Contour-improved versus fixed-order perturbation theory in hadronic tau decays

The hadronic decay rate of the tau lepton serves as one of the most precise determinations of the QCD coupling alpha_s. The dominant theoretical source of uncertainty at present resides in the seeming disparity of two approaches to improving the perturbative expansion with the help of the renormalisation group, namely fixed-order and contour-improved perturbation theory. In this work it is demonstrated that in fact both approaches yield compatible results. However, the fixed-order series is found to oscillate around the contour-improved result with an oscillation frequency of approximately six perturbative orders, approaching it until about the 30th order, after which the expansion reveals its asymptotic nature. Additionally, the renormalisation scale and scheme dependencies of the perturbative series for the tau hadronic width are investigated in detail.Comment: 20 pages, 5 eps-figures; discussion on scale and scheme dependence added as compared to published journal version JHEP 09 (2005) 05

Determination of |V_us| from hadronic tau decays

The recent update of the strange spectral function and the moments of the invariant mass distribution by the OPAL collaboration from hadronic tau decay data are employed to determine |V_us| as well as m_s. Our result, |V_us|=0.2208\pm0.0034, is competitive to the standard extraction of |V_us| from K_e3 decays and to the new proposals to determine it. Furthermore, the error associated to our determination of |V_us| can be reduced in the future since it is dominated by the experimental uncertainty that will be eventually much improved by the B-factories hadronic tau data. Another improvement that can be performed is the simultaneous fit of both |V_us| and m_s to a set of moments of the hadronic tau decays invariant mass distribution, which will provide even a more accurate determination of both parameters.Comment: 6 pages. Invited talk given by E.G. at the XXXXth Rencontres de Moriond on Electroweak Interactions and Unified Theories, La Thuile, Italy, 5-12 Mar 200

Quantum Loops in the Resonance Chiral Theory: The Vector Form Factor

We present a calculation of the Vector Form Factor at the next-to-leading order in the 1/N_C expansion, within the framework of Resonance Chiral Theory. The calculation is performed in the chiral limit, and with two dynamical quark flavours. The ultraviolet behaviour of quantum loops involving virtual resonance propagators is analyzed, together with the kind of counterterms needed in the renormalization procedure. Using the lowest-order equations of motion, we show that only a few combinations of local couplings appear in the final result. The low-energy limit of our calculation reproduces the standard Chiral Perturbation Theory formula, allowing us to determine the resonance contribution to the chiral low-energy couplings, at the next-to-leading order in 1/N_C, keeping a full control of their renormalization scale dependence.Comment: 27+1 pages, 9 figure

V(us) Determination from Hyperon Semileptonic Decays

We analyze the numerical determination of the quark mixing factor V(us) from hyperon semileptonic decays. The discrepancies between the results obtained in two previous studies are clarified. Our fits indicate sizeable SU(3) breaking corrections, which unfortunately can only be fully determined from the data at the first order. The lack of a reliable theoretical calculation of second-order symmetry breaking effects translates into a large systematic uncertainty, which has not been taken into account previously. Our final result, V(us) = 0.226 +/- 0.005, is not competitive with the existing determinations from K(l3), K(l2) and \tau decays.Comment: 16 pages, no figures. References added and other minor change

Strange Quark Mass from the Invariant Mass Distribution of Cabibbo-Suppressed Tau Decays

Quark mass corrections to the tau hadronic width play a significant role only for the strange quark, hence providing a method for determining its mass. The experimental input is the vector plus axial-vector strange spectral function derived from a complete study of tau decays into strange hadronic final states performed by ALEPH. New results on strange decay modes from other experiments are also incorporated. The present analysis determines the strange quark mass at the Mtau mass scale using moments of the spectral function. Justified theoretical constraints are applied to the nonperturbative components and careful attention is paid to the treatment of the perturbative expansions of the moments which exhibit convergence problems. The result obtained, m_s(Mtau^2) = (120 +- 11_exp +- 8_Vus +- 19_th) MeV = (120^+21_-26) MeV, is stable over the scale from Mtau down to about 1.4 GeV. Evolving this result to customary scales yields m_s(1 GeV^2) = (160^+28_-35) MeV and m_s(4 GeV^2) = (116^+20_-25) MeV.Comment: LaTex, 8 pages, 4 figures (EPS

Determination of the Chiral Couplings L_10 and C_87 from Semileptonic Tau Decays

Using recent precise hadronic tau-decay data on the V-A spectral function, and general properties of QCD such as analyticity, the operator product expansion and chiral perturbation theory, we get accurate values for the QCD chiral order parameters L_10^r(M_rho) and C_87^r(M_rho). These two low-energy constants appear at order p^4 and p^6, respectively, in the chiral perturbation theory expansion of the V-A correlator. At order p^4 we obtain L_10^r(M_rho) = -(5.22\pm 0.06)10^{-3}. Including in the analysis the two-loop (order p^6) contributions, we get L_10^r(M_rho) = -(4.06\pm 0.39)10^{-3} and C_87^r(M_rho) = (4.89\pm 0.19)10^{-3}GeV^{-2}. In the SU(2) chiral effective theory, the corresponding low-energy coupling takes the value \overline l_5 = 13.30 \pm 0.11 at order p^4, and \overline l_5 = 12.24 \pm 0.21 at order p^6.Comment: 17 pages, 3 figures, v2: Added reference, published versio

The Standard Model prediction for eps'/eps

We review fundamental aspects of a new Standard Model analysis of eps'/eps which takes into account the strong enhancement induced by final state interactions.Comment: 3 pages, LaTex, uses ws-p10x7.cls. Talk given by E.P. at ICHEP2000, Osaka, July 27-August 2, 200

epsilon'/eps in the Standard Model

We overview the detailed analysis of epsilon'/epsilon within the Standard Model, presented in ref. 1. When all sources of large logarithms are considered, both at short and long distances, it is possible to perform a reliable Standard Model estimate of epsilon'/epsilon. The strong S-wave rescattering of the final pions has an important impact on this observable. The Standard Model prediction is found to be Re(epsilon'/epsilon) =(1.7 +- 0.9)*10^{-3}, in good agreement with the most recent experimental measurements. A better estimate of the strange quark mass would reduce the uncertainty to about 30%.Comment: Talk presented at the International Europhysics Conference on High Energy Physics, July 12-18, 2001,Budapest, Hungar