Examining The Crossover From Hadronic To Partonic Phase In QCD And A Monte Carlo Study On The Identification And Property Of Instanton-induced Final State In E~+p Deep-inelastic Scattering

In the 1970s of last century,T.D.Lee et al.predicted that through high energy heavy ion collisions it is possible to form a high energy and/or high density environment in space so that a new state of matter—quark gluon plasma consisting of a large amount of deconfined quarks, antiquarks and gluons will be produced.The quark-deconfinement is accompanied by a change of vacuum.Since then,great theoretical and experimental progress has been made in the field.At present,relativistic heavy ion experiments,in particular,the Relativistic Heavy Ion Collider(RHIC)in Brookheaven National Laboratory in U.S.with colliding energy above several hundred GeV has observed parton(quark and gluon)degree of freedom in a volume about thousands times that of hadrons.Combining all the experimental results,people believe that a strongly coupled quark gluon plasma(sQGP)has been formed.The discovery of sQGP is a milestone in physics.However,the development in this field has just started and far from being completed.Especially,the microscopic structure of this new state of matter is totally unknown,challenging theory and experiment.The QCD phase diagram predicted by lattice gauge theory is the following:in the region with high baryon chemical potential and low temperature the transition between normal hadronic matter and QGP is a first order phase transition.As the increase of temperature and decrease of baryon chemical potential,the first order phase-transition line ends at the critical point and at even higher temperature and lower baryon chemical potential,there is a crossover.It is estimated that at the RHIC and the coming LHC energy regions,a high temperature and low baryon chemical potential environment will be created,and thus falls in the crossover region.We point out the serious problem,for the first time,that most of the current models on the market which claimed crossover violate the color confinement principle of QCD.After careful study,we found that this problem is not an accident,but has deep reason,closely related to the property of QCD vacuum.Aiming at solving this problem,we propose a basic assumption as the following.The aggregation of hadrons could be of two forms:gas-like and molecule-like.In the first case all the hadrons in an aggregation are melted together,forming a big "bag" with quarks and gluons moving inside,which is usually referred to as a quark-gluon-plasma droplet.This is the typical characteristic of first order phase transition.Current transport models adopt this kind of picture and lead to the violation of color confinement principle.The resulting matter is weakly coupled QGP,which is inconsistent with the picture of sQGP discovered at RHIC experiment and obtained in lattice QCD calculation.In the second case,bonds between hadrons(or cells after bond-formation)connect the hadrons to cluster.The cells connected by bonds do not fuse to form a big bag,but like the atoms in a molecule,in spite of being no longer color singlets, still keep their individuality,i.e.,still occupy separate space regions,connected with each other by bonds,forming a color singlet cluster.Molecule-like aggregation of hadrons results in a grape shape quark matter,which is the right way for the crossover from hadron phase to quark matter, and the quark matter obtained as such is a strongly coupled fluid.Grape shape quark gluon plasma(gQGP)means that the space occupied by quark matter is of a grape shape.It is a particular form of sQGP.It is well known that electromagnetic interaction can produce hydrogen molecule,water molecule,...,up to large organic-molecule,while the idea that strong interaction can also produce molecule-type structure is a new concept.We innovatively propose that it is possible to produce grape-shape quark matter(gQGP)through molecule-type aggregation.According to this picture, we construct a percolation model with dynamic basis.The dynamics of bond formation is quark delocalization,which in essence is the tunnelling through the barrier in between hadrons.Defining a percolation rule and using a temperature dependent parameter as the control parameter for bond-formation,we construct a new percolation model,which successfully describe the crossover consistent with color confinement.Pair distribution function is used to study the matter form of gQGP and that during crossover.It is found that liquid behavior is more and more notable when the system is approaching gQGP state.The structure of quark matter possessing this liquid behavior is of a grape shape,with quark groups connected by bonds.As temperature increases further,bonds break up,quarks exist in groups inside perturbative vacuum,which fits the picture of normal liquid,and is similar to the colored-bound-state picture of sQGP proposed in literature.Our main conclusion is:hadrons can aggregate to form molecule-like clusters.When all the hadrous are connected by bonds and form a big cluster,quark matter is formed in grape shape.Molecule-like aggregation,leading to grape-shape quark matter,is the right way for the crossover from hadronic to partonic phase,compatible with the principle of color confinement. To demonstrate this idea,we construct a toy model.The results obtained are independent on the concrete dynamics and percolation used,but are directly resulted from the molecule-like aggregation.The tunnelling through barriers are instantons.In our model,quark delocalization(or tunnelling)play an important part in the crossover process from hadronic to partonic phase. Hence,instantons are expected to play an important role in the crossover.We studied the instanton-induced e~+p deep-inelastic scattering.Final state produced from instanton-induced quark-gluon fusion process are called IFS.Different methods to reconstruct IFS and current jet are tried.A comparison of these methods are performed and a good method is found which can reconstruct well the energies of current jet and IFS as well as the mass of the latter.The isotropy property of the IFS obtained in this way is discussed.The molecule-like aggregation is proposed in this thesis for the first time.It is shown that this is the right way for the QCD crossover.A quantitative calculation for this qualitative picture relies on the finite temperature field theory.How to calculate the tunnelling between cells in finite temperature field theory is a first-line problem.Furthermore,combining the molecule-like and gas-like aggregation to give a unified dynamical model which can describe both the crossover and the first order phase transition and predict the property of critical point is worth further study.