Rare B decays with leptons at Belle

We present a new measurement of the purely leptonic decay B- -> tau- nu_tau-bar with a semileptonic B tagging method, using a data sample containing 657*10^6 BB-bar pairs collected with the Belle detector at the KEKB asymmetric e+e- collider. A sample of BB-bar pairs are tagged by reconstructing one B meson decaying semileptonically. We detect the B- -> tau- nu_tau-bar candidate in the recoil. We obtain a signal with a significance of 3.8 standard deviations including systematics, and measure the branching fraction to be B(B- -> tau- nu_tau-bar) = 1.65+0.38-0.37(stat)+0.35-0.37(syst))*10^-4. This result confirms the evidence for B- -> tau- nu_tau-bar obtained in the previous Belle measurement with a hadronic B tagging method. The B meson decay constant f_B and constraint on charged Higgs are obtained using the measured branching fraction.

K. Hara · 2008