arXiv subjects
Publications and source records attributed to A. Gokalp.
As an extension of our previous work, the growth of density fluctuations in the spinodal region of charge asymmetric nuclear matter is investigated in the basis of the stochastic mean-field approach in the non-relativistic framework. A complete treatment of density correlation functions are presented by including collective modes and non-collective modes as well.
Nucleon exchange mechanism is investigated in central collisions of symmetric heavy-ions in the basis of the stochastic mean-field approach. Quantal diffusion coefficients for nucleon exchange are calculated by including non-Markovian effects and shell structure. Variances of fragment mass distributions are calculated in central collisions of ${}^{40}$Ca + ${}^{40}$Ca, ${}^{48}$Ca + ${}^{48}$Ca and ${}^{56}$Ni + ${}^{56}$Ni systems.
Early growth of density fluctuations of nuclear matter in spinodal region is investigated employing the stochastic mean-field approach. In contrast to the earlier treatments in which only collective modes were included in the calculations, in the present work non-collective modes are also included, thus providing a complete treatment of the density correlation functions. Calculations are carried out for symmetric matter in non-relativistic framework using a semi-classical approximation.
Spinodal instabilities and early growth of baryon density fluctuations in symmetric nuclear matter are investigated in the basis of stochastic extension of relativistic mean-field approach in the semi-classical approximation. Calculations are compared with the results of non-relativistic calculations based on Skyrme-type effective interactions under similar conditions. A qualitative difference appears in the unstable response of the system: the system exhibits most unstable behavior at higher baryon densities around $ρ_{b}=0.4 ~ρ_{0}$ in the relativistic approach while most unstable behavior occurs at lower baryon densities around $ρ_{b}=0.2 ~ρ_{0}$ in the non-relativistic calculations
Quantal effects on growth of spinodal instabilities in charge asymmetric nuclear matter are investigated in the framework of a stochastic mean field approach. Due to quantal effects, in both symmetric and asymmetric matter, dominant unstable modes shift towards longer wavelengths and modes with wave numbers larger than the Fermi momentum are strongly suppressed. As a result of quantum statistical effects, in particular at lower temperatures, magnitude of density fluctuations grows larger than those calculated in semi-classical approximation.
We study the radiative $ϕ\to K^0\bar{K}^0γ$ decay within a phenomenological framework by considering the contributions of the $f_{0}(980)$ and $a_{0}(980)$ scalar resonances. We calculate the branching ratio $B(ϕ\to K^0\bar{K}^0γ)$ by employing the coupling constants $g_{f_0K^{+}K^{-}}$ and $g_{a_0K^{+}K^{-}}$ as determined by different experimental groups.
In order to describe the fusion of two very heavy nuclei at near barrier energies, a generalized Langevin approach is proposed, which incorporates the quantum statistical fluctuations in accordance with the fluctuation and dissipation theorem. It is illustrated that the quantum statistical effects introduce an enhancement of the formation of compound nucleus, though the quantum enhancement is somewhat less pronounced as indicated in the previous calculations.
We investigate f_0-sigma mixing in the framework of light cone QCD sum rules and by employing the experimental results about the decay widths and the masses of these scalar mesons we estimate the scalar mixing angle by using sigma meson data and f_0 meson data. The two values we thus obtain of the scalar mixing angle are not entirely consistent with each other, possibly indicating that the structure of these mesons cannot be simple quark-antiquark states.
We investigate the $a_0K^+K^-$-vertex in the framework of light cone QCD sum rules. We estimate the coupling constant g$_{a_0K^+K^-}$ which is an essential ingredient in the analysis of physical processes involving $a_0(980)$ meson. Our result is somewhat larger than the previous determinations of this coupling constant.
We calculate the branching ratio of omega --> pi^+ pi^- gamma decay in a phenomenological framework in which the contributions of VMD, chiral loops, sigma-meson intermediate state amplitudes and the effects of omega-rho mixing are considered. We conclude that the sigma-meson intermediate state amplitude and omega-rho mixing make substantial contribution to the branching ratio.
In an attempt to explain the latest experimental result about the branching ratio of $ω\to π^0π^0γ$ decay we reexamine the decay mechanism of this decay in a phenomenological framework in which the contributions of VMD, chiral loops, $σ$-meson intermediate state amplitudes and the effects of $ω-ρ$ mixing are considered. We conclude that in order to obtain the experimental value of the branching ratio $B(ω\to π^0π^0γ)$ $σ$-meson amplitude which makes a substantial contribution should be included into the reaction mechanism and the effects of $ω-ρ$ mixing should be taken into account. We also estimate the coupling constant $g_{ωσγ}$ as $g_{ωσγ}=0.11$ which is much smaller than the values suggested by light cone QCD sum rules calculations.
We study the radiative $ρ^0\to π^+π^-γ$ and $ρ^0\to π^0π^0γ$ decays and we calculate their branching ratios using a phenomenological approach by adding to the amplitude calculated within the framework of chiral perturbation theory and vector meson dominance the amplitude of $σ$-meson intermediate state. Our results for the branching ratios are in good agreement with the experimental values.
We study the radiative decay $ϕ\toπ^{0}ηγ$ within the framework of a phenomenological approach in which the contributions of $ρ$-meson, chiral loop and $a_0$-meson are considered. We analyze the interference effects between different contributions and utilizing the experimental branching ratio and invariant $π^0η$ mass spectrum for $ϕ\toπ^{0}ηγ$ decay we estimate the branching ratio of $ϕ\to a_0γ$ decay.
The dipole response function of nuclear matter at zero and finite temperatures is investigated by employing the linearized version of the extended TDHF theory with a non-Markovian binary collision term. Calculations are carried out for nuclear dipole vibrations by employing the Steinwedel-Jensen model and compared with experimental results for $^{120}Sn$ and $^{208}Pb$.
We investigate the $ωσγ$-vertex and estimate the coupling constant g$_{ωσγ}$ in the framework of the light cone QCD sum rules. We compare our result with the values of this coupling constant deduced from a phenomenological analysis of $ω\toππγ$ decays.
We study $ϕa_0γ$- and $ϕσγ$-vertices in the framework of the light cone QCD sum rules and we estimate the coupling constants g$_{ϕa_0γ}$ and g$_{ϕσγ}$ utilizing $ωϕ$-mixing. We compare our results with the previous estimations of these coupling constants in the literature obtained from phenomenological considerations.
The $ω$-meson photoproduction, $γ+p\to p+ω$, is studied in the framework of a model, containing $π$-meson exchange in t-channel and nucleon-exchange in s- and u-channels. Considering both $ωNN$-coupling constants in the region of time-like meson four momenta as the free parameters, we find different sets of solutions for these constants from the existing data on the t-dependence of the differential cross sections, $dσ(γ+p\to p+ω)/dt$, in the near threshold region $E_γ\leq 2$ GeV. These sets of $ωNN$-coupling constants, corresponding to destructive and constructive $π\bigotimes N$-interference contributions to $dσ/dt$ can be well distinguished by measurements of beam asymmetry, induced by linear photon polarization.
We investigate the possibility that the process of $ρ^{0}$-meson photoproduction on proton, $γ+p\to p+ρ^{0}$, in the near threshold region $E_γ< 2$ GeV, can be considered in the framework of model with $π$-, $σ$- and N-exchanges. This suggestion is based on a study of the t-dependence of differential cross section, $dσ(γp \to p ρ^{0})/dt$, which has been measured by SAPHIR Collaboration. We find that the suggested model provides a good description of the experimental data with new values of $ρNN$-coupling constants in the region of the time-like $ρ^{0}$-meson momentum. Our results suggest that such model can be considered as a suitable nonresonant background mechanism for the future discussion of possible role of nucleon resonance contributions. Our predictions for $ρ^{0}$-meson photoproduction on neutron target and for beam asymmetry on both proton and neutron targets are presented.