Search Results (11)

We establish a statistical two-body fractal (STF) model to study the spectrum of $J/\psi$. $J/\psi$ serves as a reliable probe in heavy-ion collisions. The distribution of $J/\psi$ in hadron gas is influenced by flow, quantum and strong interaction effects. Previous models have predominantly focused on one or two of these effects while neglecting the others, resulting in the inclusion of unconsidered effects in the fitted parameters. Here, we study the issue from a new point of view by analyzing the fact that all three effects induce a self-similarity structure, involving a $J/\psi$-$\pi$ two-meson state and a $J/\psi$, $\pi$ two-quark state, respectively. We introduce modification factor $q_{TBS}$ and $q_2$ into the probability and entropy of charmonium. $q_{TBS}$ denotes the modification of self-similarity on $J/\psi$, $q_2$ denotes that of self-similarity and strong interaction between c and $\overline{c}$ on quarks. By solving the probability and entropy equations, we derive the values of $q_{TBS}$ and $q_2$ at various collision energies and centralities. Substituting the value of $q_{TBS}$ into distribution function, we successfully obtain the transverse momentum spectrum of low-$p_T$ $J/\psi$, which demonstrates good agreement with experimental data. The STF model can be employed to investigate other mesons and resonance states. Full article

We explore the emergence of the collisional broadening of hadrons under the influence of different media using the hadronic transport approach SMASH (Simulating Many Accelerated Strongly interacting Hadrons), which employs vacuum properties and contains no a priori information about in-medium effects. In this context, we define collisional broadening as a decrease in the lifetime of hadrons, and it arises from an interplay between the cross-sections for inelastic processes and the available phase space. We quantify this effect for various hadron species, in both a thermal gas in equilibrium and in nuclear collisions. Furthermore, we distinguish the individual contribution of each process and verify the medium response to different vacuum assumptions; we see that the decay width that depends on the resonance mass leads to a larger broadening than a mass-independent scenario. Full article

In this study, we calculate the pressure of the interacting pion gas using the Beth–Uhlenbeck approach to the relativistic virial expansion with Breit–Wigner phase shifts for the $\sigma$- and $\varrho$-meson resonances. The repulsive phase shift ${\delta }_0^2$ is taken from the quark interchange model of Barnes and Swanson, which is in very good agreement with the experimental data. In this work we show that the cancellation of the attractive (I = 0) and repulsive (I = 2) isospin channel contributions to the scalar $\pi \pi$ interaction in the low-energy region that is known for the vacuum phase shifts also takes place at a finite temperature. This happens despite the strong medium dependence of these phase shifts that enters our model by the temperature dependence of the $\sigma$-meson and constituent quark masses, because for these masses the relation $M_{\sigma }\left(T\right)\approx 2m_q\left(T\right)$ holds and the scattering length approximation is valid as long as the strong decay channel $\sigma \to \pi \pi$ is open. Exploiting the Nambu–Jona-Lasinio model for describing the dynamical breaking of chiral symmetry in the vacuum and its restoration at a finite temperature, we justify with our approach that the $\sigma$-meson should be absent from the hadron resonance gas description at low temperatures because the above cancellation holds. However, since this cancellation breaks down in the vicinity of the hadronization transition, where due to chiral symmetry restoration the decay channel $\sigma \to \pi \pi$ closes and the $\sigma$-meson becomes a good resonance, the latter should be included into the statistical model description of chemical freeze-out in heavy-ion collisions. Full article

We analyzed the transverse momentum spectra ($p_T$) reported by the NA61/SHINE and NA49 experiments in inelastic proton–proton ($pp$) and central Lead–Lead ($Pb-Pb$), Argon–Scandium ($Ar-Sc$), and Beryllium–Beryllium ($Be-Be$) collisions with the Blast-wave model with Boltzmann–Gibbs (BWBG) statistics. The BGBW model was in good agreement with the experimental data. We were able to extract the transverse flow velocity (${\beta }_T$), the kinetic freeze-out temperature ($T_0$), and the kinetic freeze-out volume (V) from the $p_T$ spectra using the BGBW model. Furthermore, we also obtained the initial temperature ($T_i$) and the mean transverse momentum (<$p_T$>) by the alternative method. We observed that $T_0$ increases with increasing collision energy and collision cross-section, representing the colliding system’s size. The transverse flow velocity was observed to remain invariant with increasing collision energy, while it showed a random change with different collision cross-sections. In the same way, the kinetic freeze-out volume and mean transverse momentum increased with an increase in collision energy or collision cross-section. The same behavior was also seen in the freeze-out temperature, which increased with increasing collision cross-sections. At chemical freeze-out, we also determined both the chemical potential and temperature and compared these with the hadron resonance gas model (HRG) and different experimental data. We report that there is an excellent agreement with the HRG model and various experiments, which reveals the ability of the fit function to manifest features of the chemical freeze-out. Full article

We calculated the shear viscosity of hot and dense nuclear matter produced in a symmetric system of central gold–gold collisions at energies of BES RHIC, FAIR and NICA. For calculations of the collisions, the transport model UrQMD was employed. The shear viscosity was obtained within the Green–Kubo formalism. The hadron resonance gas model was used to determine temperature and chemical potentials of baryon charge and strangeness out of microscopic model calculations. In contrast to our previous works, we determined the partial viscosity of the main hadron species, such as nucleons, pions, kaons and Lambdas, via the nucleon–nucleon, pion–pion and so forth, correlators. A decrease in the beam energy from $E_{lab}=40$ to 10 AGeV leads a to rise in baryon shear viscosity accompanied by a drop in the shear viscosity of mesons. The ratio of total shear viscosity to entropy density also decreases. Full article

In this work, we present a unified approach to the thermodynamics of hadron–quark–gluon matter at finite temperatures on the basis of a quark cluster expansion in the form of a generalized Beth–Uhlenbeck approach with a generic ansatz for the hadronic phase shifts that fulfills the Levinson theorem. The change in the composition of the system from a hadron resonance gas to a quark–gluon plasma takes place in the narrow temperature interval of 150–190 MeV, where the Mott dissociation of hadrons is triggered by the dropping quark mass as a result of the restoration of chiral symmetry. The deconfinement of quark and gluon degrees of freedom is regulated by the Polyakov loop variable that signals the breaking of the $Z\left(3\right)$ center symmetry of the color $SU\left(3\right)$ group of QCD. We suggest a Polyakov-loop quark–gluon plasma model with $\mathcal{O}\left({\alpha }_s\right)$ virial correction and solve the stationarity condition of the thermodynamic potential (gap equation) for the Polyakov loop. The resulting pressure is in excellent agreement with lattice QCD simulations up to high temperatures. Full article

The study of hadronic resonance production is an essential part of the physical programs of many heavy-ion experiments. Detailed measurement of the resonance properties is also foreseen in the future Multi-Purpose Detector (MPD) experiment at the NICA collider. In this report, we focus on the experimental challenges for the reconstruction of resonances in heavy-ion experiments and examine the MPD capabilities for the reconstruction of ρ(770)⁰, K*(892)^0,±, φ(1020), Λ(1520), Σ(1385)^± and Ξ(1530)⁰. Full article

We review recent advances in the understanding of the Quantum Chromodynamics (QCD) transition and its nature, paying special attention to the analysis of chiral symmetry restoration within different approaches based on effective theories. After presenting some of the main aspects of the current knowledge of the phase diagram from the theoretical, experimental and lattice sides, we discuss some recent problems where approaches relying on effective theories have been particularly useful. In particular, the combination of ideas such as Chiral Perturbation Theory, unitarity and Ward Identities allows us to describe successfully several observables of interest. This is particularly relevant for quantities expected to be dominated by the light meson components of the hadron gas such as the scalar and topological susceptibilities. In addition, ward identities and effective Lagrangians provide systematic results regarding chiral and $U{\left(1\right)}_A$ partner degeneration properties which are of great importance for the interplay between those two transitions and the nature of chiral symmetry restoration. Special attention is paid to the connection of this theoretical framework with lattice simulations. Full article

Motivated by the recent lattice study by FASTSUM collaboration, effective masses of the baryon parity-doublers are shown for various pion masses. A general trend of the nucleon and delta parity-doublers is consistent with the lattice Quantum Chromodynamics (QCD) observation, whereas the hyperon masses exhibit a qualitatively different behavior, traced back to the lattice set-up with the heavy pion comparable to the kaon. As an application to hot QCD, we demonstrate the fluctuations and correlations involving baryon number in hot hadronic matter with modified masses of negative-parity baryons, in the context of the hadron resonance gas. Confronting the baryon number susceptibility, baryon–charge and baryon–strangeness correlations as well as their ratios with the lattice QCD data for the physical pion mass, we find that the strong downward mass shift in the hyperons can accidentally reproduce some correlation ratios, however it also tends to overshoot the individual fluctuations and correlations of lattice simulations. Another application of nucleon parity doubling is the physics of neutron stars. Under beta equilibrium and charge neutrality, hadronic matter with unbroken chiral symmetry can be favored in the core of the neutron stars. Full article

We have extended the hadron resonance gas (HRG) model by including the effect of both attractive and repulsive interaction in the scattering matrix (S-matrix) formalism. The attractive part of the interaction is calculated using K-matrix formalism while the repulsive part is included by fitting to experimental phase shifts. We have calculated various thermodynamics quantities like pressure, energy density, entropy density etc. A good agreement between our calculations and the hadronic phase of the lattice QCD (LQCD) simulations is observed. We have also calculated fluctuations and correlations for various conserved charges like baryon, strangeness and electric charge. In the present model, ${\chi }_B^2$ , ${\chi }_{BS}^{11}$ and $C_{BS}$ agree well with the LQCD data. Full article

We outline an approach to a unified equation of state for quark-hadron matter on the basis of a $\mathsf{\Phi }-$ derivable approach to the generalized Beth-Uhlenbeck equation of state for a cluster decomposition of thermodynamic quantities like the density. To this end we summarize the cluster virial expansion for nuclear matter and demonstrate the equivalence of the Green’s function approach and the $\mathsf{\Phi }-$ derivable formulation. As an example, the formation and dissociation of deuterons in nuclear matter is discussed. We formulate the cluster $\mathsf{\Phi }-$ derivable approach to quark-hadron matter which allows to take into account the specifics of chiral symmetry restoration and deconfinement in triggering the Mott-dissociation of hadrons. This approach unifies the description of a strongly coupled quark-gluon plasma with that of a medium-modified hadron resonance gas description which are contained as limiting cases. The developed formalism shall replace the common two-phase approach to the description of the deconfinement and chiral phase transition that requires a phase transition construction between separately developed equations of state for hadronic and quark matter phases. Applications to the phenomenology of heavy-ion collisions and astrophysics are outlined. Full article