Evidence for the decay $B^{0}to eta eta$

We report a search for $B^{0}to eta eta $ with a data sample corresponding to an integrated luminosity of $698 ,{rm fb}^{-1}$ containing $753 times 10^{6}$ $Bbar{B}$ pairs collected at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. The branching fraction is measured to be $mathcal{B}(B^{0} to eta eta ) = (7.6^{+2.7 +1.4}_{-2.3 -1.6}) times 10^{-7}$ at the level of 3.3 standard deviations above zero, which provides the first evidence for the decay $B^{0} to eta eta$.