Two-gluon emission and interference in a thin QCD medium: insights into jet formation

Abstract

In heavy-ion collisions, an abundant production of high-energy QCD jets allows to study how these multiparticle sprays are modified as they pass through the quark-gluon plasma. In order to shed new light on this process, we compute the inclusive two-gluon rate off a hard quark propagating through a color deconfined medium at first order in medium opacity. We explicitly impose an energy ordering of the two emitted gluons, such that the "hard" gluon can be thought of as belonging to the jet substructure while the other is a "soft" emission (which can be collinear or medium-induced). Our analysis focusses on two specific limits that clarify the modification of the additional angle-and formation time-ordering of splittings. In one limit, the formation time of the "hard" gluon is short compared to the "soft" gluon formation time, leading to a probabilistic formula for production of and subsequent radiation off a quark-gluon antenna. In the other limit, the ordering of formation is reverted, which automatically leads to the fact that the jet substructure is resolved by the medium. We observe in this case a characteristic delay: the jet radiates as one color current (quark) up to the formation of the "hard" gluon, at which point we observe the onset of radiation of the new color current (gluon). Within our kinematic constraints, our computation supports a picture in which the in-medium jet dynamics are described as a collection of subsequent antennas which are resolved by the medium according to their transverse extent.

Groomed jets in heavy-ion collisions: sensitivity to medium-induced bremsstrahlung

“…In the opposite, incoherent case, the colour correlation of the constituents is broken. See also [48,49] for similar conclusions in the context of the two-gluon emission spectrum. These findings imply that energy loss should be applied only to substructures of the jet that are resolved by the medium [50].…”

“…Besides the trivial case, when the jet consists only of one parton, this is explicitly seen in the case when the jet consists of two partons, see e.g. [48]. This implies that, even if we include splittings, we can treat the jet as if it only consists of a parent parton carrying the total energy and colour charge of the jet.…”

Parton energy loss in QCD matter

“…While the full splitting function was first calculated in [34] at first order in opacity, two limits of the spectrum, relevant for jet fragmentation in medium, were meticulously analysed [35]. For simplification, one of the gluons was treated as "hard", i.e.…”

“…A significant fraction of typical jets in heavy-ion collisions could remain completely unresolved by the medium however they are still affected by energy loss effects due to the total (quark/gluon) colour charge of the jet. Corrections to this picture also can also account for the gradual eradication of angular ordering of the jet constituents and Figure 1: Sketch of the two kinematic limits of the double emission rate calculated in [35]. In both panels the hard gluon is blue while the soft gluon is red and the blob represents all possible placements of the in-medium exchange.…”

“…(b) The soft limit, right panel: in this limit the formation time of the hard gluon is very short compared to the soft gluon one and the angles of emission of both gluons are comparable. Figures taken from [35].…”