Properties of mesons and quarks under extreme conditions from Gauge-Gravity duality

Abstract

This thesis is concerned with the properties of mesons and quarks which live in the strongly coupled plasma of certain gauge theories which are similar to QCD. To study these plasmas we exploit gauge gravity duality which has been particularly useful for understanding QCD at temperatures above, but not far above, that at which quarks and gluons are deconfined. For example we will show analytically that mesons propagating through these plasmas have a subluminal limiting velocity at non zero temperatures. This limiting velocity decreases with increasing temperature towards the dissociation temperature. We then argue that this behavior will be universal in gauge theories with a gravity dual. If this result applies in QCD it would have observable effects in future heavy ion collisions at RHIC and the LHC. We also study the width of these mesons by exploiting nonperturbative string effects which can destabilize them, a result we attribute on the gauge theory side to thermal fluctuations. We show that the lifetime of these mesons, described via nonperturbative string effects, decreases rapidly above the momentum at which the meson speed approaches its limiting velocity. This is further evidence for the universality of the limiting velocity and it sharpens the signature expected in heavy ion collisions, especially when the LHC starts colliding ions. Finally the system that was used to study these mesons has an interesting phase structure in the plane of temperature and quark chemical potential. We find a third order phase transition line which ends at a tricritical point. We argue that this phase transition is driven by the same nonperturbative physics which contributed to the meson lifetime.