The x and Q^2 dependence of q̂, quasi-particles and the JET puzzle

We present the first attempt to extract the "x" dependence of the parton distribution function (PDF) of the quark gluon plasma (QGP). In the absence of knowledge regarding the mass of a QGP constituent, we define the new variable x_N, the nucleonic momentum fraction, which represents the ratio of the momentum of the parton to that of a self-contained section of the plasma that has the mass of a nucleon. Calculations are compared to data for single hadron suppression in terms of the nuclear modification factor R_AA and the azimuthal anisotropy parameter v_2, as a function of transverse momentum p_T, centrality and energy of the collision. It is demonstrated that the scale evolution of the QGP-PDF is responsible for the reduction in normalization of the jet transport coefficient q̂, between fits to Relativistic Heavy-Ion Collider (RHIC) and Large Hadron Collider (LHC) data; a puzzle, first discovered by the JET collaboration. Best fits to data are suggestive of the presence of quasi-particles with wide dispersion relations in the QGP.