Jet radiation in a longitudinally expanding medium

Abstract

In a series of previous papers, we have presented a new approach, based on perturbative QCD, for the evolution of a jet in a dense quark-gluon plasma. In the original formulation, the plasma was assumed to be homogeneous and static. In this work, we extend our description and its Monte Carlo implementation to a plasma obeying Bjorken longitudinal expansion. Our key observation is that the factorisation between vacuum-like and medium-induced emissions, derived in the static case, still holds for an expanding medium, albeit with modified rates for medium-induced emissions and transverse momentum broadening, and with a modified phase-space for vacuum-like emissions. We highlight a scaling relation valid for the energy spectrum of medium-induced emissions, through which the case of an expanding medium is mapped onto an effective static medium. We find that scaling violations due to vacuum-like emissions and transverse momentum broadening are numerically small. Our new predictions for the nuclear modification factor for jets RAA, the in-medium fragmentation functions, and substructure distributions are very similar to our previous estimates for a static medium, maintaining the overall good qualitative agreement with existing LHC measurements. In the case of RAA, we find that the agreement with the data is significantly improved at large transverse momenta pT ≳ 500 GeV after including the effects of the nuclear parton distribution functions.

Medium-induced cascade in expanding media

“…All these observables have been analyzed in various frameworks before. However, in this paper we present a novel implementation of the effects of expanding medium which allows to quantify the sensitivity to the plasma evolution (for related work, see also [32]), and we present the calculation of the inclusive jet suppression evaluated differentially in the full set of basic kinematic quantities: transverse momentum, rapidity, and the azimuth.…”

Quenching effects in the cumulative jet spectrum

“…In the current work, the medium is treated as a static brick with fixed q0 and length L, see table 1 for details that is generally a good approximation even for expanding media, see ref. [56]. There is a notable change of the curves at high p T due to the inclusion of nPDFs (see also in figure 10 in appendix C for only the nPDF effects).…”

Dynamically groomed jet radius in heavy-ion collisions

“…that works for processes dominated by the most energetic mediuminduced emissions (ω ∼ ωc). As shown in Ref [81],. it is also violated by VLEs via a change of the phase space boundaries that we neglect in this study.…”

“…Previous studies showed that a simple Bjorken-like expansion of the medium is well captured by rescaling the jet quenching parameter q of a homogeneous brick [80,81], q ≡ ||q(t)|| 1/2 , where ||f (t)|| 1/2 stands for the 1/2-norm of the function f (t) with compact support. This scaling is a consequence of the local nature of the medium-induced emissions in the multiple soft scattering regime ω ω c 6 .…”