We report measurements of the photon beam asymmetry Σ for the reactions γp → pπ 0 and γp → pη from the GLUEX experiment using a 9 GeV linearly-polarized, tagged photon beam incident on a liquid hydrogen target in Jefferson Lab's Hall D. The asymmetries, measured as a function of the proton momentum transfer, possess greater precision than previous π 0 measurements and are the first η measurements in this energy regime. The results are compared with theoretical predictions based on t-channel, quasi-particle exchange and constrain the axial-vector component of the neutral meson production mechanism in these models.

Study of the rare and forbidden decays of η/η′ offers a sensitive probe to test fundamental symmetries of quantum chromodynamics and search for new physics beyond the Standard Model. To study the rare decays of η/η′ to π+π−e+e−, π+π−μ+μ− and e+e−μ+μ− at the BESIII detector, we developed several event generators based on the vector meson dominant model with finite-width corrections and the pseudoscalar mesons mixing theory. The various distributions from event generators are in good agreement with the theoretical predictions, which indicates that the event generators work very well after implemention in the BESIII Monte Carlo simulation package. In the BESIII physics analysis, the performance of the event generators will be improved in accordance with the distributions of different variables of η/η′ from data and the improvement on the theoretical calculations.

Instead of the usual zero-width approximation, the finite-width approximation for the resonance, the ρ-ω mesons, is used to investigate the light-cone local QCD sum rules for the form factors of the transitions, ργ*→π0and γγ*→π0. To be more certain, a product of two weight functions is chosen in the method of the analytic continuation by duality to annihilate the uncertainty in the continuum region and the integrand in the integration range not covered by vector-meson dominance and where the theoretical expression may least be trusted. The resultant form factors in the cases for the zero- and finite-widths are almost the same, both agree well with the experimental measurements, and comparison with the results from the Laplacian transformed light-cone sum rules and etc. is discussed.

The light unflavoured meson η/η decays are valuable for testing non-perturbative quantum chromodynamics and exploring new physics beyond the Standard Model. This paper describes a series of event generators, including η/η → γl, which have been developed for investigating η/η decay dynamics. For most of these generators, their usability has been validated in BESIII analyses for determining the detection efficiency, and background studies. The consistency between data and Monte Carlo shows that these generators work well in the BESIII simulation, and will also be useful for ongoing BESIII analyses and other experiments for studying η/η physics.