Information about the energies of the various particles produced in each collision is crucial to understanding what occurred at the collision point. This information is collected from two kinds of “calorimeters” in CMS.

The Electromagnetic Calorimeter (ECAL) is the inner layer of the two and measures the energy of electrons and photons by stopping them completely. Hadrons, which are composite particles made up of quarks and gluons, fly through the ECAL and are stopped by the outer layer called the Hadron Calorimeter (HCAL).