Nucleon-nucleon bremsstrahlung: Anomalous magnetic moment effects

Background: Two soft-photon amplitudes, the two-u-two-t special (TuTts) amplitude and the Low amplitude, are known to produce quantitatively similar npγ cross sections, but they predict quite different ppγ cross sections for those kinematic conditions in which the nucleon scattering angles are small (less than 25 deg.). Purpose: These two amplitudes have been applied to systematically investigate three different nucleon-nucleon bremsstrahlung (NNγ) processes: ppγ,npγ, and nnγ. The nnγ process is explored for the first time. The primary focus of this work is to investigate the contribution of the proton and the neutron anomalous magnetic moments to all three NNγ processes for projectile energies above 150 MeV and for laboratory scattering angles (θ1 and θ2) lying between 8 deg. and 40 deg.. Method: A special soft-photon expansion in which the TuTts amplitude is expanded in terms of the Low amplitude plus additional amplitudes is utilized to explore the relationship between the TuTts and Low amplitudes and the reasons why they agree and disagree. We also used the TuTts amplitude to calculate the NNγ cross section with and without the anomalous magnetic moment contributions to explore the importance of that element of the electromagnetic current. Results: The TuTts amplitude describes well the available ppγ cross-section data. The anomalous magnetic moment contribution is (i) significant in the ppγ process when each scattering angle is less than 25 deg. but insignificant when each scattering angle is 40 deg. or greater and (ii) insignificant in the npγ process for all scattering angles. The nnγ cross sections for the TuTts and Low amplitudes differ substantially for the kinematics investigated. Conclusions: In general, the Low amplitude agrees well with the TuTts amplitude when anomalous magnetic moment effects are not significant, but the two amplitudes can yield quite different predictions when such effects are significant. These findings have enhanced our understanding of the fundamental emission mechanism governing NNγ processes, and they explain why the TuTts amplitude should be used to describe all three NNγ processes