QSSR estimate of the BBB_B parameter at next-to-leading order

We compute the leading $\alpha_s$ corrections to the two-point correlator of the $O_{\Delta B=2}$ operator which controls the $B^0 \bar B^0$ mixing. Using this result within the QCD spectral sum rules approach and some phenomenologically reasonable assumptions in the parametrization of the spectral function, we conclude that the vacuum saturation values $B_B\simeq B_{B^*}\simeq 1$ are satisfied within 15\%.Comment: 8 pages, LaTeX, CERN-TH.7140/94, PM 93/16, and KEK Preprint 93-184, two figures appended as a PS fil

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

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

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

The two-pseudoscalar-meson decay of χcJ\chi_{cJ} with twist-3 corrections

The decays of $\chi_{cJ} \to \pi^+\pi^-, K^+K^-$ $(J=0,2)$ are discussed within the standard and modified hard scattering approach when including the contributions from twist-3 distribution amplitudes and wave functions of the light pseudoscalar meson. A model for twist-2 and twist-3 distribution amplitudes and wave functions of the pion and kaon with BHL prescription are proposed as the solution to the end-point singularities. The results show that the contributions from twist-3 parts are actually not power suppressed comparing with the leading-twist contribution. After including the effects from the transverse momentum of light meson valence-quark state and Sudakov factors, the decay widths of the $\chi_{cJ}$ into pions or kaons are comparable with the their experimental data.Comment: 31 pages, 5 figures, 3 table

Towards a determination of the chiral couplings at NLO in 1/N(C): L_8(mu) and C_38(mu)

We present a dispersive method which allows to investigate the low-energy couplings of chiral perturbation theory at the next-to-leading order (NLO) in the 1/N(C) expansion, keeping full control of their renormalization scale dependence. Using the resonance chiral theory Lagrangian, we perform a NLO calculation of the scalar and pseudoscalar two-point functions, within the single-resonance approximation. Imposing the correct QCD short-distance constraints, one determines their difference Pi(t)=Pi_S(t)-Pi_P(t) in terms of the pion decay constant and resonance masses. Its low momentum expansion fixes then the low-energy chiral couplings L_8 and C_38. At mu_0=0.77 GeV, we obtain L_8(mu_0)^{SU(3)} = (0.6+-0.4)10^{-3} and C_{38}(mu_0)^{SU(3)}=(2+-6)10^{-6}.Comment: Extended version published at JHEP01(2007)039. A NLO prediction for the O(p6) chiral coupling C_38 has been added. The original L_8 results remain unchange

Charm-quark mass from weighted finite energy QCD sum rules

The running charm-quark mass in the $\bar{MS}$ scheme is determined from weighted finite energy QCD sum rules (FESR) involving the vector current correlator. Only the short distance expansion of this correlator is used, together with integration kernels (weights) involving positive powers of $s$, the squared energy. The optimal kernels are found to be a simple {\it pinched} kernel, and polynomials of the Legendre type. The former kernel reduces potential duality violations near the real axis in the complex s-plane, and the latter allows to extend the analysis to energy regions beyond the end point of the data. These kernels, together with the high energy expansion of the correlator, weigh the experimental and theoretical information differently from e.g. inverse moments FESR. Current, state of the art results for the vector correlator up to four-loop order in perturbative QCD are used in the FESR, together with the latest experimental data. The integration in the complex s-plane is performed using three different methods, fixed order perturbation theory (FOPT), contour improved perturbation theory (CIPT), and a fixed renormalization scale $\mu$ (FMUPT). The final result is $\bar{m}_c (3\, {GeV}) = 1008\,\pm\, 26\, {MeV}$, in a wide region of stability against changes in the integration radius $s_0$ in the complex s-plane.Comment: A short discussion on convergence issues has been added at the end of the pape

Semi-inclusive tau decays involving the vector or axial-vector hadronic currents

The theoretical predictions for the semi-inclusive hadronic tau decays into an even/odd number of pions (and kaons) are up-dated, and compared with the corresponding sums of exclusive tau-decay modes. The value of alpha(s)(M(tau)2) obtained from these semi-inclusive widths agree with the one obtained from the total inclusive hadronic width. The experimental e+ e- --> hadrons data are also used to perform additional tests in the vector sector. Using the e+ e- data and varying the value of M(tau), we show that there is a good agreement among the results obtained at different mass scales