Flavour (Particle Physics)

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In particle physics, **flavour** or **flavor** is a quantum number of elementary particles. In quantum chromodynamics, flavour is a global symmetry. In the electroweak theory, on the other hand, this symmetry is broken, and Flavour changing processes exist, such as quark decay or neutrino oscillations.

## Overview

If there are two or more particles which have identical interactions, then they may be interchanged without affecting the physics. Any (complex) linear combination of these two particles give the same physics, as long as they are orthogonal or perpendicular to each other. In other words, the theory possesses symmetry transformations such as <math>Mleft(right)</math>, where *u* and *d* are the two fields, and *M* is any unitary matrix with a unit determinant. Such matrices form a Lie group called SU (see special unitary group). This is an example of flavour symmetry.

The term "flavour" was first coined for use in the quark model of hadrons in 1968.

## Flavour quantum numbers

### Leptons

All leptons carry a lepton number *L* = 1. In addition, leptons carry weak isospin, *T*<sub>3</sub>, which is − for the three charged leptons (i.e. electron, muon and tau) and + for the three associated neutrinos. Each doublet of a charged lepton and a neutrino consisting of opposite *T*<sub>3</sub> are said to constitute one generation of leptons. In addition, one defines a quantum number called weak hypercharge,...

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The term "flavour" was first coined for use in the quark model of hadrons in 1968.

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