Search for a massive invisible particle X⁰ in B⁺ → e⁺X⁰ and B⁺ → #mu#⁺X⁰ decays

We present a search for a non-Standard-Model invisible particle X⁰ in the mass range 0.1–1.8 GeV/c² in B⁺ → e⁺X⁰ and B⁺ → #mu#⁺X⁰ decays. The results are obtained from a 711 fb⁻¹ data sample which corresponds to 772 × 10⁶$B\overline{B}$ pairs, collected at the Υ(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.