Probing the Frontiers of QCD

Abstract

With the energy scales opened up by RHIC and LHC the age of high-pT physics is upon us. This has created new opportunities and novel mysteries, both of which will be explored in this thesis. The possibility now exists experimentally to exploit these high momentum particles to uniquely probe the unprecedented state of matter produced in heavy ion collisions. At the same time naive theoretical expectations have been dashed by data. The first puzzle we confront is that of the enormous intermediate-pT azimuthal anisotropy, or v2, of jets observed at RHIC. The second puzzle is the surprisingly similar suppression of light mesons and nonphotonic electrons, which precludes perturbative predictions predicated on gluon bremsstrahlung radiation as the dominant energy loss channel. Near qualitative agreement results from including collisional energy loss and integrating over the fluctuating jet pathlengths. Another conjecture for heavy quark energy loss comes via explicit construction using the AdS/CFT correspondence; the momentum loss of a hanging dragging string moving through the deconfined plasma leads to qualitative agreement with heavy quark decay data. We propose a robust test to experimentally differentiate these two competing ideas: the ratio of charm to bottom suppression rapidly approaches 1 for pQCD but is independent of momentum and well below 1 for AdS/CFT. Finally as a warmup problem to calculating the photon bremsstrahlung associated with jet energy loss we quantify improvements to the perturbative estimates of the Ter-Mikayelian effect.