The radiation pattern of a QCD antenna in a dense medium

TL;DR

This paper analyzes the radiation pattern of a quark-antiquark pair in a dense medium, identifying regimes of decoherence and coherence, and deriving the relevant hard scales affecting medium-induced gluon radiation.

Contribution

It introduces a detailed framework for understanding medium-induced radiation regimes for a QCD antenna, including the concepts of dipole and decoherence regimes, and the associated hard scales.

Findings

Identifies the hard scale Qhard = max(r_perp^{-1}, Qs) for large angle radiation.

Defines two regimes: dipole and decoherence, based on the transverse size and medium effects.

Shows coherence is restored at transverse momenta greater than Qhard.

Abstract

We calculate the radiation spectrum off a qq-bar pair of a fixed opening angle theta_qq-bar traversing a medium of length L. Multiple interactions with the medium are handled in the harmonic oscillator approximation, valid for soft gluon emissions. We discuss the time-scales relevant to the decoherence of correlated partons traversing the medium and demonstrate how this relates to the hard scale that govern medium-induced radiation. For large angle radiation, the hard scale is given by Qhard = max(r_perp^{-1}, Qs), where r_perp = theta_qq-bar L is the probed transverse size and Qs is the maximal transverse momentum accumulated by the emitted gluon in the medium. These situations define in turn two distinct regimes, which we call "dipole" and "decoherence" regimes, respectively, and which are discussed in detail. A feature common to both cases is that coherence of the radiation is restored at large transverse momenta, k_\perp > Qhard.