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.
We compare two non-relativistic (NR) reduction schemes (heavy-fermion and Foldy-Wouthuysen) that are used to derive low-energy effective-field-theory Lagrangians. We give the explicit transformation between the two types of fields to O(1/m^2), derive
d from a quite general, relativistic Lagrangian. Beyond leading order the NR reductions always involve the smaller components of the Dirac spinors that are to be integrated out to formulate the NR theory. Even so, the transformation between the NR Lagrangians can be carried out explicitly to O(1/m^2) using a field renormalization, as long as the lower components of the Lagrangian are known. The fixed coefficient corrections to some low-energy constants at O(1/m^2) will depend on the particular scheme chosen, but will match after the field renormalization.
