Daniel Pablos

A Hybrid Strong/Weak Coupling Approach to Jet Quenching

A bstractWe 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-Q2 processes associated with the QCD evolution of the jet from its production as a single hard parton through its fragmentation, up to but not including hadronization, are treated perturbatively following DGLAP evolution, to which we ascribe a spacetime structure. 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 itself (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 inferred 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 experimental results for a number of observables that have been measured in high energy jet data from heavy ion collisions at the LHC, including jet RAA as a function of transverse momentum, the dijet asymmetry, and the jet fragmentation function ratio, all as functions of collision centrality. 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 N=4

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

\mathcal{N}=4

Jet quenching within a hybrid strong/weak coupling approach

supersymmetric Yang-Mills theory. For comparison, we also construct analogous alternative models in which we assume that 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.

Erratum to: A hybrid strong/weak coupling approach to jet quenching

A bstractWithin the context of a hybrid strong/weak coupling model of jet quenching, we study the modification of the angular distribution of the energy within jets in heavy ion collisions, as partons within jet showers lose energy and get kicked as they traverse the strongly coupled plasma produced in the collision. To describe the dynamics transverse to the jet axis, we add the effects of transverse momentum broadening into our hybrid construction, introducing a parameter K≡q^/T3

A Simultaneous Description of Hadron and Jet Suppression in Heavy Ion Collisions

K\equiv \widehat{q}/{T}^3

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

that governs its magnitude. We show that, because of the quenching of the energy of partons within a jet, even when K ≠ 0 the jets that survive with some specified energy in the final state are narrower than jets with that energy in proton-proton collisions. For this reason, many standard observables are rather insensitive to K. We propose a new differential jet shape ratio observable in which the effects of transverse momentum broadening are apparent. We also analyze the response of the medium to the passage of the jet through it, noting that the momentum lost by the jet appears as the momentum of a wake in the medium. After freezeout this wake becomes soft particles with a broad angular distribution but with net momentum in the jet direction, meaning that the wake contributes to what is reconstructed as a jet. This effect must therefore be included in any description of the angular structure of the soft component of a jet. We show that the particles coming from the response of the medium to the momentum and energy deposited in it leads to a correlation between the momentum of soft particles well separated from the jet in angle with the direction of the jet momentum, and find qualitative but not quantitative agreement with experimental data on observables designed to extract such a correlation. More generally, by confronting the results that we obtain upon introducing transverse momentum broadening and the response of the medium to the jet with available jet data, we highlight the importance of these processes for understanding the internal, soft, angular structure of high energy jets.