In addition to lepton number, lepton family numbers are defined as[4] Prominent examples of lepton flavor conservation are the muon decays and In these decay reactions, the creation of an electron is accompanied by the creation of an electron antineutrino, and the creation of a positron is accompanied by the creation of an electron neutrino.
Numerous searches for physics beyond the Standard Model incorporate searches for lepton number or lepton flavor violation, such as the hypothetical decay[7] Experiments such as MEGA and SINDRUM have searched for lepton number violation in muon decays to electrons; MEG set the current branching limit of order 10−13 and plans to lower to limit to 10−14 after 2016.
[8] Some theories beyond the Standard Model, such as supersymmetry, predict branching ratios of order 10−12 to 10−14.
[7] The Mu2e experiment, in construction as of 2017, has a planned sensitivity of order 10−17.
[9] Because the lepton number conservation law in fact is violated by chiral anomalies, there are problems applying this symmetry universally over all energy scales.
However, the quantum number B − L is commonly conserved in Grand Unified Theory models.
nor would be conserved, e.g. in neutrinoless double beta decay, where two neutrinos colliding head-on might actually annihilate, similar to the (never observed) collision of a neutrino and antineutrino.
When following the electric-charge-sign convention, the lepton number (shown with an over-bar here, to reduce confusion) of an electron, muon, tauon, and any neutrino counts as
the lepton number of the positron, antimuon, antitauon, and any antineutrino counts as
When this reversed-sign convention is observed, the baryon number is left unchanged, but the difference B − L is replaced with a sum: B + L , whose number value remains unchanged, since and