Quark branching in QCD matter to any order in opacity beyond the soft gluon emission limit

TL;DR

This paper develops a formalism to compute the gluon spectrum from quark jets in nuclear matter at any order in opacity, extending beyond the soft gluon approximation and applicable to various quark types.

Contribution

It introduces a method to calculate the full gluon spectrum for quark jets in medium to arbitrary order in opacity, surpassing previous first-order limited approaches.

Findings

Calculates gluon spectrum to any order in opacity.

Extends beyond soft gluon emission limit.

Applicable to light and heavy quark branching.

Abstract

Cold nuclear matter effects in reactions with nuclei at a future electron-ion collider (EIC) lead to a modification of semi-inclusive hadron production, jet cross sections, and jet substructure when compared to the vacuum. At leading order in the strong coupling, a jet produced at an EIC is initiated as an energetic quark, and the process of this quark splitting into a quark-gluon system underlies experimental observables. The spectrum of gluons associated with the branching of this quark jet is heavily modified by multiple scattering in a medium, allowing jet cross sections and jet substructure to be used as a probe of the medium's properties. We present a formalism that allows us to compute the gluon spectrum of a quark jet to an arbitrary order in opacity, the average number of scatterings in the medium. This calculation goes beyond the simplifying limit in which the gluon radiation is soft and can be interpreted as energy loss of the quark, and it significantly extends previous work which computes the full gluon spectrum only to first order in opacity. The theoretical framework demonstrated here applies equally well to light parton and heavy quark branching, and is easily generalizable to all in-medium splitting processes.