Editing Lepton number (section)

{{Short description|Difference between number of leptons and antileptons}}
{{Flavour quantum numbers}}
 
In [[particle physics]], '''lepton number''' (historically also called '''lepton charge''')<ref name=Gribov-Pontecorvo-1969-01-20-PhL-B/>
is a [[conservation law|conserved]] [[quantum number]] representing the difference between the number of [[lepton]]s and the number of [[Antiparticle|antileptons]] in an elementary particle reaction.<ref name=Griffiths-1987-txbk-Tipler-Llewellyn-2002-txbk/>
Lepton number is an additive [[quantum number]], so its sum is preserved in interactions (as opposed to multiplicative [[quantum number]]s such as parity, where the product is preserved instead). The lepton number <math>L</math> is defined by
<math display=block>L = n_\ell - n_{\overline\ell},</math>
where
* <math>n_\ell             \quad </math> is the number of [[lepton]]s and
* <math>n_{\overline\ell } \quad </math> is the number of [[antilepton]]s.

Lepton number was introduced in 1953 to explain the absence of reactions such as
: {{math| {{subatomic particle|antineutrino}} + {{subatomic particle|neutron}} → {{subatomic particle|proton}} + {{subatomic particle|electron}} }}
in the [[Cowan–Reines neutrino experiment]], which instead observed
: {{math| {{subatomic particle|antineutrino}} + {{subatomic particle|proton}} → {{subatomic particle|neutron}} + {{subatomic particle|positron}} }}.<ref name=Konopinski-Mahmoud-1953-11-15-PhR/>
This process, [[inverse beta decay]], conserves lepton number, as the incoming [[antineutrino]] has lepton number&nbsp;−1, while the outgoing [[positron]] (antielectron) also has lepton number&nbsp;−1.