5,801 research outputs found

    Charmonium decays: J/psi --> e+e- and eta_c --> gamma gamma

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    We compute the O(alpha_s^2) correction to the decay rate eta_c --> gamma gamma and discuss its implications for precision quarkonium physics. We study the suitability of the ratio Gamma(J/psi --> e+e-)/Gamma(eta_c --> gamma gamma), in which the non-perturbative or soft effects cancel at O(alpha_s^{0,1}), for extracting fundamental parameters of QCD at low energies. We show that the QCD-based theory of charmonia is not capable of predicting this ratio with any degree of confidence.Comment: 6 page

    Computing Techniques for Two-Loop Corrections to Anomalous Magnetic Moments of Leptons

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    Two-loop electroweak corrections to the electron, muon, and tau lepton anomalous magnetic moments have been computed recently. Effects of hadronic contributions to the photon propagator in two-loop QED corrections have also been reanalysed. The common technique used in both calculations was asymptotic expansion of Feynman diagrams in the ratio of lepton mass to some heavy mass scale. In this talk we present some details of this method paying particular attention to the expansion of multi-scale diagrams.Comment: Updated Higher Order Hadronic Corrections. Talk given at the 5th International Workshop ``New Computing Techniques in Physics Research'', Lausanne, September 1996. 7 pages, late

    Techniques for computing two-loop QCD corrections to b-->c transitions

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    We have recently presented the complete O(alpha_s^2) corrections to the semileptonic decay width of the b quark at maximal recoil. Here we discuss various technical aspects of that calculation and further applications of similar methods. In particular, we describe an expansion which facilitates the phase space integrations and the treatment of the mixed real-virtual corrections, for which Taylor expansion does not work and the so-called eikonal expansion must be employed. Several terms of the expansion are given for the O(alpha_s^2) QCD corrections to the differential semileptonic decay width of the b --quark at maximal recoil. We also demonstrate how the light quark loop corrections to the top quark decay rate can be obtained using the same methods. We briefly discuss the application of these techniques to the calculation of the O(alpha_s^2) correction to zero recoil sum rules for heavy flavor transitions.Comment: 22 pages, revte

    The muon anomalous magnetic moment in QED: three-loop electron and tau contributions

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    We present an analytic calculation of electron and tau O(alpha^3) loop effects on the muon anomalous magnetic moment. Computation of such three-loop diagrams with three mass scales is possible using asymptotic and eikonal expansions. An evaluation of a new type of eikonal integrals is presented in some detail.Comment: 9 pages, late

    Cutting description of trivial 1-cohomology

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    A characterisation of trivial 1-cohomology for a broad class of metric spaces is presented. The condition ties cohomology and connectedness properties of open sets

    Two-loop QCD corrections to semileptonic b decays at maximal recoil

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    We present a complete O(αs2)O(\alpha_s^2) correction to the differential width of the inclusive semileptonic decay bclνlb\to cl\nu_l at the kinematical point of vanishing invariant mass of the leptons, q2=0q^2=0. Together with the recently computed O(αs2)O(\alpha_s^2) correction at the upper boundary of the lepton invariant mass spectrum, this new information permits an estimate of the O(αs2)O(\alpha_s^2) effect in the total inclusive semileptonic decay width bclνlb\to cl\nu_l. We argue that the non-BLM part of the O(αs2)O(\alpha_s^2) correction gives at most 1% correction to the inclusive semileptonic decay width bclνlb\to cl\nu_l. This significantly improves the credibility of extracting Vcb|V_{cb}| from the inclusive semileptonic decays of the b-hadrons.Comment: 8 pages, revte