Function In Nuclear Matter Research Articles

Transverse isospin response function of asymmetric nuclear matter from a local isospin density functional

The time dependent local isospin density approximation (TDLIDA) has been extended to the study of the transverse isospin response function in nuclear matter with an arbitrary neutron-proton asymmetry parameter $\ensuremath{\xi}$. The energy density functional has been chosen in order to fit existing accurate quantum Monte Carlo calculations with a density dependent potential. The evolution of the response with $\ensuremath{\xi}$ in the $\mathrm{\ensuremath{\Delta}}{T}_{z}=\ifmmode\pm\else\textpm\fi{}1$ channels is quite different. While the strength of the $\mathrm{\ensuremath{\Delta}}{T}_{z}=+1$ channel disappears rather quickly by increasing the asymmetry, the $\mathrm{\ensuremath{\Delta}}{T}_{z}=\ensuremath{-}1$ channel develops a stronger and stronger collective mode that in the regime typical of neutron star matter at $\ensuremath{\beta}$ equilibrium almost completely exhausts the excitation spectrum of the system. The neutrino mean free paths obtained from the TDLIDA responses are strongly dependent on $\ensuremath{\xi}$ and on the presence of collective modes, leading to a sizable difference with respect to the prediction of the Fermi gas model.

Three-body force effect on off-shell mass operator and spectral functions in nuclear matter

Within the framework of the Brueckner theory, the off-shell behaviors of the mass operator $M(k,\omega)=V(k,\omega)+iW(k,\omega)$, i.e., its dependence upon the momentum $k$ and upon the nucleon frequency $\omega$, are investigated by including nuclear three-body force (TBF). The first two terms of the hole-line expansion of the mass operator are taken into account. The TBF effects on their off-shell properties are discussed. A comparison is made between the on-shell and off-shell values of $M_{1}$. The nucleon spectral function and nucleon momentum distribution are also calculated, and the calculation shows that they are hardly affected by the TBF effect at the saturation density. At a high density two times greater than the saturation density, inclusion of the TBF may lead to a visible effect on the spectral function and may enhance the depletion of the hole states.

A New Expression for Radial Distribution Function of Nuclear Matter

A Variational Monte Carlo Method (VMC) is used for the calculations of radial distribution function of nuclear matter. Urbana 14 v potential is used for the nucleon-nucleon interactions in the calculations. The new expression for radial distribution function of nuclear matter is obtained by fitting of Monte Carlo simulations results to a function.

Formation spectra of light kaonic nuclei by in-flight(K¯,N)reactions with a chiral unitary amplitude

We study theoretically the in-flight $({K}^{\ensuremath{-}},N)$ reactions for the formation of light kaonic nuclear systems to get deeper physical insights on the spectra and to investigate the formation spectra of the reaction that will be observed at new facilities like the Japan Proton Accelerator Research Complex (J-PARC). We show the expected spectra for the formation of the ${K}^{\ensuremath{-}}\mathit{pp}$, ${K}^{\ensuremath{-}}\mathit{pn}$, ${K}^{\ensuremath{-}}\mathit{nn}$, and ${K}^{\ensuremath{-}}$-$^{11}\mathrm{B}$ systems that are accessible by the $({K}^{\ensuremath{-}},N)$ experiments. By considering the conversion part of the Green's function, we show the missing mass spectra of the $({K}^{\ensuremath{-}},N)$ reactions in coincident with the particle emissions due to $\overline{K}$ absorption. To calculate the cross sections, we use the so-called $T\ensuremath{\rho}$ approximation to evaluate the optical potential. As for the amplitude $T$, we adopt the chiral unitary amplitude of $\overline{K}N$ channel in vacuum for simplicity. The effects of the $p$-wave optical potential of $\ensuremath{\Sigma}(1385)$ channel and the contributions from ${\overline{K}}^{0}$ mixing in $^{3}\mathrm{He}$$({K}^{\ensuremath{-}},n)$ reaction are also evaluated numerically. We also study the behavior of the poles of kaon Green's function in nuclear matter. We conclude that $^{3}\mathrm{He}$$({K}^{\ensuremath{-}},n)$ and $^{3}\mathrm{He}$$({K}^{\ensuremath{-}},p)$ reaction spectra in coincident with the $\ensuremath{\pi}\ensuremath{\Sigma}$ emission may show the structure in the kaon bound region indicating the existence of the unstable kaonic nuclear states. As for the $^{12}\mathrm{C}$$({K}^{\ensuremath{-}},p)$ spectra with the $\ensuremath{\pi}\ensuremath{\Sigma}$ emission, we may also observe the structure in the bound region, however, we need to evaluate the medium effects carefully for larger nuclei.

Isospin dependent nucleon-nucleus optical potential with Skyrme interactions

In this paper, the isospin dependent nucleon-nucleus optical potential theory is introduced on the basis of the effective Skyrme interactions. From the view of the many-body theory, the nucleon optical potential can be identified with the mass operator of the one-particle Green function. The first and second order mass operators of the one particle Green function in nuclear matter are derived, and the real and imaginary parts of the optical potential for finite nuclei are obtained as usual by applying a local density approximation. Our results, in most cases, can be derived and expressed analytically. From an extensive comparison with experimental data of various quantities of interests, it is concluded that for certain versions of Skyrme interactions without parameter adjusting the agreement between theory and experiments can be obtained satisfactorily. We define the ratio of the difference and summation of the neutron and proton nonelastic cross sections as the isospin effect value. The calculated results show that the isopin effect value decreases as the incident energy increases and increases as the asymmetric parameter ${\ensuremath{\alpha}}_{0}=(N\text{\ensuremath{-}}Z)/A$ of the target nucleus increases.

Variational calculations of nuclei with low-momentum potentials

Variational calculations of the deuteron and the triton illustrate that simple wave function ansätze become more effective after evolving the nucleon–nucleon potential to lower momentum (“Vlowk”). This is consistent with many-body wave functions becoming much less correlated at lower cutoffs, as shown by two-particle wave functions and pair-distribution functions in nuclear matter. These results motivate a program to explore variational many-body calculations of binding energies and other low-energy nuclear properties using low-momentum potentials.

Two-body correlation functions in nuclear matter with neutron-proton condensate

The density, spin, and isospin correlation functions in nuclear matter with a neutron-proton condensate are calculated to study the possible signatures of the BEC-BCS crossover in the low-density region. It is shown that the criterion of the crossover (Phys. Rev. Lett. 95, 090402 (2005)), consisting in the change of the sign of the density correlation function at low momentum transfer, fails to describe correctly the density-driven BEC-BCS transition at finite isospin asymmetry or finite temperature. The presence (BCS regime) or absence (BEC regime) of the singularity in the momentum distribution of the quasiparticle density of states can be used as an unambiguous signature of the BEC-BCS transition.

Spin-Dependent Structure Functions in Nuclear Matter and the Polarized EMC Effect

An excellent description of both spin-independent and spin-dependent quark distributions and structure functions has been obtained with a modified Nambu--Jona-Lasinio model, which is free of unphysical thresholds for nucleon decay into quarks--hence incorporating an important aspect of confinement. We utilize this model to investigate nuclear medium modifications to structure functions and find that we are readily able to reproduce both nuclear matter saturation and the experimental F2N(A)/F2N ratio, that is, the European Muon Collaboration (EMC) effect. Applying this framework to determine g1p(A), we find that the ratio g1p(A)/g1p differs significantly from unity, with the quenching caused by the nuclear medium being about twice that of the spin-independent case. This represents an exciting result, which, if confirmed experimentally, will reveal much about the quark structure of nuclear matter.

Dynamical response functions in correlated fermionic systems

Response functions in nuclear matter at finite temperature are considered beyond the usual Hartree–Fock plus random phase approximation (RPA) scheme. The contributions due to the propagator for the dressed nucleons and the corresponding vertex corrections are treated in a consistent way. For that purpose a semi-realistic Hamiltonian is developed with parameters adjusted to reproduce the nucleon self-energy as derived from realistic nucleon–nucleon interactions. For a scalar residual interaction the resulting response functions are very close to the RPA response functions. However, the collective modes, if present, get an additional width due to the coupling to multi-pair configurations. For isospin-dependent residual interactions we find strong modifications of isospin response functions due to multi-pair contributions in the response function. Such a modification can lead to the disappearance of collective spin or isospin modes in a correlated system and shall have an effect on the absorption rate of neutrinos in nuclear matter.

Spectral function at high missing energies and momenta

The nuclear spectral function at high missing energies and momenta has been determined from a self-consistent calculation of the Green's function in nuclear matter using realistic nucleon-nucleon interactions. The results are compared with recent experimental data derived from ($e,e'p$) reactions on $^{12}C$. A rather good agreement is obtained if the Green's functions are calculated in a non-perturbative way.

Hadronic spectral functions in nuclear matter

We study the in-medium properties of mesons ( π, η, ρ) and baryon resonances in cold nuclear matter within a coupled-channel analysis. The meson self energies are generated by particle–hole excitations. Thus multi-peak spectra are obtained for the mesonic spectral functions. In turn this leads to medium-modifications of the baryon resonances. Special care is taken to respect the analyticity of the spectral functions and to take into account effects from short-range correlations both for positive and negative parity states. Our model produces sensible results for pion and Δ dynamics in nuclear matter. We find a strong interplay of the ρ meson and the D 13(1520), which moves spectral strength of the ρ spectrum to smaller invariant masses and leads to a broadening of the baryon resonance. The optical potential for the η meson resulting from our model is rather attractive whereas the in-medium properties modifications of the S 11(1535) are found to be quite small.

Sum rules and short-range correlations in nuclear matter at finite temperature

The nucleon spectral function in nuclear matter fulfills an energy weighted sum rule. Comparing two different realistic potentials, these sum rules are studied for Green's functions that are derived self-consistently within the $T$ matrix approximation at finite temperature.

THE DIELECTRIC FUNCTION EXCITED BY ρNN TENSOR COUPLING IN NUCLEAR MATTER

The dielectric function of nuclear matter excited by ρNN tensor coupling has been studied in the framework of finite temperature field theory. The induced current mechanism has been introduced to explain the three extrema on the dielectric function curve, of which one is in the space-like region and the other two are in the time-like region. It points out that the tensor coupling contributes much more large amplitude than the vector coupling and plays a more important role on the time-like region compared with its effect on the space-like region.

Vector Mesons in Nuclear Matter

We present a relativistic and unitary approach to pion- and photonnucleon scattering taking into account the πN, ρN, щN, хN, πΔ, KΛ and KΣ channels. Our scheme dynamically generates the s- and d-wave baryon resonances N (1535), N (1650), N (1520) and N (1700) and as well as Δ (1620) and Δ (1700) in terms of quasi-local two-body interaction terms. We obtain a fair description of the experimental data relevant for the properties of slow vector mesons in nuclear matter. The resulting s-wave ρ- and ω-meson nucleon scattering amplitudes, which define the leading density modification of the ρ-and ω-meson spectral functions in nuclear matter, are presented.

Dielectric function excited byρmeson polarization in nuclear matter

The dielectric function of nuclear matter has been studied in the framework of finite temperature field theory based on the quantum hadrodynamics II model. We find that it has two extrema and one singularity in terms of ${K}_{0}∕k$. One of the extrema reflects Landau damping and the other is related to the resonance phenomenon.

From Meson– and Photon–Nucleon Scattering to Vector Mesons in Nuclear Matter

We present a relativistic and unitary approach to pion- and photon-nucleon scattering taking into account the $\pi N$, $\rho N$, $\omega N$, $\eta N$, $\pi\Delta$, $K \Lambda$ and $K \Sigma$ channels. Our scheme dynamically generates the s- and d-wave nucleon resonances N(1535), N(1650), N(1520) and N(1700) and isobar resonances $\Delta(1620)$ and $\Delta(1700)$ in terms of quasi-local two-body interaction terms. We obtain a fair description of the experimental data relevant for slow vector-meson propagation in nuclear matter. The s-wave $\rho $- and $\omega $-meson nucleon scattering amplitudes, which define the leading density modification of the $\rho$- and $\omega $-meson spectral functions in nuclear matter, are predicted.

From Meson- and Photon-Nucleon Scattering to Vector Mesons in Nuclear Matter

We present a relativistic and unitary approach to pion- and photon-nucleon scattering taking into account the πN, ρN, ωN, ηN, πΔ, KΛ and KΣ channels. Our scheme dynamically generates the s- and d-wave baryon resonances N(1535), N(1650), N(1520) and N(1700) and as well as Δ(1620) and Δ(1700) in terms of quasi-local two-body interaction terms. We obtain a fair description of the experimental data relevant to the properties of slow vector mesons in nuclear matter. The resulting s-wave ρ- and ω-meson-nucleon scattering amplitudes which define the leading density modification of the ρ- and ω-meson spectral functions in nuclear matter, are presented.

Quark distribution functions in nuclear matter

The convolution formalism is applied to calculate thedistribution function for valence quarks in the nuclearmedium. A manifestly covariant expression for the quarkdistribution function is obtained and applied to studythe case of nuclear matter. A covariant and chiralinvariant model of the nucleon is used to producenumerical results.

Two-nucleon spectral function in infinite nuclear matter

The two-nucleon spectral function in nuclear matter is studied using Correlated Basis Function perturbation theory, including central and tensor correlations produceded by a realistic hamiltonian. The factorization property of the two-nucleon momentum distribution into the product of the two single nucleon distributions shows up in an analogous property of the spectral function. The correlated model yields a two-hole contribution quenched whith respect to Fermi gas model, while the peaks acquire a quasiparticle width that vanishes as the two momenta approach $k_F$. In addition, three-hole one-particle and more complicated intermediate states give rise to a background, spread out in energy and absent in the uncorrelated models. The possible connections with one- and two-nucleon emission processes are briefly discussed.

Transport theoretical approach to the nucleon spectral function in nuclear matter

The nucleon spectral function in infinite nuclear matter is calculated in a quantum transport theoretical approach. Exploiting the known relation between collision rates and correlation functions the spectral function is derived self-consistently. By re-inserting the spectral functions into the collision integrals the description of hard processes from the high-momentum components of wave functions and interactions is improved iteratively until convergence is achieved. The momentum and energy distributions and the nuclear matter occupation probabilities are in very good agreement with the results obtained from many-body theory.