Meson-exchange Potential Models Research Articles

Numerical Exact Ab Initio Four-Nucleon Scattering Calculations: from Dream to Reality

In the present manuscript we review the work of the last ten years on the pursuit to obtain numerical exact solutions of the four-nucleon scattering problem using the most advanced force models that fit two nucleon data up to pion production threshold with a $$\chi ^2$$ per data point approximately one, together with the Coulomb interaction between protons; three- and four-nucleon forces are also included in the framework of a meson exchange potential model where NN couples to N $$\varDelta $$ . Failure to describe the world data on four-nucleon scattering observables in the framework of a non relativistic scattering approach falls necessarily on the force models one uses. Four-nucleon observables pose very clear challenges, particular in the low energy region where there are a number of resonances whose position and width needs to be dynamically generated by the nucleon–nucleon (NN) interactions one uses. In addition, our calculations constitute the most advance piece of work where observables for all four-nucleon reactions involving isospin $$I=0$$ , $$I=0$$ coupled to $$I=1$$ and isospin $$I=1$$ initial states are calculated at energies both below and above breakup threshold. We also present a very extensive comparison between calculated results and data for cross sections and spin observables. Therefore the present work reveals both the shortcomings and successes of some of the present NN force models in describing four-nucleon data and serve as a benchmark for future developments.

Neutron star properties and the equation of state of neutron-rich matter

We calculate total masses and radii of neutron stars (NS) for pure neutron matter and nuclear matter in beta-equilibrium. We apply a relativistic nuclear matter equation of state (EOS) derived from Dirac-Brueckner-Hartree-Fock (DBHF) calculations. We use realistic nucleon-nucleon (NN) interactions defined in the framework of the meson exchange potential models. Our results are compared with other theoretical predictions and recent observational data. Suggestions for further study are discussed.

ELASTIC PROTON-PROTON SCATTERING AT GEV ENERGIES: ANALYZING POWERS ANAND SPIN CORRELATION PARAMETERS ANN, ASS, AND ASL

The analyzing powers ANand spin correlation parameters ANN, ASS, and ASLof the elastic proton-proton scattering have been measured with an internal polarized atomic beam target at the cooler synchrotron COSY. Excitation functions were taken continuously during the acceleration and deceleration of the internal beam for kinetic energies ranging between 0.45 and 2.5 GeV and scattering angles 30°≤Θc.m.≤90°. The analyzing power excitation functions provide a new polarization standard in the GeV region. The spin correlation data on ASSdisagree strikingly with existing phase shift solutions. The addition of the new polarization data to the world database removes some of the ambiguities in the phase shift solutions and the direct reconstruction of scattering amplitudes. The data are compared with a recent extension of a meson-exchange potential model to higher energies.

NONMESONIC WEAK DECAYS OF SINGLE-Λ AND DOUBLE-Λ HYPERNUCLEI OF LIGHT MASS SYSTEMS

The nucleon-induced and hyperon-induced nonmesonic weak decay rates of the single-Λ and double-Λ hypernuclei are evaluated in the meson-exchange potential model. The roles of K-meson exchange are discussed. The ΛΛ → nΛ decay rate of [Formula: see text] is estimated to be fair and might be detectable in a future experiment.

Study of odd-mass N = 82 isotones with realistic effective interactions

The microscopic quasiparticle-phonon model (MQPM) is used to study the energy spectra of the odd Z = 53–63, N = 82 isotones. The results are compared with experimental data, with the extreme quasiparticle-phonon limit and with the results of an unrestricted 2s1d0g 7 2 0h 11 2 shell model (SM) calculation. The interaction used in these calculations is a realistic two-body G-matrix interaction derived from modern meson-exchange potential models for the nucleon-nucleon interaction. For the shell model all the two-body matrix elements are renormalized by the Q ̂ - box method, whereas for the MQPM the

Extended shell model calculation for even N = 82 isotones with a realistic effective interaction

The shell model within the 2s1d0g 7 2 0h 11 2 shell is applied to calculate nuclear structure properties of the even Z = 52−62, N = 82 isotones. The results are compared with experimental data and with the results of a quasiparticle random-phase approximation (QRPA) calculation. The interaction used in these calculations is a realistic two-body G-matrix interaction derived from modern meson-exchange potential models for the nucleon-nucleon interaction. For the shell model all the two-body matrix elements are renormalized by the Q -box method whereas for the QRPA the effective interaction is defined by the G-matrix.

Asymmetric Nuclear Matter and Neutron Star Properties

In this work we calculate the total mass, radius, moment of inertia, and surface gravitational redshift for neutron stars using various equations of state (EOS). Modern meson-exchange potential models are used to evaluate the $G$-matrix for asymmetric nuclear matter. We calculate both a non-relativistic and a relativistic EOS. Of importance here is the fact that relativistic Brueckner-Hartree-Fock calculations for symmetric nuclear matter fit the empirical data, which are not reproduced by non-relativistic calculations. Relativistic effects are known to be important at high densities, giving an increased repulsion. This leads to a stiffer EOS compared to the EOS derived with a non-relativistic approach. Both the non-relativistic and the relativistic EOS yield moments of inertia and redshifts in agreement with the accepted values. The relativistic EOS yields, however, too large mass and radius. The implications are discussed.

Emissivities of neutrinos in neutron star cores.

In this work we consider various URCA processes relevant for neutrino emissions in the core region of neutron stars. The parameters which enter these processes, such as effective masses, proton pairing gaps, proton fractions, and electron and muon chemical potentials, are calculated in a self-consistent Brueckner-Hartree-Fock approach for $\ensuremath{\beta}$-stable nuclear matter. Modern meson-exchange potential models are employed. We find that the emissivities from modified URCA processes are reduced due to the presence of superconducting protons at densities below $0.4{\mathrm{fm}}^{\ensuremath{-}3}$.

Neutron-star properties from modern meson-exchange potential models

In this work we calculate the total mass, radius, moment of inertia and surface gravitational redshift for neutron stars using various equations of state (EOS). The latter are derived from the recent meson-exchange potential models of the Bonn group, and we derive both a non-relativistic and a relativistic EOS. Of importance here is the fact that relativistic Brueckner-Hartree-Fock calculations for symmetric nuclear matter meet the empirical data, which are not reproduced by non-relativistic calculations. Relativistic effects are known to be important at high densities, giving an increased repulsion. This leads to a stiffer EOS compared to the EOS derived with a nonrelativistic approach. Both the non-relativistic and the relativistic EOS yield values for moment of inertia and redshifts in agreement with the accepted values. The relativistic EOS yields however too large mass and radius. The implications are discussed.

Perturbative many-body approaches to finite nuclei

In this work we discuss various approaches to the effective interaction appropriate for finite nuclei. The methods we review are the folded-diagram method of Kuo and co-workers and the summation of folded diagrams as advocated by Lee and Suzuki. Examples of applications to sd-shell nuclei from previous works are discussed together with hitherto unpublished results for nuclei in the pf-shell. Since we find the method of Lee and Suzuki to yield the best converged results, we apply this method to calculate the effective interaction for nuclei in the pf-shell. To calculate this effective interaction we have used three recent versions of the Bonn meson-exchange potential model. These versions are fitted to the same set of data and differ only in the strength of the tensor force. The importance of the latter for finite nuclei is also discussed.

First measurement of the depolarization parameter D0n0n of the charge-exchange reaction

The depolarization parameter D0n0n in the charge-exchange reaction pp ↑→ nn ↑ has been measured for the first time at the CERN Low Energy Antiproton Ring (LEAR) at 875 MeV/c antiproton beam momentum, in the forward hemisphere. The measured values of D0n0n are always smaller than ±0.3, indicating that the two-spin amplitudes dominate the scattering matrix as suggested by the meson exchange potential models.

Relativistic effects in proton-proton bremsstrahlung.

The {ital pp}{gamma} reaction at intermediate energies is investigated using a modern relativistic meson-exchange potential model. Both the single-scattering and rescattering terms are treated including relativistic corrections. It is shown that for energetic photons in the region near the end-point energy the relativistic spin correction is very large and reduces the {ital pp}{gamma} cross section by a factor of {similar to}2. This large correction is dynamical in origin and can be traced in a very simple way to the strong energy dependence of the {sup 1}{ital S}{sub 0} state contribution. It is also shown that, under these kinematical conditions, the rescattering term {ital reduces} the cross section, which amplifies the relativistic effects on the {ital pp}{gamma} reaction. For the kinematical conditions of the recent TRIUMF experiment at {ital T}{sub lab}=280 MeV the rescattering contribution {ital enhances} the cross section and reduces the discrepancy between the data and the calculation.

Hard photon production in proton-deuteron reactions at intermediate energies.

Energetic photon production cross sections in proton-deuteron reactions at intermediate proton incident energies are calculated by using a meson-exchange potential model for describing the elementary nucleon-nucleon bremsstrahlung amplitude. The convection, magnetization, and exchange current contributions as well as the kinematics prescribed by the momentum distribution of the deuteron are included explicitly. The results are compared with the recent data of the Grenoble and Michigan State groups. The absolute cross sections are consistently underpredicted by \ensuremath{\sim}(20--40)% which indicates that further work is required for a better understanding of the elementary pn bremsstrahlung process.

On the role of third- and higher-order contributions to the effective interaction for pf shell nuclei

In this work we have studied the inclusion of third- and higher-order contributions in perturbation theory to the effective interaction appropriate for the pf shell. We conclude that these contributions are important and ought to be carefully analyzed when approximating the effective interaction to a given order. To calculate the nuclear G-matrix we have employed a recent version of the Bonn meson-exchange potential model (potential A) and the Paris potential. We find the Paris potential to yield too little attraction, a result which agrees with findings in the sd shell. The Bonn A potential reproduces fairly well the data for most JT channels in the A = 42 system. For JT = 10 in the same mass system, however, too much attraction is introduced and the agreement with the data is poor. The implications are discussed.

Effective interactions for valence-hole nuclei with modern meson-exchange potential models

Within the framework of the folded-diagram theory, we have studied the effective interaction appropriate for hole-hole nuclei in the mass regions of 16O and 40Ca, using the Bonn and Paris potential models. To sum up the folded diagrams we have employed the renormalization procedure of Lee and Suzuki, using a so-called Q ̌ - box in which were included all one-body and two-body irreducible valence-linked diagrams up to third order in perturbation theory. We report discrepancies for the mass dependence of the effective interaction for several JT configurations with respect to empirically deduced mass dependencies. The role of core-polarization processes up-to third order were found to be one of the mechanisms behind these discrepancies. Compared to the results obtained with the Paris potential, more attraction is introduced by the Bonn potential for all matrix elements of concern, a result which agrees well with previous findings for the particle-particle interaction in the same mass regions. A qualitative agreement with experimental data is obtained.

Folded-diagram effective interactions with the Bonn meson-exchange potential model

A modern version of the Bonn one-boson-exchange potential has been used in a folded-diagram expansion of the effective interaction appropriate for sd-shell nuclei. Two different techniques for the summation of the folded diagrams have been studied, including all one-body and two-body valence-linked diagrams through third order and using both a harmonicoscillator (HO) basis and a Brueckner-Hartree-Fock (BHF) basis. The folded-diagram expansion of Kuo and co-workers shows a slow convergence when the HO choice is made, whereas the vertex renormalization procedure of Lee and Suzuki results in a rapidly converging expansion for the effective interaction. The convergence is very good for both iteration techniques when the self-consistent BHF basis is used.

Folded-diagram effective interactions with the Bonn meson-exchange potential model: M. Hjorth-Jensen and E. Osnes. Department of Physics, University of Oslo, Box 1048 Blindern, N-0316 Oslo 3, Norway; and H. Müther. Institut für Theoretische Physik, Universität Tübingen, Auf der Morgenstelle 14, D-7400 Tübingen, Germany

Folded-diagram effective interactions with the Bonn meson-exchange potential model: M. Hjorth-Jensen and E. Osnes. Department of Physics, University of Oslo, Box 1048 Blindern, N-0316 Oslo 3, Norway; and H. Müther. Institut für Theoretische Physik, Universität Tübingen, Auf der Morgenstelle 14, D-7400 Tübingen, Germany

Measurement of the analyzing power of the charge-exchange [formula omitted] reaction in the momentum range 546-875 MeV/ c at LEAR

Results are given for the analysing power of the p ̄ p→ n ̄ n charge-exchange reaction at four incident beam momenta. The measurement is part of an experimental programme to study the spin structure of this reaction at LEAR. The analyzing power shows strong angular and energy dependence, which at present is not reproduced by the existing meson-exchange potential models.

Nucleon-nucleon bremsstrahlung at intermediate energies.

Energetic photon production in proton-neutron as well as in proton-proton collisions at intermediate bombarding energies are investigated in a meson-exchange potential model calculation in an effort to provide a more reliable basis for calculations of hard photon production in heavy-ion collisions. Special attention is paid to the role of the one-body rescattering contribution, which has been neglected in previous work. It is shown that this contribution enhances the cross section for photon energies only near the maximum value allowed kinematically. As a function of photon energy, both pn and pp bremsstrahlung inclusive cross sections are shown to be rather flat except in the low photon energy region. The shape of the angular distribution is, to a large extent, determined by the convection current contribution, which in the soft-photon limit has a well-known dipole shape in the case of pn bremsstrahlung and a quadrupole shape in the case of pp bremsstrahlung. For photon energies above \ensuremath{\sim}150 MeV where the magnetic current contribution dominates over the convection contribution, the shape of the angular distribution is roughly of a ${\mathrm{cos}}^{2}$\ensuremath{\theta} form. The internal current gives rise to a nearly isotropic angular distribution. The pp\ensuremath{\gamma} cross section is shown to be very small compared to the pn\ensuremath{\gamma} cross section, even for considerably high-energy photons, because of the two-body current contribution, which is mostly absent in pp bremsstrahlung. The present calculation yields np bremsstrahlung cross sections which still tend to underpredict the data, although the measurements have large uncertainties.

Hard photons in proton-nucleus collisions.

A calculation of energetic photon production in proton-nucleus reactions at intermediate bombarding energies is performed assuming the first chance nucleon-nucleon collisional mechanism. The elementary nucleon-nucleon bremsstrahlung amplitude is calculated within a meson exchange potential model which includes the one-body as well as the two-body current contributions. The results are in reasonable agreement with the recent data by the Grenoble group at the proton incident energy of ${T}_{\mathrm{lab}}$=168 MeV and indicate that these energetic photons are created in a relatively low-density region in the nucleus (\ensuremath{\sim}40% of the nuclear matter normal density). The present model, however, cannot account for the strong angular dependence observed at ${T}_{\mathrm{lab}}$=72 MeV.