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Register a new userSearch for the rare decay of $B^+ to ell^{,+} u_{ell} gamma$ with improved hadronic tagging
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We present the result of the search for the rare $B$ meson decay of $B^+ to ell^{,+} u_{ell} gamma$ with $ell =e,mu$. For the search the full data set recorded by the Belle experiment of $711 , mathrm{fb}^{-1}$ integrated luminosity near the $Upsilon (4S)$ resonance is used. Signal candidates are reconstructed for photon energies $E_{gamma}$ larger than $1 , mathrm{GeV}$ using a novel multivariate tagging algorithm. The novel algorithm fully reconstructs the second $B$ meson produced in the collision using hadronic modes and was specifically trained to recognize the signal signature in combination with hadronic tag-side $B$ meson decays. This approach greatly enhances the performance. Background processes that can mimic this signature, mainly charmless semileptonic decays and continuum processes, are suppressed using multivariate methods. The number of signal candidates is determined by analyzing the missing mass squared distribution as inferred from the signal side particles and the kinematic properties of the tag-side $B$ meson. No significant excess over the background-only hypothesis is observed and upper limits on the partial branching fraction $ Delta mathcal{B} $ with $E_{gamma}> 1 , mathrm{GeV}$ individually for electron and muon final states as well as for the average branching fraction of both lepton final states are reported. We find a Bayesian upper limit of $Delta mathcal{B}( B^{+} to ell^{, +} u_{ell} gamma) < 3.0 times 10^{-6}$ at 90% CL and also report an upper limit on the first inverse moment of the light-cone distribution amplitude of the $B$ meson of $lambda_B$ at 90% CL.
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We present measurements of partial branching fractions of inclusive semileptonic $B to X_u , ell^+, u_{ell}$ decays using the full Belle data set of 711 fb$^{-1}$ of integrated luminosity at the $Upsilon(4S)$ resonance and for $ell = e, mu$. Inclusive semileptonic $B to X_u , ell^+, u_{ell}$ decays are CKM suppressed and measurements are complicated by the large background from CKM-favored $B to X_c , ell^+, u_{ell}$ transitions, which have a similar signature. Using machine learning techniques, we reduce this and other backgrounds effectively, whilst retaining access to a large fraction of the $B to X_u , ell^+, u_{ell}$ phase space and high signal efficiency. We measure partial branching fractions in three phase-space regions covering about $31%$ to $86%$ of the accessible $B to X_u , ell^+, u_{ell}$ phase space. The most inclusive measurement corresponds to the phase space with lepton energies of $E_ell^B > 1 $ GeV, and we obtain $Delta mathcal{B}(B to X_u ell^+ , u_ell) = left( 1.59 pm 0.07 pm 0.16 right) times 10^{-3}$ from a two-dimensional fit of the hadronic mass spectrum and the four-momentum-transfer squared distribution, with the uncertainties denoting the statistical and systematic error. We find $left| V_{ub} right| = left( 4.10 pm 0.09 pm 0.22 pm 0.15 right) times 10^{-3}$ from an average of four calculations for the partial decay rate with the third uncertainty denoting the average theory error. This value is higher but compatible with the determination from exclusive semileptonic decays within 1.3 standard deviations. In addition, we report charmless inclusive partial branching fractions separately for $B^+$ and $B^0$ mesons as well as for electron and muon final states. No isospin breaking or lepton flavor universality violating effects are observed.
We present the measurement of the first to fourth order moments of the four-momentum transfer squared, $q^2$, of inclusive $B rightarrow X_c ell^+ u_{ell}$ decays using the full Belle data set of 711 $mathrm{fb}^{-1}$ of integrated luminosity at the $Upsilon(4S)$ resonance where $ell = e, mu$. The determination of these moments and their systematic uncertainties open new pathways to determine the absolute value of the CKM matrix element $V_{cb}$ using a reduced set of matrix elements of the heavy quark expansion. In order to identify and reconstruct the $X_c$ system, we reconstruct one of the two $B$-mesons using machine learning techniques in fully hadronic decay modes. The moments are measured with progressively increasing threshold selections on $q^2$ starting with a lower value of 3.0 $mathrm{GeV}^2$ in steps of 0.5 $mathrm{GeV}^2$ up to a value of 10.0 $mathrm{GeV}^2$. The measured moments are further unfolded, correcting for reconstruction and selection effects as well as QED final state radiation. We report the moments separately for electron and muon final states and observe no lepton flavor universality violating effects.
We present a search for the lepton-flavor-violating decays $B^{0} to tau^pm ell^mp$, where $ell=(e,,mu)$, using the full data sample of $772 times 10^6$ $B overline{B}$ pairs recorded by the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We use events in which one $B$ meson is fully reconstructed in a hadronic decay mode. The $tau^pm$ lepton is reconstructed indirectly using the momentum of the reconstructed $B$ and that of the $ell^mp$ from the signal decay. We find no evidence for $B^{0} to tau^pm ell^mp$ decays and set upper limits on their branching fractions at 90% confidence level of $cal B$($B^{0} to tau^pm mu^{mp}$)$ < 1.5 times 10^{-5}$ and $cal B$($B^{0} to tau^pm e^{mp}$)$ < 1.6 times 10^{-5}$.
We predict the amplitude of the $Bto K ell^+ell^-$ decay in the region of the dilepton invariant mass squared $0<q^2leq m_{J/psi}^2$, that is, at large hadronic recoil. The $Bto K$ form factors entering the factorizable part of the decay amplitude are obtained from QCD light-cone sum rules. The nonlocal effects, generated by the four-quark and penguin operators combined with the electromagnetic interaction, are calculated at $q^2<0$, far below the hadronic thresholds. For hard-gluon contributions we employ the QCD factorization approach. The soft-gluon nonfactorizable contributions are estimated from QCD light-cone sum rules. The result of the calculation is matched to the hadronic dispersion relation in the variable $q^2$, which is then continued to the kinematical region of the decay. The overall effect of nonlocal contributions in $Bto Kell^+ell^-$ at large hadronic recoil is moderate. The main uncertainty of the predicted $Bto K ell^+ell^-$ partial width is caused by the $Bto K$ form factors. Furthermore, the isospin asymmetry in this decay is expected to be very small. We investigate the deviation of the observables from the Standard Model predictions by introducing a generic new physics contribution to the effective Hamiltonian.
The branching fractions of the decays $B^{+} to eta ell^{+} u_{ell}$ and $B^{+} to eta^{prime} ell^{+} u_{ell}$ are measured, where $ell$ is either an electron or a muon, using a data sample of $711,{rm fb}^{-1}$ containing $772 times 10^6 Bbar{B}$ pairs collected at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. To reduce the dependence of the result on the form factor model, the measurement is performed over the entire $q^2$ range. The resulting branching fractions are ${cal B}(B^{+} rightarrow eta ell^{+} u_{ell}) = (2.83 pm 0.55_{rm (stat.)} pm 0.34_{rm (syst.)}) times 10^{-5}$ and ${cal B}(B^{+} rightarrow eta ell^{+} u_{ell}) = (2.79 pm 1.29_{rm (stat.)} pm 0.30_{rm (syst.)}) times 10^{-5}$.
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