Precision extraction of a_nn from pi- d --> n n gamma using chiral perturbation theory

The neutron-neutron scattering length a_nn provides a sensitive probe of charge-symmetry breaking in the strong interaction. Here we summarize our recent efforts to use chiral perturbation theory in order to systematically relate a_nn to the shape of the neutron spectrum in the reaction pi- d --> n n gamma. In particular we show how the chiral symmetry of QCD relates this process to low-energy electroweak reactions such as p p --> d e+ nu_e. This allows us to reduce the uncertainty in the extracted a_nn (mainly due to short-distance physics in the two-nucleon system) by a factor of more than three, to <0.05 fm. We also report first results on the impact that two-nucleon mechanisms of chiral order P^4 have on the pi- d --> n n gamma neutron spectrum.