Observation of the production of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>η</mml:mi></mml:math>mesons in two-photon collisions

Using 2674 ${\mathrm{nb}}^{\ensuremath{-}1}$ of data taken at $\sqrt{s}$ from 5.00 to 7.25 GeV with a trigger sensitive to decays of lower-mass particles produced in two-photon collisions, we have observed 56\ifmmode\pm\else\textpm\fi{}12 events consistent with the reaction ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}\ensuremath{\eta}$, $\ensuremath{\eta}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$. Background has been subtracted using separated-beam data. We obtain ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}(\ensuremath{\eta})=0.56\ifmmode\pm\else\textpm\fi{}0.16$ keV and the pseudoscalar-nonet mixing angle ${\ensuremath{\theta}}_{P}=\ensuremath{-}17.6\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}3.6\ifmmode^\circ\else\textdegree\fi{}$.