Relation between Scattering and Absorption in the Pais-Piccioni Phenomenon

The expressions for the ${\theta }_1$ and ${\theta }_2$ amplitudes in a beam of neutral $\theta$ mesons traversing an absorber are put in terms of forward-scattering amplitudes. It is discovered that a phase-shift term as well as an absorption term is needed to describe the regeneration of ${\theta }_1$ mesons in the unscattered beam.