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We have searched for neutrinoless tau lepton decays into l h h or l V0, where l stands for an electron or muon, h for a charged light hadron, pi or K, and V0 for a neutral vector meson, rho, K*(892) and phi, using a 158 /fb data sample collected with the Belle detector at the KEKB e+e- collider. Since the number of events observed are consistent with the expected background, we set upper limits on the branching fractions in the range of 1.6-8.0 x 10-7 for various decay modes at the 90% confidence level.
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Neutrinoless tau-lepton decays into either three leptons (tau- -> l1- l2 l3) or one lepton and one K0S meson(tau- -> l- K0S) where lepton l means either an electron or muon, have been searched for using 48.6 fb^{-1} of data collected with the Belle detector at the KEKB e+e- collider. No evidence for candidate decays are found in any channel. Therefore we set 90% confidence level upper limits on the branching fraction for 8 different decay modes. These limits are more stringent than those set previously and reach to the 10^{-7} level.
We have searched for neutrinoless tau lepton decays into three charged leptons using an 87.1 fb^{-1} data sample collected with the Belle detector at the KEKB e^+e^- collider. Since the number of signal candidate events is compatible with that expected from the background, we set 90% confidence level upper limits on the branching fractions in the range (1.9-3.5) x 10^{-7} for various decay modes tau -> l l l where l represents e or mu.
A search for the rare decays $B_s^0totau^+tau^-$ and $B^0totau^+tau^-$ is performed using proton--proton collision data collected with the LHCb detector. The data sample corresponds to an integrated luminosity of 3fb$^{-1}$ collected in 2011 and 2012. The $tau$ leptons are reconstructed through the decay $tau^-topi^-pi^+pi^- u_{tau}$. Assuming no contribution from $B^0totau^+tau^-$ decays, an upper limit is set on the branching fraction $mathcal{B}(B_s^0totau^+tau^-) < 6.8times 10^{-3}$ at 95% confidence level. If instead no contribution from $B_s^0totau^+tau^-$ decays is assumed, the limit is $mathcal{B}(B^0totau^+tau^-)< 2.1 times 10^{-3}$ at 95% confidence level. These results correspond to the first direct limit on $mathcal{B}(B_s^0totau^+tau^-)$ and the worlds best limit on $mathcal{B}(B^0totau^+tau^-)$.
We report on a search for the lepton flavor violating tau- -> mu- gamma and tau- -> e- gamma decays based on 535/fb of data accumulated at the Belle experiment. No signal is found and we set 90% confidence level upper limits on the branching ratios Br(tau- -> mu- gamma) < 4.5x10^-8 and Br(tau- -> e- gamma) < 1.2x10^-7.
The evolution of the determination of the strong coupling constant $alpha_s$ from the leptonic branching ratios, the lifetime, and the invariant mass distributions of the hadronic final state of the $tau$ lepton over the last two decades is briefly reviewed. The improvements in the latest ALEPH update are described in some detail. Currently this is one of the most precise $alpha_s$ determinations. Together with the other determination at the $Z$ boson mass pole, they constitutes the most accurate test of the asymptotic freedom in QCD.
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