Narrow Results

Selecting the optimal topology of a neural network for a particular application is a difficult task. Genetic Algorithm (GA) has been used to find the optimal neural network (NN) solution (i.e., hybrid technique) to calculate the pseudo-rapidity distribution of the shower particles for C¹², O¹⁶, Si²⁸, and S³² on nuclear emulsion. An efficient NN has been designed by GA to predict the distributions that are not present in the training set and matched them effectively. The proposed method shows a better fitting with experimental data. The hybrid technique GA–ANN simulation results prove a strong presence modeling in heavy ion collisions.

High Energy Physics (HEP), due to the vast and complex data expected from current and future experiments, is in need of powerful and efficient techniques for various analysis tasks. Genetic Programing (GP) is a powerful technique that can be used such complex tasks. In this paper, Genetic programing is used for modeling the functions that describe the pseudo-rapidity distribution of the shower particles for ¹²C, ¹⁶O, ²⁸Si and ³²S on nuclear emulsion and also to predict the distributions that are not present in the training set and matched them effectively. The proposed method shows a better fitting with experimental data. The GP prediction results prove a strong presence modeling in heavy ion collisions.

The contributions of δ-isovector-scalar and ρ-isovector-vector meson-exchanges to nucleon–nucleon elastic cross-sections (ECS) are studied based on QHD-type effective Lagrangian within the framework of the RBUU transport theory. The detailed expressions of ECS for symmetric nuclear system have also obtained. The medium correction of ρ-meson mass and a proper symmetry energy coefficient are selected in order to fix a set of suitable coupling constants for δ and ρ exchanges with the constraint of free ECS as a function of energy. The respective δ and ρ contributions to free ECS as a function of energy are also shown and it is found they both affect the neutron–proton and neutron–neutron (proton–proton) ECS on the contrary way.

In this brief review, we summarize the new developments on the description of gluon radiation by energetic quarks traversing a medium as well as the observable consequences in high-energy heavy-ion collisions. Information about the initial state is essential for a reliable interpretation of the experimental results and will also be reviewed. Comparison with experimental data from RHIC and expectation for the future LHC will be given.

Large elliptic flow at RHIC seems to indicate that ideal hydrodynamics provides a good description of Au–Au collisions, at least at the maximum RHIC energy. The medium formed has been interpreted as a nearly perfect (low-viscosity) liquid, and connections have been made to gravitation through string theory. Recently, claimed observations of large flow fluctuations comparable to participant eccentricity fluctuations seem to confirm the ideal hydro scenario. However, determination of the azimuth quadrupole with 2D angular autocorrelations, which accurately distinguish "flow" (quadrupole) from "nonflow" (minijets), contradicts conventional interpretations. Centrality trends may depend only on the initial parton geometry, and methods used to isolate flow fluctuations are sensitive instead mainly to minijet correlations. The results presented in this paper suggest that the azimuth quadrupole may be a manifestation of gluonic multipole radiation.

We discuss energy–momentum tensor and the second law of thermodynamics for a system of relativistic diffusing particles. We calculate the energy and entropy flow in this system. We obtain an exact time-dependence of energy, entropy and free energy of a beam of photons in a reservoir of a fixed temperature.

A mean-field potential version of the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model is used to investigate the production of strange (anti-)baryons, especially the Λs and s, from heavy ion collisions at SPS energies. It is found that, with the consideration of both formed and pre-formed hadron potentials in UrQMD, the transverse mass and longitudinal rapidity distributions of experimental data of both Λs and s can be quantitatively explained fairly well. Our investigation also shows the equal importance of both the production mechanism and the rescattering process of hadrons for the final yield of strange baryons.

We carry out hydrodynamical simulation of the evolution of fluid in relativistic heavy-ion collisions with random initial fluctuations. The time evolution of power spectrum of momentum anisotropies shows very strong correspondence with the physics of cosmic microwave anisotropies as was earlier predicted by us. In particular, our results demonstrate suppression of superhorizon fluctuations and the correspondence between the location of the first peak in the power spectrum of momentum anisotropies and the length scale of fluctuations and expected freeze-out time-scale (more precisely, the sound horizon size at freeze-out).

The difference in elliptic flow between protons and antiprotons, produced in ${}^{197}\mathrm{\text{Au}}+{}^{197}\mathrm{\text{Au}}$ collisions at center-of-mass energies $\sqrt{s_{NN}}=5\text{–}12\mathrm{\text{GeV}}$, is studied within a modified version of the ultra-relativistic quantum molecular dynamics (UrQMD) model. Two different model scenarios are compared: the cascade mode and the mean field mode which includes potential interactions for both formed and pre-formed hadrons. The model results for the elliptic flow of protons and the relative elliptic flow difference between protons and antiprotons obtained from the mean field mode agree with the available experimental data, while the elliptic flow difference is near zero for the cascade mode. Our results show that the elliptic flow splitting, observed for particles and antiparticles, can be explained by the inclusion of proper hadronic interactions. In addition, the difference in elliptic flow between protons and antiprotons depends on the centrality and the rapidity window. With smaller centrality and/or rapidity acceptance, the observed elliptic flow splitting is more sensitive to the beam energy, indicating a strong net baryon density dependence of the effect. We propose to confirm this splitting at the upcoming experiments from Beam Energy Scan (BES) Phase-II at Relativistic Heavy Ion Collider (RHIC), the Compressed Baryonic Matter (CBM) at Facility for Antiproton and Ion Research (FAIR), High Intensity heavy ion Accelerator Facility (HIAF) and Nuclotron-based Ion Collider fAcility (NICA).

The strong electromagnetic fields in peripheral heavy ion collisions give rise to photon–photon and photon–nucleus interactions. I present a general survey of the photon–photon and photon-hadron physics accessible in these collisions. Among these processes I discuss the nuclear fragmentation through the excitation of giant resonances, the Coulomb dissociation method for application in nuclear astrophysics, and the production of particles.

In heavy nuclei the spatial distribution of protons and neutrons is different. At CERN SPS energies production of π⁺ and π^- differs for pp, pn, np and nn scattering. These two facts lead to an impact parameter dependence of the π⁺ to π^- ratio in ²⁰⁸Pb+²⁰⁸Pb collisions. A recent experiment at CERN seems to confirm qualitatively these predictions. It may open a possibility for determination of neutron density distribution in nuclei.

We present results on pentaquark searches from nuclear collisions at RHIC with the STAR detector system. An intriguing peak has been observed in the invariant mass distribution of from 18.6 Million d+Au collision events at . The peak centers at a mass 1528 ± 2 ± 5 MeV/c² and the FWHM ~15 MeV/c² limited by detector responses. The statistical significance of the peak is 4.2σ. Such a state if confirmed is manifestly exotic and implies a family of isospin one states. A weak signal of less statistical significance (~3σ) has been observed in 5.6M Au+Au collision events at 62.4 GeV. Searches in 10.7M Au+Au collision events at 200 GeV yield no significant signal. The Au+Au results neither confirm nor rule out the d+Au observation as a possible state.

The High Acceptance Di-Electron Spectrometer HADES¹ has been constructed at the SIS accelerator (GSI Darmstadt) to investigate electron-positron pairs produced in proton, pion and heavy ion induced reactions. The main goal of these studies is to explore properties of hadrons in nuclear matter. The apparatus and the experimental results from C+C at 2.0 AGeV and 1.0 AGeV and p+p at 2.2 GeV compared with Monte-Carlo events from a generator based on known cross-sections and branching ratios are presented.

The production of e⁺e^- - pairs has been measured for C + C collisions at a beam energy of 1 AGeV. Preliminary invariant pair mass spectra are constructed utilizing a probabilistic analysis method for the identification of single e⁺/e^- tracks. The results are compared to predictions based on sources with known production cross sections, and to earlier measurements.

The production of the sigma (σ) meson is investigated in the quantum molecular dynamics model for the p+A reactions with the nuclei A being ¹²C, ⁴⁰Ca and ²⁰⁸Pb at the incident proton energies E_p = 1.50 GeV. The simulation results indicates a distinctive A dependence of the sigma production, that is, the increase of A is followed by an increase of the production cross-sections. The σ meson production in proton induced reactions is strongly medium-dependent, and the produced σ mesons decaying in a denser medium experience a stronger mass shift towards lower masses. This mass shift is an experimentally accessible observable in the final state pion pairs which did not suffer reabsorption by the surrounding nucleons. It is pointed out that the ratio of the simulation results of measurable σ mesons (in the form of two pions) cross-sections as a function of the sigma invariant-mass from various reactions opens the possibility to address experimentally the mass shift of the σ in a dense nuclear environment.

The HADES collaboration studied dielectron production in C+C, p+p, and d+p reactions, with the main goal to investigate properties of vectors mesons through their dielectron decay. Production of e⁺e^- pairs in Ar+KCl collisions at a beam energy of 1.756 A GeV was measured recently by the collaboration and preliminary results of the experimental data analysis will be reported. Pair spectra will be compared with a prediction of a thermal model based on the Monte Carlo event generator Pluto.

Results from the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) in nucleus–nucleus and proton–proton collisions at c.m. energy are presented in the context of the methods of single and two-particle inclusive reactions which were used in the discovery of hard-scattering in p–p collisions at the CERN ISR in the 1970's. These techniques are used at RHIC in A + A collisions because of the huge combinatoric background from the large particle multiplicity. Topics include J/Ψ suppression, jet quenching in the dense medium (sQGP) as observed with π0 at large transverse momentum, thermal photons, collective flow, two-particle correlations, suppression of heavy quarks at large p_T and its possible relation to Higgs searches at the LHC. The differences and similarities of the measurements in p–p and A + A collisions are presented. The two discussion sessions which followed the lectures on which this article is based are included at the end.

We review the color glass condensate effective theory, that describes the gluon content of a high energy hadron or nucleus, in the saturation regime. The emphasis is put on applications to high energy heavy ion collisions. After describing initial state factorization, we discuss the glasma phase, that precedes the formation of an equilibrated quark–gluon plasma. We end this review with a presentation of recent developments in the study of the isotropization and thermalization of the quark–gluon plasma.

We present a brief review of recent theoretical developments and related phenomenological approaches for understanding the initial state of heavy ion collisions, with emphasis on the Color Glass Condensate formalism.

In this paper, we review recent progress toward understanding the nature of quarkonia in the quark gluon plasma. We review the theory necessary to understand the melting of bound states due to color-screening, including lattice results for the heavy quark potential, lattice results on the correlation functions related to the relevant spectral functions, and the emergence of a complex-valued potential in high-temperature quantum chromodynamics. We close with a brief survey of phenomenological models of quarkonium suppression in relativistic heavy ion collisions.