Meson exchange currents in neutron-proton bremsstrahlung

Abstract

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.