On the early stage of nucleus-nucleus collisions

A statistical model of the early stage of central nucleus--nucleus (A+A) collisions is developed. We suggest a description of the confined state with several free parameters fitted to a compilation of A+A data at the AGS. For the deconfined state a simple Bag model equation of state is assumed. The model leads to the conclusion that a Quark Gluon Plasma is created in central nucleus--nucleus collisions at the SPS. This result is in quantitative agreement with existing SPS data on pion and strangeness production and gives a natural explanation for their scaling behaviour. The localization and the properties of the transition region are discussed. It is shown that the deconfinement transition can be detected by observation of the characteristic energy dependence of pion and strangeness multiplicities, and by an increase of the event--by--event fluctuations. An attempt to understand the data on J/psi production in Pb+Pb collisions at the SPS within the same approach is presented

Holographic superconductivity in the presence of dark matter: basic issues

The holographic approach to study strongly coupled superconductors in the presence of dark matter is reviewed. We discuss the influence of dark matter on the superconducting transition temperature of both s-wave and p-wave holographic superconductors. The upper critical field, coherence length, penetration depth of holographic superconductors as well as the metal-insulator transitions have also been analysed. Issues related to the validity of AdS/CFT correspondence for the description of superconductors studied in the laboratory and possible experiments directed towards the detection of dark matter are discussed. In doing so we shall compare our assumptions and assertions with those generally accepted in the elementary particle experiments aimed at the detection of dark matter particles.Comment: 5+ pages, 1 figure, National Conference on Superconductivity 2015, Karpacz, Polan

Charm estimate from the dilepton spectra in nuclear collisions

A validity of a recent estimate of an upper limit of charm production in central Pb+Pb collisions at 158 AGeV is critically discussed. Within a simple model we study properties of the background subtraction procedure used for an extraction of the charm signal from the analysis of dilepton spectra. We demonstrate that a production asymmetry between positively and negatively charged background muons and a large multiplicity of signal pairs leads to biased results. Therefore the applicability of this procedure for the analysis of nucleus-nucleus data should be reconsidered before final conclusions on the upper limit estimate of charm production could be drawn

Holographic calcualtion of the magneto-transport coefficients in Dirac semimetals

Based on the gauge/gravity correspondence we have calculated the thermoelectric kinetic and transport characteristics of the strongly interacting materials in the presence of perpendicular magnetic field. The 3+1 dimensional system with Dirac-like spectrum is considered as a strongly interacting one if it is close to the particle-hole symmetry point. Transport in such system has been modeled by the two interacting vector fields. In the holographic theory the momentum relaxation is caused by axion field and leads to finite values of the direct current transport coefficients. We have calculated conductivity tensor in the presence of mutually perpendicular electric and magnetic fields and temperature gradient. The geometry differs from that in which magnetic field lies in the same plane as an electric one and temperature gradient.Comment: 26 pages, 7 figure

Magnetotransport of Weyl semimetals with Z2\mathbb{Z}_2 topological charge and chiral anomaly

We calculate the magnetoconductivity of the Weyl semimetal with $\mathbb{Z}_2$ symmetry and chiral anomaly utilizing the recently developed hydrodynamic theory. The system in question will be influenced by magnetic fields connected with ordinary Maxwell and the second $U(1)$-gauge field, which is responsible for anomalous topological charge. The presence of chiral anomaly and $\mathbb{Z}_2$ anomalous charge endow the system with new transport coefficients. We start with the linear perturbations of the hydrodynamic equations and calculate the magnetoconductivity of this system. The holographic approach in the probe limit is implemented to obtain the explicit dependence of the longitudinal magnetoconductivities on the magnetic fields.Comment: 35 pages, 4 figures, LaTex, the title was changed, the version meets the printed one

Holographic vortices in the presence of dark matter sector

The {\it dark matter} seem to be an inevitable ingredient of the total matter configuration in the Universe and the knowledge how the {\it dark matter} affects the properties of superconductors is of vital importance for the experiments aimed at its direct detection. The homogeneous magnetic field acting perpendicularly to the surface of (2+1) dimensional s-wave holographic superconductor in the theory with {\it dark matter} sector has been modeled by the additional $U(1)$-gauge field representing dark matter and coupled to the Maxwell one. As expected the free energy for the vortex configuration turns out to be negative. Importantly its value is lower in the presence of {\it dark matter} sector. This feature can explain why in the Early Universe first the web of {\it dark matter} appeared and next on these gratings the ordinary matter forming cluster of galaxies has formed.Comment: 23 pages, JHEP-styl