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 Γ1s 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 Γ1s=(1171-49+23) meV yielding α=(252-11+5) μb.