This document summarizes thoughts on opportunities from high-energy nuclear collisions.
In a suitably chosen back-to-back kinematics, four-jet production in hadronic collisions is known to be dominated by contributions from two independent partonic scattering processes, thus giving experimental access to the structure of generalized two
-parton distributions 2GPDs. Here, we show that a combined measurement of the double hard four-jet cross section in proton-proton and proton-nucleus collisions will allow one to disentangle different sources of two-parton correlations in the proton, that cannot be disentangled with 4-jet measurements in proton-proton collisions alone. To this end, we analyze in detail the structure of 2GPDs in the nucleus (A), we calculate in the independent nucleon approximation all contributions to the double hard four-jet cross section in pA, and we determine corrections arising from the nuclear dependence of single parton distribution functions. We then outline an experimental strategy for determining the longitudinal two-parton correlations in the proton.
Global perturbative QCD analyses, based on large data sets from electron-proton and hadron collider experiments, provide tight constraints on the parton distribution function (PDF) in the proton. The extension of these analyses to nuclear parton dist
ributions (nPDF) has attracted much interest in recent years. nPDFs are needed as benchmarks for the characterization of hot QCD matter in nucleus-nucleus collisions, and attract further interest since they may show novel signatures of non-linear density-dependent QCD evolution. However, it is not known from first principles whether the factorization of long-range phenomena into process-independent parton distribution, which underlies global PDF extractions for the proton, extends to nuclear effects. As a consequence, assessing the reliability of nPDFs for benchmark calculations goes beyond testing the numerical accuracy of their extraction and requires phenomenological tests of the factorization assumption. Here we argue that a proton-nucleus collision program at the LHC would provide a set of measurements allowing for unprecedented tests of the factorization assumption underlying global nPDF fits.
We determine the energy it takes to move a test quark along a circle of radius L with angular frequency w through the strongly coupled plasma of N=4 supersymmetric Yang-Mills (SYM) theory. We find that for most values of L and w the energy deposited
by stirring the plasma in this way is governed either by the drag force acting on a test quark moving through the plasma in a straight line with speed v=Lw or by the energy radiated by a quark in circular motion in the absence of any plasma, whichever is larger. There is a continuous crossover from the drag-dominated regime to the radiation-dominated regime. In the crossover regime we find evidence for significant destructive interference between energy loss due to drag and that due to radiation as if in vacuum. The rotating quark thus serves as a model system in which the relative strength of, and interplay between, two different mechanisms of parton energy loss is accessible via a controlled classical gravity calculation. We close by speculating on the implications of our results for a quark that is moving through the plasma in a straight line while decelerating, although in this case the classical calculation breaks down at the same value of the deceleration at which the radiation-dominated regime sets in.
