Thermal Field Theory in Equilibrium

Thermal field theory has applications in many areas of physics and one of those is the early Universe. There is an excess of matter over antimatter in the present Universe. Unless this was an initial condition in the Big Bang Scenario, this baryon asymmetry must have been created during the evolution of the Universe. According to Sakarov’s three criteria for baryogenesis, the Universe must have been out of equilibrium, there must be baryon-number violating processes, and there must be CP violation. Although baryonnumber violating processes are exponentially suppressed at T = 0, they are significant at high temperatures. Moreover, CP violation is known from the kaon system, and the Universe was out of equilibrium during a cosmological phase transition if it was first order. Hence, all the necessary ingredients of baryosynthesis may have been present in the early Universe and has been subject of intense investigation in recent years. Another important application of thermal field theory is heavy-ion collisions. QCD is expected to undergo a phase transition at high temperature and/or high density, where chiral symmetry is restored, and quarks and gluons are deconfined. Hadrons are no longer the relevant degrees of freedom, but matter is described in terms of a plasma of interacting quarks and gluons. A quark-gluon plasma is expected to be created in heavy-ion collisions at RHIC and LHC. Signatures of the formation of a quark-gluon plasma includes photon and dilepton production, and J/ψ suppression, In order to understand such complicated nonequilibrium phenomena, we need to have equilibrium field theory under control. In this talk, I would like to give an overview of recent developments in equilibrium thermal field theory.