Measurement of the two-photon width of theηandη′mesons

Formation of the $\eta$ and ${\eta }^{\prime }$ mesons in two-photon interactions has been observed with the ASP detector at the SLAC $e^{+}{e}^{-}$ storage ring PEP ($\sqrt{s}=29\mathrm{}$ GeV). In a data sample of 108 ${\mathrm{pb}}^{-1\mathrm{}}$, a total of $2287 \eta$ and $547 {\eta }^{\prime }$ events have been detected in the $\gamma \gamma$ decay mode. The two-photon widths are determined to be ${\Gamma }_{\gamma \gamma }\left(\eta \right)=0.490\mathrm{}\pm 0.010\pm 0.048$ keV and ${\Gamma }_{\gamma \gamma }\left({\eta }^{\prime }\right)=4.96\mathrm{}\pm 0.23\pm 0.72$ keV. The SU(3) pseudoscalar mixing angle deduced from these values is ${\theta }_p=-19.8\mathrm{}°\pm 2.2°$.