  Two-nucleon effective field theory (EFT) beyond momenta of order the pion
mass is studied for both cutoff regularization and dimensional regularization
with power divergence subtraction (PDS). Models with two mass scales illustrate
how effects of long-distance pion physics must be removed from the coefficients
that encode short-distance physics. The modified effective range expansion
shows that treating pions perturbatively, as in the PDS power counting, limits
the radius of convergence of the EFT. Predictions from both regularization
schemes with one-pion contributions are compared to data. The breakdown of the
effective field theory occurs for momenta of order 300 MeV, even using the
modified effective range expansion. This behavior is shown to be consistent
with that expected from two-pion contributions.
