New photodisintegration threshold observable in3He

Measurements of the cross section, vector and tensor analyzing powers, and linear gamma-ray polarization in the radiative capture reactions $D(p,\ensuremath{\gamma}{)}^{3}\mathrm{He}$ and $p(d,\ensuremath{\gamma}{)}^{3}\mathrm{He}$ at c.m. energies in the range 0--53 keV allow the determination of the reduced matrix elements (RMEs) relevant for these transitions. From these RMEs the value of the integral which determines the Gerasimov-Drell-Hearn sum rule for ${}^{3}\mathrm{He}$ is obtained in the threshold region, corresponding to two-body breakup, and compared with the results 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 2 when two-body currents are taken into account. This factor of 2 is due to an almost exact cancellation between the dominant $E1$ RMEs in the theoretical calculation. The excess $E1$ strength observed experimentally could provide useful insights into the nuclear interaction at low energies.