2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/178234We 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.REVTeX4, 4 pages (5 pages since the arXiv doesn't collapse refs), 3 eps figures; revised version to appear in PRL, conclusions unchangedNuclear Theorytext