A time of flight detector is a particle detector which can discriminate between a lighter and a heavier elementary particle of same momentum using their time of flight between two scintillators. The first of the scintillators activates a clock upon being hit while the other stops the clock upon being hit. If the two masses are denoted by m₁and m₂ and have velocities v₁ and v₂ then the time of flight difference is given by

where L is the distance between the scintillators. The approximation is in the relativistic limit at momentum p and c denotes the speed of light in vacuum.