Search for the decay $B_s^0\to \eta \eta$

We report results from a search for the decay $B_s^0\to \eta \eta$ using $121.4{\mathrm{fb}}^{-1}$ of data collected at the $\mathrm{\Upsilon }(5S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We do not observe any signal and set an upper limit on the branching fraction of $14.3\times {10}^{-5}$ at 90% confidence level. This result represents a significant improvement over the previous most stringent limit.

Figure 1

Feynman diagrams for $B_s^0\to \eta \eta$.

Figure 2

Fit results for the $B_s^0\to {\eta }_{\gamma \gamma }{\eta }_{\gamma \gamma }$ (top), $B_s^0\to {\eta }_{\gamma \gamma }{\eta }_{3\pi }$ (middle) and $B_s^0\to {\eta }_{3\pi }{\eta }_{3\pi }$ (bottom) modes. The projections are shown for events inside a signal region in $M_{\mathrm{bc}}$, $\mathrm{\Delta }E$ and ${\mathcal{C}}_{\mathrm{NN}}^{\prime }$, except for the variable plotted. The signal region is defined as $5.39<M_{\mathrm{bc}}<5.43\mathrm{GeV}/c^2$, $-0.27<\mathrm{\Delta }E<0.12\mathrm{GeV}$, and ${\mathcal{C}}_{\mathrm{NN}}^{\prime }>2.91.$ Points with the error bars show the data, solid black curves show the total fit function, and dotted red (dashed blue) curves show the signal (background) contribution.