Precision Determination of the dπ↔NN Transition Strength at Threshold

An unusual but effective way to determine at threshold the dπ↔NN transition strength α is to exploit the hadronic ground-state broadening Γ_1s in pionic deuterium, accessible by x-ray spectroscopy. The broadening is dominated by the true absorption channel dπ^-→nn, which is related to s-wave pion production pp→dπ⁺ by charge symmetry and detailed balance. Using the exotic atom circumvents the problem of Coulomb corrections to the cross section as necessary in the production experiments. Our dedicated measurement finds Γ_1s=(1171_-49⁺²³)meV yielding α=(252_-11⁺⁵)μb.