Abstract
We propose and explore a new hybrid approach to jet quenching in a strongly coupled medium. The basis of this phenomenological approach is to treat physics processes at different energy scales differently. The high-$Q^2$ processes associated with the QCD evolution of the jet from production as a single hard parton through its fragmentation, up to but not including hadronization, are treated perturbatively. The interactions between the partons in the shower and the deconfined matter within which they find themselves lead to energy loss. The momentum scales associated with the medium (of the order of the temperature) and with typical interactions between partons in the shower and the medium are sufficiently soft that strongly coupled physics plays an important role in energy loss. We model these interactions using qualitative insights from holographic calculations of the energy loss of energetic light quarks and gluons in a strongly coupled plasma, obtained via gauge/gravity duality. We embed this hybrid model into a hydrodynamic description of the spacetime evolution of the hot QCD matter produced in heavy ion collisions and confront its predictions with jet data from the LHC. The holographic expression for the energy loss of a light quark or gluon that we incorporate in our hybrid model is parametrized by a stopping distance. We find very good agreement with all the data as long as we choose a stopping distance that is comparable to but somewhat longer than that in ${\cal N}=4$ supersymmetric Yang-Mills theory. For comparison, we also construct alternative models in which energy loss occurs as it would if the plasma were weakly coupled. We close with suggestions of observables that could provide more incisive evidence for, or against, the importance of strongly coupled physics in jet quenching.
Highlights
We propose and explore a new hybrid approach to jet quenching in a strongly coupled medium
In the temperature range explored by current colliders, namely T ∼ 150 − 600 MeV, we know from the comparison of more and more precisely measured experimental observables to more and more sophisticated calculations of relativistic viscous hydrodynamics that the quark-gluon plasma produced in heavy ion collisions is a droplet of strongly coupled liquid that expands and flows collectively, hydrodynamically
Gauge/gravity duality has made it possible to use holographic calculations to analyze the way in which varied energetic probes have their energy degraded, and are otherwise modified, as they propagate through strongly coupled plasma [17,18,19,20,21,22,23,24,25,26]. (For a review, see ref. [5].) These computations provide detailed dynamical information on the energy loss processes in this limit
Summary
As we have stressed in the preceding Introduction, no single theoretical framework is currently available within which controlled calculations of all important aspects of jet quenching in heavy ion collisions can reliably be carried out. We shall assume that no hard radiative processes occur between the DGLAP vertices and that the dynamics of these partons in the plasma is analogous to that of energetic objects propagating through the strongly coupled plasma in a gauge theory with a dual gravitational description. Any of the partons of the jet which propagate in plasma may suffer a hard splitting, governed by the DGLAP equations In addition to these hard splittings, these partons possess associated soft fields that interact strongly with the medium. These have a natural interpretation in a dual gravitational representation: they are strings trailing behind the quark, which is represented by the end point of the string. We will explore the phenomenological consequences of these ideas in a simplified model implementation which we hope captures the main features of some future complete computation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
