The Response of Hot QCD Matter to Hard Partons

Abstract

The quark gluon plasma (QGP) forms when matter governed by quantum chromodynamics (QCD) undergoes a transition at high temperature or high density from hadronic bound states to deconfined quarks and gluons. The QGP at high temperature is believed to be experimentally accessible in relativistic heavy-ion collisions, such as those done at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Lab and in the near future at the Large Hadron Collider (LHC) at CERN. The results obtained so far reveal the production of energetic (hard) partons in the early stages of a heavy-ion collision which propagate through the plasma. Results also show that the QGP produced at RHIC is a nearly ideal fluid and that hard partons may generate conical, Mach-like, disturbances in the QGP. This thesis uses theoretical methods to address how the QGP responds to a hard parton that propagates through the plasma and contains the first rigorous derivation of how a hard parton deposits energy and momentum in a QGP which lead to the formation of a Mach cone. A comparison of experimental results with the theory introduced in this thesis could shed light on important properties of the QGP such as its equation of state and transport coefficients like viscosity. I investigate this problem by evaluating the source of energy and momentum generated by the hard parton in the QGP. Formalisms are developed and applied for evaluating the source of energy and momentum in perturbation theory with three different methods: classical kinetic theory, finite temperature field theory, and by including the energy lost by the hard parton to radiation. Having obtained the source of energy and momentum generated by the hard parton, I evaluate the medium response using linearized hydrodynamics. My results show Mach cone formation in the medium. I compare the medium response for different viscosities and speeds of sound, from which I find the Mach cone weakens and broadens as viscosity is increased. By studying the time evolution of the medium response once the source of energy and momentum is turned off, which occurs in a heavy-ion collision during the hadronic phase, I find that the conical disturbance is enhanced relative to diffusive contributions over a time period of several fm/c.