Angular structure of the in-medium QCD cascade

TL;DR

This paper analyzes how gluons in a medium-induced QCD cascade spread out angularly, deriving and solving an evolution equation that reveals two regimes of transverse momentum growth depending on medium size and gluon softness.

Contribution

It introduces an analytical framework for the angular broadening of in-medium QCD cascades, identifying two distinct regimes of transverse momentum evolution.

Findings

Transverse momentum grows with medium size in standard broadening regime.

For large media and soft gluons, transverse momentum saturates, independent of medium size.

Results qualitatively agree with recent dijet asymmetry data.

Abstract

We study the angular broadening of a medium-induced QCD cascade. We derive the equation that governs the evolution of the average transverse momentum squared of the gluons in the cascade as a function of the medium length, and we solve this equation analytically. Two regimes are identified. For a medium of a not too large size, and for not too soft gluons, the transverse momentum grows with the size of the medium according to standard momentum broadening. The other regime, visible for a medium of a sufficiently large size and very soft gluons, is a regime dominated by multiple branchings: there, the average transverse momentum saturates to a value that is independent of the size of the medium. This structure of the in-medium QCD cascade is, at least qualitatively, compatible with the recent data on dijet asymmetry.