Heavy quark

A heavy quark has a mass large compared with the characteristic low-energy scale of quantum chromodynamics. Charm quarks, bottom quarks and top quarks are usually called heavy; treating charm as asymptotically heavy is a less accurate approximation than the corresponding treatment of bottom.

The separation of scales permits an expansion in inverse quark mass for many properties of a hadron containing one heavy quark. In the leading limit, the light degrees of freedom respond chiefly to the heavy quark's color field, with reduced sensitivity to its spin and precise flavor. This helps relate patterns in heavy-hadron spectroscopy and decays. The top quark is a special case: it decays before forming ordinary long-lived hadrons, so not every bound-state argument for charm and bottom applies to it.