle, a calorimeter's whole purpose is to stop it and soak up everything it carries.

When a high-energy particle slams into the dense material, it does not simply stop; it triggers a cascade. An electron or photon striking heavy material produces more photons and electron-positron pairs, which produce still more, building an avalanche called an electromagnetic shower. A hadron such as a proton or pion sets off a messier nuclear cascade, a hadronic shower. Either way, the original energy is shared among a growing swarm of low-energy particles that the calorimeter detects, often by the faint light (scintillation) or ionization they produce; the total signal is proportional to the incoming energy. Detectors therefore use two kinds in sequence: an electromagnetic calorimeter tuned to absorb electrons and photons, and behind it a hadronic calorimeter sized to contain the deeper, bulkier showers of hadrons.

Calorimeters are essential because they measure something a tracker cannot — total energy — and, crucially, they work for neutral particles too. A photon or a neutron leaves no track, but it still showers and deposits its energy, so the calorimeter sees it. A useful caveat about precision: calorimeter energy resolution improves with energy (the relative error shrinks as the particle gets more energetic), the opposite trend from a tracker, whose momentum measurement degrades at very high energy. The two are complementary, which is why a full detector uses both.