Quark

According to the Standard Model of particle physics quarks are one of the two fundamental building blocks of matter, the other being leptons. There are six known flavours of quarks: up, down, strange, charm, bottom and top. The up, charm and top quarks have a positive electrical charge with a magnitude two thirds that of an electron whereas the down, strange and bottom quarks have a negative charge with a magnitude one third that of an electron.

In addition to an electric charge all quarks also carry a colour charge and so will interact via the strong nuclear force. The properties of this strong force leads to a property called confinement whereby, at low energies, quarks are bound into states, called hadrons, with no net colour charge. Two types hadrons are known to exist: mesons which are a quark bound with an anti-quark and baryons which are three quarks bound together. In the early twenty first century some experimental data suggesting four and five quark bound states was published but firm evidence of these states remains to be found.

The one exception to the confinement rule is the top quark which decays with a lifetime less than 5x10^-25s. This is so rapid that there is insufficient time for the quark to hadronize i.e. form bound hadronic states. Thus top quarks are unique in decaying as a free quark and so offer a unique opportunity to measure the properties of a free quark directly.

Just like the leptons quarks are divided into three generations, each consisting of two quarks: one up-like quark and one down-like quark. Only the first generation, containing the up and down quarks, is stable. Only the weak force is capable of mixing generations of quarks.