Suppressed Amplitudes in np → dγ

The low energy radiative capture process np → dγ 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 S0 channel via the isovector magnetic moment of the nucleon. In this work we use effective field theory to compute the isoscalar M1, isoscalar E2 and isovector E1 amplitudes that are strongly suppressed for cold neutron capture. The actual value of the isoscalar E2 amplitude is expected to be within ∼ 10% 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.