Photos: MENU-2019, the 15th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon

The neutron-neutron scattering length ann 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 ann to the shape of the neutron spectrum in the reaction π d→ nnγ. In particular we show how the chiral symmetry of QCD relates this process to low-energy electroweak reactions such as pp → deνe. This allows us to reduce the uncertainty in the extracted ann (mainly due to shortdistance 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 πd→ nnγ neutron spectrum.