Search for B→K^{(*)}νν̅ and Invisible Quarkonium Decays

We search for the flavor-changing neutral-current decays B→K^{()}νν¯, and the invisible decays J/ψ→νν̅ and ψ(2S)→νν̅ via B→K^{()}J/ψ and B→K^{()}ψ(2S), respectively, using a data sample of 471×10⁶ BB̅ pairs collected by the BABAR experiment. We fully reconstruct the hadronic decay of one of the B mesons in the Ψ(4S)→ BB̅ decay, and search for the B→K^{()}νν̅ decay in the rest of the event. We observe no significant excess of signal decays over background and report branching fraction upper limits of B(B⁺→K⁺νν̅) < 3.7×10⁻⁵, B(B⁰→K⁰ νν̅) <8.1×10⁻⁵, B(B⁺→K⁺ νν̅) < 11.6×10⁻⁵, B(B⁰→K⁰ νν̅) < 9.3×10⁻⁵, and combined upper limits of B(B→Kνν̅) <3.2×10⁻⁵ and B(B→Kνν̅) < 7.9×10⁻⁵, all at the 90% confidence level. For the invisible quarkonium decays, we report branching fraction upper limits of B(J/ψ→νν̅) < 3.9×10⁻³ and B(ψ(2S)→νν̅) < 15.5×10⁻³ at the 90% confidence level. Using the improved kinematic resolution achieved from hadronic reconstruction, we also provide partial branching fraction limits for the B→K^{()}νν̅ decays over the full kinematic spectrum.