Bogolyubov Institute for Theoretical Physics

List of Departments Topmost Scientific Results

Department of High-Density Energy Physics

2024
2023
2022
2021
2020
2019
2018
2017
2016

The fluctuations of the number of particles in the crossover region near the critical point of the first-order phase transition were studied using the simulation of a classical Lennard-Jones fluid (3D Ising universality class) by means of molecular dynamics. The previous study was extended using the collective motion of particles. The results obtained were analyzed within the framework of heavy ion collisions
M.I. Gorenstein, V.O. Kuznetsov
Within the framework of the realistic iHKM and LQTH models, the dependence of the pion-pion femtoscopic radii on the transverse mass of the pair mTm_TmT in the region of its lowest values (< 200 MeV/s) for central collisions of lead nuclei at the Large Hadron Collider was studied. It was found that in both models the behavior of the radii in the region of ultralow momentum deviates from the power law characteristic of higher momentum values. Instead, it is possible to describe the dependence of the radii on the transverse velocity of the pair for all corresponding values of the pair momentum with a single formula. The unusual behavior for pions with ultralow momentum may indicate a peculiar separation of these particles from the rest of the system and the formation of a peculiar "trap" for ultrasoft pions
Y.M. Sinyukov, V.M. Shapoval
The electric and magnetic screening masses in the deconfinement phase at a finite baryon density were calculated analytically and numerically for the first time using Monte Carlo simulations
O.A. Borisenko, V. Chelnokov
Within the limits of the 't Hufta-Venetian three-dimensional effective model of Polyakov loops, the correlation functions of Polyakov loops were calculated. This allowed us to investigate the mass spectrum of the SU(N) model at a non-zero baryon chemical potential. The mass spectrum turns out to be complex and leads to an exponential decay of correlations modulated by the oscillating factor
O.A. Borisenko, S. Voloshin, V. Chelnokov
Linear perturbations of static spherically symmetric background configurations of general relativity with a real scalar field, which is minimally connected to gravity due to the presence of a self-action potential, are studied. The background solutions have a naked singularity in the center of the configuration
O. Savchuk
Correlations involving seven conserved quantities, namely energy, baryon number, electric charge, strangeness, and three momentum components, give rise to correlations in heavy ion collisions. Using a simple one-dimensional hydrodynamic model, the evolution of the entire 7×7 correlation matrix as a function of the relative spatial velocity is calculated
O. Savchuk
The quantum van der Waals model of quarkionic matter is extended to account for nonzero isospin asymmetry using a two-component van der Waals equation with generalized volume exclusion. The isospin dependence of the interaction parameters is determined using constraints related to the symmetry energy, its derivative, and the properties of the ground state of nuclear matter. The results demonstrate a peak in the speed of sound for all values of asymmetry, indicating a transition to quarkionic matter. Including lepton components, the equation of state for neutron stars is analyzed, showing that quarkionic matter can support neutron stars with a maximum mass of at least 2.6 solar masses
R.V. Poberezhnyuk
The chemical freezing curve in heavy-ion collisions is investigated in the context of searching for the critical point (CP) of QCD at high baryon densities. The chemical freezing points at a given baryon chemical potential provide a lower limit on the possible temperature of the QCD QD. We find the QD position slightly above the freezing curve, which indicates that the QCD QD can be located very close to the chemical freezing in nucleus-nucleus collisions for the energy at the center of mass of a nucleon pair of 3.5-6 GeV
M.I. Gorenstein, A. Lysenko
The exact solutions of the one-dimensional lattice gauge theory at finite temperatures and nonzero chemical potential are considered. The statistical sum, free energy, average Polyakov loop, baryon density, quark condensate, meson and baryon correlation functions are calculated. In the limit of large Nf, a Roberger-Weiss phase transition is found and its traces at finite Nf are discussed. The critical behavior of the models is described in detail, and the phase structure is studied. A comparison of these limits with QCD for the U(3) and SU(3) groups is made
O.A. Borisenko, S. Voloshin, V. Chelnokov
The integrated hydrokinetic model (iHKM) is modified to describe relativistic collisions of nuclei in the energy range of 1-50 GeV per nucleon pair. Compared to ultrarelativistic energies, such collisions are characterized by significantly longer times of dense matter formation, which affects the stages of the system evolution, in particular, partial thermalization, hydrodynamic expansion and transition to the hadron cascade. An extended version of the model (iHKMe) is presented, which provides a description of spectra, elliptical flows and interferometric lengths in a wide range of energies
M.D. Adzhymambetov, Y.M. Sinyukov
Preliminary data from the Beam-Energy Scan II measurements conducted by the STAR collaboration at the relativistic heavy-ion collider indicate a drop in the fourth-to-second-order cumulative ratio as a function of collision energy. In the same energy range where this structure appears, a transition from hadrons to quarks, i.e., a deconfinement transition, is expected. This paper considers the role of quark deconfinement in establishing fluctuations at the early stages of a collision. Two models are compared: one with a halt occurring through baryons, and the other where the halt occurs through quark degrees of freedom. In the latter model, the fluctuations in the baryon number are significantly reduced, and this signal is found to persist after recombination into hadrons and subsequent diffusion
O. Savchuk
Transverse momentum correlations have recently been measured by the ALICE collaboration at the LHC. A long-range structure of the relative pseudovelocity of particle pairs is observed, which may indicate an initial state signal due to the scattering of correlations. However, fluctuations within the thermally equilibrated medium must be taken into account. Using the constraints of quantum chromodynamics on the lattice, we have predicted the development and propagation of balancing correlations induced by energy-momentum conservation. At the same time, we have propagated the initial correlation using hydrodynamics to assess its impact. Our findings indicate that the resulting correlation, known as the "backbone", is sensitive to both fluctuations in the equilibrated medium and the pre-equilibrium stage. This may provide important information about the early stages of the collision
O. Savchuk
The observed value of the energy-momentum tensor of a quantum scalar field in the locally equilibrium approximation of a quasi-equilibrium statistical operator has been calculated. It is shown that the observed value of the energy-momentum tensor has additional terms compared to the corresponding value in the state of global thermodynamic equilibrium. In particular, it is established that the effective equation of state, i.e. the relation between pressure and energy density, is modified compared to the case of homogeneous thermodynamic equilibrium. The obtained corrections have a non-equilibrium origin and can be significant for systems formed in relativistic collisions of nuclei and particles
S.V. Akkelin
To solve the problem of data normalization, a new approach to the procedure of minimizing the total squared deviation of theoretical cross sections from experimental ones is proposed. Initially, all data were filtered to ensure their self-consistency and compliance with the data contained in the HEPData Base. For most datasets, from ISR to LHC, the parameters of the cross-section shift (all points of a given set) up or down have been determined depending on the cross-section normalization inaccuracy specified for each dataset in the HEPData Base. Usually, these parameters are not taken into account when comparing theoretical and experimental data
E.S. Martynov, G. Tersimonov
During the analysis of cosmological simulations within the framework of the CAMELS project, it was demonstrated that multilayer fully connected neural networks are able to effectively recover the cosmological parameters of the model from the observed quantities. The estimates of the cosmological parameters have been significantly improved by applying an additional procedure of combining data from simulations conducted for close values of the cosmological parameters. The results obtained were presented in the conference abstracts
M.D. Adzhymambetov

Using the method of molecular dynamics, the effect of the liquid-gas mixed phase on fluctuations in the number of particles was investigated. The metastable region of the mixed phase is modeled as a system of non-interacting clusters. Large fluctuations were detected in the region of spinodal instability.
M.I. Gorenstein, R.V. Poberezhnyuk, O.V. Savchuk
In a simple analytically solvable model of the thermal expansion system, two-boson momentum correlations were investigated with a fixed limitation of the number of particles in proton-proton collisions at energies characteristic of the Large Hadron Collider. It is shown that an increase in the rate of expansion, as well as an increase in the multiplicity of particles, increases the contribution of the ground state to the momentum spectra of particles and leads to the suppression of the Bose-Einstein momentum correlation. These conclusions are directly related to the recently experimentally discovered dependence of the Bose-Einstein momentum correlation on the multiplicity.
Y.M. Sinyukov, M.D. Adzhimambetov, S.V. Akkelin
An exact solution is obtained for a wide class of models of Polyakov loops on a lattice in arbitrary dimensions with gauge groups U(N) and SU(N) in the limit when the number of colors (N) and the number of quark flavors (N_f) go to infinity. Analytical expressions for the shielding chromoelectric and chromomagnetic masses of gluons in the deconfinement phase were obtained. It was found that in a certain range of parameters in the deconfinement phase, the correlation functions have an exponential decline modulated by the oscillatory functions. This means that the chromoelectric masses are complex.
O. Borisenko, V. Chelnokov, S. Voloshin
A model of the star as a system of pi mesons in the Bose condensate state is formulated. Calculated mass-radius ratios for the pi-meson part and parameters of its lepton environment.
M.I. Gorenstein, O. Savchuk
A dual representation for SU(N) gauge theories with dynamical quarks is constructed. This representation was used for numerical Monte Carlo simulations of the effective Polyakov loop model at finite baryon density. Various local observables such as energy density, baryon density, quark condensate were calculated and the phase diagram of the model was detailed.
O. Borisenko, S. Voloshin
Two mysterious femtoscopic observations in collisions of ultrarelativistic heavy ions are considered in detail. First, it is the proximity of the maximum times of pion emission at completely different collision energies: from the highest energies of RHIC to the upper energies of LHC. Another paradoxical effect is that, despite the rather long duration of the post-hydrodynamic cascade stage, the times of maximum emission observed are close to the times of particleization - the transformation of the quark-gluon liquid into hadrons. A detailed analysis of the features of the hydrodynamic evolution of the system at different energies of the colliding nuclei and the structure of the emission function of pions and kaons in the hadronic stage sheds light on these paradoxical results.
Y.M.Sinyukov, M.D. Adzhimambetov, S.V. Akkelin

A system of bosons studied within the mean-field framework has two fascinating phenomena: a liquid-gas first order phase transition and Bose-Einstein condensation. Depending on the mean-field potential parameters one can observe two types of critical points (CP), that belong to different universality classes with distinct sets of critical exponents. As examples the pion and α matter are considered.
V. Kuznietsov, O. Savchuk, and M.I. Gorenstein
Two-boson momentum correlations at fixed particle number constraint are studied in a simple analytically solvable model of a thermal expanding system. It was shown that the increase of expansion rate, as well as increase of particle multiplicity, enhances the ground-state contribution to particle momentum spectra and leads to suppression of the Bose-Einstein momentum correlations. These findings are directly related to the recently found phenomena as for multiplicity-dependence of the Bose-Einstein momentum correlations in high-multiplicity p+p collision events at the LHC.
M.D. Adzhymambetov, S.V. Akkelin, Yu.M. Sinyukov
An exact solution is obtained for a wide class of Polyakov loop models on a lattice in arbitrary dimension with U(N) and SU(N) gauge groups in the limit when N and Nf go to infinity, where N is the number of colors, and Nf is the number of quark flavors. This made it possible to obtain analytical expressions for screening chromo-electric and chromo-magnetic masses of gluons in the deconfinement phase. It was also established that in a certain region of parameters in the deconfinement phase the exponential decay of correlation functions is modulated by an oscillating function. This means that the chromo-electric masses are complex.
O. Borisenko, V. Chelnokov, S. Voloshin
A model of the vertex of three-Froissarton interaction has been constructed, which provides small corrections to the original Dyson-Schwinger equation and allows to solve the problem of breaking unitarity in the diffraction production of many massive hadron beams (the Finkelstein-Kajanti problem).
E. Martynov, G. Tersimonov

The analysis of parton scattering is performed taking into account multiparton distribution functions. The properties of these functions are investigated. The obtained results provide new unique information about the structure of hadrons and the dynamics of their interaction.
Cor.-member of the NAS of Ukraine G.M. Zinoviev
The equation of state for the interacting pi-meson system that includes the Bose-Einstein condensation and first order phase transition was obtained. The search for such physical systems by measuring the electric charge fluctuations was proposed.
M.I. Gorenstein, R.V. Poberezhnyuk, O.V. Savchuk
Two-particle momentum correlations in the system of N identical bosons are studied on the basis of a quantum canonical ensemble. New effects related to high-temperature Bose-Einstein condensation of pions in small systems are identified.
Y.M. Sinyukov, M.D. Adzhymambetov, S.V. Akkelin
Using variational principle, we show that a condition of the spatial collapse in imperfect Bose gas is absolutely not determined either by the value or by the sign of the scattering length, in the contrary to the result following from the Gross-Pitaevskii equation, where the collapse is predicted at negative scattering length.
B.E. Grinyuk, K.A. Bugaev

The integrated Hydrokinetic Model is updated by incorporating the additional mechanisms of photon radiation to analyze the spectra of direct photons, as well as elliptic and triangular flows at energies of nuclear collisions at colliders RHIC and LHC for various collision centralities. It was found that the selfconsistent description of these data requires the inclusion of photon radiation associated with confinement processes ("hadronization photons") and entanglement effects.
Cor. Member of the NAS of Ukraine G.M. Zinovjev, Yu.M. Sinyukov
New formalism of self-consistent treatment of whole excluded volume of the mixture of hadrons with an arbitrary number of hard-core radii and such clusters as light (anti-, hyper-) nuclei is suggested. It was possible to derive a realistic equation of state for such mixtures that allows one to go beyondthe Van der Waals approximation. Deaing wth new equation of state, we developed hadron resonance gas model providing the very accurate description of multiplicities of hadrons and nuclei measured in the central nuclear collisions by the ALICE CERN Collaboration at the center-of-mass 2760 GeV and by the STAR BNL Collaboration at 200 GeV.
Cor. Member of the NAS of Ukraine G.M. Zinovjev, K.O. Bugaev, B.E. Grinyuk, V.V. Sagun, O.I. Ivanytskyi
It is developed the subensemble acceptance method that quantifies the effect of global conservation laws and is an important step toward a direct comparison between cumulants of conserved charges measured in central heavy ion collisions and theoretical calculations of grand-canonical fluctuations, such as lattice QCD. As an example, we apply our formalism to net-baryon fluctuations at vanishing baryon chemical potentials as encountered in collisions at the LHC.
R.V. Poberezhnyuk, M.I. Gorenstein
Using the Laplace-Fourier transformation technique, the method to exactly account for the particle number fluctuations for the equations of state with induced surface and curvature tensions is suggested. Such an approach allows one to generalize the classical concept of morphological thermodynamics to the dense mixtures of both the Boltzmann particles and the quantum ones with the hard-core repulsion. In contrast to the standard formulation, in the new approach the coefficients of induced surface and curvature tensions are suppressed at hight densities not by the exponential, but by the power-like functions of system pressure.
K.O. Bugaev
Simultaneous effects of finite system size and global charge conservation on thermal fluctuations in the vicinity of a critical point are investigated. A finite interacting system that exchanges particles with a finite reservoir (thermostat) is considered. It is shown that such a statistical ensemble differs from the common canonical and grand canonical ensembles, and global charge conservation effects strongly influence the cumulants of particle number distribution. If the system size is sufficiently large, the global charge conservation effects can be accurately described analytically within a recently developed subensemble acceptance method. It is found that the finite size effects start to play a significant role when the correlation length grows large due to the proximity of the critical point or when the system is small enough to be comparable to an eigenvolume of an individual particle.
R.V. Poberezhnyuk., М.І. Gorenstein

A modification of the collinear evolution equations as an appropriate approach to improving the behavior of parton distribution functions in the region of small longitudinal momentum fractions and to finding more theoretical arguments to clarify the possible appearance of saturation regime is suggested.
Cor. Member of NAS of Ukraine G.M. Zinovjev
Basing on the theorems of analytic S-matrix theory and the dispersion relations for the elastic scattering amplitudes it is shown that contribution of the odderon, crossing-odd component of the amplitude, dominates at high energies in the ratio of real to imaginary part of the amplitude. Asymptotically, these ratios have opposite signs for proton-proton and proton-antiproton scatterings.
E.S. Martynov, G.S. Tersimonov
A single component quantum Van der Waals equation of state of hard spheres from quantum partition function was derived for the first time. The suggested analytical method of self-consistent calculation of the excluded particle volume in a dense medium is generalized for the hard sphere mixture of arbitrary number of different hard-core radii. The quantum equation of state is extrapolated to high densities and generalized for the mixtures of convex hard particles of arbitrary shape for spatial dimensions D ≥ 2.
K.A. Bugaev
Backward nucleon production by heavy baryonic resonances in proton-nucleus collisions is proposed.
M.I. Gorenstein
On the basis of the integrated hydrokinetic model the full similarity of "soft physics" processes in nuclei collisions of gold at the energies per nucleon pair of 200 GeV at RHIC, lead at the LHC energies 2.76 and 5.02 TeV, and xenon at 5.44 TeV has been found.
Yu.M. Sinyukov, V.M. Shapoval, M.D. Adzhymambetov
The thermodynamically consistent mean-field model is applied to describe dense boson systems for high temperatures and zero chemical potential. It is shown that if the attractive component is strong enough to violate the system-stability condition (the effective particle mass is greater than or equal to zero), then the first-order phase transition occurs in the multi-boson system and scalar field condensate is formed. The allowed states of the system with the condensate are shown to exist under the condition that the effective mass particle vanishes, i.e., the boson quasi-particles become massless.
D.V. Anchishkin

The equation of state with the induced surface tension is generalized to quantum gases with the mean-field interaction. Such an approach allows one to go beyond the Van der Waals approximation for quantum systems. Explicit expressions for the quantum virial coefficients of arbitrary order are found in the low density limit. All virial coefficients for the quantum Van der Waals equation of state are found as well.
K.A. Bugaev, A.I. Ivanytskyi, V.V. Sagun, corr. member of NAS of Ukraine G.M. Zinovjev
The spectrum of photons with large transverse momentum is calculated and compared with the experimental data within the framework of the model of synchrotron radiation, which results from quark interactions with the collective confining color field in relativistic heavy ion collisions. It makes it possible to demonstrate the feasibility of this type of radiation due to distinctive features of lepton pair production in the corresponding experiments.
corr. member of NAS of Ukraine G.M. Zinovjev
On the basis of Froissaron and Maximal Odderon model for proton and antiproton interaction at zero transferred momenta it is shown that the latest data of the TOTEM experiment at CERN give evidence for the discovery of odderon contribution, which was predicted almost 50 years ago. Moreover, the generalization of the model for differential cross-sections of nucleon elastic scattering confirmed the presence of odderon effects in the interaction of high-energy protons and antiprotons at non-zero transferred momenta.
E.S. Martynov
Statistical model of quark-gluon bags with a continuous transition to the high temperature phase is developed. Thermodynamical functions and fluctuations of conserved charges are calculated. The results are in agreement with those obtained within QCD lattice calculations.
M.I. Gorenstein
Within the framework of the evolution model of high ion collisions at LHC energies it is found that the inelastic reactions after matter hadronization influence on the relation of different particle number formation. Such reactions are important (compensated) factor of the hadron production, which restricts a direct studying the equation of quark-gluon plasma state and initial conditions of superdense matter formation.
Yu.M. Sinyukov, V.M. Shapoval

Significantly alternative mechanism for thermal radiation of photons and di-leptons is suggested. It is based on an effect of synchrotron radiation that arises from the interaction of quarks with collective color field providing a confinement. The intensity of such a radiation for the hot medium of size 1-10 fermi (that just is expected in the collisions of relativistic heavy ions) tuгns out quantitatively very close to standard volume photon radiation at temperatures T=200-300 MeV. The most striking feature of such a mechanism is the high degree of polarization of photons. It has been found out the virtual photons develop the noticeable specific anisotropy in the angle distribution of leptons with respect to the three-momentum of pair and is shown such a radiation is will be non-isotropic for the non-central collisions. Such an angular anisotropy is absent in the Drell-Yan mechanism and the other "standard volumetric" mechanisms and could be (if discovered experimentally) quite reliable signal of creating quark-gluon plasma in the relativistic collisions of hadrons and heavy ions.
Corr. Member of the NAS of Ukraine G.M. Zinovjev
The newest data of the TOTEM Collaboration at 13 TeV and all the present data on the total cross sections together with ratios of real to imaginary part of elastic forward scattering amplitudes at energies higher than 5 GeV have been analyzed. As a result the serious arguments have been presented, and the surprisingly small value of this ratio measured in TOTEM experiment сlearly showed the first experimental observation of an "odderon" in its maximal form.
Ye.S. Martynov
The model of hadron resonance gas that includes an effect of interaction between baryons has been developed and used to calculate the baryon number fluctuations, in particular, the cumulants of third and fourth order in order to describe the results of the central nuclei-nuclei collisions.
M.I. Gorenstein
Using the proposed equation of state with the induced surface tension which allows one to go beyond the Van der Waals approximation the thermodynamic properties of hadronic matter at chemical freez-out are fixed with high confidence for the temperature Т=50-170 МеV and baryonic chemical potential μ = 0-770 MeV. Fitting the experimental data on hadronic multiplicities the equation of phase state that is created at the center of mass collision energies of nuclei in the interval = 4.9-9.2 GeV/nucl has been found for the first time, and the strong arguments in favor of this phase as a phase of nearly massless hadrons have been given. Thus, it leads to the conclusion about existence of a tri-critical endpoint, but not a critical one, in the QCD matter.
K.A. Bugaev, V.V. Sagun, A.I. Ivanytskyi, Corr. Member of the NAS of Ukraine G.M. Zinovjev
The meson resonance K * (892) with the lifetime of 4-5 fm/s are used as a probe to analyze a spaсe-temporal picture of nuclear-nuclear collision processes. It is found that almost half of these mesons are born inside a hadron environment and 70% of their decay products interact with the medium. It testifies to the existence of dense hadron matter with a life time not less than 5 fm/c after the quark-gluon plasma hadronization. The numerical calculations of this picture within the integrated hydrokinetic model lead to the results fitting well the LHC experimental data.
Yu.M. Sinyukov, V.M. Shapoval

A novel formulation of lattice SU(2) gluodynamics in terms of geometrical clusters of two types formed by the Polyakov loops was developed and the equivalent thermodynamic description of lattice gluodynamics in the vicinity of deconfinement phase transition was obtained. For the first time the temperature dependence of physical surface tension of geometrical clusters is found directly from the lattice QCD. It is also found that the surface tension of these clusters can be used as a new order parameter for the corresponding phase transition.
K.A. Bugaev, A.I. Ivanytskyi, V.V. Sagun, D.R. Oliinychenko, G.M. Zinovjev
An extension of the hadron resonance gas model which includes the attractive and repulsive van der Waals interactions between baryons is constructed. This model yields the nuclear liquid-gas 1st order phase transition at low temperatures and high baryon densities. A behavior of fluctuations and correlations of baryon number, electric charge, and strangeness in the so-called crossover region T = (140-190) MeV resembles closely the results obtained from lattice QCD simulations.
M.I. Gorenstein
Analysing two models of four-quark (Nambu-Jona-Lasinio model and Keldysh model) interactions which are of intrinsic difference in the behaviors of their correlation lengths some issues of quark condensations are considered. It is demonstrated that the quark condensates substantially are not sensitive to the details of those interactions in the range of coupling constants interesting for applications.
G.M. Zinovjev
Within the integrated hydrokinetic model, it is analyzed the transverse momentum pT-dependence of the pair correlations of strange mesons and baryons that are created in nucleus-nucleus collisions at Large Hadron Collider (LHC) and Relativistic Heavy Ion Collider (RHIC). By using the correlation functions for different pair species (kaon-kaon, Lambda-proton, Cascade - proton), the femtoscopic spatiotemporal picture of the pairs emission is restored. At that, the pT -scaling behavior of kaon and pion femto-scales is revealed. The result is confirmed now in the experiments at RHIC and LHC.
Yu.M. Sinyukov, V.M. Shapoval

Bogolyubov Institute for Theoretical Physics of the National Academy of Sciences of Ukraine

Bogolyubov Institute for Theoretical Physics
of the National Academy of Sciences of Ukraine