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K. A. Bugaev

Publications and source records attributed to K. A. Bugaev.

At least 55 records · Page 3Linked to original sources
arXiv2014

Strangeness enhancement at the hadronic chemical freeze-out

The chemical freeze-out of hadrons created in the high energy nuclear collisions is studied within the realistic version of the hadron resonance gas model. The chemical non-equilibrium of strange particles is accounted via the usual $γ_{s}$ factor which gives us an opportunity to perform a high quality fit with $χ^2/dof \simeq 63.5/55 \simeq 1.15$ of the hadronic multiplicity ratios measured from the low AGS to the highest RHIC energies. In contrast to previous findings, at low energies we observe the strangeness enhancement instead of a suppression. In addition, the performed $γ_{s}$ fit allows us to achieve the highest quality of the Strangeness Horn description with $χ^2/dof=3.3/14$. For the first time the top point of the Strangeness Horn is perfectly reproduced, which makes our theoretical horn as sharp as an experimental one. However, the $γ_{s}$ fit approach does not sizably improve the description of the multi-strange baryons and antibaryons. Therefore, an apparent deviation of multi-strange baryons and antibaryons from chemical equilibrium requires further explanation.

hep-ph↗
arXiv2014

Separate chemical freeze-out of strange particles with conservation laws

The Hadron Resonance Gas Model with two chemical freeze-outs, connected by conservation laws is considered. We are arguing that the chemical freeze-out of strange hadrons should occur earlier than the chemical freeze-out of non-strange hadrons. The hadron multiplicities measured in the heavy ion collisions for the center of mass energy range 2.7 - 200 GeV are described well by such a model. Based on a success of such an approach, a radical way to improve the Hadron Resonance Gas Model performance is suggested. Thus, we suggest to identify the hadronic reactions that freeze-out noticeably earlier or later that most of the others reactions (for different collision energies they may be different) and to consider a separate freeze-out for them.

hep-ph↗
arXiv2014

Bimodality as a signal of the nuclear liquid-gas phase transition

Here we present an explicit counterexample to a bimodality concept as the unique signal of first order phase transition. Using an exact solution of the simplified version of the statistical multifragmentation model we demonstrate that the bimodal distributions can naturally appear in infinite system without a phase transition in the regions of the negative values of the surface tension coefficient. Also we propose a new parameterization for the compressible nuclear liquid which is consistent with the L. van Hove axioms of statistical mechanics. As a result the proposed model does not lead to the irregular behaviour of the isotherms in the mixed phase region which is typical for mean-field models. Peculiarly, the suggested approach to account for the nuclear liquid compressibility automatically leads to an appearance of an additional state that in many respects resembles the physical antinuclear matter.

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arXiv2013

Chemical freeze-outs of strange and non-strange particles and residual chemical non-equilibrium

We propose an elaborate version of the hadron resonance gas model with the combined treatment of separate chemical freeze-outs for strange and non-strange hadrons and with an additional γ_{s} factor which accounts for the remaining strange particle non-equilibration. Two sets of chemical freeze-outs parameters are connected by the conservation laws of entropy, baryonic charge, isospin projection and strangeness. The developed approach enables us to perform a high-quality fit of the hadron multiplicity ratios for AGS, SPS and RHIC energies with total χ^2/dof \simeq 1.05. A special attention is paid to a complete description of the Strangeness Horn. A well-known \bar p, \bar Λand \bar Ξselective suppression problem is also discussed.

hep-ph↗
arXiv2013

Bimodality Phenomenon in Finite and Infinite Systems Within an Exactly Solvable Statistical Model

We present a few explicit counterexamples to the widely spread belief about an exclusive role of the bimodal nuclear fragment size distributions as the first order phase transition signal. In thermodynamic limit the bimodality may appear at the supercritical temperatures due to the negative values of the surface tension coefficient. Such a result is found within a novel exactly solvable formulation of the simplified statistical multifragmentation model based on the virial expansion for a system of the nuclear fragments of all sizes. The developed statistical model corresponds to the compressible nuclear liquid with the tricritical endpoint located at one third of the normal nuclear density. Its exact solution for finite volumes demonstrates the bimodal fragment size distribution right inside the finite volume analog of a gaseous phase. These counterexamples clearly demonstrate the pitfalls of Hill approach to phase transitions in finite systems.

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arXiv2013

Chemical Freeze-out of Strange Particles and Possible Root of Strangeness Suppression

Two approaches to treat the chemical freeze-out of strange particles in hadron resonance gas model are analyzed. The first one employs their non-equillibration via the usual γ_s factor and such a model describes the hadron multiplicities measured in nucleus-nucleus collisions at AGS, SPS and RHIC energies with χ^2/dof = 1.15. Surprisingly, at low energies we find not the strangeness suppression, but its enhancement. Also we suggest an alternative approach to treat the strange particle freeze-out separately, but with the full chemical equilibration. This approach is based on the conservation laws which allow us to connect the freeze-outs of strange and non-strange hadrons. Within the suggested approach the same set of hadron multiplicities can be described better than within the conventional approach with χ^2/dof = 1.06. Remarkably, the fully equilibrated approach describes the strange hyperons and antihyperons much better than the conventional one.

hep-ph↗
arXiv2013

Is bimodality a sufficient condition for a first order phase transition existence?

Here we present two explicit counterexamples to the widely spread beliefs about an exclusive role of bimodality as the first order phase transition signal. On the basis of an exactly solvable statistical model generalizing the statistical multifragmentation model we demonstrate that the bimodal distributions can naturally appear both in infinite and in finite systems without a phase transition. In the first counterexample a bimodal distribution appears in an infinite system at the supercritical temperatures due to the negative values of the surface tension coefficient. In the second counterexample we explicitly demonstrate that a bimodal fragment distribution appears in a finite volume analog of a gaseous phase. In contrast to the statistical multifragmentation model, the developed statistical model corresponds to the compressible nuclear liquid with the tricritical endpoint located at one third of the normal nuclear density. The suggested parameterization of the liquid phase equation of state is consistent with the L. van Hove axioms of statistical mechanics and it does not lead to an appearance of the non-monotonic isotherms in the macroscopic mixed phase region which are typical for the classical models of the Van der Waals type. Peculiarly, such a way to account for the nuclear liquid compressibility automatically leads to an appearance of an additional state that in many respects resembles the physical antinuclear matter.

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arXiv2013

The statistical multifragmentation model for liquid-gas phase transition with a compressible nuclear liquid

We propose a new formulation of the statistical multifragmentation model based on the analysis of the virial expansion for a system of the nuclear fragments of all sizes. The developed model not only enables us to account for short-range repulsion, but also to calculate the surface free energy which is induced by the interaction between the fragments. We propose a new parameterization for the liquid phase pressure which allows us to introduce a compressible nuclear liquid into the statistical multifragmentation model. The resulting model is exactly solvable and has no irregular behavior of the isotherms in the mixed phase region that is typical for mean-field models. The general conditions for the 1-st and 2-nd (or higher) order phase transitions are formulated. It is shown that all endpoints of the present model phase diagram are the tricritical points, if the Fisher exponent $τ$ is in the range $\{3}{2} \le τ\le 2$. The treatment of nuclear liquid compressibility allows us to reduce the tricritical endpoint density of the statistical multifragmentation model to one third of the normal nuclear density. A specific attention is paid to of the fragment size distributions in the region of a negative surface tension at supercritical temperatures.

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arXiv2013

Can bimodality exist without phase transition?

Here we present an explicit counterexample to the widely spread beliefs about an exclusive role of bimodality as the first order phase transition signal. On the basis of an exactly solvable statistical model generalizing the statistical multifragmentation model of nuclei we demonstrate that the bimodal nuclear fragment size distributions can naturally appear in infinite system without a phase transition. It appears at the supercritical temperatures due to the negative values of the surface tension coefficient. The developed statistical model corresponds to the compressible nuclear liquid with the tricritical endpoint located at one third of the normal nuclear density.

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arXiv2012

Adiabatic Chemical Freeze-out and Wide Resonance Modification in a Thermal Medium

Here we develop a model equation of state which successfully parameterizes the thermodynamic functions of hadron resonance gas model at chemical freeze-out and which allows us to naturally explain the adiabatic chemical freeze-out criterion. The present model enables us to clearly demonstrate that at chemical freeze-out the resulting hadronic mass spectrum used in the hadron resonance gas model is not an exponential-like, but a power-like. We argue that such a property of hadronic mass spectrum at chemical freeze-out can be explained by the two new effects found here for wide resonances existing in a thermal environment: the near threshold thermal resonance enhancement and the near threshold resonance sharpening. The effect of resonance sharpening is studied for a sigma meson and our analysis shows that for the temperatures well below 92 MeV the effective width of sigma meson is about 50 to 70 MeV. Thus, the effect of resonance sharpening justifies the usage of the sigma-like field-theoretical models for the strongly interacting matter equation of state at such temperatures. Also we argue that the most optimistic hope to find the quark gluon bags experimentally may be related to their sharpening and enhancement in a thermal medium. In this case the wide quark gluon bags may appear directly or in decays as narrow resonances that are absent in the tables of elementary particles and that have the width about 50-150 MeV and the mass about or above 2.5 GeV.

hep-ph↗
arXiv2012

Critical exponents of the quark-gluon bags model with the critical endpoint

The critical indices α', β, γ' and δof the Quark Gluon Bags with Surface Tension Model that has the critical endpoint are calculated and compared with the exponents of other models. These indices are expressed in terms of the most general parameters of the model. Despite the usual expectations the found critical indices do not depend on the Fisher exponent τand on the parameter \varkappa which relates the mean bag surface to its volume. The scaling relations for the obtained critical exponents are verified and it is demonstrated that for the standard definition of the index α' the Fisher and the Griffiths scaling inequalities are not fulfilled in general case, whereas the Liberman scaling inequality is always obeyed. This is not surprising for the phase diagram with the asymmetric properties of pure phases, but the present model also provides us with the first and explicit example that the specially defined index α'_s does not recover the scaling relations as well. Therefore, here we suggest the physically motivated definition of the index α' = α'_c and demonstrate that such a definition recovers the Fisher scaling inequality, while it is shown that the Griffiths inequality should be generalized for the phase diagram with the asymmetric properties. The critical exponents of several systems that belong to different universality classes are successfully described by the parameters of the present model and hence its equation of state can be used for a variety of practical applications.

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arXiv2012

Simple Solution to the Strangeness Horn Description Puzzle

We propose to use the thermal model with the multi-component hard-core radii to describe the hadron yield ratios from the low AGS to the highest RHIC energies. It is demonstrated that the variation of the hard-core radii of pions and kaons enable us to drastically improve the fit quality of the measured mid-rapidity data and for the first time to completely describe the Strangeness Horn behavior as the function of the energy of collision without spoiling the fit quality of other ratios. The best global fit is found for the vanishing hard-core radius of pions and for the hard-core radius of kaons being equal to 0.35 fm, whereas the hard-core radius of all other mesons is fixed to 0.3 fm and that one of baryons is fixed to 0.5 fm. It is argued that the multi-component hadron resonance gas model opens us a principal possibility to determine the second virial coefficients of hadron-hadron interaction.

hep-ph↗
arXiv2011

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_σ= 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_σdefines the QCD (tri)critical endpoint temperature and claim that a negative value of surface tension coefficient recently discovered is not a sole result, but should also exist in ordinary liquids at the supercritical temperatures.

hep-ph↗
arXiv2011

The Role of Bag Surface Tension in Color Confinement

We discuss here the novel view at the color confinement which, on the one hand, allows us to find out the surface tension coefficient of quark gluon bags and, under a plausible assumption, to determine the endpoint temperature of the QCD phase diagram, on the other hand. The present model considers the confining color tube as the cylindrical quark gluon bag with non-zero surface tension. A close inspection of the free energies of elongated cylindrical bag and the confining color tube that connects the static quark-antiquark pair allows us to find out the string tension in terms of the surface tension, thermal pressure and the bag radius. Using the derived relation it is possible to estimate the bag surface tension at zero temperature directly from the lattice QCD data and to estimate the (tri)critical endpoint temperature. In the present analysis the topological free energy of the cylindrical bag is accounted for the first time. The requirement of positive entropy density of such bags leads to negative values of the surface tension coefficient of quark gluon bags at the cross-over region, i.e. at the continuous transition to deconfined quarks and gluons. We argue that the cross-over existence at supercritical temperatures in ordinary liquids is also provided by the negative surface tension coefficient values. It is shown that the confining tube model naturally accounts for an existence of a very pronounced surprising maximum of the tube entropy observed in the lattice QCD simulations, which, as we argue, signals about the fractional surface formation of the confining tube. In addition, using the developed formalism we suggest the gas of free tubes model and demonstrate that it contains two phases.

hep-ph↗
arXiv2011

The critical exponents of the QCD (tri)critical endpoint within exactly solvable models

The critical indices alpha', beta, gamma' and delta of the Quark Gluon Bags with Surface Tension Model with the tricritical and critical endpoint are calculated as functions of the usual parameters of this model and two newly introduced parameters (indices). The critical indices are compared with that ones of other models. The universality class of the present model with respect to values of the model parameters is discussed. The scaling relations for the found critical exponents are verified and it is demonstrated that for the standard definition of the index alpha' some of them are not fulfilled in general case. Although it is shown that the specially defined index alpha'_s recovers the scaling relations, another possibility, an existence of the non-Fisher universality classes, is also discussed.

hep-ph↗
arXiv2011

Can We Rigorously Define Phases in a Finite System?

Here we propose the generalized statistical multifragmentation model which includes the liquid phase pressure of the most general form. This allows us to get rid of the absolute incompressibility of the nuclear liquid. Also the present model employes a very general form the surface tension coefficient of nuclear fragments. Such a model is solved analytically for finite volumes by the Laplace-Fourier transform method to isobaric ensemble. A complete analysis of the isobaric partition singularities of this model is also done for finite volumes. It is shown that the real part of any simple pole of the isobaric partition defines the free energy of the corresponding state, whereas its imaginary part, depending on the sign, defines the inverse decay/formation time of this state. The developed formalism allows us to exactly define the finite volume analogs of gaseous, liquid and mixed phases of the class of similar models from the first principles of statistical mechanics and demonstrate the pitfalls of earlier works. The finite width effects for large nuclear fragments and quark gluon bags are also discussed.

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arXiv2011

Directed, Elliptic and Triangular Flows in Asymmetric Heavy Ion Collisions

In this paper we propose to thoroughly investigate asymmetric nuclear collisions both in the fixed target mode at the laboratory energy below 5 GeV per nucleon and in the collider mode with a center of mass energy below 11 GeV per nucleon. Using the UrQMD transport model, we demonstrate a strong enhancement of directed and elliptic flow coefficients for the midcentral asymmetric nuclear collisions compared to symmetric collisions. We argue that such an enhancement is due to the disappearance of the nuclear shadowing effect on the side of the smaller projectile nucleus. An analysis of the energy and centrality dependencies of the directed, elliptic and triangular flows at midrapidity shows us their sensitivity to the details of the employed model of hadronic interaction. In general, the flow patters found for asymmetric nuclear collisions have a very rich and complicated structure of energy and centrality dependencies compared to the flows found for symmetric collisions and are worth to be investigated experimentally. The directed, elliptic, and triangular flow coefficients are computed for target nuclei containing high density fluctuations and thoroughly compared with that ones obtained in the absence of such fluctuations.

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