The Dissociation Of Hadron In Relativistic Heavy Ion Collision

The states of matter,their features and the transitions between them have been one of the major subjects among the most challenging problems of Modern physics.In Standard Model,quarks,gluons and bosons make up our universe.And the hadrons make up most of the observable matter in our universe.So investigating their intrinsic properties and structure is very helpful for us to know our world.It is believed that hadron is composed of quark and gluon propogating interaction.Duing to confinement,free quark and gluons can not be found in nature.In Big Bang theory,it is believed that there may be free quarks and gluons in the universe during the initial very short time of the Big Bang.The theory of quantum chromodynamics(QCD)predicts that at high temperature and/or density the interacting matter should undergo a phase transition from hadron to quark-gluon plasma(QGP).Now,the relativistic heavy-ion collisions may be the only way to produce QGP on the earth,which means the collsions allow us to study this transition in the laboratory.Howevery,the volume of fireballs they form is estimated to be only a few to a few hundred cubic femtometres,and its lifetime is very short.So it is impossible for us to observe the QGP directly.We can only infer its nature through the product of the collisions.The suppression of the heavy quarkonium is one of the cleanest signatures of QGP formation produced by the relativistic heavy-ion collisions.At high temperatures and/or density,QCD medium screens the interaction between the heavy quark and antiquark pairs().The Debye screening radius characterizes the distance beyond which significant thermal modification arise in the interquark potential.One may believe that the heavy quarkonium dissociates in the medium when the Debye screening radius becomes smaller than the radius of heavy quarkonium.To determine the dissociation temperatures and the erergy density of the heavy quarkonium,we should investigate their behavior in hot QCD medium.Recently,there have been some finite temperature studies of quarkonium spectra in lattice QCD.There are also many investigation on the behavior of quarkonium in the Schr?dinger equation formalism.On the other hand,it is now generally believed that the transition from QGP to hadron phase occurs as a“rapid cross-over”and not as a genuine phase transition.So determing the“phase transition temperature”is an extremely fine process.In the past,people usually used the susceptibility in the equivalent quark model,such as NJL model,to determine the phase transition temperature T_c.What is the physical meaning of T_c?This thesis discusses this problem from the perspective of solving the dissociation of nucleons at finite temperature.The dissociation temperatures of quarkonium and nucleon in a thermal medium are obtained with the help of Gaussian Expansion Method(GEM).At finite temperature and vanishing density,the binding energy and radius,which can be used as a criterion of dissociation,are obtained by solving the corresponding Schr?dinger equation.Based on previous works,the GEM is used to study the dissociation of quarkonium in an impenetrable QGP sphere.And we argue the finite volume effects on quarkonium dissociation temperatures.The results show that the dissociation temperatures of the ground state and excited states decrease as the cavity radius decreases when the radius is less then 5fm,which means a very obvious finite volume effect.We also find that the bigger radius or the less mass of the constituents of the quarkonium lead more obvious finite volume effects.According to the relation between the interactions of three-quark system and that of quark-antiquark system,the temperature-dependent interquark potential of nucleon is obtained.By solving the corresponding Schr?dinger equation,the dissociation temperature of nucleon we calculate is slightly higher than T_c.It shows that the true physical meaning of T_cis the dissociation temperature of the most basic baryons that make up the natural matter,i.e.nucleon.