Meson-nucleon Coupling Research Articles

RECENT PROGRESS IN RELATIVISTIC MANY-BODY APPROACH

The recent progress of the relativistic many-body approach by the group at Peking University will be reviewed. In particular, the adiabatic and configuration-fixed constrained triaxial RMF approaches, triaxial RMF approach with time-odd components, a Shell-model-Like APproach (SLAP), a Reflection ASymmetric RMF (RAS-RMF) approach, and a new relativistic Hartree-Fock (RHF) approach with density-dependent σ, ω, ρ and π meson-nucleon couplings for finite nuclei and nuclear matter, will be highlighted.

Density-dependent relativistic Hartree–Fock approach

A new relativistic Hartree-Fock approach with density-dependent $\sigma$, $\omega$, $\rho$ and $\pi$ meson-nucleon couplings for finite nuclei and nuclear matter is presented. Good description for finite nuclei and nuclear matter is achieved with a number of adjustable parameters comparable to that of the relativistic mean field approach. With the Fock terms, the contribution of the $\pi$-meson is included and the description for the nucleon effective mass and its isospin and energy dependence is improved.

Parity nonconservation in elasticp→pscattering

By looking at the parity-nonconserving (PNC) asymmetries for different energies in (p) over right arrowp scattering, it is in principle possible to determine the PNC rho NN and omega NN couplings of a single-meson-exchange model of the PNC NN force. Analysis of the experimental data at 13.6, 45, and 221 MeV was performed by Carlson , [Phys. Rev. C 65, 035502 (2002)] who concluded the data were in agreement with the uncertainties accorded the original DDH estimates for the PNC meson-nucleon couplings. In this work it is shown first that a comparison with updated hadronic predictions of these couplings suggests the existence of some discrepancy for the PNC omega NN coupling. The effect of varying the strong coupling constants and introducing cutoffs in the one-boson-exchange weak potential is then investigated. As expected, the resulting asymmetry is quite sensitive to these parameters regardless of the energy. However, the above mentioned discrepancy persists. The dependence of this conclusion on various ingredients entering an improved description of the PNC NN force is also examined. Additional mechanisms include the two-pion resonance nature of the rho meson and some momentum dependence of the isoscalar PNC rho NN vertex. None of these corrections removes or even alleviates the above discrepancy. Their impact on the theoretical determination of the vector meson-nucleon couplings, the description of the PNC force in terms of single-meson exchange, and the interpretation of measurements are examined.

Nucleon–meson coupling constants and form factors in the quark model

Abstract We demonstrate the calculation of the coupling constants and form factors required by effective hadron Lagrangians using the quark model. These relations follow from equating expressions for strong transition amplitudes in the two approaches. As examples we derive the NNm nucleon–meson coupling constants and form factors for m = π , η , η ′ , σ , a 0 , ω and ρ, using harmonic oscillator quark model meson and baryon wavefunctions and the 3P0 decay model; these results are relevant to quark-based descriptions of the NN force. This technique should be useful in the application of effective Lagrangians to processes in which the lack of data precludes the direct determination of coupling constants and form factors from experiment.

EFFECTIVE MESON MASSES, EFFECTIVE MESON–NUCLEON COUPLINGS AND NEUTRON STAR RADII

Using a generalized mean field theory, we have studied relations among effective meson masses, effective meson–nucleon couplings and equations of state (EOS) in asymmetric nuclear matter. If the effective ω-meson mass becomes smaller at high density, the EOS becomes stiffer. However, if we require that the ω-meson mean field is proportional to baryon density, the effective ω–nucleon coupling also becomes smaller at the same time as the effective ω-meson mass becomes smaller. Consequently, the EOS becomes softer. A similar relation is found for the effective ρ-meson mass and the effective ρ–nucleon coupling. We have also studied relations among effective meson masses, effective meson–nucleon couplings and a radius R of a neutron star. R depends somewhat on the value of the effective ω-meson mass and the effective ω–nucleon coupling.

Relativistic model for nuclear matter and atomic nuclei with momentum-dependent self-energies

The Lagrangian density of standard relativistic mean-field models with density-dependent meson-nucleon coupling vertices is modified by introducing couplings of the meson fields to derivative nucleon densities. As a consequence, the nucleon self-energies that describe the effective in-medium interaction become momentum dependent. In this approach it is possible to increase the effective (Landau) mass of the nucleons, that is related to the density of states at the Fermi energy, as compared to conventional relativistic models. At the same time the relativistic effective (Dirac) mass is kept small to obtain a realistic strength of the spin-orbit interaction. Additionally, the empirical Schr\odinger-equivalent central optical potential from Dirac phenomenology is reasonably well described. A parametrization of the model is obtained by a fit to properties of doubly magic atomic nuclei. Results for symmetric nuclear matter, neutron matter, and finite nuclei are discussed.

Relativistic mean-field models with medium-dependent meson-nucleon couplings

Microscopic structure models based on the relativistic mean-field approximation have been extended to include effective Lagrangians with explicit density-dependent meson-nucleon couplings. In a number of recent studies it has been shown that this class of global effective interactions provides an improved description of asymmetric nuclear matter, neutron matter and finite nuclei far from stability.

Self-consistent relativistic QRPA studies of soft modes and spin-isospin resonances in unstable nuclei

The excitation phenomena in unstable nuclei are investigated in the framework of the relativistic quasiparticle random-phase approximation (RQRPA) in the relativistic Hartree-Bogoliubov model (RHB) which is extended to include effective interactions with explicit density-dependent meson-nucleon couplings. The properties of the pygmy dipole resonance (PDR) are examined in 132Sn and within isotopic chains, showing that already at moderate proton-neutron asymmetry the PDR peak energy is located above the neutron emission threshold. A method is suggested for determining the size of the neutron skin within an isotopic chain, based on the measurement of the excitation energies of the Gamow-Teller resonance relative to the isobaric analog state. In addition, for the first time the relativistic RHB + RQRPA model, with tensor ω meson-nucleon couplings, is employed in calculations of β-decay half-lives of nuclei of the relevance for the r-process.

New relativistic mean-field interaction with density-dependent meson-nucleon couplings

We adjust a new improved relativistic mean-field effective interaction with explicit density dependence of the meson-nucleon couplings. The effective interaction DD-ME2 is tested in relativistic Hartree-Bogoliubov and quasiparticle random-phase approximation (QRPA) calculations of nuclear ground states and properties of excited states, in calculation of masses, and it is applied to the analysis of very recent data on superheavy nuclei.

Parity-nonconserving observables in thermal neutron capture on a proton

We calculate parity nonconserving observables in the processes where a neutron is captured on a proton at the threshold energy radiating a photon. Various potential models such as Paris, Bonn and Argonne $v18$ are used for the strong interactions, and the meson-exchange description is employed for the weak interactions between hadrons. The photon polarization $P_\gamma$ in the unpolarized neutron capture process and photon asymmetry $A_\gamma$ in the polarized neutron capture process are obtained in terms of the weak meson-nucleon coupling constants. $A_\gamma$ turns out to be basically insensitive to the employed strong interaction models and thus can be uniquely determined in terms of the weak coupling constants, but $P_\gamma$ depends significantly on the strong interaction models.

Parity violation in nuclear systems

Experimental measurements of Parity Non-Conserving (PNC) asymmetries in simple nuclear systems represent always a key-tool for the study of the weak nucleon-nucleon interaction and consequently an accurate experimental method for the determination of the meson-nucleon weak coupling constants of the underlying theory. Recent theoretical analysis on the deuteron photodisintegration with polarized photons, a few MeV above threshold, have drastically improved previous theoretical estimates. Based on that, the feasibility of measuring the photon asymmetry A γ in the reaction \( \bar \gamma + d \to n + p \) with the 10-MeV CW Linac at the Institute of Accelerating Systems and Applications (IASA) is considered here. A brief review on previous experimental results obtained in the deuteron photodisintegration and in the thermal-neutron radiative capture on protons (inverse reaction) is given. The most important parameters in the design of a nuclear parity experiment are presented and the crucial factors, such as beam intensity, beam polarization and neutron detection techniques with the required high accuracy are outlined.

Covariant density functional theory for isospin properties of nuclei far from stability

The standard relativistic mean-field density functionals based on non-linear meson exchange terms are extended to include density dependent meson-nucleon coupling constants. Special care is taken for the density dependence in the isovector channel. This provides not only an improved description of the equation of state for neutron matter and asymmetric nuclear matter but also for isovector properties of finite nuclei far from stability such as the neutron skin thickness. In particular it improves nuclear binding energies considerably as compared earlier applications of relativistic density functional theory to nuclear mass tables. An average root mean square deviation of 900 keV is found.

P- andT-odd two-nucleon interaction and the deuteron electric dipole moment

The nuclear physics relevant to the electric dipole moment (EDM) of the deuteron is addressed. The general operator structure of the $P$- and $T$-odd nucleon-nucleon interaction is discussed and applied to the two-body contributions of the deuteron EDM, which can be calculated in terms of $P$- and $T$-odd meson-nucleon coupling constants with only small model dependence. The one-body contributions, the EDMs of the proton and the neutron, are evaluated within the same framework. Although the total theoretical uncertainties are sizable, we conclude that, compared to the neutron, the deuteron EDM is competitive in terms of sensitivity to $\mathit{CP}$ violation, and complementary with respect to the microscopic sources of $\mathit{CP}$ violation that can be probed.

Vacuum polarization effects in relativistic nuclear pairing

In the present work we discuss the contribution of the vacuum polarization on the nuclear pairing, in the context of the relativistic Hartree-Fock-Bogoliubov (HFB) approximation. The polarization effects on the pairing, as a function of Fermi momentum, is shown with the scalar and vector meson-nucleon coupling constants scaled by a parameter x. We have obtained that the nuclear pairing is affected by the vacuum polarization.

Quasiparticle random phase approximation based on the relativistic Hartree-Bogoliubov model. II. Nuclear spin and isospin excitations

The proton-neutron relativistic quasiparticle random-phase approximation (PN-RQRPA) is formulated in the canonical single-nucleon basis of the relativistic Hartree-Bogoliubov model, for an effective Lagrangian characterized by density-dependent meson-nucleon couplings. The model includes both the $T=1$ and $T=0$ pairing channels. Pair configurations formed from the fully or partially occupied states of positive energy in the Fermi sea, and the empty negative-energy states from the Dirac sea, are included in PN-RQRPA configuration space. The model is applied to the analysis of charge-exchange modes: isobaric analog resonances and Gamow-Teller resonances.

Ground-state properties of rare-earth nuclei in the relativistic Hartree-Bogoliubov model with density-dependent meson-nucleon couplings

The relativistic mean-field effective interaction with density-dependent meson-nucleon couplings DD-ME1 is tested in the calculation of deformed nuclei. Ground-state properties of six isotopic chains ($60\le Z \le 70$) in the region of rare-earth nuclei are calculated by using the relativistic Hartree-Bogoliubov (RHB) model with the DD-ME1 mean-field interaction, and with the Gogny D1S force for the pairing interaction. Results of fully self-consistent RHB calculations for the total binding energies, charge isotope shifts and quadrupole deformation parameters are compared with the available empirical data.

New effective interactions in relativistic mean field theory with nonlinear terms and density-dependent meson-nucleon coupling

New parameter sets for the Lagrangian density in the relativistic mean field (RMF) theory, PK1 with nonlinear sigma- and omega-meson self-coupling, PK1R with nonlinear sigma-, omega- and rho-meson self-coupling and PKDD with the density-dependent meson-nucleon coupling, are proposed. They are able to provide an excellent description not only for the properties of nuclear matter but also for the nuclei in and far from the valley of beta-stability. For the first time in the parametrization of the RMF Lagrangian density, the center-of-mass correction is treated by a microscopic way, which is essential to unify the description of nuclei from light to heavy regions with one effective interaction.

Radial, sideward and elliptic flow at AGS energies

We study the baryon transverse in-plane (sideward) and elliptic flow from SIS to AGS energies for AuAu collisions in a relativistic dynamical simulation model that includes all baryon resonances up to a mass of 2 GeV as well as string degrees of freedom for the higher mass continuum. There are two factors which dominantly determine the baryon flow at these energies: the momentum dependence of the scalar and vector potentials and the resonance-string degrees of freedom. We fix the explicit momentum dependence of the nucleon-meson couplings of NL3(hard) equation of state (EoS) by the nucleon optical potential up to 1 GeV of kinetic energy. We simultaneously reproduce the sideward flow, the elliptic flow and the radial transverse mass distribution of protons data at AGS energies. In order to study the sensitivity of different mean-field EoS, we use NL2(soft) and NL23(medium) along with NL3(hard) momenta-dependent mean-field EoS. We find that to describe data on both sideward and elliptic flow, NL3 model is better at 2 A·GeV, while NL23 model is at 4–8 A·GeV.

Parity violation in proton-proton scattering at 221 MeV

pp elastic scattering at 221.3 MeV incident proton energy. This comprehensive paper includes details of the corrections, some of magnitude comparable to Az itself, required to arrive at the final result. The largest correction was for the effects of first moments of transverse polarization. The addition of the result, Az = (0.84 ± 0.29(stat.) ± 0.17(syst.)) × 10 7 , to the ~ pp parity violation experimental data base greatly improves the experimental constraints on the weak meson-nucleon coupling constants h pp and h pp , and also has implications for the interpretation of electron parity violation experiments.

Relativistic predictions of spin observables for exclusive proton knockout reactions

We demonstrate the ability of complete sets of exclusive $(\stackrel{\ensuremath{\rightarrow}}{p},2\stackrel{\ensuremath{\rightarrow}}{p})$ polarization transfer observables to discriminate between different model ingredients of the relativistic distorted wave impulse approximation (DWIA). Spin observables are identified, which are sensitive to Dirac versus Schr\"odinger dynamical equations of motion, different distorting optical potentials, finite-range versus zero-range approximations to the DWIA, as well as medium-modified meson-nucleon coupling constants and meson masses. In particular, we consider the knockout of protons from the ${3s}_{1/2},$ ${2d}_{3/2},$ and ${2d}_{5/2}$ states in ${}^{208}\mathrm{Pb},$ at an incident laboratory kinetic energy of 202 MeV, and for coincident coplanar scattering angles (28.0\ifmmode^\circ\else\textdegree\fi{}, -54.6\ifmmode^\circ\else\textdegree\fi{}). The reaction kinematics are chosen so as to maximize the influence of distortion effects, while still maintaining the validity of the impulse approximation, and also avoiding complications associated with the inclusion of recoil corrections in the relativistic Dirac equation.