The up and down quarks do not have equal masses, and this difference gives rise to violations of isospin symmetry in hadronic systems. In the two-nucleon system one manifestation of the quark-mass difference is that the neutron-neutron scattering length, ann, is different from the proton-proton scattering length, app. But, despite much work and many years of controversy, the value of ann-app is still not well-established. In this talk I will first explain how information on ann can be extracted from the capture of stopped pions by deuterium nuclei. I will then argue that chiral perturbation theory provides a sytematic tool with which to perform such an extraction. I will describe a recent calculation which computed the spectrum of neutrons obtained from pi- d->gamma nn up to O(Q^3) in the chiral expansion, and I will identify one important source of uncertainty in the calculation at that order. I will then explain how that source of uncertainty can be removed using information from low-energy weak processes in the two-nucleon system, such as proton-proton fusion. When combined with other improvements in our O(Q^3) calculation this will yield a computation of pi- d->nn gamma with a theoretical error bar that is more than a factor of two smaller than that of any previous model.
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