PrimEx-Eta: A Precision Measurement of the Eta Meson Radiative Decay Width

The $\eta$ meson is a unique probe of QCD symmetry breaking. Of particular importance is the $\eta\rightarrow\gamma\gamma$ decay, as it proceeds via the chiral anomaly. In the chiral limit, the amplitude for the two-photon decay of the pure SU(3) states, $\eta_{0}$ and $\eta_{8}$, is exactly calculable, and therefore a precision measurement of the $\eta$ radiative decay width provides both a precision test of this chiral anomaly prediction as well as information about the $\eta$-$\eta'$ mixing angle. In the past, this 2$\gamma$ decay width has been measured both in a fixed target experiment utilizing the Primakoff effect and in $e^{+}e^{-}$ collider experiments. However, a large discrepancy between the results of the two types of experiments remains unresolved. The PrimEx-$\eta$ experiment in Hall D at Jefferson Lab will perform a precision measurement of $\Gamma_{\eta\rightarrow\gamma\gamma}$ via the Primakoff method to address this discrepancy, and to reduce the overall uncertainty. Additionally, it will allow to significantly reduce uncertainties on partial widths for all other $\eta$ decays. In this talk the motivation and experimental techniques will be discussed along with a presentation of data from the experiment's first phase which was completed in the Spring of 2019.