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The neutron-neutron scattering length a_nn provides a sensitive probe ofcharge-symmetry breaking in the strong interaction. Here we summarize ourrecent efforts to use chiral perturbation theory in order to systematicallyrelate a_nn to the shape of the neutron spectrum in the reaction pi- d --> n ngamma. In particular we show how the chiral symmetry of QCD relates thisprocess to low-energy electroweak reactions such as p p --> d e+ nu_e. Thisallows us to reduce the uncertainty in the extracted a_nn (mainly due toshort-distance physics in the two-nucleon system) by a factor of more thanthree, to <0.05 fm. We also report first results on the impact that two-nucleonmechanisms of chiral order P^4 have on the pi- d --> n n gamma neutronspectrum.

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