While chiral perturbation theory for mesons is characterized by a momentum expansion in $Q/{\mathrm{\Lambda }}_{\chi }$ with ${\mathrm{\Lambda }}_{\chi }\approx 1\mathrm{GeV}$, existing formulations of effective theory for nucleon-nucleon scattering deviate from data at $Q\approx 300\mathrm{MeV}$ or lower. We offer heuristic evidence that unsuspected nonanalytic structure exists in the complex momentum plane obstructing the effective field theory expansion in the spin-triplet channels, associated with the peak of the angular-momentum barrier whose energy in low partial waves satisfies $k=\sqrt{ME}\approx 300\mathrm{MeV}$. With this motivation, we construct a meromorphic function of $k^2$ we call the $C$-matrix, for which the radius of convergence of its Taylor expansion in $k^2$ is equivalent to that of the momentum expansion of the effective field theory. Thus the range of validity of the effective theory is directly related to the pole structure of the $C$-matrix. We uncover that pole structure and confirm that it is the source of the obstruction. The systematic inclusion of dimer fields as propagating degrees of freedom in the effective theory to account for those poles results in cutoff insensitive fits at order $Q^0$ to most of the lower partial-wave phase shifts up to the pion production threshold, using only the one pion exchange part of the long-range nucleon-nucleon interaction. Our theory should be applicable to the singular potentials regularly found in atomic physics as well.

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• Effective field theory
• Few-body systems
• Nuclear forces