In the context of high-energy
hadron collisions,
quarks and
gluons are collectively called
partons. The jets emerging from the collisions originally consist of partons, which quickly combine to form hadrons, a process called
hadronization. Only the resulting hadrons can be directly observed. The hot, dense medium produced in the collisions is also composed of partons; it is known as a
quark–gluon plasma (QGP). In this realm, the laws of physics that apply are those of
quantum chromodynamics (QCD). High-energy nucleus–nucleus collisions make it possible to study the properties of the
QGP medium through the observed changes in the jet fragmentation functions as compared to the unquenched case. According to
QCD, high-momentum
partons produced in the initial stage of a nucleus–nucleus collision will undergo multiple interactions inside the collision region prior to
hadronization. In these interactions, the energy of the partons is reduced through collisional energy loss and medium-induced gluon radiation, the latter being the dominant mechanism in a QGP. The effect of jet quenching in QGP is the main motivation for studying jets as well as high-momentum particle spectra and particle correlations in heavy-ion collisions. Accurate jet reconstruction will allow measurements of the jet fragmentation functions and consequently the degree of quenching and therefore provide insight on the properties of the hot dense QGP medium created in the collisions. == Experimental evidence of jet quenching ==