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 $0.1\text{-}1.8\mathrm{G}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2$ in $B^{+}\to e^{+}{X}^0$ and $B^{+}\to {\mu }^{+}{X}^0$ decays. The results are obtained from a $711~{\rm fb}^{-1}$datasamplethatcorrespondsto$772 \times 10^{6} B\bar{B}$ pairs, collected at the $\mathrm{{\rm Y}}(4S)$ 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 ${10}^{-6}$.