5,801 research outputs found

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

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

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

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

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

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

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

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