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We observe a net beam-excess of $8.7 pm 6.3$ (stat) $pm 2.4$ (syst) events, above 160 MeV, resulting from the charged-current reaction of $ u_{mu}$ and/or $bar u_{mu}$ on C and H in the LSND detector. No beam related muon background is expected in this energy regime. Within an analysis framework of $pi^0 to u_{mu}bar u_{mu}$, we set a direct upper limit for this branching ratio of $Gamma(pi^0 to u_mu bar u_mu) / Gamma(pi^0 to all) < 1.6 times 10^{-6}$ at 90% confidence level.
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We present the first measurements of branching fractions of rare tau-lepton decays, $tau^- rightarrow pi^- u_{tau} ell^+ ell^-$ ($ell = e$ or $mu$), using a data sample corresponding to 562 fb$^{-1}$ collected at a center-of-mass energy of 10.58 GeV with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. The $tau^- rightarrow pi^- u_tau e^+ e^-$ decay is observed for the first time with 7.0$sigma$ significance. The partial branching fraction determined by the structure-dependent mechanisms mediated by either a vector or an axial-vector current for the mass region $M_{pi e e}>1.05$ GeV/$c^2$ is measured to be $mathcal{B}(tau^-rightarrow pi^- u_tau e^+ e^-)[M_{pi^- e^+ e^-}>1.05~{rm GeV}/c^2] = (5.90 pm 0.53 pm 0.85 pm 0.11) times 10^{-6}$, where the first uncertainty is statistical, the second is systematic, and the third is due to the model dependence. In the full phase space, due to the different detection efficiencies for the structure-dependent mechanisms mediated by axial-vector and vector currents, the branching fraction varies from $mathcal{B}_{A}(tau^-rightarrow pi^- u_tau e^+ e^-) = (1.46 pm 0.13 pm 0.21) times 10^{-5}$ to $mathcal{B}_{V}(tau^-rightarrow pi^- u_tau e^+ e^-) = (3.01 pm 0.27 pm 0.43) times 10^{-5}$, respectively. An upper limit is set on the branching fraction of the $tau^- rightarrow pi^- u_tau mu^+ mu^-$ decay, $mathcal{B}(tau^-rightarrow pi^- u_tau mu^+ mu^-) < 1.14 times 10^{-5}$, at the 90% confidence level.
A high statistics data sample of the $K^{+}tomu^{+} u_{mu}$ decay was accumulated by the OKA experiment in 2012. The missing mass analysis was performed to search for the decay channel $K^{+}tomu^{+} u_{H}$ with a hypothetic stable heavy neutrino in the final state. The obtained missing mass spectrum does not show peaks which could be attributed to existence of stable heavy neutrinos in the mass range (220 $< m_{ u_{H}} <$ 375) MeV$/c^{2}$. As a result, we obtain upper limits on the branching ratio and on the value of the mixing element $|U_{mu H}|^{2}$.
By analyzing 482 pb$^{-1}$ of $e^+e^-$ collision data collected at the center-of-mass energy $sqrt s=4.009$ GeV with the BESIII detector, we measure the %absolute branching fractions for the semi-leptonic decays $D_{s}^{+}to phi e^{+} u_{e}$, $phi mu^{+} u_{mu}$, $eta mu^{+} u_{mu}$ and $etamu^{+} u_{mu}$ to be ${mathcal B}(D_{s}^{+}tophi e^{+} u_{e})=(2.26pm0.45pm0.09)$%, ${mathcal B}(D_{s}^{+}tophi mu^{+} u_{mu})=(1.94pm0.53pm0.09)$%, ${mathcal B}(D_{s}^{+}toeta mu^{+} u_{mu})=(2.42pm0.46pm0.11)$% and ${mathcal B}(D_{s}^{+}toetamu^{+} u_{mu}) = (1.06pm0.54pm0.07)$%, where the first and second uncertainties are statistical and systematic, respectively. The branching fractions for the three semi-muonic decays $D_s^+tophi mu^+ u_mu, eta mu^+ u_mu$ and $eta mu^+ u_mu$ are determined for the first time and that of $D^+_sto phi e^+ u_e$ is consistent with the world average value within uncertainties.
The ArgoNeuT collaboration reports the first measurement of neutral current $pi^{0}$ production in $ u_{mu}$-argon and $bar{ u}_{mu}$-argon scattering. This measurement was performed using the ArgoNeuT liquid argon time projection chamber deployed at Fermilabs NuMI neutrino beam with an exposure corresponding to 1.2$times 10^{20}$ protons-on-target from the Fermilab Main Injector and a mean energy for $ u_{mu}$ of 9.6~GeV and for $bar{ u}_{mu}$ of 3.6~GeV. We compare the measured cross section and kinematic distributions to predictions from the GENIE and NuWro neutrino interaction event generators.
The Cabibbo-suppressed semileptonic decay $B^{+}to poverline{p}mu^{+} u_{mu}$ is observed for the first time using a sample of $pp$ collisions corresponding to an integrated luminosity of 1.0, 2.0 and 1.7fb$^{-1}$ at centre-of-mass energies of 7, 8 and 13TeV, respectively. The differential branching fraction is measured as a function of the $poverline{p}$ invariant mass using the decay mode $B^{+}to J/psi K^{+}$ for normalisation. The total branching fraction is measured to be begin{align*} mathcal{B}(B^{+}to poverline{p}mu^{+} u_{mu}) = (5.27 ^{+0.23}_{-0.24} pm 0.21 pm 0.15)times 10^{-6}, end{align*} where the first uncertainty is statistical, the second systematic and the third is from the uncertainty on the branching fraction of the normalisation channel.
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