Explicit Δ(1232) degrees of freedom in Compton scattering off the deuteron

We examine elastic Compton scattering off the deuteron for photon energies between 50 MeV and 100 MeV in the framework of chiral effective field theories to next-to-leading order. We compare one theoretical scheme with only pions and nucleons as explicit degrees of freedom to another in which the Delta(1232) resonance is treated as an explicit degree of freedom. Whereas pion degrees of freedom suffice to describe the experimental data measured at about 70 Mev, the explicit A(1232) gives important contributions that help to reproduce the angular dependence at higher energies. The static isoscalar dipole polarizabilities alpha(E)(s) and beta(M)(s) are fitted to the available data, giving results for the neutron polarizabilities, alpha(E)(n) = (14.6 +/- 2.0(stat) 1.1(wf)) x 10(-4) fm(3), beta(M)(n)(1.4 +/- 2.2(stat) +/- 0.1 (wf)) x 10(-4) fm(3) with "wf" denoting the error due to the dependence on the deuteron wave function. These values are in good agreement with previous experimental analyses. Comparing them to the currently accepted proton values we conclude that there is no evidence for significant differences between the proton and neutron electromagnetic dipole polarizabilities. (C) 2004 Elsevier B.V. All rights reserved.