ABSTRACT
We present the results of a search for the bâdâ^{+}â^{-} flavor-changing neutral-current rare decays B^{+,0}â(η,ω,π^{+,0},ρ^{+,0})e^{+}e^{-} and B^{+,0}â(η,ω,π^{0},ρ^{+})µ^{+}µ^{-} using a 711 fb^{-1} data sample that contains 772×10^{6} BB[over ¯] events. The data were collected at the Ï(4S) resonance with the Belle detector at the KEKB asymmetric-energy e^{+}e^{-} collider. We find no evidence for signal and set upper limits on branching fractions at the 90% confidence level in the range (3.8-47)×10^{-8} depending on the decay channel. The obtained limits are the world's best results. This is the first search for the channels B^{+,0}â(ω,ρ^{+,0})e^{+}e^{-} and B^{+,0}â(ω,ρ^{+})µ^{+}µ^{-}.
ABSTRACT
We measure the tau-to-light-lepton ratio of inclusive B-meson branching fractions R(X_{τ/â})≡B(BâXτν)/B(BâXâν), where â indicates an electron or muon, and thereby test the universality of charged-current weak interactions. We select events that have one fully reconstructed B meson and a charged lepton candidate from 189 fb^{-1} of electron-positron collision data collected with the Belle II detector. We find R(X_{τ/â})=0.228±0.016(stat)±0.036(syst), in agreement with standard-model expectations. This is the first direct measurement of R(X_{τ/â}).
ABSTRACT
We present the first comprehensive tests of the universality of the light leptons in the angular distributions of semileptonic B^{0}-meson decays to charged spin-1 charmed mesons. We measure five angular-asymmetry observables as functions of the decay recoil that are sensitive to lepton-universality-violating contributions. We use events where one neutral B is fully reconstructed in Ï(4S)âBB[over ¯] decays in data corresponding to 189 fb^{-1} integrated luminosity from electron-positron collisions collected with the Belle II detector. We find no significant deviation from the standard model expectations.