Observation of the exclusive reaction $e^{+}{e}^{-}\to \varphi \eta$ at $\sqrt{s}=10.58\mathrm{GeV}$

We report the observation of $e^{+}{e}^{-}\to \varphi \eta$ near $\sqrt{s}=10.58\mathrm{GeV}$ with $6.5\sigma$ significance in the $K^{+}{K}^{-}\gamma \gamma$ final state in a data sample of $224{\mathrm{fb}}^{-1}$ collected by the BABAR experiment at the PEP-II $e^{+}{e}^{-}$ storage rings. We measure the restricted radiation-corrected cross section to be $\sigma (e^{+}{e}^{-}\to \varphi \eta )=2.1\pm 0.4(\mathrm{stat})\pm 0.1(\mathrm{syst})\mathrm{fb}$ within the range $|\cos {\theta }^{*}|<0.8$, where ${\theta }^{*}$ is the center-of-mass polar angle of the $\varphi$ meson. The $\varphi$ meson is required to be in the invariant mass range of $1.008<m_{\varphi }<1.035\mathrm{GeV}/c^2$. The radiation-corrected cross section in the full $\cos {\theta }^{*}$ range is extrapolated to be $2.9\pm 0.5(\mathrm{stat})\pm 0.1(\mathrm{syst})\mathrm{fb}$.

Figure 1

Possible Feynman diagram for $e^{+}{e}^{-}\to \varphi \eta$.

Figure 2

(a) Distribution of the invariant mass (${\rm Y}(4S)$ data) for the $K^{+}{K}^{-}\gamma \gamma$ final state near the $\varphi \eta$ region. The accepted signal region is indicated by the lines. (b) Scatter plot of the invariant masses of the $K^{+}{K}^{-}$ and $\gamma \gamma$ pairs for those events in the accepted signal region.

Figure 3

Mass projections for (a) $K^{+}{K}^{-}$ pairs and (b) $\gamma \gamma$ pairs in $K^{+}{K}^{-}\gamma \gamma$ events.

Figure 4

 Cross section extrapolations based on BABAR’s measurement at $\sqrt{s}=10.58\mathrm{GeV}$ assuming $1/s^3$ (black) or $1/s^4$ (red) energy dependence. The bands show 1 standard deviation uncertainties in the extrapolations. The CLEO measurement at $\sqrt{s}=3.67\mathrm{GeV}$ is also shown.