Isoscalar M1 and E2 amplitudes in np → dγ

The low energy radiative capture process np --> dy provides a sensitive probe of the two-nucleon system. The cross section for this process is dominated by the isovector M1 amplitude for capture from the S-1(0) channel via the isovector magnetic moment of the nucleon. In this work we use effective field theory to compute the isoscalar M1 and isoscalar E2 amplitudes that are strongly suppressed for cold neutron capture. The actual value of the isoscalar E2 amplitude is expected to be within similar to 15% of the value computed 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 detail the deuteron quadrupole form factor and S-3(1)-D-3(1) mixing. (C) 1999 Elsevier Science B.V. All rights reserved.