Search for a massive invisible particle $X^0$ in $B^{+}to e^{+}X^{0}$ and $B^{+}to mu^{+}X^{0}$ decays

We present a search for a non-Standard-Model invisible particle $X^0$ in the mass range $0.1textrm{-}1.8 ,{rm GeV}/{c^2}$ in $B^{+}to e^{+} X^{0}$ and $B^{+}to mu^{+} X^{0}$ decays. The results are obtained from a $711~{rm fb}^{-1}$ data sample that corresponds to $772 times 10^{6} Bbar{B}$ pairs, collected at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. One $B$ meson is fully reconstructed in a hadronic mode to determine the momentum of the lepton of the signal decay in the rest frame of the recoiling partner $B$ meson. We find no evidence of a signal and set upper limits on the order of $10^{-6}$.