Predictions for boson-jet observables and fragmentation function ratios from a hybrid strong/weak coupling model for jet quenching

Abstract

Abstract:We have previously introduced a hybrid strong/weak coupling model for jet quenching in heavy ion collisions in which we describe the production and fragmentation of jets at weak coupling, using Pythia, and describe the rate at which each parton in the jet shower loses energy as it propagates through the strongly coupled plasma, dE/dx, using an expression computed holographically at strong coupling. The model has a single free parameter that we fit to a single experimental measurement. We then confront our model with experimental data on many other jet observables, focusing in this paper on boson-jet observables, finding that it provides a good description of present jet data. Next, we provide the predictions of our hybrid model for many measurements to come, including those for inclusive jet, dijet, photon-jet and Z-jet observables in heavy ion collisions with energy √ s = 5.02 ATeV coming soon at the LHC. As the statistical uncertainties on near-future measurements of photon-jet observables are expected to be much smaller than JHEP03(2016)053those in present data, with about an order of magnitude more photon-jet events expected, predictions for these observables are particularly important. We find that most of our preand post-dictions do not depend sensitively on the form we choose for the rate of energy loss dE/dx of the partons in the shower. This gives our predictions considerable robustness. To better discriminate between possible forms for the rate of energy loss, though, we must turn to intrajet observables. Here, we focus on ratios of fragmentation functions. We close with a suggestion for a particular ratio, between the fragmentation functions of inclusive and associated jets with the same kinematics in the same collisions, which is particularly sensitive to the x-and E-dependence of dE/dx, and hence may be used to learn which mechanism of parton energy loss best describes the quenching of jets.

Angular structure of jet quenching within a hybrid strong/weak coupling model

“…[148,149] we introduced a phenomenological hybrid strong/weak coupling approach to analyzing jet quenching. In this approach, we treat different physics processes of relevance for the interaction of developing jet showers with the quark gluon plasma differently.…”

“…[143,145]. The Monte Carlo implementation of this hybrid model has been successful in describing LHC measurements of a large suite of single jet, dijet and photon+jet observables [148,149] and has been used to make predictions for more such observables and for Z+jet observables [149]. To date, the model has contained only a single free parameter, with all its successes and predictions having been obtained after fitting this parameter to a single measured data point.…”

“…Upon making this assumption, the modification of jet showers is only due to the interaction of each of the partons in the jet with the strongly coupled medium. After associating each parton in the jet with a life-time determined via a formation time argument [148,149,163], we compute the energy lost by each parton as it propagates in the strongly coupled plasma. Since the interactions of each…”

“…We shall further assume that the most salient differences between the strongly coupled limit of N = 4 SYM theory and QCD can be incorporated via varying the value of κ sc , which becomes the only fitting parameter of the hybrid model formulated in refs. [148,149]. We have implemented this hybrid model into a Monte Carlo simulation in which hard jets, showering as described by Pythia [164], are embedded within a droplet of hot matter produced in a heavy ion collision, expanding and cooling as described by relativistic viscous hydrodynamics.…”

“…Flow effects are taken into account by evaluating the rate of energy loss in the local fluid rest frame, as explained in ref. [149]. In refs.…”