Victor Tomas MARI SURKAU

PhD Student

Research group:  Particle Physics

Thesis:

"Improvements of the Polyakov loop potential and phenomenological applications to Quantum Chromodynamics."
Advisor: Urko Reinosa

Research

Quantum Chromodynamics, Field theory at finite temperatures, confinement/deconfinement transition, Quark-Gluon Plasma

Quantum Chromodynamics (QCD) has been very successful in describing nuclear interactions, but some of the most fascinating fundamental principles of the strong interaction theory are still poorly understood. For example, the phenomenon of confinement, which binds quarks inside hadrons remains a major open question and is the main investigative aim of this thesis. At extreme temperatures or densities, as in the primordial Universe or dense stellar bodies such as neutron stars, it is expected that QCD presents phase transitions, with, for example, the formation of a plasma of deconfined quarks and gluons. The understanding of the phase diagram of nuclear matter is a very active research topic and the subject of large-scale experiments, such as the ultra-relativistic heavy ion collisions at RHIC and CERN.

Unfortunately, these phenomena occur at energy scales for which the usual methods of perturbative QCD are inapplicable, complicating their theoretical description. Typical non-perturbative approaches like Lattice QCD or Schwinger-Dyson cannot be applied to all cases or rely on hard-to-control approximations. A more phenomenological approach is based on the use of low-energy QCD models, such as the Nambu-Jona-Lasinio model, which aims at capturing the aspects of QCD associated with the (breaking of) chiral symmetry. They can be improved at finite temperatures by including confinement through the coupling to the Polyakov loop, the order parameter for the confinement/deconfinement transition. The benefit of working within low-energy QCD models is that one can study the impact of various external control parameters in particular regarding the phase structure (temperature, chemical potential, external fields) at a lower computational cost than in full QCD.

The Polyakov loop and its associated thermodynamical potential are difficult to evaluate, mainly due to the intensity of the strong interaction. Recently, a new proposal has been made to access the Polyakov loop potential, based on the Curci-Ferrari (CF) action, a massive extension of the QCD action in the Landau gauge that also aims to deal with Gribov ambiguities. Usage of this setup has revealed, that the CF action provides an accurate grasp of many properties of QCD while requiring relatively simple perturbative calculations. Since the above-mentioned low-energy QCD models are coupled to the Polyakov loop potential this means that one can hope to improve them by evaluating the Polyakov loop potential perturbatively within the CF setup. The impact on predictions by this interplay between the Polaykov loop potential and the low-energy QCD models will be studied during the thesis. New features such as renormalization group improvements or the resummation of Hard-Thermal-Loops for higher temperatures will also be included. Other improvements to be made include the evaluation of higher-order loop corrections.