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[en] We report the results of search for the neutrinoless lepton-flavor violating decay of the tau lepton into three charged leptons, performed using 376 fb-1 of data collected at an e+e- center-of-mass energy around 10.58 GeV with the BaBar detector at the PEP-II. In all six decay modes considered, the number of events found in data are compatible with the background expectations. Upper limits on the branching fractions are set in the range (4-8) x 10-8 at 90% confidence level.
[en] The study of νμ→ντ oscillations and the explicit observation of ντ through the identification of the final-state τ lepton ('direct appearance search') represent the most straightforward test of the oscillation phenomenon. It is, nonetheless, the most challenging from the experimental point of view. In this paper, we discuss current empirical evidence for the direct appearance of tau neutrinos at the atmospheric scale and the perspectives for the next few years, up to the completion of the CNGS physics programme. We investigate the relevance of this specific oscillation channel for gaining insights into neutrino physics within the standard three-family framework. Finally, we discuss the opportunities offered by precision studies of νμ→ντ transitions for the occurrence of more exotic scenarios, emerging from additional sterile neutrinos or non-standard interactions.
[en] In this paper the author discusses the issue of universality with respect to the tau particle. Present work is trying to attack this by looking at τντW and Zτ bar τ couplings. Some models predict that non-universality will be such a small effect that it may be only tractable with a device such as a tau-charm factory. The issue of universality has impacts in other electroweak reactions, and in many models. The observation of such effects may require very high luminosity however
[en] This work presents simultaneous branching fraction measurements of the decay modes τ- → K- nπ0ντ with n = 0,1,2,3 and τ- → π- nπ0ντ with n = 3,4. The analysis is based on a data sample of 427 x 106 τ+τ- pairs recorded with the BABAR detector, which corresponds to an integrated luminosity of 464.4 fb-1. The measured values are Β(τ- → K-ντ) = (6.57 ± 0.03 ± 0.11) x 10-3, Β(τ- → K-π0ντ) = (4.61 ± 0.03 ± 0.11) x 10-3, Β(τ- → K- π0π0ντ) = (5.05 ± 0.17 ± 0.44) x 10-4, Β(τ- → K-π0π0π0ντ) = (1.31 ± 0.43 ± 0.40) x 10-4, Β(τ0 → π0π0π0π0ντ) = (1.263 ± 0.008 ± 0.078) x 10-2 and Β(τ0 → π0π0π0π0π0ντ) = (9.6 ± 0.5 ± 1.2) x 10-4, where the uncertainties are statistical and systematic, respectively. All measurements are compatible with the current world averages whereas the uncertainties are significantly smaller by a factor of up to five. The determination of Β(τ0 → π-π0π0π0π0ντ) is the first measurement of this branching fraction. The measured branching fractions are combined with the current world averages. Using the new averages, an updated determination of |Vus| from hadronic τ decays yields |Vus| = 0.2146 ± 0.0025, which improves previous measurements by 19%. Its uncertainty is comparable to the one of the current world average from semileptonic kaon decays
[en] R-parity stands as an ad hoc assumption in the most popular version of the supersymmetric standard model. More than fifteen years' studies of R-parity violations have been restricted to various limiting scenarios. We illustrate how the single-VEV parametrization provides a workable framework to analyze the phenomenology of the complete theory of supersymmetry without R-parity. In our comprehensive study of various aspects of the resulting leptonic phenomenology at tree-level, we find that the physical τ lepton could actually bear substantial gaugino and higgsino components, making it very different from the e and the μ
[en] I propose an explanation of the current discrepancy between the measured world average value and the theoretical value of the τ lepton lifetime. I argue that there exist common systematic errors in many of the experiments that use three-prong τ decays for the lifetime measurement. These systematic effects always shift the measurement towards longer lifetimes, and are caused by the small opening angle of three-prong τ decays and limited tracking chamber ability to resolve nearby hits. The theoretical τ lifetime agrees well with the measured world average value from the one-prong τ decays