Quark

:For other uses of this term, see: Quark (disambiguation)

Flavour

Each quark is assigned a baryon number, B  =  1/3, and a vanishing lepton number L  =  0. They have fractional electric charge, Q, either Q  =  +2/3 or Q  =  ?1/3. The former are called up-type quarks, the latter, down-type quarks. Each quark is assigned a weak isospin: Tz  =  +1/2 for an up-type quark and Tz  =  ?1/2 for a down-type quark. Each doublet of weak isospin defines a generation of quarks. There are three generations, and hence six flavours of quarks ? the up-type quarks have flavours u, c and t, the down-type quark flavours are d, s, b.

Related Topics:
Baryon number - Lepton number - Charge - Weak isospin - Flavour

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The number of generations of quarks and leptons are equal in the standard model. The number of generations of leptons is strongly constrained by experiments at the LEP in CERN and by observations of the abundance of helium in the universe. Precision measurement of the lifetime of the Z boson at LEP constrains the number of generations to be three. Astronomical observations of helium abundance give consistent results. Results of direct searches for a fourth generation give limits on the mass of the lightest possible fourth generation quark. The most stringent limit comes from analysis of results from the Tevatron collider at Fermilab, and shows that the mass of a fourth-generation quark must be greater than 190 GeV.

Related Topics:
LEP - CERN - Helium - Z boson - Tevatron - Fermilab - GeV

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Each flavour defines a quantum number which is conserved under the strong interactions, but not the weak interactions. The magnitude of flavour changing in the weak interaction is encoded into a structure called the CKM matrix. This also encodes the CP violation allowed in the Standard Model. The flavour quantum numbers are described in detail in the article on flavour.

Related Topics:
Strong interaction - Weak interaction - CKM matrix - CP violation - Flavour

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