Fluctuation study of pionisation in ultrarelativistic nucleus-nucleus interaction

The scaled factorial moments and the multifractal moments have been investigated in different??-intervals to study the dynamical fluctuation of pions produced in 200 AGeV³²S-Ag/Br interaction. In order to investigate the detail characteristics of intermittency behaviour, theF-moments are extracted up to the eighth order of moments in differentM-intervals. The analysis indicates a non-thermal phase transition and different regime of particle production during the hadronisation process.     

Baryon distributions in ultrarelativistic nucleus-nucleus collisions

We have measured distributions in transverse momentum and rapidity of protons from interactions of 14.6 GeV/nucleon²⁸Si projectiles with targets of Al and Pb. The transverse momentum spectra exhibit a thermal shape with a rapidity dependent temperature parameter. For very central or violent collisions the proton rapidity distributions exhibit the large rapidity shifts and (for Si+Al) a peak at midrapidity as required for full stopping.     

Λ0 polarization in ultrarelativistic nucleus-nucleus collisions

We study the polarization of Λ⁰’s produced in ultrarelativistic heavy-ion collisions. We argue that when the density of participants in the reaction is below the critical value for the production of a Quark-Gluon Plasma (QGP), recombination processes dominate the Λ⁰ production cross section and hence, Λ⁰’s are polarized. However, when the density of participants is above the critical value for QGP formation, coalescence processes dominate Λ⁰ production, giving rise to a reduction in the Λ⁰ polarization. For densities below the critical density for QGP production, we describe Λ⁰ polarization in terms of the DeGrand-Miettinen model and correct for the effects introduced by multiple scattering of the produced Λ⁰ within the nuclear environment.     

Properties of light mesons in ultrarelativistic nucleus-nucleus collisions

The PHENIX experiment at the Relativistic Heavy Ion Collider is designed for studying the new state of nuclear matter, which is presumably created in central ultrarelativistic collisions of heavy nuclei: the so-called quark-gluon plasma. Light mesons are an important tool in this research. This paper reviews the recent experimental results on the light meson invariant yields, nuclear modification factors, and the yield ratios for the vector and pseudoscalar mesons in the p + p, d + Au, and Au + Au collisions at the energy of √s _NN = 200 GeV.     

Mass dependence of hadron distributions in ultrarelativistic nucleus-nucleus collisions

(Anti-) baryon and kaon distributions in nucleus-nucleus reactions at 200 AGeV are studied in the framework of the transport model RQMD. Production mechanisms for strangeness and baryon pairs are tested by comparing their projectile and target mass dependence to available experimental data. RQMD contains two collective production processes, fusion of overlapping strings into highly charged chromoelectric ropes and hadronic rescattering. It turns out that both rope formation and hadronic rescattering are of importance for creation-and annihilation-of strangeness and antibaryons.     

Bose-einstein correlations of soft pions in ultrarelativistic nucleus-nucleus collisions

A detailed analysis of pair correlations of positive pions in the target fragmentation region is presented. Data on nuclear collisions at 200 GeV per nucleon were measured with the Plastic Ball in the WA80 experiment at the CERN SPS. The correlation functions are compared with analytical functions and with simulations incorporating Bose-Einstein symmetrization, final-state interactions and detector resolution. Source radii are shown to increase with increasing target size and with centrality. For central collisions the radii are larger than the geometrical sizes of the involved nuclei.     

Production and rescattering of heavy quarks in ultrarelativistic nucleus-nucleus collisions

Contributions of various mechanisms to the heavy-quark-production cross section in ultrarelativistic nucleus-nucleus collisions, including the direct pair production $b\bar b$ and $c\bar c$ as a result of hard primary collisions and pair production in showers induced by hard-produced partons in the initial and final states, are investigated. The sensitivity of the muon pair spectra with large invariant masses (from semileptonic $B\bar B$ decays) and spectra of the secondary J/ψ (from individual B decays) to the multiple scattering and the energy losses of b quarks in dense quark-gluon matter is studied. The formation of such matter is expected in heavy-ion collisions at the LHC.     

Kaon and pion spectra in relativistic and ultrarelativistic nucleus-nucleus collisions

The formation of kaon and pion spectra is considered within the relativistic hydrodynamic model of interaction of heavy nuclei. The terms describing the formation of quarks and gluons are included in the state equation of the hydrodynamic model. This approach allowed consideration of nucleus-nucleus collisions in the range from 4 to 160 GeV/nucleon and reproduce the increase in temperature of the kaons formed, which is not reproduced within the ultrarelativistic quantum molecular-dynamics model and other microscopic approaches.     

Diabatic interaction potential for nucleus-nucleus collisions

Within a refined method for the construction of diabatic states allowing for the treatment of the full spin-orbit coupling, characteristic features of the diabatic potential for nucleus-nucleus collisions are investigated. Approximately 90% of the strong repulsion results from diabatic particle-hole excitations, while only 10% is due to compression. The diabatic interaction potential describes a physical situation intermediate between adiabatic and sudden approximations.     

The real part of the nucleus-nucleus interaction

The real part of the interaction potential for several pairs of magic nuclei has been derived from the Skyrme interaction density functional. The matter density of each nucleus is described by a Fermi distribution adjusted to reproduce the Skyrme-Hartree-Fock densities. Exchange effects due to antisymmetrization are taken into account in an approximate way. The tail of the resulting potential can be accurately approximated with a Woods-Saxon shape beyond the inflexion point of the calculated potential. The parameters of these Woods-Saxon potentials show regularities with respect to the masses of the target and projectile. We have tested the validity of the real part of the potential against elastic scattering data by choosing an imaginary part with the same geometry and a variable strength. For the energy range we consider the calculated grazing angles are somewhat larger than the experimental ones.     

On the observability of direct photons in ultrarelativistic nucleus-nucleus collisions by means of inclusive photon and hadron spectra

A method is presented to search for a direct photon signal in collisions between nuclei at ultrarelativistic energies. In the proposed method the analysis is performed in the center of momentum system of the radiating system, being the cms of the colliding nucleons in case of collisions of symmetric systems. This results in a maximal strength of a possible direct photon signal in the observed energy spectra, since the smearing of the signal due to the Lorentz transformation between the cms and the laboratory frame in case of fixed target experiments is absent. The feasibility of detecting a direct photon signal is examined by comparison of calculated photon and hadron spectra for central and peripheral ³²S on S collisions at a nucleon-nucleon cms energy of about 20 GeV, being the energy domain of the CERN-SPS fixed target experiments.     

Conditions for the appearance of gravitational ultrarelativistic spin-orbital interaction

A qualitative analysis of the energy and momentum integrals for the Mathisson-Papapetrou equations in a Schwarzschild field is employed to determine the conditions under which all solutions of the exact Mathisson-Papapetrou equations corresponding to fixed initial coordinate and velocity values differ significantly from the solution of the abbreviated equations normally used for the same initial data. Numerical computer calculations provide additional indication of a difference in the world lines of the exact Mathisson-Papapetrou equations from solutions of the geodesic equations at ultrarelativistic velocity values. For high energy particles entering into the composition of cosmic rays one can expect appearance of gravitational ultrarelativistic spin-orbital interaction upon motion of such particles in the field of a neutron star.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 8–12, October, 1985.     

Coherent final state interaction in jet production in nucleus-nucleus collisions

The coherent final state interaction of an energetic parton produced in AA collisions is studied. This interaction is due to the change in the cutoff scale and in the running coupling constant when the parton passes from a vacuum to a quark-gluon plasma. It is demonstrated that the contribution of this new mechanism to the energy loss may be of the same order of magnitude as the induced gluon radiation. However, an accurate evaluation of this medium effect is a difficult task, because there is a strong cancellation between the cutoff and running coupling constant effects. The uncertainties in the contribution of the coherent final state interaction restrict strongly the accuracy of jet tomographic analyses of the matter density produced in AA reactions.     

Fluctuation interaction of Ising subsystems

A phase diagram of two Ising subsystems σ and s has been constructed on a Bethe lattice with a coordination number 4 (a simplified analog of a square lattice). In contrast to the known Ashkin-Teller model, the interaction between these two subsystems is of a purely fluctuational nature; i.e., it does not manifest itself in the ground state and nullifies the sums of the products of average spins 〈σ〉〈s〉 (interactions of this type are realized in lattice-type adsorbed systems with dipolelike intermolecular interactions and strong azimuthal angular dependence of the adsorption potential of symmetry C₄). Apart from conventional states, i.e., a high-temperature disordered state (〈σ〉=〈s〉=0) and a low-temperature ordered state (〈σ〉 and 〈s〉 =? 0), this system can also exist in a correlated state (〈σs〉 =? 0 at 〈σ〉=〈s〉=0). In the theory of orientational phase transitions, this state corresponds to a fundamentally different, intermediate (on the temperature axis) phase in which a preferred direction of long molecule axes arises in the absence of spontaneous polarization. The results of Monte Carlo simulation on a square lattice agree with the conclusions obtained on a Bethe lattice. The characteristics of the orientational phase transition in a 2 × 1 monolayer of CO molecules adsorbed on the NaCl(100) surface are discussed.     

Isotopic and neutron-excess effects in nucleus-nucleus interaction and fusion cross sections

The interaction potentials and fusion cross sections for various nickel and tin isotopes are calculated. The fusion cross sections are computed with allowance for coupling to the channels of low-lying 2⁺ and 3^? vibrational states in the interacting nuclei. Good agreement with the experimental data is obtained. The isotopic dependences of interaction potentials and fusion cross sections of nuclei are analyzed in detail.     

Nuclear rainbow in scattering and reactions and nucleus-nucleus interaction at small distances

A review of recent experiments on the study of the nuclear rainbow phenomenon in scattering and some reactions induced by light heavy ions is given. It includes (i) the study of the differential cross sections of the ¹⁶O + ¹²C elastic scattering at seven ¹⁶O energies between 130 and 281 MeV; (ii) finding of the phenomenological potential deeper than that of folding model; (iii) the first data on ¹⁶O + ¹⁴C elastic scattering; (iv) dispersion relation analysis of the obtained data and observation of abnormal nuclear dispersion; and (v) use of the charge-exchange ¹⁴C(⁶Li,⁶He)¹⁴N reaction to search for pion-condensation effects.     

Effect of the Pauli exclusion principle on the potential of nucleus-nucleus interaction

The dependence of the potentials of nucleus-nucleus interaction on taking into account the antisymmetrization of nucleons and the contribution of the nucleon kinetic energy to the potential is studied within approaches based on the energy-density functional, double-folding model, and the two-center shell model. It is shown that the contribution of the nucleon kinetic energy in colliding nuclei leads to the appearance of a significant core at short distances between the nuclei involved.