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Register a new userImproved search for two body muon decay ${mu}^+{rightarrow}e^+X_H$
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Charged lepton flavor violating muon decay ${mu}^+{rightarrow}e^+X_H$, where $X_H$ is a massive neutral boson, was sought by searching for extra peaks in the muon decay ${mu}^+{rightarrow}e^+{ u}bar{ u}$ energy spectrum in the $m_{X_H}$ mass region $47.8-95.1$ MeV/$c^2$. No signal was found and 90% confidence level upper limits were set on the branching ratio ${Gamma}({mu}^+{rightarrow}e^+X_H)/{Gamma}({mu}^+{rightarrow}e^+{ u}bar{ u})$ at the level of $10^{-5}$ for this region.
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Lepton family number violation is tested by searching for $mu^+to e^+X^0$ decays among the 5.8$times 10^8$ positive muon decay events analyzed by the TWIST collaboration. Limits are set on the production of both massless and massive $X^0$ bosons. The large angular acceptance of this experiment allows limits to be placed on anisotropic $mu^+to e^+X^0$ decays, which can arise from interactions violating both lepton flavor and parity conservation. Branching ratio limits of order $10^{-5}$ are obtained for bosons with masses of 13 - 80 MeV/c$^2$ and with different decay asymmetries. For bosons with masses less than 13 MeV/c$^{2}$ the asymmetry dependence is much stronger and the 90% limit on the branching ratio varies up to $5.8 times 10^{-5}$. This is the first study that explicitly evaluates the limits for anisotropic two body muon decays.
A search for the rare leptonic decay $B^{+} rightarrow {mu}^{+}{mu}^{-}{mu}^{+}{ u}_{{mu}}$ is performed using proton-proton collision data corresponding to an integrated luminosity of $4.7$ fb$^{-1}$ collected by the LHCb experiment. The search is carried out in the region where the lowest of the two ${mu}^{+}{mu}^{-}$ mass combinations is below $980$MeV/c$^{2}$. The data are consistent with the background-only hypothesis and an upper limit of $1.6 times 10^{-8}$ at 95% confidence level is set on the branching fraction in the stated kinematic region.
Koike et al. proposed the charged lepton flavor violation (CLFV) decay of the muonic atom $mu^-e^-rightarrow e^-e^-$ as one of the promising processes to search for new physics beyond the standard model. It was found that the attractive interaction of leptons with the nucleus enhances the transition rate of the $mu^-e^-rightarrow e^-e^-$ process. We report on our improved analysis of this process by taking into account the distortion of the out-going electrons in the nuclear Coulomb potential and the relativistic treatment of the muon and the electrons. As results, we found significant enhancement of the transition rate. The transition rate for $^{208}$Pb becomes about 7 times larger than the previous estimation, which enhances the sensitivity of this process to discover the CLFV interaction. We also report on the energy spectrum of the out-going electron.
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 search for the rare radiative leptonic decay $D_s^+togamma e^+ u_e$ is performed for the first time using electron-positron collision data corresponding to an integrated luminosity of 3.19 fb$^{-1}$, collected with the BESIII detector at a center-of-mass energy of 4.178 GeV. No evidence for the $D_s^+togamma e^+ u_e$ decay is seen and an upper limit of $mathcal B(D_s^+togamma e^+ u_e)<1.3times 10^{-4}$ is set on the partial branching fraction at a 90% confidence level for radiative photon energies $E_{gamma}^*>0.01$~GeV.
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