Unitarity Constraint for Threshold Coherent Pion Photoproduction on the Deuteron and Chiral Perturbation Theory

The contribution of the two-step process fld ! pn ! ß 0 d to the imaginary part of the amplitude for coherent pion production on the deuteron is calculated exploiting unitarity constraints. The result shows that this absorptive process is not negligible and has to be considered in an extraction of the elementary neutron production amplitude from the fld ! ß 0 d cross section at threshold. In addition, it is argued that a consistent calculation of fld ! ß 0 d in baryon chiral perturbation theory beyond next-to-leading order requires the inclusion of this absorptive process. PACS numbers: 21.45.+v, 25.20.Lj Typeset using REVT E X Recently there has been considerable interest in the coherent electromagnetic production of pions from the deuteron near threshold. The main motivation thereby is to gain information on the elementary neutron amplitude fln ! ß 0 n which is experimentally not directly accessible. A first measurement of the ed ! eß 0 d reaction near q 2 ¯ = \Gamma0:0..