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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]
Conserved' means that the sum of the baryon number of all incoming particles is the same as the sum of the baryon numbers of all particles resulting from the reaction. The one exception is the hypothesized Adler–Bell–Jackiw anomaly in electroweak interactions ; [ 4 ] however, sphalerons are not all that common and could occur at high energy ...
Exact conservation laws include conservation of mass-energy, conservation of linear momentum, conservation of angular momentum, and conservation of electric charge. There are also many approximate conservation laws, which apply to such quantities as mass , parity , [ 1 ] lepton number , baryon number , strangeness , hypercharge , etc.
Absolutely conserved quantum numbers in the Standard Model are: electric charge (Q) weak isospin (T 3) baryon number (B) lepton number (L) In some theories, such as the grand unified theory, the individual baryon and lepton number conservation can be violated, if the difference between them (B − L) is conserved (see Chiral anomaly).
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]
Conservation of angular momentum. Conservation of total (i.e. net) lepton number , which is the number of leptons (such as the electron) minus the number of antileptons (such as the positron); this can be described as a conservation of (net) matter law.
As energy must be conserved, for pair production to occur, the incoming energy of the photon must be above a threshold of at least the total rest mass energy of the two particles created. (As the electron is the lightest, hence, lowest mass/energy, elementary particle, it requires the least energetic photons of all possible pair-production ...
The anomalies that would break baryon number conservation and lepton number conservation individually cancel in such a way that B – L is always conserved. One hypothetical example is proton decay where a proton (B = 1, L = 0) would decay into a pion (B = 0, L = 0) and positron (B = 0, L = –1). The weak hypercharge Y W is related to B – L via