Chiral Phase Transition In An Electric Field Background

Investigation on Quantum Chromodynamics(QCD)matter is essential for a deep understanding of the properties of the strong interaction,and the properties of hadronic matter under finite temperature and chemical potential are important topics in particle physics.In recent years,because of the possible strong electromagnetic fields in heavy ion collision experiments,the influence of electromagnetic field has also become a hot topic.Extensive reseaches have focused on the properties of hadronic matter under magnetic field background,while the effects of electric fields,especially under finite temperature and chemical potential,are less studied,so it needs to be deeply explored.Given the color confinement of QCD,perturbation theory can not be applied in low-energy regions.Instead effective models or lattice QCD are usually used in low-energy regions.Based on the first principle,lattice QCD is an important method in dealing with nonperturbative problems.But when the chemical potential is included,it is difficult to use because of the sign problem.Effective models(for example,NJL models)can be viewed in some extent as QCD low-energy effective theories,they have been extensively used in the case of zero temperature and finite temperature(and/or chemical potential)due to the relatively easy calculations.NJL models include quark degree of freedom only,and possess the important property of QCD—spontaneous breaking of the chiral symmetry,so it is very convenient to use them to investigate the chiral phase transition;in addition,combining with Polyakov loop,the PNJL model can also be used to study the deconfinement phase transition.When calculation is performed in effective models,the perturbation method is usually necessary.For example,for NJL models,the lowest approximation is the mean field approximation,and the perturbation calculations that exceed the mean field approximation are usually diffcult to handle.The optimization perturbation theory(OPT)method used in this paper is an important calculation technique,which is essentially a variational method,managing to account for part of the higher order perturbation effects through lower order perturbation calculations.In this thesis,the chiral phase transition under uniform electric field is studied,and the influence of temperature and chemical potential on chiral phase transition is considered in the same time.We use a two-flavor NJL model and take into an approximation beyond the mean field approximation by using the OPT method.The results show that: When the electric field is enhanced,(1)the effective mass and quark condensation decrease,and the system changescontinuously from the chiral symmetry breaking phase to the chiral symmetry restoring phase,(2)the splitting of the u,d quark condensation becomes larger,(3)the sensitivity of the effective mass and quark condensation to chemical potential and temperature decreases.OPT method does not change the qualitative behavior of the phase transition.