Precision extraction of $a_{nn}$ from $\pi^-d\to nn\gamma$ using $\chi$PT

We present an updated chiral perturbation theory calculation of the $\pi^-d\to nn\gamma$ reaction. By using other observables to constrain the unknown short-distance physics, we can drastically reduce the theoretical error in the extracted neutron-neutron scattering length. Further consequences of the results will be discussed.