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We search for the rare flavor-changing neutral current process $B^- to Lambda {overline p} u{overline{ u}}$ using data from the BABAR experiment. A total of 424 fb$^{-1}$ of $e^+e^-$ collision data collected at the center-of-mass energy of the $Upsilon$(4S) resonance is used in this study, corresponding to a sample of ${(471 pm 3) times 10^{6}}$ $Boverline{B}$ pairs. Signal $B^- to Lambda {overline p} u{overline{ u}}$ candidates are identified by first fully reconstructing a $B^+$ decay in one of many possible exclusive decays to hadronic final states, then examining detector activity that is not associated with this reconstructed $B^+$ decay for evidence of a signal $B^- to Lambda {overline p} u{overline{ u}}$ decay. The data yield is found to be consistent with the expected background contribution under a null signal hypothesis, resulting in an upper limit of ${{cal B} (B^- to Lambda {overline p} u{overline{ u}}) < 3.0times 10^{-5}}$ at the $90%$ confidence level.
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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.
We evaluate long-distance electromagnetic (QED) contributions to $bar{B}{}^0 to D^+ tau^{-} bar{ u}_{tau}$ and $B^- to D^0 tau^{-} bar{ u}_{tau}$ relative to $bar{B}{}^0 to D^+ mu^{-} bar{ u}_{mu}$ and $B^- to D^0 mu^{-} bar{ u}_{mu}$, respectively, in the standard model. We point out that the QED corrections to the ratios $R(D^{+})$ and $R(D^{0})$ are not negligible, contrary to the expectation that radiative corrections are almost canceled out in the ratio of the two branching fractions. The reason is that long-distance QED corrections depend on the masses and relative velocities of the daughter particles. We find that theoretical predictions for $R(D^{+})^{tau/mu}$ and $R(D^{0})^{tau/mu}$ can be amplified by $sim4%$ and $sim3%$, respectively, for the soft-photon energy cut in range $20$-$40$ MeV.
We report a measurement of the branching fraction ratios R(D(*)) of Bbar -> D(*) tau- nubar_tau relative to Bbar -> D()* l- nubar_l (where l = e or mu) using the full Belle data sample of 772 x 10^6 BBbar pairs collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. The measured values are R(D)= 0.375 +- 0.064(stat.) +- 0.026(syst.) and R(D*) = 0.293 +- 0.038(stat.) +- 0.015(syst.). The analysis uses hadronic reconstruction of the tag-side B meson and purely leptonic tau decays. The results are consistent with earlier measurements and do not show a significant deviation from the standard model prediction.
We report the results of a search for the decay $K^+ to pi^+ u bar u$ in the kinematic region with $pi^+$ momentum $140 < P < 195$ MeV/c using the data collected by the E787 experiment at BNL. No events were observed. When combined with our previous search in this region, one candidate event with an expected background of $1.22 pm 0.24$ events results in a 90% C.L. upper limit of $2.2 times 10^{-9}$ on the branching ratio of $K^+ to pi^+ u bar u$. We also report improved limits on the rates of $K^+ to pi^+ X^0$ and $K^+ to pi^+ X^1 X^2$ where $X^0, X^1, X^2$ are hypothetical, massless, long-lived neutral particles.
We present the results of searches for nucleon decay via bound neutron to antineutrino plus pizero and proton to antineutrino plus piplus using data from a combined 172.8 kiloton-years exposure of Super-Kamiokande-I, -II, and -III. We set lower limits on the partial lifetime for each of these modes. For antineutrino pizero, the partial lifetime is >1.1x10^{33} years; for antineutrino piplus, the partial lifetime is >3.9x10^{32} years at 90% confidence level.
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