Why the hydrodynamics is valid at early stage of heavy-ion collisions?

Evolution of hot and dense nuclear matter produced in central Au+Au collisions at energies of NICA, FAIR, and SPS is studied within two transport models. Two interesting features in the matter behaviour are observed almost from the very beginning of the collisions, at t ≥ 2 fm/c, for all studied reactions. (i) Expansion of the system proceeds with constant entropy-per-baryon ratio. (ii) Effective equation of state has a linear form, P = aε, a sime const. Both observations support the formal application of hydrodynamics at the early stages of heavy- ion collisions, when the system is very far from the equilibrium

Relaxation to equilibrium and EOS in ultra-relativistic heavy-ion collisions

We study relaxation to equilibrium of hot and dense hadron-string matter produced in the central zone of central heavy-ion collisions at energies 11.6AGeV ≤ Elab ≤ 160AGeV. Two microscopic transport models, UrQMD and QGSM, are employed. The analysis is performed for the central cubic cell with volume V = 125 fm3. To check how close the system is to the equilibrium, its hadron yields and hadron energy spectra are compared with those of the statistical model of ideal hadron gas. For all collision energies it was found that the matter in the cell was approaching the equilibrium state, which lasted about 10 - 20 fm/c. After that the matter became very dilute and the thermal contact between the hadrons was lost. Equation of state is well fitted to linear dependence P/ε = a = c2 s, where the square of the sonic velocity c2 s increases from 0.12 at Elab = 11.6 AGeV to 0.145 at Elab = 160 AGeV. These results are valid also for very early times of the system evolution when the matter is still out of equilibrium. Together with the isentropic expansion, the linear dependence of P on ε supports the application of hydrodynamic description to early stages of heavy-ion collisions

Relaxation to equilibrium in relativistic heavy ion collisions

Relaxation to equilibrium of hot and dense matter produced in central area of relativistic heavy ion collisions at energies ranging from several AGeV to hundreds AGeV is studied within two Monte Carlo transport models. The analysis was performed for three different areas: (i) fixed cubic cell with volume V = 125 fm3, (ii) fixed asymmetric cell with volume V = 4 x 4 x 1 = 16 fm3, and (iii) expanding cell. In the last case the cell volume follows the growth of the area with uniformly distributed energy. To check whether or not the system is equilibrated, its hadron yields and their energy spectra are compared with those of the statistical model of ideal hadron gas. For all cells and for all collision energies it was found that the matter in the cell was approaching the equilibrium state. The higher the collision energy, the shorter the time of equilibration. The equilibration phase lasts about 10 - 20 fm/c, after that the matter becomes very dilute and the thermal contact between hadrons is lost. Equation of state is well fitted to linear dependence P/ɛ = a = c2s , where the square of the sonic velocity c2s increases from 0.12 at Elab = 11.6AGeV to 0.145 at Elab = 160AGeV. The characteristic kinks observed in the T - μB phase diagrams are linked to inelastic freeze-out in the expanding fireball

Total and Partial Shear Viscosity in Heavy-Ion Collisions at Energies of BES, FAIR and NICA

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 Elab=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

On the origin of forward?backward multiplicity correlations in pp collisions at ultrarelativistic energies

We study multiplicity correlations of hadrons in forward and backward hemispheres in pp inelastic interactions at energies 200 GeV ≤ √s ≤ 13 TeV within the microscopic quark–gluon string model. The model correctly describes (i) the almost linear dependence of average multiplicity in one hemisphere on the particle multiplicity in other hemisphere in the centre-of-mass frame; (ii) the increase of the slope parameter bcorr with rising collision energy; (iii) the quick falloff of the correlation strength with increase of the midrapidity gap; (iv) saturation of the forward–backward correlations at very high multiplicities. Investigation of the sub-processes on partonic level reveals that these features can be attributed to shortrange partonic correlations within a single string and superposition of several sub-processes containing different numbers of soft and hard Pomerons with different mean multiplicities. If the number of Pomerons in the event is fixed, no forward–backward correlations are observed. Predictions are made for the top LHC energy √s = 13 TeV

Unruh effect in heavy ion collisions

We apply the idea of the Unruh effect to the description of particle production in relativistic heavy ion collisions. Because the full information about all particles is needed, we employ the spectra of hadrons generated by the UrQMD model for pp and Au+Au collisions at various energies. Particles are considered as generated straight at the Unruh horizon thus allowing estimation of the Unruh temperature of the source. The analysis is provided for different types of mesons and their charges

Unruh effect in heavy ion collisions

We apply the idea of the Unruh effect to the description of particle production in relativistic heavy ion collisions. Because the full information about all particles is needed, we employ the spectra of hadrons generated by the UrQMD model for pp and Au+Au collisions at various energies. Particles are considered as generated straight at the Unruh horizon thus allowing estimation of the Unruh temperature of the source. The analysis is provided for different types of mesons and their charges

Unruh Entropy of a Schwarzschild Black Hole

The entropy produced by Unruh radiation is estimated and compared to the entropy of a Schwarzschild black hole. We simulate a spherical system of mass M by a set of Unruh horizons and estimate the total entropy of the outgoing radiation. Dependence on the mass and spin of the emitted particles is taken into account. The obtained results can be easily extended to any other intrinsic degrees of freedom of outgoing particles. The ratio of Unruh entropy to the Schwarzschild black hole entropy is derived in exact analytical form. For large black holes, this ratio exhibits high susceptibility to quantum numbers, e.g., spin s, of emitted quanta and varies from 0% for s=0 to 19.0% for s=5/2

Calculation of shear viscosity in Au+Au collisions at NICA energies

Shear viscosity of hot and dense nuclear matter, produced in the central zone of central gold-gold collisions at energies of NICA, is calculated within the UrQMD model. Besides the microscopic simulations of heavy ion collisions, the procedure assumes the application of statistical model to determine the temperature and chemical potentials in the system, and study of the relaxation process within the UrQMD box with periodic boundary conditions. The latter is used for calculation of the correlator which enters the Green-Kubo formula for shear viscosity. The fluctuations at early and late stages of the system evolution are studied. Results are compared to predictions of other models