Searching for P- and CP- odd effects in heavy ion collisions

TL;DR

This thesis explores the possibility of parity and CP violation in QCD at high temperatures and densities, using effective models and analyzing potential experimental signatures in heavy ion collisions.

Contribution

It introduces a detailed theoretical framework for parity violation in QCD via an axial chemical potential and predicts observable effects in vector meson decay and dilepton production.

Findings

Parity breaking could occur in dense QCD systems.

Overproduction of lepton pairs near rho-omega resonance is predicted.

Angular distributions of dileptons can reveal parity-violating effects.

Abstract

In this thesis we study the possibility that QCD breaks parity at high temperatures and densities, a scenario that may be tested in heavy ion collisions. Analytical studies with effective models suggest that QCD may break parity in dense systems. Besides, P- and CP- odd bubbles may appear in a finite volume due to local large topological fluctuations in a hot medium. The last effect may be treated in a quasi-equilibrium description by means of a non-trivial axial chemical potential. We consider the 'two flavour' Nambu--Jona-Lasinio model in the presence of a vector and an axial external chemical potentials and study the phase structure of the model at zero temperature. These magnitudes could trigger a phase where parity could be broken in QCD. Analogies and differences with the expected behaviour in QCD are discussed besides the limitations of the model. We also consider the low energy realization of QCD in terms of mesons when an axial chemical potential is present. The eigenstates of strong interactions do not have a definite parity and interactions that would otherwise be forbidden compete with the familiar ones. We focus on scalars and pseudoscalars that are described by a generalized linear sigma model. Finally, we investigate how local parity breaking may affect vector physics. A modified dispersion relation is derived for the lightest vector mesons rho and omega. This effect predicts a natural overproduction of lepton pairs in the vicinity of the rho-omega resonance peak as well as a polarization asymmetry around this peak. The dilepton excess seems relevant to explain the anomalous dielectron yield quoted by PHENIX/STAR. We present a detailed analysis of the angular distribution associated to the lepton pairs created from these mesons searching for polarization dependencies. Two angular variables are found to carry the main information related to the parity-breaking effect.