Flavour quantum number

In particle physics, flavour or flavor refers to a species of an elementary particle. The Standard Model counts six flavours of quarks and six flavours of leptons. They are conventionally parameterized with flavour quantum numbers that are assigned to all subatomic particles.

In classical mechanics, forces can only change a particle's momentum. Quantum theory, however, allows forces that can alter other facets of a particle's nature described by quantum numbers. In particular, the behavior of the weak force is such that it allows the conversion of quantum numbers describing mass and electric charge of both quarks and leptons from one discrete type to another. This is known as a flavor change, or flavor transmutation. Due to their quantum description, flavour states may also undergo quantum superposition.

In atomic physics the principal quantum number of an electron specifies the electron shell in which it resides, which determines the energy level of the whole atom. Analogously, the five flavour quantum numbers (isospin, strangeness, charm, beauty or truth) can characterize the quantum state of quarks (and leptons), by the degree to which it exhibits six distinct flavours (u, d, s, c, b, t).

Composite particles can be created from multiple quarks, forming hadrons, such as mesons and baryons, each possessing unique aggregate characteristics, such as different masses, electric charges, and decay modes. For hadrons, these quantum numbers depend on the numbers of constituent quarks of each particular flavour.

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