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Register a new userSearch for $B^+ to e^+ u$ and $B^+ to mu^+ u$ decays using hadronic tagging
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We present a search for the rare leptonic decays $B^+ to e^+ u_e$ and $B^+ to mu^+ u_mu$, using the full $Upsilon(4S)$ data sample of $772times 10^6$ $Bbar{B}$ pairs collected with the Belle detector at the $smalltextbf{KEKB}$ asymmetric-energy $e^+ e^-$ collider. One of the $B$ mesons from the $Upsilon(4S) to Bbar{B}$ decay is fully reconstructed in a hadronic mode, while the recoiling side is analyzed for the signal decay. We find no evidence of a signal in any of the decay modes. Upper limits of the corresponding branching fractions are determined as ${cal B}(B^+ to e^+ u_e)<3.5times 10^{-6}$ and ${cal B}(B^+ to mu^+ u_mu)<2.7 times 10^{-6}$ at 90$%$ confidence level.
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We present a search for the rare leptonic decays $B^+ to e^+ u_e$ and $B^+ to mu^+ u_mu$, using the full $Upsilon(4S)$ data sample of $772times 10^6$ $Bbar{B}$ pairs collected with the Belle detector at the $rm {small KEKB}$ asymmetric-energy $e^+ e^-$ collider. One of the $B$ mesons from the $Upsilon(4S) to Bbar{B}$ decay is fully reconstructed in a hadronic mode, while the recoiling side is analyzed for the signal decay. We find no evidence of a signal in any of the decay modes. Upper limits of the corresponding branching fractions are determined as ${cal B}(B^+ to e^+ u_e)<3.5times 10^{-6}$ and ${cal B}(B^+ to mu^+ u_mu)<2.7 times 10^{-6}$ at 90$%$ confidence level.
A search for the flavor-changing neutral-current decay $B^{+}to K^{+} ubar{ u}$ is performed at the Belle II experiment at the SuperKEKB asymmetric energy electron-positron collider. The results are based on a data sample corresponding to an integrated luminosity of $63,mbox{fb}^{-1}$ collected at the $Upsilon{(4S)}$ resonance and a sample of $9,mbox{fb}^{-1}$ collected at an energy $60mathrm{,Mekern -0.1em V}$ below the resonance. A novel measurement method is employed, which exploits topological properties of the $B^{+}to K^{+} ubar{ u}$ decay that differ from both generic bottom-meson decays and light-quark pair production. This inclusive tagging approach offers a higher signal efficiency compared to previous searches. No significant signal is observed. An upper limit on the branching fraction of $B^{+}to K^{+} ubar{ u}$ of $4.1 times 10^{-5}$ is set at the 90% confidence level.
We report the result for a search for the leptonic decay of $B^+ to mu^+ , u_{mu}$ using the full Belle data set of 711 fb${}^{-1}$ of integrated luminosity at the $Upsilon(4S)$ resonance. In the Standard Model leptonic $B$-meson decays are helicity and CKM suppressed. To maximize sensitivity an inclusive tagging approach is used to reconstruct the second $B$ meson produced in the collision. The directional information from this second $B$ meson is used to boost the observed $mu$ into the signal $B$ meson rest-frame, in which the $mu$ has a monochromatic momentum spectrum. Though its momentum is smeared by the experimental resolution, this technique improves the analysis sensitivity considerably. Analyzing the $mu$ momentum spectrum in this frame we find $mathcal{B}(B^+ to mu^+ , u_mu) = left( 5.3 pm 2.0 pm 0.9 right) times 10^{-7}$ with a one-sided significance of 2.8 standard deviations over the background-only hypothesis. This translates to a frequentist upper limit of $mathcal{B}(B^+ to mu^+ , u_{mu}) < 8.6 times 10^{-7}$ at 90% CL. The experimental spectrum is then used to search for a massive sterile neutrino, $B^+ to mu^+ , N$, but no evidence is observed for a sterile neutrino with a mass in a range of 0 - 1.5 GeV. The determined $B^+ to mu^+ , u_{mu}$ branching fraction limit is further used to constrain the mass and coupling space of the type II and type III two-Higgs-doublet models.
This contribution illustrates a new search for the flavor-changing neutral-current decay ${B^+ to K^+ u bar u}$ performed by the Belle II experiment at the SuperKEKB asymmetric-energy electron-positron collider. In this study, a sample corresponding to an integrated luminosity of $63, rm{fb}^{-1}$ collected at the $rm Upsilon(4S)$ resonance and an additional sample of $9, rm{fb}^{-1}$ collected at an energy $60, rm MeV$ below the resonance are used. A novel technique, based on an inclusive tagging method and exploiting the topological features of the ${B^+ to K^+ u bar u}$ decay, is employed and it provides a higher signal efficiency with respect to the methods used in the previous searches. No significant signal is observed. An upper limit of $4.1 times 10^{-5}$ is set on the ${B^+ to K^+ u bar u}$ branching fraction at the $90, %$ confidence level.
The rare pion decays ${pi}^+{rightarrow}{mu}^+{ u}_{mu}{ u}bar{ u}$ and ${pi}^+{rightarrow}e^+{ u}_{e}{ u}bar{ u}$ are allowed in the Standard Model but highly suppressed. These decays were searched for using data from the PIENU experiment. A first result for ${Gamma}({pi}^+{rightarrow}{mu}^+{ u}_{mu}{ u}bar{ u})/{Gamma}({pi}^+{rightarrow}{mu}^+{ u}_{mu})<8.6{times}10^{-6}$, and an improved measurement ${Gamma}({pi}^+{rightarrow}{e}^+{ u}_{e}{ u}bar{ u})/{Gamma}({pi}^+{rightarrow}{mu}^+{ u}_{mu})<1.6{times}10^{-7}$ were obtained.
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