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

Published for SISSA by Springer Received: September 14, 2015 Revised: December 23, 2015 Accepted: February 19, 2016 Published: March 9, 2016 Jorge Casalderrey-Solana,a Doga Can Gulhan,b José Guilherme Milhano,c,d Daniel Pablosa and Krishna Rajagopalb,e a Departament d’Estructura i Constituents de la Matèria and Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona, Martı́ i Franquès 1, 08028 Barcelona, Spain b Laboratory for Nuclear Science and Department of Physics, Massachusetts Institute of Technology (MIT), Cambridge, MA 02139 U.S.A. c CENTRA, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal d Physics Department, Theory Unit, CERN, CH-1211 Genève 23, Switzerland e Center for Theoretical Physics, MIT, Cambridge, MA 02139, U.S.A. E-mail: , , , , 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 Open Access, c The Authors. Article funded by SCOAP3 . doi:10.1007/JHEP03(2016)053 JHEP03(2016)053 Predictions for boson-jet observables and fragmentation function ratios from a hybrid strong/weak coupling model for jet quenching Keywords: Heavy Ion Phenomenology, Jets ArXiv ePrint: 1508.00815 JHEP03(2016)053 those 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. Contents 1 2 Description of the model and its implementation 2.1 The hybrid model approach 2.2 The effects of flow on the rate of energy loss 2.3 The effects of flow on single-jet and dijet observables 2.4 Species dependence of jet suppression 5 5 8 11 13 √ 3 Boson-jet correlations, including predictions for s = 5.02 ATeV collisions and for Z-jet correlations 15 3.1 Generation and selection of Monte Carlo events 17 √ 3.2 Photon-jet observables: comparison with experimental results at s = √ 2.76 ATeV and predictions for s = 5.02 ATeV 19 √ 3.3 Z-jet observables: predictions for s = 5.02 ATeV 24 4 Fragmentation functions 4.1 Fragmentation functions of the associated jets in photon-jet and Z-jet pairs 4.2 Fragmentation functions of the associated jets in dijet pairs 27 28 34 5 Conclusions and outlook 37 A Energy Loss in a boosted fluid 41 B Update on single-jet and dijet observables at √ s = 2.76 ATeV C Predictions for single-jet and dijet observables at D Model dependence of boson-jet correlations 1 √ s = 5.02 ATeV 42 44 47 Introduction The LHC has ushered in a new era in the exploration of the properties of matter under extreme conditions. By colliding Pb ions at center of mass energies in the multi-TeV regime, the LHC has provided us with droplets of the hottest matter ever produced in the laboratory and a diverse suite of copious high energy probes with which to explore the microscopic properties of the strongly coupled, liquid, quark-gluon plasma (QGP) discovered at RHIC [1–4]. For jet probes in particular, the results of the successful LHC Run 1 have shown that many of the properties of these fundamental QCD objects are substantially modified when the jets are produced in Pb-Pb collisions as compared to –1– JHEP03(2016)053 1 Introduction –2– JHEP03(2016)053 when they are produced in p-p collisions [5–20]. These modifications are results of the final state interaction between the jets and the droplets of hot QCD matter formed in heavy ion collisions. As the principal underlying effect is the energy loss suffered by each of the components of the jet showers on their way out of the hot matter, the various modifications of jet properties observed in heavy ion collisions are referred to, in sum, as jet quenching. The phenomenon of jet quenching was first discovered without reconstructing individual jets via the strong reduction in the number of intermediate-pT hadrons in heavy ion collisions at RHIC [21, 22]. Precisely because varied modifications of jet properties can now be well measured, this suite of probes has the potential to provide us with unique and important information about the properties of QGP and about the interaction between energetic partons and this strongly coupled liquid. The imminent start of the LHC heavy ion Run 2, which will increase both the center of mass energy and luminosity for heavy ion collisions and hence will substantially increase the production rates of jets and all other hard probes including high energy photons and Z-bosons, makes many much more quantitative analyses of the striking phenomena observed in the first LHC run imminent. Turning this opportunity into precise extractions of QGP properties from the experimental data to come requires a diverse suite of theoretical tools. In this paper, we shall present substantial advances in the development of one such tool, introduced in ref. [23]. It is the discovery that the QGP of QCD is a strongly coupled liquid, with intense collective phenomena, no apparent quasiparticle structure, and a very rich phenomenology that makes it of such interest. But, at the same time, it makes the theoretical description of its properties and dynamics much more challenging, since many of the perturbative tools available to describe weakly coupled hard QCD processes become inapplicable in the strongly coupled liquid plasma produced in the range of temperat (...truncated)