Non-perturbative analysis on thermal radiations of photons and dileptons from quark-gluon plasma

Abstract

It is expected that a colored matter fulfills the early universe although the colored state cannot be observed on the present earth. Owing to a development of an accelerator and huge efforts, physicists succeeded to produce the colored matter on the earth by ultra-relativistic heavy ion collisions. By investigations on several observables, it is found that the produced colored matter seems to be strongly coupled. The colored matter is called quark-gluon plasma. However, among several observables, photon and dilepton production rates from the colored medium are still evaluated by a perturbative method. It is desirable to estimate these rates beyond a perturbation theory and this thesis is devoted to a non-perturbative analysis of photons and dileptons. Thermal radiations of photons and dileptons from the deconfined medium make important roles in ultra-relativistic heavy ion collisions. Photons and dileptons emitted from the quark-gluon plasma are direct signals of the deconfined matter and include the information on the medium. In this study, I evaluate production rates of thermal photons and dileptons from quark-gluon plasma with a non-perturbative method. Electromagnetic radiations are proportional to the imagi- nary part of a photon self energy. I analyze the photon self energy with a quark propagator obtained by a lattice QCD numerical simulation and a gauge invariant vertex. Numerical simulation on the lat- tice QCD is a first-principle calculation, and thus the photon self energy constructed with the lattice quark propagator is expected to include non-perturbative effects. Moreover, the obtained self energy respects the gauge invariance which makes a significantly important role in the quantum field theory. In this study, the lattice quark propagator is composed of two quasi-particle states. Quasi-particles are particle-like thermal excited states of quarks. On the lattice, one of the quasi-particles called plasmino has a peculiar nature that its dispersion has the minimum at finite momentum and it exists even in the space-like region. Owing to these characteristics, production rates evaluated with the lattice quark propagator also obtain peculiar structures. I obtained suggestive results of production rates. A photon spectrum shows non-smooth variations and the almost same slope as perturbative one. The former structure is understood by kinematics of quasi-particles and I verify that the slope is determined by a thermal factor. A dilepton spectrum shows a characteristic structure called van Hove singularity. This structure is also understood by kinematics of quasi-particles. Surprisingly, obtained spectra of photons and dileptons show compa- rable or slightly smaller rates to perturbative results although production processes are completely different. Finally, I discuss effects of obtained