Deciphering the 𝓏g distribution in ultrarelativistic heavy ion collisions

Abstract

Within perturbative QCD, we develop a new picture for the parton shower generated by a jet propagating through a dense quark-gluon plasma. This picture combines in a simple, factorised, way multiple medium-induced parton branchings and standard vacuum-like emissions, with the phasespace for the latter constrained by the presence of the medium. We implement this picture as a Monte Carlo generator that we use to study two phenomenologically important observables: the jet nuclear modification factor R AA and the z g distribution reflecting the jet substructure. In both cases, the outcome of our Monte Carlo simulations is in good agreement with the LHC measurements. We provide basic analytic calculations that help explaining the main features observed in the data. We find that the energy loss by the jet is increasing with the jet transverse momentum, due to a rise in the number of partonic sources via vacuum-like emissions. This is a key element in our description of both R AA and the z g distribution. For the latter, we identify two main nuclear effects: incoherent jet energy loss and hard medium-induced emissions. As the jet transverse momentum increases, we predict a qualitative change in the ratio between the z g distributions in PbPb and pp collisions: from increasing at small z g , this ratio becomes essentially flat, or even slightly decreasing.We want to dedicate this paper to the 80 th birthday of Al Mueller, who pioneered the studies of jet quenching in perturbative QCD and was also our collaborator on a previous paper which introduced the general physical picture that we further develop in this work. arXiv:1907.04866v1 [hep-ph]

The soft drop groomed jet radius at NLL

“…At NNLO, the clustering happens when the distance between the emissions inside and outside R g is smaller than the distance between the groomed jet axis and the radiation inside R g . Therefore, to we need to insert the constraint 36) and the phase space which generates the NGLs will thus be reduced. We thus have the following modified expression compared to eq.…”

“…Both on the experimental [5][6][7][8] and the theoretical side [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24], significant progress has been made recently in improving our understanding of soft drop groomed jet observables. In the heavy-ion community, soft drop groomed jet substructure observables have also received increasing attention from both experiment [25][26][27][28] and theory [29][30][31][32][33][34][35][36][37][38][39]. Jet grooming techniques can be used to isolate different aspects of jet quenching and may help to discriminate between different model assumptions [40].…”

Nuclear modification factors for jet fragmentation

“…The theoretical framework that we use to address this (and related) observable(s) is the pQCD approach recently developed in refs. [9,10], in which vacuum-like emissions (VLEs) and medium-induced emissions (MIEs) are factorised from each other via controlled approximations at weak coupling. This simple description is manifestly probabilistic, hence allowing for an efficient Monte-Carlo implementation.…”

“…This simple description is manifestly probabilistic, hence allowing for an efficient Monte-Carlo implementation. In [10], we already successfully applied it to two observables measured at the LHC: the jet R AA (the nuclear modification factor for inclusive jet production) and the z g -distribution (reflecting the jet substructure in terms of relatively hard splittings).…”

“…The paper is organised as follows: in section 2 we provide a brief reminder of our physical picture, introduced in refs. [9,10]. Section 3 gives our Monte Carlo results for the fragmentation function and discusses the physical mechanisms at play.…”