Di-jet asymmetry and wave turbulence

We describe a new physical picture for the fragmentation of an energetic jet propagating through a dense QCD medium, which emerges from perturbative QCD and has the potential to explain the di-jet asymmetry observed in Pb-Pb collisions at the LHC. The central ingredient in this picture is the phenomenon of wave turbulence, which provides a very efficient mechanism for the transport of energy towards the medium, via many soft particles which propagate at large angles with respect to the jet axis.Comment: 6 pages, 3 figures. Invited plenary talk at the 6th International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions (Hard Probes 2013), Stellenbosch, South Africa, Nov. 4-8, 201

The non-linear evolution of jet quenching

We construct a generalization of the JIMWLK Hamiltonian, going beyond the eikonal approximation, which governs the high-energy evolution of the scattering between a dilute projectile and a dense target with an arbitrary longitudinal extent (a nucleus, or a slice of quark-gluon plasma). Different physical regimes refer to the ratio $L/\tau$ between the longitudinal size $L$ of the target and the lifetime $\tau$ of the gluon fluctuations. When $L/\tau \ll 1$, meaning that the target can be effectively treated as a shockwave, we recover the JIMWLK Hamiltonian, as expected. When $L/\tau \gg 1$, meaning that the fluctuations live inside the target, the new Hamiltonian governs phenomena like the transverse momentum broadening and the radiative energy loss, which accompany the propagation of an energetic parton through a dense QCD medium. Using this Hamiltonian, we derive a non-linear equation for the dipole amplitude (a generalization of the BK equation), which describes the high-energy evolution of jet quenching. As compared to the original BK-JIMWLK evolution, the new evolution is remarkably different: the plasma saturation momentum evolves much faster with increasing energy (or decreasing Bjorken's $x$) than the corresponding scale for a shockwave (nucleus). This widely opens the transverse phase-space for the evolution and implies the existence of large radiative corrections, enhanced by the double logarithm $\ln^2(LT)$, with $T$ the temperature of the medium. This confirms and explains from a physical perspective a recent result by Liou, Mueller, and Wu (arXiv:1304.7677). The dominant corrections are smooth enough to be absorbed into a renormalization of the jet quenching parameter $\hat q$. This renormalization is controlled by a linear equation supplemented with a saturation boundary, which emerges via controlled approximations from the generalized BK equation alluded to above.Comment: 54 pages plus 4 appendices, 6 figure

Non-perturbative aspects of hot QCD

I discuss some non-perturbative aspects of hot gauge theories as related to the unscreened static magnetic interactions. I first review some of the infrared divergences which cause the breakdown of the perturbation theory. Then I show that kinetic theory, as derived from quantum field theory, is a powerful tool to construct effective theories for the soft modes, which then can be treated non-perturbatively. The effective theory at the scale $gT$ follows from a collisionless kinetic equation, of the Vlasov type. The effective theory at the scale $g^2T$ is generated by a Boltzmann equation which includes the collision term for colour relaxation.Comment: 14 pages, 2 figures. Invited talk at SEWM'98, Copenhagen, Dec 9

Partons and jets at strong coupling from AdS/CFT

Calculations using the AdS/CFT correspondence can be used to unveil the short-distance structure of a strongly coupled plasma, as it would be seen by a `hard probe'. The results of these calculations admit a natural physical interpretation in terms of parton evolution in the plasma: via successive branchings, essentially all partons cascade down to very small values of the longitudinal momentum fraction x and to transverse momenta smaller than the saturation momentum Q_s\sim T/x. This scale Q_s controls the energy loss and the transverse momentum broadening of an energetic jet propagating through the plasma. This picture has some striking consequences, like the absence of jets in electron-proton annihilation at strong coupling, of the absence of particle production at forward and backward rapidities in hadron-hadron collisions, which look very different from the corresponding predictions of perturbative QCD and also from the known experimental situation.Comment: 10 pages, 4 figures. Based on the talk presented at the YITP International Symposium Fundamental Problems in Hot and/or Dense QCD (YITP, Kyoto, March 3-6, 2008