%0 Journal Article
%T How low-energy weak reactions can constrain three-nucleon forces and the neutron-neutron scattering length
%A A. Gardestig
%A D. R. Phillips
%J Physics
%D 2006
%I arXiv
%R 10.1103/PhysRevLett.96.232301
%X We show that chiral symmetry and gauge invariance enforce relations between the short-distance physics that occurs in a number of electroweak and pionic reactions on light nuclei. Within chiral perturbation theory this is manifested via the appearance of the same axial isovector two-body contact term in pi- d -> n n gamma, p-wave pion production in NN collisions, tritium beta decay, pp fusion, nu d scattering, and the hep reaction. Using a Gamow-Teller matrix element obtained from calculations of pp fusion as input we compute the neutron spectrum obtained in pi- d -> n n gamma. With the short-distance physics in this process controlled from p p -> d e+ nu_e the theoretical uncertainty in the nn scattering length extracted from pi- d -> n n gamma is reduced by a factor larger than three, to <~0.05 fm.
%U http://arxiv.org/abs/nucl-th/0603045v2