Editing Lepton number (section)

== Violations of the lepton number conservation laws ==
Lepton flavor is only approximately conserved, and is notably not conserved in [[neutrino oscillation]].<ref name=Fukuda-Hayakawa-Ichihara-1998-08-24-PRL/>
However, both the total lepton number and lepton flavor are still conserved in the Standard Model.

Numerous searches for [[physics beyond the Standard Model]] incorporate searches for lepton number or lepton flavor violation, such as the hypothetical decay<ref name=Adam-Bai-Baldini-etal-2011-10-21/>
: {{math| {{subatomic particle|link=y|muon}} → {{subatomic particle|link=y|electron}} + {{subatomic particle|photon|link=y}} }}.

Experiments such as MEGA and SINDRUM have searched for lepton number violation in muon decays to electrons; [[Mu to E Gamma|MEG]] set the current branching limit of order {{10^|−13}} and plans to lower to limit to {{10^|−14}} after 2016.<ref name=Baldini-2016-05-arXiv/>
Some theories beyond the Standard Model, such as [[supersymmetry]], predict branching ratios of order {{10^|−12}} to {{10^|−14}}.<ref name=Adam-Bai-Baldini-etal-2011-10-21/> The [[Mu2e]] experiment, in construction as of 2017, has a planned sensitivity of order {{10^|−17}}.<ref Name=Kwon-2015-04-21-Fermilab-PR/>

Because the lepton number conservation law in fact is violated by [[chiral anomaly|chiral anomalies]], there are problems applying this symmetry universally over all energy scales. However, the quantum number [[B − L|{{mvar|B − L}}]] is commonly conserved in [[Grand Unified Theory]] models.

If neutrinos turn out to be [[Majorana fermions]], neither individual lepton numbers, nor the total lepton number
<math display=block>L \equiv L_\mathrm{e} + L_\mathrm{\mu} + L_\mathrm{\tau},</math>
nor
: [[B − L|{{math|''B'' − ''L''}}]]
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.