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In particle physics, lepton number (historically also called lepton charge) [1] is a conserved quantum number representing the difference between the number of leptons and the number of antileptons in an elementary particle reaction. [2]
The name lepton comes from the Greek λεπτός leptós, "fine, small, thin" (neuter nominative/accusative singular form: λεπτόν leptón); [14] [15] the earliest attested form of the word is the Mycenaean Greek 𐀩𐀡𐀵, re-po-to, written in Linear B syllabic script. [16] Lepton was first used by physicist Léon Rosenfeld in 1948: [17]
All leptons carry a lepton number L = 1. In addition, leptons carry weak isospin, T 3, which is − 1 / 2 for the three charged leptons (i.e. electron, muon and tau) and + 1 / 2 for the three associated neutrinos. Each doublet of a charged lepton and a neutrino consisting of opposite T 3 are said to constitute one generation of ...
They include the quarks and leptons, as well as any composite particles consisting of an odd number of these, such as all baryons and many atoms and nuclei. Fermions have half-integer spin; for all known elementary fermions this is 1 / 2 .
There are also many approximate conservation laws, which apply to such quantities as mass, parity, [1] lepton number, baryon number, strangeness, hypercharge, etc. These quantities are conserved in certain classes of physics processes, but not in all.
This quantum number is the charge of a global/gauge U(1) symmetry in some Grand Unified Theory models, called U(1) B−L.Unlike baryon number alone or lepton number alone, this hypothetical symmetry would not be broken by chiral anomalies or gravitational anomalies, as long as this symmetry is global, which is why this symmetry is often invoked.
The tau (τ), also called the tau lepton, tau particle or tauon, is an elementary particle similar to the electron, with negative electric charge and a spin of 1 / 2 .Like the electron, the muon, and the three neutrinos, the tau is a lepton, and like all elementary particles with half-integer spin, the tau has a corresponding antiparticle of opposite charge but equal mass and spin.
bosons can decay to a lepton and antilepton (one of them charged and another neutral) [d] or to a quark and antiquark of complementary types (with opposite electric charges ± + 1 / 3 and ∓ + 2 / 3 ).