Medium-induced gluon branching

Abstract

We 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 mediuminduced 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.

Angular distribution of medium-induced QCD cascades

“…First, the typical duration of the branching process is assumed to be small compared to the total time spent by the gluon in the medium. This allows to treat the branchings as effectively instantaneous [18]. Second, the transverse momentum broadening that takes place during a branching is ignored (corrections involving the small transverse momentum induced during the splitting can be absorbed in corrections toq [9,19] (see also [20,21] for related discussions of radiative corrections to p ⊥ broadening).…”

“…This allows to treat the branchings as effectively instantaneous [18]. Second, the transverse momentum broadening that takes place during a branching is ignored (corrections involving the small transverse momentum induced during the splitting can be absorbed in corrections toq [9,19] (see also [20,21] for related discussions of radiative corrections to p ⊥ broadening). The branching is then treated as effectively collinear: After the splitting, the two new gluons carry fractions z and 1−z of both the initial energy and the initial transverse momentum.…”

“…The angular distribution that we determine may be a useful ingredient to be implemented in Monte Carlo event generators that are developed by several groups [5,6,7,8]. The present work builds on the probabilistic equation that describes the in-medium QCD cascade, and which was derived recently [9]. It controls the evolution with the size of the medium, L, of the inclusive distribution of partons produced in the cascade as a function of the energy and the transverse momentum of the observed gluon.…”

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

“…The spectrum is regulated in the infrared when t br is of the order of the mean free path λ mfp , which corresponds to the Bethe-Heitler (BH) frequency ω BH =qλ 2 mfp . We model this regime by regularizing the branching time, i.e., t br → t br + λ mfp and refer to [11,17] for further details on the derivation of Eq. (3).…”

“…In our case Q s = 3.6 GeV severely restricts the amount of soft induced radiation that is allowed within the cone. The description of broadening will be discussed in more detail in a forthcoming work, see also [17].…”