A time-of-flight (TOF) 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 m 1 {\displaystyle m_{1}} and m 2 {\displaystyle m_{2}} and have velocities v 1 {\displaystyle v_{1}} and v 2 {\displaystyle v_{2}} then the time of flight difference is given by

Δ t = L ( 1 v 1 − 1 v 2 ) ≈ L c 2 p 2 ( m 1 2 − m 2 2 ) {\displaystyle \Delta t=L\left({\frac {1}{v_{1}}}-{\frac {1}{v_{2}}}\right)\approx {\frac {Lc}{2p^{2}}}(m_{1}^{2}-m_{2}^{2})}

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