127 research outputs found
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 between the longitudinal size
of the target and the lifetime of the gluon fluctuations. When , meaning that the target can be effectively treated as a shockwave, we
recover the JIMWLK Hamiltonian, as expected. When , 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
) 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 , with
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 . 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 follows from
a collisionless kinetic equation, of the Vlasov type. The effective theory at
the scale 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
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