A Precision Measurement of the η Radiative Decay Width via the Primakoff Effect

We propose to perform a measurement of the η → γγ decay width at a 3% precision via the Primakoff effect in Hall D with the GlueX standard setup. This experiment will not only resolve a long standing puzzle of the experimental discrepancy between the Primakoff and collider measurement results, but will also significantly reduce the overall error bar on this important quantity, resulting in a direct improvement on all other partial η decay widths. A high precision measurement of the η → γγ decay width will have significant impact on the experimental determination of fundamental parameters, such as the ratios of the light quark masses (mu, md, ms) and the η − η ′ mixing angle, and will provide a sensitive test of QCD symmetries in the chiral limit. This experiment will also deliver the first cross section measurement on the γp→ ηp elementary process at the forward angles in the 10 GeV energy range.