Flavour (particle physics)

Flavour or Flavor (English for flavor or taste) is one of the quantum numbers of elementary particles ( quarks and leptons ) in the context of the weak interaction. In the theory of electroweak interactions, this symmetry is broken, however, so that is no flavor conservation number, and there are flavor changing processes. In quantum chromodynamics, however, there is a global symmetry, and flavor remains in all processes that are subject only to the strong interaction.

The flavor quantum numbers of the quarks (even beauty ) designated by the respective quarks as isospin ( for up and down quarks ), Charm, Strangeness, Topness (also Truth ) and Bottom Ness.

The term was first used in 1968 flavor associated with the quark model of hadrons. The name is said to have been invented by Murray Gell-Mann and Harald Fritzsch, as they passed on the way to lunch at an ice cream shop ( Baskin -Robbins ), which 31 different flavors offered.

Quark flavors

There are a total of 6 different quark flavors ( 2 per generation):

Here B is the baryon number, Q is the electric charge ( in units of e), I3 the isospin 3- component, S is the strangeness, C of Charm, B ' the Bottom Ness ( the apostrophe in B' is used to distinguish the baryon number B ), T is the Topness, Y is the hypercharge YW and the weak hypercharge.

The flavor quantum numbers of the respective numbers of quarks are defined:

The sign convention is chosen so that for quarks from the up- type (u, c, t) are the respective flavor quantum number is positive, whereas for quarks from down - type (d, s, b ) is negative. For antiquarks, the sign is always just the other way round as for the respective quark, for all other elementary particles, the respective flavor 0

Hadrons get their flavor of the valence quarks, this is the basis of the Eightfold Way and the quark model.

For hadrons and quarks, the Gell-Mann - Nishijima formula applies:



Ordinary matter, which consists of protons and neutrons, is the isospin, and the two quark flavors described up (u) and down (d). Odd Matter later made the introduction of the s-quark and of the equivalent quantum number strangeness necessary. According to the isospin symmetry assumed in 1964 Bjørken and Glashow, that there must be a partner to the strangeness another quantum number, which they called charm. The postulated Orthocharmonium of them (analogous to the orthopositronium ) was discovered in 1974 at BNL as J and the SLAC under the name ψ ( J / ψ meson ).

Lepton Flavours

Leptons also occur in six flavors ( two per Leptonenfamilie ):

Lf is here the respective Leptonenfamilienzahl for the families f = e, μ and τ. Their sum gives the lepton number L.

Antiparticles have opposite quantum numbers compared to the corresponding particles. For example, the positron ( the anti- electron), the quantum numbers Le = -1 and Q = 1


If you look at ( quark ) and generations ( lepton ) families in principle equivalent, then, it is also the leptons in up- like ( neutrinos ) and down -type (mass- prone leptons ) divided. The difference in charges between an up -type and down -type flavor in each case 1 is thus the quarks and leptons in the three families or generations, each with one up and one down -like - like particles can be classified:

The number of families of quarks and leptons must correspond to chiral anomalies to prevent.

A fermion of the respective Flavours is an eigenstate of the weakly interacting part of the Hamiltonian: Each particle interacts in a characteristic manner with the vector bosons W ± and Z0. On the other hand, a fermion with finite mass ( ie an eigenstate of the kinetic part of the Hamiltonian ) is a superposition of different flavor states. It follows that the flavor state of a particle can change while it moves freely. The transformation from the flavor basis to mass basis for quarks is carried by the Cabibbo - Kobayashi - Maskawa matrix ( CKM matrix ). For leptons exist analogously, the Maki - Nakagawa- Sakata matrix ( MNS matrix).

At least three families the CKM matrix allows a violation of CP invariance.

Conserved quantities

Absolutely preserved, for example:

  • The electric charge Q
  • The weak hypercharge Yw and the third component of the weak isospin T3 = Q - Yw / 2
  • The difference of baryon and lepton number: B - L and X = 5 ( B -L) - 2YW.

Under the strong interaction all flavor quantum numbers are preserved.