Jet Tomography in Heavy-Ion Collisions—Challenges, Results, and Open Problems

Abstract

Over the past 30 years, jet observables have proven to provide important information about the quark-gluon plasma created in heavy-ion collisions. I review the challenges, results, and open problems of jet physics in heavy-ion collisions, discussing the main ideas as well as some most recent results focussing on two major jet observables, the nuclear modification factor and the high-p T elliptic flow. Jets in heavy-ion collisionsRelativistic high-energy heavy-ion collisions offer the unique possibility to study matter experimentally under extreme conditions of high temperature and densities in the laboratory. One of the main challenges is to probe the quark-gluon plasma (QGP) created in such heavy-ion collisions. One set of observables is based on jets, sprays of particles that are produced back-to-back due to the conservation of energy and momentum. Those jets propagate through the dense matter formed while depositing energy. As this jet-energy loss inevitably leads to an attenuation of the jet [1,2,3,4,5] this concept is referred to as jet quenching.The breakthrough of studying jets in heavy-ion collisions came with the start of the Relativistic Heavy Ion Collider (RHIC) in 2000 [6, 7, 8, 9]. By studying the azimuthal distribution of the back-to-back jets in Au+Au collisions at RHIC, it could be shown that this part of the jet which propagates through the hot and dense matter is suppressed (or quenched) as compared to measurements in proton+proton (p+p) or deuteron+gold (d+Au) collisions [10,11]. This result is considered as a clear signal that at RHIC energies the hot and dense QGP medium is only created in heavy-ion collisions.