In principle we can calculate the mass of a particle if we know its momentum and either its speed or its energy. However, for a particle moving close to the speed of light any small uncertainty in momentum or energy makes it difficult to determine its mass from these two, so we need to measure speed too.

A multi-layer detector is used to identify particles. Each layer gives different information about the "event." Computer calculations based on the information from all the layers reconstruct the positions of particle tracks and identify the momentum, energy, and speed of as many as possible of the particles produced in the event.