Editing Muon (section)

=== Prohibited decays ===
Certain neutrino-less decay modes are kinematically allowed but are, for all practical purposes, forbidden in the [[Standard Model]], even given that neutrinos have mass and oscillate. Examples forbidden by lepton flavour conservation are:
: {{subatomic particle|muon}} → {{subatomic particle|electron}} + {{subatomic particle|photon}}
and
: {{subatomic particle|muon}} → {{subatomic particle|electron}} + {{subatomic particle|positron}} + {{subatomic particle|electron}} .

Taking into account neutrino mass, a decay like {{subatomic particle|muon}} → {{subatomic particle|electron}} + {{subatomic particle|photon}} is technically possible in the Standard Model (for example by [[neutrino oscillation]] of a virtual muon neutrino into an electron neutrino), but such a decay is extremely unlikely and therefore should be experimentally unobservable. Fewer than one in 10<sup>50</sup> muon decays should produce such a decay.

Observation of such decay modes would constitute clear evidence for theories [[beyond the Standard Model]]. Upper limits for the branching fractions of such decay modes were measured in many experiments starting more than {{rounddown|{{age|format=raw|1964|1|1}}|-1}} years ago. The current upper limit for the {{subatomic particle|antimuon}} → {{subatomic particle|positron}} + {{subatomic particle|photon}} branching fraction was measured 2009–2013 in the [[Mu to E Gamma|MEG]] experiment and is {{val|4.2e-13}}.<ref name=Baldini>
{{cite arXiv
 | last1 = Baldini | first1 =A.M.
 | collaboration=MEG collaboration
 | display-authors=etal
 | title =Search for the lepton flavour violating decay μ<sup>+</sup> → e<sup>+</sup>γ with the full dataset of the MEG experiment
 | date =May 2016
 | eprint=1605.05081
 | class =hep-ex
}}</ref>