Role Of Meson Exchange Currents Research Articles

Muon capture on the deuteron and3He

We plan to investigate the role of meson exchange currents in the description of the μ − + d → νμ + n + n and μ − + 3 He → νμ + 3 H reactions. They both are treated as the decay of the corresponding muonic atoms, with the muon initially on the lowest K shell. The muon binding energy in these atoms can be safely neglected and in the initial state we deal essentially with the deuteron (or 3 He) and muon at rest. These two reactions are interesting for several reasons. First of all, they offer a testing ground for the nuclear wave functions, which for any nucleon-nucleon (NN) and three-nucleon (3N) forces can be constructed for such light systems with great accuracy. In these reactions few-nucleon weak current operators are an important dynamical ingredient. In the current operators apart from the relatively well known single nucleon contributions, two-nucleon parts (generated by various meson exchanges) play an important role. Their details are not well known and several models should be considered. We present our formalism for dealing with these reactions and a simple method for partial wave decomposition of the two-nucleon operators. The crucial nuclear matrix elements of the corresponding weak current operators will be calculated in the momentum space and using partial wave decomposition. The effect of meson exchanges will be investigated in the energy spectrum of the emitted neutrinos (in the deuteron case) and in the total decay rates for the two reactions. We will employ various models of NN and 3N forces, such as the Bonn B or chiral NNLO potentials. Our results with the single nucleon currents look already very promising and we hope for the improvement in the description of the experimental data, when dominant two-nucleon current operators are included in our framework.

Meson exchange currents in electromagnetic one-nucleon emission

The role of meson exchange currents (MEC) in electron- and photon-induced one-nucleon emission processes is studied in a nonrelativistic model including correlations and final state interactions. The nuclear current is the sum of a one-body and of a two-body part. The two-body current includes pion seagull, pion-in-flight and the isobar current contributions. Numerical results are presented for the exclusive 16O(e,e'p)15N and 16O(\gamma,p)15N reactions. MEC effects are in general rather small in (e,e'p), while in (\gamma,p) they are always large and important to obtain a consistent description of (e,e'p) and (\gamma,p) data, with the same spectroscopic factors. The calculated (\gamma,p) cross sections are sensitive to short-range correlations at high values of the recoil momentum, where MEC effects are larger and overwhelm the contribution of correlations.

Meson exchange currents in a relativistic model for electromagnetic one nucleon emission

We analyze the role of meson exchange currents (MEC) in photon- and electron-induced one nucleon emission reactions in a fully relativistic model. The relativistic mean field theory is used for the bound state and the Pauli reduction for the scattering state. Direct 1-body and exchange 2-body terms in the nuclear current are considered. Results for the 12C(gamma,p) and 16O(gamma,p) differential cross sections and photon asymmetries are displayed in an energy range between 60 and 196 MeV. The 2-body seagull current affects the cross section less than in nonrelativistic analyses. In the case of the 16O(gamma,n) differential cross section MEC effects are large but not sufficient to reproduce the data. MEC have a small effect on (e,e'p) calculations.

Fast neutron capture reactions in nuclear astrophysics

The cross sections of the neutron capture reaction on light nuclei, protons,12C and16O, were measured at astrophysically relevant energies between 10 and 300 keV. They are not only important for estimating yields of elements in the primordial nucleosynthesis, stellar nucleosynthesis of s-, r-, and p-processes, but also to study the role of meson exchange currents, the nuclear structure and the reaction mechanism. In the measurement, we used a prompt γ-ray detection method, combined with a pulsed neutron beam, and a recently developed Monte-Carlo code, TIME-MULTI, to correct for neutron multiple-scattering effects in a sample.

Isovector M1 transitions in 28Si and the role of meson exchange currents.

At the newly installed 180\ifmmode^\circ\else\textdegree\fi{} Darmstadt electron scattering facility we have measured the isovector M1 transition strengths in $^{28}\mathrm{Si}$ in the excitation energy range from 10 to 18 MeV. Overall agreement with a shell-model calculation using the unified sd-shell interaction and effective g factors is obtained. Comparison with Gamow-Teller strength deduced from (p,n) data reveals the presence of meson exchange current contributions of the order of 25% to the summed M1 transition strength. \textcopyright{} 1996 The American Physical Society.

Meson-exchange current effects in nucleon photoemission

The role of meson-exchange currents in (γ, p) and (γ, n) reactions at intermediate energies is investigated in the frame of a direct knockout model where the photon interacts with a pair of correlated nucleons. Realistic short-range correlations and final-state interactions are included in the calculations. Direct and exchange terms are considered. The results for 16O(γ, p) and 16O(γ, n) indicate that the two-body seagull current significantly affects the cross sections. In both cases the size and the shape of the experimental angular distributions are fairly well reproduced.

Meson exchange currents and high-resolution ( gamma,p) reactions.

The role of one-pion-exchange currents in intermediate-energy photonuclear reactions of the type A(\ensuremath{\gamma},p)B is studied within a mean-field approach. Calculations are presented for the $^{12}\mathrm{C}$(\ensuremath{\gamma},p) reaction leading to the low-lying states of $^{11}\mathrm{B}$ including states with a predominant hole character and states with a more complicated two-hole--one-particle (2h-1p) structure. The level of agreement with the data for decay to the 2h-1p states suggests the important role of meson exchange currents in (\ensuremath{\gamma},p) reactions at intermediate energies.

Delayed muon induced fission of209Bi and the role of meson-exchange currents

The probability for delayed muon induced fission of209Bi has been determined from a (μ−,f1f2) measurement. The measured fission probability Pf=(4.2±0.7)×10−5 is compared with theoretical predictions. The high fission threshold reaction seems well suited for studying the influence of two-body meson-exchange currents in nuclear muon capture.

The 6 −T=1 resonance in 28si via high-resolution inelastic electron scattering

High-resolution (e,e′) was used to measure the form factor of the 6 −, T=1 resonance in 28Si. The results disagree with previous experimental results and with theoretical calculations. The role of meson-exchange currents in producing the observed quenching of magnetic strength, and the relevance of (e,e′) to other reactions are briefly discussed.

Exchange currents and the radial distribution of valence nucleons as determined from elastic electron scattering

Elastic magnetic electron scattering of high multipolarity has previously been used to determine the radial distribution of valence nucleons. In the present work, the role of meson exchange currents in the analysis of such measurements is discussed.

Meson exchange effects on electron scattering form factors in 12C

The role of meson-exchange currents in the excitation of nuclear states by electron scattering is investigated in the favourable case of 12C. The description uses the non-covariant perturbative model based on the expansion of the mesic S-matrix in the interaction representation. Only the one-pion-exchange contributions to the long-range part of the two-body electromagnetic interaction are retained. When coming to the derivation of the multipole operators, the two-body local current densities are expanded around the c.m. of the pair of nucleons. The approximation is introduced chiefly to simplify the practical calculations, but it also allows the model to fulfil the requirement of charge conservation. Application of the model is made to the strongly excited isospin-one levels lying above 14MeV excitation in 12C. The electron scattering form factors are calculated with wave functions of the single particle-hole model and compared with data for momentum transfers extending up to 700 MeV/ c. The exchange correction is found to be small for low values of the transfer but to grow substantially as the transfer increases; so much as to overcome the contribution of the one-body operators. The effect is always an increase of the form factors with a similar dependence on transfer as the one-body form factors. Comparison of the results for the impulse form factors with other recent theoretical calculations show a reasonable degree of model independence. The spin-orbit relativistic corrections to the one-body currents are also examined but are found to play a negligible role, particularly in the transverse interaction.