Matter under extreme conditions: Theoretical studies in two energy regimes.

摘要

We apply the methods of statistical mechanics and field theory at finite temperature to understand phenomena in intermediate and high energy heavy-ion collisions.; In the intermediate energy scenario we concentrate on the pervasive phenomena of multifragmentation. We introduce various extensions of the recently proposed Recursive Statistical Multifragmentation Model (RSM model). In particular, we devise a novel Monte-Carlo technique to improve the treatment of the excluded volume in the model. We consider extensions to account for Coulomb effects and inclusion of isospin degrees of freedom. We then devise a consistent decay formalism to account for the change in isotope ratios due to decay of particle unstable clusters produced in such systems. We demonstrate how, with the above mentioned extensions, populations of various intermediate mass fragments observed in experiments may be explained. We then focus on the possible observation of critical phenomena in experiments, and provide a criticism of the parametrization techniques currently used by practitioners in the field. We demonstrate how such techniques may lead to misleading interpretations and identifications of critical phenomena.; In high energy heavy-ion collisions, we focus on the ongoing search for the Quark-Gluon-Plasma (QGP). The QGP is formed for a very short time and hence its presence is inferred through indirect signatures. In this thesis, we concentrate on the electromagnetic signatures of such a plasma. We demonstrate how the explicit breaking of charge conjugation invariance by the QGP may lead to the appearance of processes hitherto considered absent due to symmetry considerations. These processes allow for gluons to fuse to form lepton pairs and turn out to be comparable, in certain regions of parameter space, to the tree level rate for lepton pair formation from quark anti-quark annihilation. We then investigate the issue of collinear and infrared divergences in two-loop dilepton production rates. This is done by calculating the imaginary part of the retarded two-loop self-energy of a static vector boson in a plasma of quarks and gluons. We recombine the various cuts of the self-energy to generate physical processes. We demonstrate how cuts containing loops may be reinterpreted in terms of interference between O(alpha) tree diagrams and the Born term along with spectators from the medium. We apply our results to the rate of dilepton production in the limit of dilepton invariant mass M T. We find that all infrared and collinear singularities cancel in the final result obtained in this limit.