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

We present a search for a non-Standard-Model invisible particle X-0 in the mass range 0.1-1.8 GeV/c(2) in B+ -> e(+)X(0) and B+ -> mu X-+(0) decays. The results are obtained from a 711 fb(-1) data sample that corresponds to 772 x 10(6) B (B) over bar 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).