lower to limit to after 2016. Some theories beyond the Standard Model, such as supersymmetry, predict branching ratios of order to . The Mu2e experiment, in construction as of 2017, has a planned sensitivity of order . 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 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 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. Reversed signs convention Some authors prefer to use lepton numbers that match the signs of the charges of the leptons involved, following the convention in use for the sign of weak isospin and the sign of strangeness quantum number (for quarks), both of which conventionally have the otherwise arbitrary sign of the quantum number match the sign of the particles' electric charges. 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 is replaced with a sum: whose number value remains unchanged, since and See also Baryon number Footnotes References Conservation laws Particle physics Leptons Flavour (particle physics) he:מספר לפטוני