Isoscalar M1 and E2 amplitudes in np -> d gamma

The low energy radiative capture process np --> dy provides a sensitive pro be of the two-nucleon system. The cross section for this process is dominat ed by the isovector M1 amplitude for capture from the S-1(0) channel via th e isovector magnetic moment of the nucleon. In this work we use effective f ield theory to compute the isoscalar M1 and isoscalar E2 amplitudes that ar e strongly suppressed for cold neutron capture. The actual value of the iso scalar E2 amplitude is expected to be within similar to 15% of the value co mputed in this work. In contrast, due to the vanishing contribution of the one-body operator at leading order and next-to-leading order, the isoscalar M1 amplitude is estimated to have a large uncertainty. We discuss in detai l the deuteron quadrupole form factor and S-3(1)-D-3(1) mixing. (C) 1999 El sevier Science B.V. All rights reserved.