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In high energy physics, the lepton number is the number of leptons minus the number of antileptons.

In equation form,

- $L\; =\; n\_\{ell\}\; -\; n\_\{overline\{ell\}\}$

so all leptons have assigned a value of +1, antileptons −1, and non-leptonic particles 0. Lepton number (sometimes also called lepton charge) is an additive quantum number, which means that its sum is preserved in interactions (as opposed to multiplicative quantum numbers such as parity, where the product is preserved instead).

Beside the leptonic number, leptonic family numbers are also defined:

- the electronic number $L\_e$ for the electron and the electronic neutrino;
- the muonic number $L\_\{mu\}$ for the muon and the muonic neutrino;
- the tauonic number $L\_\{tau\}$ for the tauon and the tauonic neutrino;

with the same assigning scheme as the leptonic number: +1 for particles of the corresponding family, −1 for the antiparticles, and 0 for leptons of other families or non-leptonic particles.

- $begin\{matrix\}$

The lepton family numbers arise from the fact that lepton number is usually conserved in each leptonic family. For example, almost 100% of the time the muon decays as:

- $begin\{matrix\}$

thus preserving the electronic and muonic numbers. This means that a lepton family number conservation law exist for each one of $L\_e$, $L\_\{mu\}$ and $L\_\{tau\}$.

- $begin\{matrix\}$

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 much more likely to work and is seen in different models such as the Pati-Salam model.

- Griffiths, David J. (1987).
*Introduction to Elementary Particles*. Wiley, John & Sons, Inc. ISBN 0-471-60386-4. - Tipler, Paul; Llewellyn, Ralph (2002).
*Modern Physics (4th ed.)*. W. H. Freeman. ISBN 0-7167-4345-0.

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Last updated on Saturday August 09, 2008 at 12:44:03 PDT (GMT -0700)

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This article is licensed under the GNU Free Documentation License.

Last updated on Saturday August 09, 2008 at 12:44:03 PDT (GMT -0700)

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