Narrow Results

A determination of the strong coupling α_s at rather low energies is possible through the analysis of hadronic decays of the τ lepton. In turn, the low energy necessitates sufficient control over perturbative QCD corrections, the nonperturbative condensate contributions in the framework of the operator product expansion (OPE), as well as corrections going beyond the OPE, the duality violations (DVs). Perturbative QCD uncertainties arise from open questions regarding the renormalization group resummation of the series. The fit quantities are moment integrals of the τ spectral function data in a certain energy window and care should be taken to have good perturbative behavior of those moments as well as control over higher-dimensional operator corrections. Furthermore, all parameters occurring in the theoretical description should be extracted from fits to the data in a self-consistent manner.

Recently, we extracted the strong coupling, ${\alpha }_s(m_{\tau }^2)$, from the revised ALEPH data for non-strange hadronic tau decays. Our analysis is based on a method previously used for the determination of the strong coupling from OPAL data. In our strategy, we employ different moments of the spectral functions both with and without pinching, including duality violations, in order to obtain fully self-consistent analyses that do not rely on untested assumptions (such as the smallness of higher dimension contributions in the operation product expansion (OPE)). Here we discuss the ${\alpha }_s$ values obtained from the ALEPH and the OPAL data, the robustness of the analysis, as well as non-perturbative contributions from DVs and the OPE. We show that, although the ${\alpha }_s$ determination is sound, non-perturbative effects limit the accuracy with which one can extract the strong coupling from tau decay data. Finally, we discuss the compatibility of the data sets and the possibility of a combined analysis.

Using the most recent release of the ALEPH $\tau$ decay data, we present a very detailed phenomenological update of the ${\alpha }_s(m_{\tau }^2)$ determination. We have exploited the sensitivity to the strong coupling in many different ways, exploring several complementary methodologies. All determinations turn out to be in excellent agreement, allowing us to extract a very reliable value of the strong coupling. We find ${\alpha }_s^{(n_f=3)}(m_{\tau }^2)=0.328\pm 0.012$ which implies ${\alpha }_s^{(n_f=5)}(M_Z^2)=0.1197\pm 0.0014$. We critically revise previous work, and point out the problems flawing some recent analyses which claim slightly smaller values.

We present a recent BABAR search for lepton flavor violation in tau decays. The search for τ → ℓℓℓ(ℓ = e, μ) decays is based on a data sample of 91.6 fb^-1. No signal is found in any of six channels and 90% CL upper limits are set in the range of (1 - 3) × 10^-7. The search for τ → μγ decay is done with a 63 fb^-1 data sample. The number of observed events is consistent with expected background level and a preliminary upper limit is set at 2 × 10^-6 at 90% CL.

We review the CP violation in the semileptonic |ΔS| = 1 τ-decays in supersymmetric extensions of the standard model (SM). We show that Within SUSY models with conserved R parity, the CP asymmetry of τ → kπν is below the current experimental limits, although it is enhanced by several order of magnitude than the SM results. We also study the impact of the lepton violation terms in SUSY models with R parity violation. We show that the CP asymmetry of τ-decay is enhanced significantly and the current experimental limits obtained by CLEO collaborations can be easily accommodated.

We present recent measurements of τ decays into final states with kaons, which are based on several hundred million τ⁺τ^- pairs produced in electron-positron annihilation data collected with the BABAR detector. These measurements can be used to determine the Cabibbo-Kobayashi-Maskawa matrix element |V_us| with a tiny systematic error and in two different methods affected by different systematic effects.

In this paper, we investigate decays in the framework of the RPV SUSY model. We discuss the tree-level contribution of the sparticles and to these decay branching ratios. In the two channels, the -mediated channel is more sensitive to the parameter product than the -mediated channel to . And the parameter product is severely constrained to the order of by the experiment data with , which is one order of magnitude more stringent than before. In the calculation of hadronic matrix elements, the resonant effects are large than those of nonresonant terms. Especially, the resonant contribution of scalar meson f₍₉₈₀₎ plays a dominate role in -mediated channel.