The studies of high energy nuclear (heavy-ion) collisions have been exploring the properties of QCD under extreme conditions. It is predicted that the matter in these conditions forms a new phase which is called the quark-gluon plasma (QGP). Many features of this matter is already observed in RHIC and LHC experiments, one of them being the jet-quenching phenomenon. The color-charged highly-energetic projectiles (quarks and gluons) lose some of their energy while traversing this medium, and form jets that are less energetic compared to those produced in proton-proton collisions. This is reflected in the observed hadron and jet spectra, as well as correlations between jets. The analyses so far, however, have not distinguished whether an observed jet originates from a quark or a gluon. This is a rather important aspect, since the color factor difference causes quarks and gluons to suffer the energy-loss differently. While setting constraints on the parameters of the energy-loss, the quark-gluon identification will also help characterization of collisions and make it possible to investigate the patterns of the quenched energy in more detail.
The distinct fragmentation features of quark jets and gluon jets make them possible to distinguish, which is a technique already practiced in pp collisions. Performing a similar method in heavy-ion collisions can shed light onto the quenching mechanism in the hot and dense QCD medium. However, more advanced methods have to be developed in order to cope for the large underlying event in the nuclear collisions.
The CMS detector at the LHC experiment at CERN has excellent capabilities for charged hadron tracking and calorimetric energy measurement, which constitute the essential elements of jet studies. In addition, with its triggering capabilities, CMS has collected large datasets of dijet and photon+jet events. These different channels, having different parton content, can be used for controlling the quark-gluon tagging performance.