Jet physics in heavy-ion collisions

Jets are expected to play a prominent role in the ongoing efforts to characterize the hot and dense QCD medium created in ultrarelativistic heavy ion collisions. The success of this program depends crucially on the existence of a full theoretical account of the dynamical effects of the medium on the jets that develop within it. By focussing on the discussion of the essential ingredients underlying such a theoretical formulation, we aim to set the appropriate context in which current and future developments can be understood.Comment: 36 pages, 5 figures, few minor corrections, references added. Final version published in IJMP

Jet coherence in QCD media: the antenna radiation spectrum

We study the radiation of a highly energetic partonic antenna in a colored state traversing a dense QCD medium. Resumming multiple scatterings of all involved constituents with the medium we derive the general gluon spectrum which encompasses both longitudinal color coherence between scattering centers in the medium, responsible for the well known Landau-Pomeranchuk-Migdal (LPM) effect, and transverse color coherence between partons inside a jet, leading, in vacuum, to angular ordering of the parton shower. We discuss shortly the onset of transverse decoherence which is reached in opaque media. In this regime, the spectrum consists of independent radiation off the antenna constituents.Comment: 15 pages, 2 figures, paper shortened and partly rewritten, references added, results unchange

Jet thermalization in QCD kinetic theory

We perform numerical studies in QCD kinetic theory to investigate the energy and angular profiles of a high energy parton - as a proxy for a jet produced heavy ion collisions - passing through a Quark-Gluon Plasma (QGP). We find that the fast parton loses energy to the plasma mainly via a radiative turbulent gluon cascade that transport energy locally from the jet down to the temperature scale where dissipation takes place. In this first stage, the angular structure of the turbulent cascade is found to be relatively collimated. However, when the lost energy reaches the plasma temperature is it rapidly transported to large angles w.r.t. the jet axis and thermalizes. We investigate the contribution of the soft jet constituents to the total jet energy. We show that for jet opening angles of about 0.3 rad or smaller the effect is negligible. Conversely, larger opening angles become more and more sensitive to the thermal component of the jet and thus to medium response. Our result showcase the importance of the jet cone size in mitigating or enhancing the details of dissipation in jet quenching observables.Comment: 41 pages, 11 figures

Stopping in central Pb + Pb collisions at SPS energies and beyond

We investigate stopping and baryon transport in central relativistic Pb + Pb and Au + Au collisions. At energies reached at the CERN Super Proton Synchrotron [sqrt(s_NN) = 6.3-17.3 GeV] and at RHIC (62.4 GeV), we determine the fragmentation-peak positions from the data. The resulting linear growth of the peak positions with beam rapidity is in agreement with our results from a QCD-based approach that accounts for gluon saturation. No discontinuities in the net-proton fragmentation peak positions occur in the expected transition region from partons to hadrons at 6-10 GeV.Comment: 5 pages, 3 figures, 1 table. Figures updated, table shortened, 1 reference adde

Mass effect and coherence in medium-induced QCD radiation off a qqˉq {\bar q} antenna

The medium-induced one-gluon radiation spectrum off a massive quark-antiquark ($q {\bar q}$) antenna traversing a colored QCD medium is calculated in this contribution. The gluon spectrum off the antenna computed at first order in the opacity expansion is collinear finite but infrared divergent, which is different from the result obtained from an independent emitter which is both infrared and collinear finite. The interference between emitters dominates the soft gluon radiation when the antenna opening angle is small and the emitted gluon is soft, whereas the antenna behaves like a superposition of independent emitters when the opening angle is large and the radiated gluon is hard. As a phenomenological consequence, we investigate the energy lost by the projectiles due to the radiation. In general, the size of the mass effects is similar in both cases.Comment: 4 pages, 1 figure, Proceedings of Quark Matter 2011, Annecy, Franc

Antiangular Ordering of Gluon Radiation in QCD Media

We investigate angular and energy distributions of medium-induced gluon emission off a quark-antiquark antenna in the framework of perturbative QCD as an attempt toward understanding, from first principles, jet evolution inside the quark-gluon plasma. In-medium color coherence between emitters, neglected in all previous calculations, leads to a novel mechanism of soft-gluon radiation. The structure of the corresponding spectrum, in contrast with known medium-induced radiation, retains some properties of the vacuum case; in particular, it exhibits a soft divergency. However, as opposed to the vacuum, the collinear singularity is regulated by the pair opening angle, leading to a strict angular separation between vacuum and medium-induced radiation, denoted as antiangular ordering. We comment on the possible consequences of this new contribution for jet observables in heavy-ion collisions.Comment: 4 pages, 2 figures; v2. a number of minor improvements, figures updated, accepted for publication in PR

Monte Carlo Tools for Jet Quenching

A thorough understanding of jet quenching on the basis of multi-particle final states and jet observables requires new theoretical tools. This talk summarises the status and propects of the theoretical description of jet quenching in terms of Monte Carlo generators.Comment: proceedings of the 22nd International Conference on Ultra-Relativistic Nucleus Nucleus Collisions (Quark Matter 2011