Quarks cannot exist in solitude. The force between quarks is the
strong nuclear force
, and at short distances, i.e. less than ~0.5fm
(less than 5x10-16
m - approximately the
of a nucleon) the attractive strong force is similar to the repulsive
electromagnetic force felt between charges.
At greater distances between a quark / anti-quark pair, the strong force
increases. Eventually a gluon (strong force carrier) will have enough energy to
spontaneously split into a new quark / anti-quark pair. This process can be
compared to a stretched elastic band - the band stretches and the energy
increases until there is enough energy to split the band in two.
The quarks and anti-quarks produced are highly energetic and decay into a
cascade of hadrons, called a
jet. The picture below shows two jets produced from a Z boson at the LEP (Large Electron-Positron Collider) at CERN in Geneva.
Some of the hadrons generated in the jets may decay themselves, meaning that the jets
will also contain leptons such as
electrons or neutrinos.