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 $0.1\textrm{-}1.8 \,{\rm GeV}/{c^2}$ 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 $772 \times 10^{6} B\bar{B}$ 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 $10^{-6}$. 
C.-S. Park, Y.-J. Kwon, I. Adachi, H. Aihara, D. M. Asner, T. Aushev,   V. Babu, I. Badhrees, A. M. Bakich, E. Barberio, P. ...
hep-ex