In particle physics, lepton number (historically also called lepton charge) is a conserved quantum number representing the difference between the number of leptons and the number of antileptons in an elementary particle reaction. Lepton number is an additive quantum number, so its sum is preserved in interactions (as opposed to multiplicative quantum numbers such as parity, where the product is preserved instead). Mathematically, the lepton number is defined by , where is the number of leptons and is the number of antileptons.
Lepton number was introduced in 1953 to explain the absence of reactions such as in the Cowan–Reines neutrino experiment, which instead observed . This process, inverse beta decay, conserves lepton number, as the incoming antineutrino has lepton number –1, while the outgoing positron (antielectron) also has lepton number –1.
Lepton flavor conservation
In addition to lepton number, lepton family numbers are defined as
- Le , the electron number, for the electron and the electron neutrino;
- Lμ , the muon number, for the muon and the muon neutrino; and
- Lτ , the tau number, for the tau and the tau neutrino.
Prominent examples of lepton flavor conservation are the muon decays and . In these, 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. Likewise, a decaying negative muon results in the creation of a muon neutrino, while a decaying positive muon results in the creation of a muon antineutrino.
Violations of the lepton number conservation laws
Numerous searches for physics beyond the Standard Model incorporate searches for lepton number or lepton flavor violation, such as the decays . 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. Some theories beyond the Standard Model, such as supersymmetry, predict branching ratios of order 10−12 to 10−14. The Mu2e experiment, in construction as of 2017, has a planned sensitivity of order 10−17.
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.
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