Measurement of exclusive π⁰ and η photoproduction in the Primakoff region with a linearly polarized photon beam at GlueX

A critical test for low-energy QCD is the decay of the π⁰. Comparing this analytic prediction with measured data thus provides a valuable test for low energy QCD and symmetry breaking. The lifetime of the π⁰ can be measured using  exclusive π⁰ photoproduction in the Coulomb field of a nucleus, referred to as the Primakoff effect. Using this technique, the PrimEx-I and II experiments found a combined radiative width measurement of  7.806 +/- 0.117 eV, which agrees with the chiral anomaly prediction within 0.72%, however there is tension with NLO calculations at the level of 2 standard deviations. Exclusive π⁰ and η photoproduction were observed during the Charged Pion Polarizability experiment at JLab’s Hall D, which utilized a linearly polarized 4.3-12 GeV photon beam incident on a Pb208 target. Exclusive π⁰ and η photoproduction yields between 0-4^° lab angle have been extracted from the data. The π⁰ yields are fitted with theoretical terms convoluted with the angular resolution and experimental acceptance, and compared with the PrimEx-I Pb208 result. We also present the first measurement of the linear polarization asymmetry for π⁰ photoproduction in the Primakoff region, which is sensitive to nuclear incoherent processes.