n+p -> d+gamma in effective field theory

The radiative capture process n + p --> d + gamma provides clear evidence f or meson exchange currents in nuclear physics. We compute this process at l ow energies using a recently developed power counting for the effective fie ld theory that describes nucleon-nucleon interactions. The leading order co ntribution to this process comes from the photon coupling to the nucleon ma gnetic moments. At subleading order there are other contributions. Among th ese are graphs where the photon couples directly to pions, i.e. meson excha nge currents. These diagrams are divergent and require the presence of a lo cal four-nucleon-one-photon counterterm. The coefficient of this operator i s determined by the measured cross section, sigma(expt) = 334.2 +/- 0.5 mb, for incident neutrons with speed \upsilon\ = 2200 m/s. (C) 1999 Elsevier S cience B.V. All rights reserved.