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According to the Standard Model, quarks exist in six flavors: up, down, charm, strange, top and bottom (indicated with the symbols u, d, c, s, t and b). Leptons occur in six other flavors: electron, muon, tau, electron neutrino, muon neutrino and tau neutrino (e−, μ−, τ−, νe, νμ and ντ). Antiparticles have −1 unit of the corresponding flavor. For example, the strange and antistrange quarks (s and ) have a strangeness of +1 and −1, respectively.
A fermion of a given flavor is an eigenstate of the weak interaction; it will interact in a definite way with the W+, W- and Z bosons. On the other hand, a fermion of a fixed mass (an eigenstate of the hamiltonian) is normally a superposition of various flavors, and this gives rise to processes that change the flavor. In the case of quarks, this is reflected in the so-called CKM matrix. The equivalent for neutrinos is the MNS matrix.