Physics around the QCD (tri)critical endpoint and new challenges for femtoscopy

On the basis of exactly solvable models with the tricritical and critical endpoints I discuss the physical mechanism of endpoints formation which is similar to the usual liquids. It is demonstrated that the necessary condition for the transformation of the 1-st order deconfinement phase transition into the 2-nd order phase transition at the (tri)critical endpoint is the vanishing of surface tension coefficient of large/heavy QGP bags. Using the novel model of the confinement phenomenon I argue that the physical reason for the cross-over appearance at low baryonic densities is the negative value of QGP bag surface tension coefficient. This implies the existence of highly non-spherical or, probably, even fractal surfaces of large and heavy bags at and above the cross-over, which, perhaps, can be observed via some correlations. The model with the tricritical endpoint predicts that at the deconfinement transition line the volume (mass) distribution of large (heavy) QGP bags acquires the power law form at the endpoint only, while in the model with the critical endpoint such a power law exists inside the mixed phase. The role of finite width of QGP bags is also discussed.     

A novel mathematical mechanism of the critical endpoint generation

We develop a novel model of the QCD matter critical endpoint by matching the deconfinement phase transition curve with the nil line of the bag surface tension coefficient. As a result, this leads to a new structure of the leading singularities of isobaric partition, and in contrast to all previous studies of such models, the deconfined phase in our approach is defined not by an essential singularity of the isobaric partition function but its simple pole. As an unexpected result, we find out that the first order phase transition in this model is the surface tension induced transition. The sufficient conditions of its existence are analyzed and the possible physical consequences are discussed.     

Model of deconfinement phase transition in baryonic quark-gluon bag system

A model of phase transition between baryonic hadron and quark-gluon matter is elaborated. The spectrum of baryonic colourless bags is found. It is shown that the constraint of bag colourlessness is decisive for the phase transition to be realized.     

Deflagrations and detonations as a mechanism of hadron bubble growth in supercooled quark-gluon plasmas

We discuss the possibility that hadron bubbles formed in quark-gluon plasmas below or slightly above the critical temperature start growing by explosive deflagration or detonation processes. In these the phase transition takes place in a thin layer of discontinuity propagating outward from the point of bubble formation. Combustion theory is written in relativistic form, and possible physical deflagration and detonation bubble solutions conserving energy and momentum, producing entropy, and satisfying correct boundary conditions are classified and numerically discussed using the bag equation of state for quark matter. The implications of these solutions to ultrarelativistic nucleus-nucleus collisions and early cosmology are discussed.     

Constraints for critical temperature of the phase transition into the quark-gluon plasma

Low temperature of the phase transition into the quark-gluon plasma correspond to low values of the bag model constant and to absolutely stable strange quark matter. Some of the observed pulsars are identified quite reliably as neutron stars. If strange matter is stable, the central density of these pulsars is to be smaller that critical density of the phase transition into the nonstrange quark matter. The nonstrange quark matter being formed turns to more stable strange matter on a weak interaction timescale converting neutron stars into strange stars. The requirement of stability of old and newly born neutron stars is used to constrain the bag model constant and the critical temperature of the phase transition into the quark-gluon plasma at zero chemical potential.     

EOS for a QGP with a temperature and baryon chemical potential dependent bag pressure

We develop an equation of state (EOS) for the quark-gluon plasma (QGP) involving the temperature and baryon chemical potential dependent bag pressure arising due to the entropy and baryon number conservation at the phase boundary together with the Gibbs’ construction for an equilibrium phase transition. We show that the bag pressure thus obtained yields a decreasing behaviour with the increasing baryon chemical potential at a fixed temperature. Further consequences of the modified bag pressure are discussed.     

Neutron Star Properties in a QCD-Motivated Model

We investigate the composition and structure of neutron-, hybrid-, and quark stars within an effective QCD-motivated model of strong interaction. The hadronic phase is described within a novel chiral SU(3) model and the deconfined quark-gluon plasma phase is formulated within the bag model. The phase transition between these phases is treated as a first order transition having two conserved charges.     

The changing scenario of the atomic nucleus—from nucleons and mesons to quarks and gluons

The general characteristics of the transition from hadronic matter of nucleons, three quark bags, mesons of quark antiquark pairs to quark gluon plasma is discussed. The phenomenological approach essentially guided by the MIT bag model and general thermodynamic criteria of first-order phase transition is elaborated. The more realistic calculations using the QCD lattice renormalization quark are touched upon. Possible signals of quark-gluon plasma are discussed. The central issue of deciphering plasma signals from the signals of hot hadronic matter is discussed in detail. The signals of the quark-gluon plasma, a subject of considerable interest in contemporary literature are focussed only on (i) dileptons (ii) photon photon pairs and (iii)J/Ψ suppression (with special emphasis on CERN experiments). The lingering shadow of “EMC” effect is also mentioned. Relics of the very early universe microseconds after the big bang in today’s universe (∼ 15 billion years later) are discussed. Finally, the outlook of this very exciting field is presented, a purely personal viewpoint, generalized eventually to poetic signals of the creation of the universe.     

Self-consistent treatment of one-gluon exchange in the soliton bag model

In the context of the soliton bag model, the quarks, soliton field and the gluon propagator are calculated self-consistently in the mean-field, one-gluon exchange approximation. The use of a confined propagator, as compared with a free propagator, is primarily to change the effective quark-gluon coupling constant by a factor of approximately two in the case of static, spherical bags. Self-consistency leads to only a small improvement in the fit to physical data, after readjustment of model parameters. Nevertheless, use of a confined propagator is expected to have an important effect on deformed bags and in dynamical calculations.     

The quark-nucleon phase diagram and quantum chromodynamics

The temperature dependence of a conjectured first-order phase transition between nuclear matter and quark-gluon matter is calculated for temperatures below T = 200 MeV. On the nuclear side a rather successful meson-nucleon mean field theory is applied while quark-gluon matter at large densities and finite temperatures is described perturbatively by quantum chromodynamics. Outside the finite volume of hot and dense quark-gluon matter the physical vacuum is characterized, by the newly determined bag parameter Λ_B = 235 MeV. We observe a dramatic drop in the density of nuclear matter at the phase transition point as the temperature increases, if the scale parameter Λ of QCD is chosen as Λ = 100 MeV. For larger values of Λ the effect is less pronounced. Further work is required to settle this problem.     

Physical mechanism of the (tri)critical point generation

We discuss some ideas resulting from a phenomenological relation recently declared between the tension of string connecting the static quark-antiquark pair and surface tension of corresponding cylindrical bag. This relation analysis leads to the temperature of vanishing surface tension coefficient of the QGP bags at zero baryonic charge density as T _?? = 152.9 ± 4.5 MeV. We develop the view point that this temperature value is not a fortuitous coincidence with the temperature of (partial) chiral symmetry restoration as seen in the lattice QCD simulations. Besides, we argue that T _?? defines the QCD (tri)critical endpoint temperature and claim that a negative value of surface tension coefficient recently discovered is not a sole result but is quite familiar for ordinary liquids at the supercritical temperatures.     

Double phase transition in hadronic matter at zero temperature

The phase transition at zero temperature between hadronic matter and quark-gluon plasma (QGP) is considered within bag model ideology with intermediate phase Q of deconfined constituent quarks (valons) taken into account. Nucleon interactions are treated within Mean Field Approximation method with several model interaction potentials. It is shown that for reasonable choice of model parameters intermediate Q phase appears at zero temperature as well.     

Methods for investigating nuclear matter under the conditions characteristic of its transition to quark-gluon plasma

We briefly consider the properties of deep inelastic nuclear reactions on dense fluctuations of nuclear matter (fluctons). We discuss the properties of the fluctons, which can be many-quark bags or “drops” of quark-gluon plasma: the characteristic parameters of nuclear matter in a flucton— temperature and density close to the critical values for a phase transition. These values can be reached or exceeded if the flucton-flucton collision events are separated out. The separation method is discussed.     

Charmed baryons in quantum chromodynamics

We consider a three-phase model of strongly interacting matter, treating each phase as an ideal gas modified by a simple phenomenological interaction feature. For nuclear matter, we take into account the baryonic repulsion; for the quark-gluon plasma, we include the bag pressure; the constituent quark phase has a non-zero effective quark mass as well as an independent bag pressure. By studying which phase dominates thermodynamically in what region of temperature and baryon number density, we obtain a phase diagram for strongly interacting matter and gain some insight on the relation between deconfinement and chiral symmetry restoration.     

Grand canonical simulations of string tension in elastic surface model

We report a numerical evidence that the string tension σ can be viewed as an order parameter of the phase transition, which separates the smooth phase from the crumpled one, in the fluid surface model of Helfrich and Polyakov-Kleinert. The model is defined on spherical surfaces with two fixed vertices of distance L. The string tension σ is calculated by regarding the surface as a string connecting the two points. We find that the phase transition strengthens as L is increased, and that σ vanishes in the crumpled phase and non-vanishes in the smooth phase.     

Effect of curvature on a statistical model of quark-gluon-plasma fireball in the hadronic medium

The free energy of a quark-gluon plasma fireball in the hadronic medium is calculated in the Ramanathan et al statistical model after incorporating the effect of curvature. The result with the inclusion of curvature is found to produce significant improvements in all the parameters we calculated with respect to the earlier results. The surface tension with this curvature effect is found to be 0.17T _c³, which is two times the earlier value of surface tension which is 0.078T _c³, and this new result is nearly close to the lattice value 0.24T _c³. As far as transition is concerned, a thermodynamic variable like entropy shows weakly first-order phase transition and it shows continuity in the behaviour of specific heat.     

The thermodynamic properties of weakly interacting quark-gluon plasma via the one-gluon exchange interaction

The thermodynamic properties of the quark-gluon plasma (QGP), as well as its phase diagram, are calculated as a function of baryon density (chemical potential) and temperature. The QGP is assumed to be composed of the light quarks only, i.e., the up and down quarks, which interact weakly, and the gluons which are treated as they are free. The interaction between quarks is considered in the framework of the one gluon exchange model which is obtained from the Fermi liquid picture. The bag model is used, with fixed bag pressure (B)for the nonperturbative part, and the quantum chromodynamics (QCD) coupling is assumed to be constant, i.e., with no dependence on the temperature or the baryon density. The effect of weakly interacting quarks on the QGP phase diagram are shown and discussed. It is demonstrated that the one-gluon exchange interaction for the massless quarks has considerable effect on the QGP phase diagram and it causes the system to reach to the confined phase at the smaller baryon densities and temperatures. The pressure of excluded volume hadron gas model is also used to find the transition phase diagram. Our results depend on the values of bag pressure and the QCD coupling constant. The latter does not have a dramatic effect on our calculations. Finally, we compare our results with the thermodynamic properties of strange quark matter and the lattice QCD prediction for the QGP transition critical temperature.