Threshold contribution to the Gerasimov-Drell-Hearn sum rule in 3He

Measurements of the cross section, vector and tensor analyzing powers, and gamma-ray polarization in the radiative capture reactions D(p, γ)3He and p(d, γ)3He at c.m. energies in the range 0-53 keV allow the determination of the reduced matrix elements (RMEs) relevant for these transitions. The RMEs provide values of the doublet and quartet M1 cross sections, as well as the asymptotic D-to-S state ratio. These quantities are in good agreement with theoretical predictions. The RMEs can also be used to compute the value of the integral which determines the Gerasimov-Drell-Hearn sum rule for 3He in the energy region of our experiment. The results are compared with the predictions of an ab initio microscopic three-body model calculation. The theoretical predictions for the value of this integral based on a 'nucleons-only' assumption are an order-of-magnitude smaller than experiment. The discrepancy is reduced to about a factor of two when meson-exchange currents are taken into account. (author)