Moments of angular distribution and beam asymmetries in ηπ0 photoproduction at GlueX

In the search for exotic mesons, the GlueX Collaboration will soon extract moments of the ηπ0 angular distribution. In the perspective of these results, we generalize the formalism of moment extraction to the case in which the two mesons are produced with a linearly polarized beam and build a model for the reaction γ→p→ηπ0p. The model includes resonant S, P, and D waves in ηπ0, produced by natural exchanges. Moments of the ηπ0 angular distribution are computed with and without the P wave, to illustrate the sensitivity to exotic resonances. Although little sensitivity to the P wave is found in moments of even angular momentum, moments of odd angular momentum are proportional to the interference between the P wave and the dominant S and D waves. We also generalize the definition of the beam asymmetry for two mesons photoproduction and show that, when the meson momenta are perpendicular to the reaction plane, the beam asymmetry enhances the sensitivity to the exotic P wave.