Chiral Phase Structure And Transport Properties Of Strongly Interacting Matter

Quantum Chromodynamics(QCD)is the fundamental theory of studying the strongly interacting matter,which successfully describes the dynamics of quarks and gluons.As it is well known,QCD exhibits two main features:1.asymptotic freedom,viz,the interaction between quarks and gluons becomes weaker and behaves as free particles at short distances or high energies.2.color confinement,viz,at large distances or small energies,the interactions get stronger,quarks and gluons are bounded in the nucleon,the degrees of freedom of system are colorless hadrons.According to QCD theory,with increasing temperature/density,there is a QCD phase transition from hadronic phase to a new deconfined state of matter consisting of free quarks and gluons.This deconfined QCD matter is called quark-gluon plasma(QGP).The first-principle results from lattice QCD simulation have indicated that with increasing temperature,the QCD phase transition is a smooth crossover at zero baryon chemical potential.At high density,lattice QCD simulation confronts a great challenge due to the fermion sign problem.In this context,the QCD effective models have been proposed to better explore the QCD phase structure.And the results from the effective model calculations show that the chiral phase transition of the strongly interacting matter is of first order at high density,and the first order phase transition ends at critical endpoint(CEP),where the phase transition is of second order.In addition,the creation of QGP and QCD phase transition also can be performed in the relativistic heavy-ion collisions(HICs)experiments.However,the signal of CEP predicted in the theoretical studies cannot be directly observed from experiment at present and can only be explored indirectly through observations of the final state.Therefore,it is a particularly important and challenging topic to study the phase transition structure,and the thermodynamic properties and dynamic transport properties of of strongly interacting matter in different phases.In non-central HICs,a huge magnetic field can be generated in the direction perpendicular to the reaction plane.With the evolution of time,the magnetic field can rapidly decay until it vanishes.The generation of magnetic field in heavy ion collision induces many new physical phenomena and has great impact on the dynamics of physical system.Therefore,it is of great significance to study the transport properties and chiral phase structure of QCD matter under the background of magnetic field.In addition,during the studies of QGP matter,the distributions of particles are usually assumed to be isotropic in the momentum space.However,in the initial expansion stage of the medium generated by HICs,the expansion rate of the system in different directions is also different due to the different pressure gradients in different directions,which ultimately leads to the deviation of the system from the local isotropic equilibrium state.This means that previous studies based on isotropic equilibrium state need to be modified to some extent,and it is necessary to understand the influence of such non-equilibrium effect on different physical quantities and QCD chiral phase transition.In the thesis.we first investigate the thermoelectric effects(Seebeck effect and Nernst effect)of QGP in the background magnetic field.Due to the uncertainty of the magnitude of magnetic field created in the HICs,we natively consider magnetic filed is constant and homogeneous in present work.Within different hierarchies of scales,the ranges of magnetic field can be roughly categorized into three scenarios:the weak magnetic case or classical case,the strong magnetic field case or higher Landau levels(hLLs)case,the strong magnetic field limit case or lowest Landau level(LLL)case.The expression of associated Seebeck coefficient(Sxx)and Nernst signal(N)can be obtained by using the relativistic Boltzmann equation under the relaxation time approximation(RTA).In an isotropic QGP,the influences of magnetic field(B)and quark chemical potential(μq)on these thermoelectric coefficients are investigated.In weak magnetic field,Sxx for a fixed μq is negative in sign,indicating that the dominant carriers for converting heat gradient to electric field are negatively charged quarks,and the absolute value of Sxx decreases with increasing temperature and eventually approaches to zero.Unlike Sxx,N is independent of charge carrier type and remains positive,and the thermal behavior of N displays a peak structure.In the strong magnetic field,due to the Landau quantization,the kinetic energy of(anti-)quarks in the transverse direction gets discreted into Landau levels,only the longitudinal Seebeck coefficient(Szz)exists.Our results show that the value of Szz at a fixed μq in the LLL approximation always remains positive.Within the effect of high Landau levels,Szz exhibits a thermal structure similar to that in the LLL approximation.As the Landau level increases further,Szz decreases and even its sign changes from positive to negative.In the early stage of HICs,the large pressure anisotropy may be existed in the created fireball,i.e.,the pressure along the beam direction is greatly lower than along transverse direction.After the rapid expansion of medium along the beam direction,the system becomes much colder in the beam direction than the transverse direction,which is finally translated into the anisotropy of distribution function in the momentum space.Therefore,we also study how this momentum anisotropy induced by the preferential expansion of medium affects the chiral phase structure,mesonic properties and transport properties of quark matter in the vicinity of the critical temperature.The performed calculations are based on the 2flavor Nambu-Jona-Lasinio model,which allow us to study the hadronic sector as well as the quark sector in one single model.The calculations of various transport coefficients also have been estimated using the kinetic theory in the RTA.Different to most existing papers,in this work the momentum anisotropy is also embedded in the estimation of the relaxation time.Our results indicate that an increase in anisotropy parameter ξ can considerably lead to a catalysis of chiral symmetry breaking.The CEP is shifted to smaller temperatures and larger quark chemical potentials as ξ increases,the impact of momentum anisotropy on temperature of CEP is almost the same as that on the quark chemical potential of CEP.The meson masses and the associated decay widths also exhibit a significant ξ-dependence.The temperature behavior of scaled shear viscosity η/T3 and scaled electrical conductivity σel/T exhibit a similar dip structure,whereas their qualitative behavior with ξ is different.Nevertheless,the minima of both η/T3 and σel/T shift toward higher temperatures with an increase of ξ.And with a rise in ξ,the value of Seebeck coefficient has a significant enhancement for the temperature below the critical temperature.In the hadronic phase,apart from QCD effective models,the thermodynamics of hadronic matter also can be described by van der Waals hadron resonance gas(VDWHRG)model,and the results in these model can be successful consistent with the lattice QCD data.However,so far most of these calculations have taken the vacuum hadron masses as inputs.As we know that spontaneous chiral symmetry breaking is an important feature in QCD vacuum,which is related to the generation of hadron masses.With the increase of temperature or baryon chemical potential,chiral symmetry will be restored,which implies that the masses of constituent quarks should be reduced to be zero.Once the constituent quark masses are relevant to temperature and baryon chemical potential,the masses of subsequent hadrons also should be dependent of temperature and baryon chemical potential naturally.In this work,an extension of the van der Waals hadron resonance gas(VDWHRG)model which includes in-medium thermal modification of hadron masses,the thermal VDWHRG(TVDWHRG)model,is considered to model more realistic hadronic matter.Based on the 2+1 flavor Polyakov Linear Sigma Model(PLSM)and the scaling mass rule of hadrons we obtain the temperature behavior of all hadron masses for different fixed baryon chemical potentials μB.We calculate various thermodynamic observables atμB=0 GeV in TVDWHRG model.An improved agreement with the lattice QCD data from the TVDWHRG model in the crossover region(T～0.16-0.19 GeV)is observed as compared to those from the VDWHRG model.We further discuss the effects of in-medium modification of hadron masses and VDW interactions between(anti)baryons on various transport coefficients at different μB.We find in contrast to the IHRG model,the TVDWHRG model leads to a qualitatively and quantitatively different behavior of transport coefficients with T and μB.