Using Jets to Probe the Quark Gluon Plasma

Accelerating and colliding heavy ions at very large energies provides a laboratory for studying nuclear matter under extreme conditions. Under such conditions, the matter is expected to undergo a phase transition, “melting” the protons and neutrons into their fundamental constituents, quarks and gluons (or partons). To study the properties of this Quark Gluon Plasma (QGP) phase, we need “probes” that carry information about the properties of the produced matter. Ideally, such probes are created early in a heavy-ion collision, allowing them to experience the full evolution of the collision, including the hottest and densest phases of the matter. Jets, which are collections of particles resulting from scatterings of energetic partons with large momentum transfer (“hard scatterings”) upon initial impact of the incoming nuclei, are among such probes. In-medium modifications of jets are quantified by comparing jet production in heavy-ion collisions to the baseline measurement in proton+proton collisions. In some hard scatterings, a photon is produced back-to-back with a jet. Direct photons, those produced during the collision rather than from decays of hadrons, are of interest because they do not interact strongly and thus are not affected significantly by the medium. With the photon energy as a good approximation for the initial energy of the recoil parton (before interaction with the medium), the study of direct-photon-tagged jets can give information about the energy loss of the recoil parton while traversing through the medium. Recent measurements of jets and photon-tagged jets will be presented and discussed.