208 research outputs found
Pade-Improved Estimate of Perturbative Contributions to Inclusive Semileptonic Decays
Pade-approximant methods are used to estimate the three-loop perturbative
contributions to the inclusive semileptonic decay rate. These
improved estimates of the decay rate reduce the theoretical uncertainty in the
extraction of the CKM matrix element from the measured inclusive
semileptonic branching ratio.Comment: 3 pages, latex, write-up of talk presented at DPF 200
The 4-loop quark mass anomalous dimension and the invariant quark mass
We present the analytical calculation of the four-loop quark mass anomalous
dimension in Quantum Chromodynamics within the minimal subtraction scheme. On
the basis of this result we find that the so-called invariant quark mass is a
very good reference mass for the accurate evolution of the running MS-bar quark
mass in phenomenological applications. We also obtain for the first time a
complete 4-th order perturbative QCD expression for a physical quantity, the
total Higgs boson decay rate into hadrons, and analyze the infrared fixed point
for this case.Comment: 11 pages, Late
Radiative corrections to muon decay in leading and next to leading approximation for electron spectrum
We have noted that the electron spectrum of muon decay in the leading
logarithmic approximation calculated in two lowest orders of the perturbation
theory in the paper of Berman (1958), can be reproduced by the parton language.
This fact permits one to generalize the result to all orders of the
perturbation theory using the structure function method.Comment: 4 pages, 1 figur
Asymptotic Pade-Approximant Methods and QCD Current Correlation Functions
Asymptotic Pade-approximant methods are utilized to estimate the
leading-order unknown (i.e., not-yet-calculated) contributions to the
perturbative expansions of two-current QCD correlation functions obtained from
scalar-channel fermion and gluon currents, as well as from vector-channel
fermion currents. Such contributions to the imaginary part of each correlator
are polynomials of logarithms whose coefficients (other than the constant term
within the polynomial) may be extracted from prior-order contributions by use
of the renormalization-group (RG) equation appropriate for each correlator. We
find surprisingly good agreement between asymptotic Pade-approximant
predictions and RG-determinations of such coefficients for each correlation
function considered, although such agreement is seen to diminish with
increasing numbers of quark flavours. The RG-determined coefficients we obtain
are then utilized in conjunction with asymptotic Pade-approximant methods to
predict the RG-inaccessible constant terms of the leading-order unknown
contributions for all three correlators. The vector channel predictions lead to
estimates for the order contribution to for three, four, and five
flavours.Comment: latex2e, 15 page
Group theory factors for Feynman diagrams
We present algorithms for the group independent reduction of group theory
factors of Feynman diagrams. We also give formulas and values for a large
number of group invariants in which the group theory factors are expressed.
This includes formulas for various contractions of symmetric invariant tensors,
formulas and algorithms for the computation of characters and generalized
Dynkin indices and trace identities. Tables of all Dynkin indices for all
exceptional algebras are presented, as well as all trace identities to order
equal to the dual Coxeter number. Further results are available through
efficient computer algorithms (see http://norma.nikhef.nl/~t58/ and
http://norma.nikhef.nl/~t68/ ).Comment: Latex (using axodraw.sty), 47 page
Quark Mass Anomalous Dimension to alpha_s**4
We present the results of analytic calculation of the quark mass anomalous
dimension to alpha_s**4.Comment: 7 pages, LaTeX; elsart.sty is used (included
Second Order QCD Corrections to
Corrections of to the decay of the top quark into a W boson
and a bottom quark are calculated. The method is based on an expansion of the
top quark propagator for small external momentum, q, as compared to the top
quark mass, M_t. The physical point q^2 = M_t^2 is reached through Pad\'e
approximations. The described method allows to take effects induced by a finite
W boson mass into account. The numerical relevance of the result is discussed.
Important cross-checks against recent results for the decay rate and the two-loop QED corrections to -decay are performed.Comment: Latex, 21 pages, 1 figure included as ps-file. Minor changes;
reference added. Version to be published in PRD. The complete paper is also
available via anonymous ftp at
ftp://ttpux2.physik.uni-karlsruhe.de/ttp99/ttp99-25/ or via www at
http://www-ttp.physik.uni-karlsruhe.de/Preprints
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