Lepton number

In particle physics, the lepton number is a conserved quantum number representing the number of leptons minus the number of antileptons in an elementary particle reaction.

In equation form,

so all leptons have assigned a value of +1, antileptons −1, and non-leptonic particles 0. Lepton number (sometimes also called lepton charge) is an additive quantum number, which means that its sum is preserved in interactions (as opposed to multiplicative quantum numbers such as parity, where the product is preserved instead).

Lepton number was introduced in 1953 and was invoked to explain the absence of reactions such as in the reactor Cowan–Reines neutrino experiment, which observed instead.

Beside the leptonic number, leptonic family numbers are also defined:

with the same assigning scheme as the leptonic number: +1 for particles of the corresponding family, −1 for the antiparticles, and 0 for leptons of other families or non-leptonic particles.