Meson Exchange Current Contributions Research Articles

Density dependence of 2p-2h meson-exchange currents

We analyze the density dependence of the contribution of meson-exchange currents to the lepton-nucleus inclusive cross section in the two-particle two-hole channel. The model is based on the Relativistic Fermi Gas, where each nucleus is characterized by its Fermi momentum $k_F$. We find that the 2p-2h nuclear response functions at their peaks scale as $A k_F^2$ for Fermi momentum going from 200 to 300 MeV/c and momentum transfer $q$ from $2k_F$ to 2 GeV/c. This behavior is different from what is found for the quasielastic response, which scales as $A/k_F$. Additionally, the deep scaling region is also discussed and there the usual scaling behavior is found to be preferable.

Magnetic structure of light nuclei from lattice QCD

Lattice QCD with background magnetic fields is used to calculate the magnetic moments and magnetic polarizabilities of the nucleons and of light nuclei with $A\le4$, along with the cross-section for the $M1$ transition $np\rightarrow d\gamma$, at the flavor SU(3)-symmetric point where the pion mass is $m_\pi\sim 806$ MeV. These magnetic properties are extracted from nucleon and nuclear energies in six uniform magnetic fields of varying strengths. The magnetic moments are presented in a recent Letter. For the charged states, the extraction of the polarizability requires careful treatment of Landau levels, which enter non-trivially in the method that is employed. The nucleon polarizabilities are found to be of similar magnitude to their physical values, with $\beta_p=5.22(+0.66/-0.45)(0.23) \times 10^{-4}$ fm$^3$ and $\beta_n=1.253(+0.056/-0.067)(0.055) \times 10^{-4}$ fm$^3$, exhibiting a significant isovector component. The dineutron is bound at these heavy quark masses and its magnetic polarizability, $\beta_{nn}=1.872(+0.121/-0.113)(0.082) \times 10^{-4}$ fm$^3$ differs significantly from twice that of the neutron. A linear combination of deuteron scalar and tensor polarizabilities is determined by the energies of the $j_z=\pm 1$ deuteron states, and is found to be $\beta_{d,\pm 1}=4.4(+1.6/-1.5)(0.2) \times 10^{-4}$ fm$^3$. The magnetic polarizabilities of the three-nucleon and four-nucleon systems are found to be positive and similar in size to those of the proton, $\beta_{^{3}\rm He}=5.4(+2.2/-2.1)(0.2) \times 10^{-4}$ fm$^3$, $\beta_{^{3}\rm H}=2.6(1.7)(0.1) \times 10^{-4}$ fm$^3$, $\beta_{^{4}\rm He}=3.4(+2.0/-1.9)(0.2) \times 10^{-4}$ fm$^3$. Mixing between the $j_z=0$ deuteron state and the spin-singlet $np$ state induced by the background magnetic field is used to extract the short-distance two-nucleon counterterm, ${\bar L}_1$, of the pionless effective theory for $NN$ systems (equivalent to the meson-exchange current contribution in nuclear potential models), that dictates the cross-section for the $np\to d\gamma$ process near threshold. Combined with previous determinations of NN scattering parameters, this enables an ab initio determination of the threshold cross-section at these unphysical masses.

Quantum Monte Carlo calculations of electromagnetic moments and transitions inA≤9nuclei with meson-exchange currents derived from chiral effective field theory

Quantum Monte Carlo calculations of electromagnetic moments and transitions are reported for $A\ensuremath{\le}9$ nuclei. The realistic Argonne ${v}_{18}$ two-nucleon and Illinois-7 three-nucleon potentials are used to generate the nuclear wave functions. Contributions of two-body meson-exchange current (MEC) operators are included for magnetic moments and $M1$ transitions. The MEC operators have been derived in both a standard nuclear physics approach and a chiral effective field theory formulation with pions and nucleons including up to one-loop corrections. The two-body MEC contributions provide significant corrections and lead to very good agreement with experiment. Their effect is particularly pronounced in the $A=9$, $T=3/2$ systems, in which they provide up to $\ensuremath{\sim}20%$ ($\ensuremath{\sim}40%$) of the total predicted value for the ${}^{9}$Li (${}^{9}$C) magnetic moment.

Meson-Exchange Currents and Quasielastic Antineutrino Cross Sections in the Superscaling Approximation

We evaluate quasielastic double-differential antineutrino cross sections obtained in a phenomenological model based on the superscaling behavior of electron scattering data and estimate the contribution of the vector meson-exchange currents in the two-particle-two-hole sector. We show that the impact of meson-exchange currents for charge-changing antineutrino reactions is much larger than in the neutrino case.

Meson-exchange currents and quasielastic neutrino cross sections in the superscaling approximation model

We evaluate the quasielastic double differential neutrino cross sections obtained in a phenomenological model based on the superscaling behavior of electron scattering data. We compare our results with the recent experimental data for neutrinos of MiniBooNE and estimate the contribution of the vector meson-exchange currents in the 2p–2h sector.

Improved transverse(e,e')response function ofHe3at intermediate momentum transfers

The transverse electron scattering response function of 3He is studied in the quasi-elastic peak region for momentum transfers between 500 and 700 MeV/c. A conventional description of the process leads to results at a substantial variation with experiment. To improve the results, the present calculation is done in a reference frame (the ANB or Active Nucleon Breit frame) which diminishes the influence of relativistic effects on nuclear states. The laboratory frame response function is then obtained via a kinematics transformation. In addition, a one-body nuclear current operator is employed that includes all leading order relativistic corrections. Multipoles of this operator are listed. It is shown that the use of the ANB frame leads to a sizable shift of the quasi-elastic peak to lower energy and, contrary to the relativistic current, also to an increase of the peak height. The additionally considered meson exchange current contribution is quite small in the peak region. In comparison with experiment one finds an excellent agreement of the peak positions. The peak height agrees well with experiment for the lowest considered momentum transfer (500 MeV/c), but tends to be too high for higher momentum transfer (10% at 700 MeV/c).

Comparison of theC12(e,e′p)cross section at low momentum transfer with a relativistic calculation

The (e,e{sup '}p{sub 0}) cross section of {sup 12}C has been measured at an energy transfer of 60 MeV and a momentum transfer of 104.4 MeV/c using a 197.5 MeV continuous electron beam. The cross section at missing momenta between 181.5 and 304.8 MeV/c obtained from the experiment is compared with theoretical calculations based on the relativistic distorted-wave impulse approximation with and without meson-exchange currents (MEC). The contribution of MEC due to the seagull current is large in the high-missing-momentum region, in particular for the longitudinal component. The cross sections calculated using three different current-conserving operators (cc1, cc2, and cc3) are similar, in contrast to the ({gamma},p) reaction, where the operators give very different results. The shape of the measured cross section is well described by the calculations, whereas its magnitude is slightly smaller than that described by the calculations.

Hadro- and photo-production of η″ in quasi-free proton and neutron processes

The η″ photoproduction process on quasi-free proton and neutron and the reaction NN → η η″NN are investigated within a relativistic effective Lagrangian approach to hadronic interactions. Resonances with spins 1/2 and 3/2 are considered together with the nucleonic and t-channel meson-exchange current contributions. In photoproduction processes, the S11 resonance is found to be responsible for the sharp rise of the cross sections near threshold. In pp → η η″pp, it is found that the S11 resonance dominates the total cross section over the entire energy region considered. The spin observables, in particular the beam and target asymmetries, are shown to be very sensitive to the reaction mechanism and will help impose more stringent constraints on the model parameters.

Contribution of meson-exchange currents to the electromagnetic structure of the deuteron at high momentum transfers

The leading term of the asymptotic expansion of the contributions of the πNN and ρπγ processes to the electromagnetic form factors of the deuteron at high momentum transfers is calculated. The resulting asymptotic behavior is compared with experimental data obtained at the Thomas Jefferson Laboratory. Constraints on the functional dependence of meson-nucleon vertex functions that ensure a correct asymptotic behavior of deuteron form factors are discussed.

Quantum Monte Carlo calculations of magnetic moments andM1transitions inA⩽7nuclei including meson-exchange currents

Green's function Monte Carlo calculations of magnetic moments and M1 transitions including two-body meson-exchange current (MEC) contributions are reported for A<=7 nuclei. The realistic Argonne v18 two-nucleon and Illinois-2 three-nucleon potentials are used to generate the nuclear wave functions. The two-body meson-exchange operators are constructed to satisfy the continuity equation with the Argonne v18 potential. The MEC contributions increase the A=3,7 isovector magnetic moments by 16% and the A=6,7 M1 transition rates by 17--34%, bringing them into very good agreement with the experimental data.

Electromagnetic processes in light nuclei: Results within a microscopic approach

Results obtained by investigating the electrodisintegration of A = 3 and 4 nuclei and the photodisintegration of A = 3, 4, 6, and 7 nuclei on the basis of precise solutions to dynamical problems involving NN and NNN forces are presented. The role of NNN forces, final-state relativistic effects, the contribution of meson-exchange currents, an anomaly in electrodisintegration at high momentum transfers and low energy transfers, sensitivity to nuclear dynamics in the exclusive electrodisintegration of 4He nuclei, and the soft-mode resonance in the photodisintegration of 6He nuclei are considered.

Meson exchange currents in neutron-proton bremsstrahlung

Background: The meson exchange current (MEC) contribution is important in the neutron-proton bremsstrahlung process ($\mathit{np}\ensuremath{\gamma}$) when the two nucleon-scattering angles are small. However, our understanding of such effects is limited, and the reason why meson exchange current effects dominate the $\mathit{np}\ensuremath{\gamma}$ cross section has not been thoroughly investigated. Purpose: The primary focus of this investigation is to understand the origin of the MEC contribution, to identify the leading MEC amplitudes, and to comprehend why these MEC amplitudes dominate the $\mathit{np}\ensuremath{\gamma}$ cross sections. Method: We used a new method that combines the one-boson-exchange (OBE) approach with the soft-photon approach to define 10 different $\mathit{np}\ensuremath{\gamma}$ amplitudes. These amplitudes are used to calculate $\mathit{np}\ensuremath{\gamma}$ cross sections at 225 MeV for nucleon laboratory scattering angles lying between ${12}^{\ifmmode^\circ\else\textdegree\fi{}}$ and ${43}^{\ifmmode^\circ\else\textdegree\fi{}}$. The results of these calculations are then compared to investigate the meson exchange current effect in $\mathit{np}\ensuremath{\gamma}$. Results: (i) The OBE amplitude ${M}_{\mathit{np}\ensuremath{\gamma},\ensuremath{\mu}}^{\mathrm{PS}}$ and the two-u-two-t special (TuTts) soft-photon amplitude ${M}_{\mathit{np}\ensuremath{\gamma},\ensuremath{\mu}}^{\mathrm{TuTts}}$ predict quantitatively similar $\mathit{np}\ensuremath{\gamma}$ cross sections. (ii) The MEC effect is found to be significant when the two nucleon-scattering angles are far from the elastic limit (${45}^{\ifmmode^\circ\else\textdegree\fi{}}$), but the effect is insignificant when the nucleon angles approach the elastic limit. (iii) The origin of the MEC effect and the leading MEC amplitudes have been identified in this investigation. Furthermore, the reason is now clear why the leading MEC amplitudes dominate the $\mathit{np}\ensuremath{\gamma}$ cross section when the nucleon-scattering angles are small. (iv) The contribution from the anomalous magnetic moments of the proton and the neutron is confirmed to be negligibly small. (v) In general, the theoretical cross sections using the amplitude ${M}_{\mathit{np}\ensuremath{\gamma},\ensuremath{\mu}}^{\mathrm{PS}}$, or the amplitude ${M}_{\mathit{np}\ensuremath{\gamma},\ensuremath{\mu}}^{\mathrm{TuTts}}$, are consistent with the triple differential cross sections recently measured at the Los Alamos National Laboratory. However, there exists an unexplained discrepancy between theory and experiment in some cases. Conclusions: The findings of this investigation have enhanced our understanding of the meson exchange current effect in $\mathit{np}\ensuremath{\gamma}$. The comparative amplitude method introduced can be used for other bremsstrahlung investigations.

Low-Energy Inelastic Neutrino Reactions onHe4

The inelastic scattering of neutrino off 4He is calculated microscopically at energies typical for core-collapse supernova environment. The calculation is carried out with the Argonne V18 nucleon-nucleon potential and the Urbana IX three-nucleon force. Full final state interaction is included via the Lorentz integral transform method. The contribution of axial meson exchange currents to the cross sections is taken into account from effective field theory of nucleons and pions to order O(Q3).

Pion double charge exchange in a composite-meson model

The pion double charge exchange amplitude is evaluated in a composite-meson model based on the four-quark interaction. The model assumes that the mesons are two-quark systems and can interact with each other only through quark loops. To evaluate the meson exchange current contribution, the form factors of the two-pion decay modes of the $\ensuremath{\rho},\ensuremath{\sigma}$, and ${f}_{0}$ mesons have been used in the calculations. The contribution of the four-quark box diagram has been taken into account as well as a contact diagram. The contributions of the $\ensuremath{\rho},\ensuremath{\sigma}$, and ${f}_{0}$ mesons increase the forward scattering cross section, which depends weakly on the energy.

Parity nonconservation in the photodisintegration of the deuteron at low energy

The parity-nonconserving asymmetry in the deuteron photodisintegration, $\vec{\gamma}+d\to n+p$, is considered with the photon energy ranged up to 10 MeV above the threshold. The aim is to improve upon a schematic estimate assuming the absence of tensor as well as spin-orbit forces in the nucleon-nucleon interaction. The major contributions are due to the vector-meson exchanges, and the strong suppression of the pion-exchange contribution is confirmed. A simple argument, going beyond the observation of an algebraic cancellation, is presented. Contributions of meson-exchange currents are also considered, but found to be less significant.

A Six-Quark Dressed-Bag Description of np→dγ Radiative Capture

The radiative capture process np→dγ is considered within the framework of a recently developed six-quark dressed-bag model for the nucleon-nucleon interaction. The calculations presented here include both the nucleon current and the meson-exchange current contributions. The latter uses short-range hadronic form factors for the pion exchange currents consistent with the soft cut-off parameter ΛπNN from the NN-potential. Contributions of the pion exchange current and Δ-isobar current to the total cross-section still cannot explain the discrepancy between the theoretical and experimental cross-sections. Possibilities for new types of meson exchange currents associated with chiral fields inside multi-quark dressed-bag states in nuclei are discussed.

P + d radiative capture

We discuss recent results on tensor polarization observables in p+d radiative capture. These observables provide a sensitive means to study the role of the D-state of the three-body bound state, and the contribution of meson exchange currents.

Quark correlations and deuteron electromagnetic form factors

The recent preliminary values of ${A(Q}^{2})$ and ${t}_{20}(70\ifmmode^\circ\else\textdegree\fi{})$ elastic electron-scattering data extracted from deuterium at larger values of ${Q}^{2}$ have placed more stringent constraints on models of the nucleon-nucleon system. Comparisons of the data with conventional hadronic models and with perturbative QCD have been made, and are strongly in favor of the hadronic picture provided isoscalar meson exchange currents are included. In this work a nonperturbative quark model of the deuteron with parameters determined from earlier work is used to investigate the role of quark correlations in describing the deuteron electromagnetic form factors. The results give a good description of the recent data and show that this model with many-body confinement has quark correlation contributions to the charge form factors which are remarkably similar to the meson exchange current contributions.

Short-range and tensor correlations in the16O(e,e′pn)reaction

The cross sections for electron-induced two-nucleon knockout reactions are evaluated for the example of the ${}^{16}\mathrm{O}{(e,e}^{\ensuremath{'}}{pn)}^{14}\mathrm{N}$ reaction leading to discrete states in the residual nucleus ${}^{14}\mathrm{N}.$ These calculations account for the effects of nucleon-nucleon correlations and include the contributions of two-body meson exchange currents as the pion seagull, pion in flight, and the isobar current contribution. The effects of short-range as well as tensor correlations are calculated within the framework of the coupled cluster method employing the Argonne V14 potential as a model for a realistic nucleon-nucleon interaction. The relative importance of correlation effects as compared to the contribution of the meson exchange currents depends on the final state of the residual nucleus. The cross section leading to specific states, such as, e.g., the ground state of ${}^{14}\mathrm{N},$ is rather sensitive to the details of the correlated wave function.

Low- and intermediate-energy nucleon-nucleon interactions and the analysis of deuteron photodisintegration within the dispersion relation technique

The nucleon-nucleon interaction in the region of the nucleon kinetic energy up to 1000 MeV is analysed together with the reaction $\gamma d \to pn$ in the photon energy range $E_{\gamma}=0-400$ MeV. Nine nucleon-nucleon $s$-channel partial amplitudes are reconstructed in the dispersion relation $N/D$ method: $^1S_0$, $^3S_1-^3D_1$, $^3P_0$, $^1P_1$, $^3P_1$, $^3P_2$, $^1D_2$, $^3D_2$ and $^3F_3$. Correspondingly, the dispersive representation of partial amplitudes $N\Delta \to pn$, $NN^* \to pn$ and $NN\pi \to pn$ is given. Basing on that, we have performed parameter-free calculation of the amplitude $\gamma d \to pn$, taking into account: $(i)$ pole diagram, $(ii)$ nucleon-nucleon final-state rescattering $\gamma d \to pn \to pn$, and $(iii)$ inelastic final-state rescatterings $\gamma d \to N\Delta(1232) \to pn$, $\gamma d \to NN^*(1400) \to pn$ and $\gamma d \to NN\pi \to pn$. The $\gamma d \to pn$ partial amplitudes for nine above-mentioned channels are found. It is shown that the process $\gamma d \to pn \to pn$ is significant for the waves $^1S_0$, $^3P_0$, $^3P_1$, at $E_{\gamma} =50 -100$ MeV, while $\gamma d \to N\Delta \to pn$ for the waves $^3P_2$, $^1D_2$,$^3F_3$ dominates at $E_{\gamma} > 300$ MeV. Meson exchange current contributions into the deuteron disintegration are estimated: they are significant at $E_\gamma =100-400$ MeV.