Energy and scale dependence of (q)over-cap and the 'JET puzzle'

摘要

We present an attempt to probe the underlying structure of the quark-gluon plasma (QGP) at high resolution, based on the extracted jet transport coefficient (q) over cap. We argue that the exchanged momentum k between the hard parton and the medium varies over a range of scales, and for k >= 1 GeV, (q) over cap can be expressed in terms of a parton distribution function (PDF). Because the mass of a QGP constituent is unknown, we define a scaling variable x(N) to represent the ratio of the parton momentum to the momentum of a self-contained section of the plasma which has a mass of 1 GeV. This scaling variable is used to parametrize the QGP-PDF. Calculations based on this reconstructed (q) over cap are compared to data sensitive to the hardcore of jets, i.e., the 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 the normalization of (q) over cap between fits to the Relativistic Heavy-Ion Collider and the Large Hadron Collider data; a puzzle, first discovered by the JET Collaboration.