Yacine Mehtar-Tani

New picture of jet quenching dictated by color coherence

We propose a new description of the jet quenching phenomenon observed in nuclear collisions at high energies in which coherent parton branching plays a central role. This picture is based on the appearance of a dynamically generated scale, the jet resolution scale, which controls the transverse resolution power of the medium to simultaneously propagating color probes. Since from the point of view of the medium all partonic jet fragments within this transverse distance act coherently as a single emitter, this scale allows us to rearrange the jet shower into effective emitters. We observe that in the kinematic regime of the LHC, the corresponding characteristic angle is comparable to the typical opening angle of high-energy jets such that most of the jet energy is contained within a non-resolvable color coherent inner core. Thus, a sizable fraction of the jets is unresolved, losing energy as a single patron without modifications of their intra-jet structure

Medium-induced QCD cascade: democratic branching and wave turbulence.

We study the average properties of the gluon cascade generated by an energetic parton propagating through a quark-gluon plasma. We focus on the soft, medium-induced emissions which control the energy transport at large angles with respect to the leading parton. We show that the effect of multiple branchings is important. In contrast with what happens in a usual QCD cascade in vacuum, medium-induced branchings are quasidemocratic, with offspring gluons carrying sizable fractions of the energy of their parent gluon. This results in an efficient mechanism for the transport of energy toward the medium, which is akin to wave turbulence with a scaling spectrum ~1/sqrt[ω]. We argue that the turbulent flow may be responsible for the excess energy carried by very soft quanta, as revealed by the analysis of the dijet asymmetry observed in Pb-Pb collisions at the LHC.

Medium-induced gluon branching

A bstractWe study the evolution of an energetic jet which radiates gluons while propagating through a dense QCD medium modeled as a random distribution of color sources. Motivated by the heavy ion experimental program at the LHC, we focus on the medium induced radiation of (relatively) soft gluons, which are abundantly emitted at large angles and thus can transport a small fraction of the jet energy far away from the jet axis. We perform a complete calculation of the medium-induced gluon branching in the regime where the gluons that take part in the branching undergo multiple soft scattering with the medium. We extend the BDMPSZ theory of radiative energy loss by including the transverse momentum dependence in the kernel that describes the branching and by analyzing the correlations between the two offspring gluons. We demonstrate that these gluons lose color coherence with respect to each other over a time scale that is comparable to the duration of the branching. It follows that interference effects between successive emissions are suppressed, a necessary ingredient for a description of multiple emission of soft gluons by a probabilistic, branching process.

The radiation pattern of a QCD antenna in a dilute medium

A bstractRadiative interferences in the multi-parton shower is the building block of QCD jet physics in vacuum. The presence of a hot medium made of quarks and gluons is expected to alter this interference pattern. To study these effects, we derive the gluon emission spectrum off a color-correlated quark-antiquark pair (antenna) traversing a colored medium to first order in the medium density. The resulting induced gluon distribution is found to be governed by the hardest scale of the problem. In our setup, this can either be the inverse antenna transverse size,

Renormalization of the jet-quenching parameter

r_{ \bot }^{{ - 1}}

Probabilistic picture for medium-induced jet evolution

, or the scale related to the transverse color correlation length in the medium, which is given by the Debye mass mD . This emerging scale opens the angular phase space of emissions off the antenna compared to the vacuum case and gives rise to a typical transverse momentum of the medium-induced emitted gluons,

Jet structure in heavy ion collisions

{\left\langle {k_{ \bot }^2} \right\rangle_{{med}}}

Jet coherence in QCD media: the antenna radiation spectrum

~ max

Angular Structure of the In-Medium QCD Cascade.

\left( {r_{ \bot }^{{ - 1}},{m_D}} \right)

Jet (de)coherence in Pb-Pb collisions at the LHC

. Above the hard scale interference effects suppress the spectrum resulting in the restoration of vacuum coherence.