The Spontaneous Breaking And Restoration Of QCD Symmetries At Finite Temperature And Density

We study the spontaneous breaking and restoration of QCD symmetries at finite temperature and density,including chiral symmetry restoration and its influence on both the properties and production cross section of pentaquark(?)~+,the spontaneous breaking of isospin symmetry in asymmetric nuclear matter with nonzero isospin density and the related meson mixing in the superfluid phase.Firstly,we adopt effective pseudoscalar and pseudovector N(?)~+K couplings to study the influence of the chiral symmetry restoration on the(?)~+ properties in medium. Considering the perturbative expansion of the(?)~+ self-energy to the lowest order and using the pole equation of the(?)~+ propagator,we solve the(?)~+ mass and width.The effect of chiral symmetry restoration in the medium is reflected on the nucleon effective mass.The(?)~+ mass decreases but the width increases with increasing temperature and/or density due to the restoration of chiral symmetry.The way and degree of the mass and width change depend on the mechanism of chiral symmetry restoration.In addition,the result without chiral symmetry restoration is very different and confirms the conclusion that the chiral symmetry controls the calculation.We also found that for any temperature and density,the mass decreasing and width increasing for positive parity(?)~+ are always stronger than that for negative parity(?)~+.Then,we consider the production process of(?)~+ induced by the partial restoration of chiral symmetry at finite temperature and density.The photoproduction cross section of processγn→(?)~+K~- is calculated with the Feynman diagrams at tree level.It is found that the threshold of the photon energy increases a lot due to the restoration of chiral symmetry in the medium.Above the threshold,the production cross section is strongly enhanced.Finally,we study meson mixing in asymmetric nuclear matter with spontaneous breaking of isospin symmetry in the framework of two flavor NJL model.In RPA approximation, mesons are constructed by the bubble summation,the meson polarization matrix has non-vanishing off-diagonal elements in the pion superfluid phase.The meson modes are linear combination of the knownσandπmesons.It is the mixing that guarantees the Goldstone mode due to the spontaneous breaking of isospin symmetry. It is found that the mixing strength develops fast with increasing isospin chemical potential. The coupling constants among mesons and quarks are also studied for detecting the possible signals of pion superfluidity and the phase transition between the normal phase and the superfluid phase.These coupling constants behave very differently in different phases.