324 research outputs found
K_l3 form factors at order p^6 in chiral perturbation theory
This paper describes the calculation of the semileptonic K_l3 decay form
factor at order p^6 of chiral perturbation theory which is the next-to-leading
order correction to the well-known p^4 result achieved by Gasser and Leutwyler.
At order p^6 the chiral expansion contains 1- and 2-loop diagrams which are
discussed in detail. The irreducible 2-loop graphs of the sunset topology are
calculated numerically. In addition, the chiral Lagrangian L^6 produces direct
couplings with the W-bosons. Due to these unknown couplings, one can always add
linear terms in q^2 to the predictions of the form factor f_-(q^2). For the
form factor f_+(q^2), this ambiguity involves even quadratic terms. Making use
of the fact that the pion electromagnetic form factor involves the same q^4
counter term, the q^4-ambiguity can be resolved. Apart from the possibility of
adding an arbitrary linear term in q^2 our calculation shows that chiral
perturbation theory converges very well in this application, as the O(p^6)
corrections are small. Comparing the predictions of chiral perturbation theory
with the recent CPLEAR data, it is seen that the experimental form factor
f_+(q^2) is well described by a linear fit, but that the slope lambda_+ is
smaller by about 2 standard deviations than the O(p^4) prediction. The
unavoidable q^2 counter term of the O(p^6) corrections allows to bring the
predictions of chiral perturbation theory into perfect agreement with
experiment.Comment: 32 pages, 7 figure
New Sum Rule Determination of the Nucleon Mass
A new QCD calculation of the mass of the nucleon is presented. It makes use
of a polynomial kernel in the dispersion integrals tailored to practically
eliminate the contribution of the unknown 1/2+ and 1/2- continuum. This
approach avoids the arbitrariness and other drawbacks attached to the Borel
kernel used in previous sum rules calculations. Our method yields stable
results for the nucleon mass and coupling for standard values of the
condensates. The prediction of the nucleon mass m_{N}=(0.945 \pm .045) GeV is
in good agreement with experiment.Comment: 7 page
Finite energy chiral sum rules in QCD
A set of well known chiral sum rules, expected to be valid in QCD, is
confronted with experimental data on the vector and axial-vector hadronic
spectral functions, obtained from tau-lepton decay by the ALEPH collaboration.
The Das-Mathur-Okubo sum rule, the first and second Weinberg sum rules, and the
electromagnetic pion mass difference sum rule are not well saturated by the
data. Instead, a modified set of sum rules having additional weight factors
that vanish at the end of the integration range on the real axis, is found to
be precociously saturated by the data to a remarkable extent.Comment: 6 pages, 6 figures. Invited talk at WIN99, 17th International
Workshop on Weak Interactions and Neutrinos, Cape Town, South Africa, January
1999. To be published in the proceedings (World Scientific
B and B_s decay constants from QCD Duality at three loops
Using special linear combinations of finite energy sum rules which minimize
the contribution of the unknown continuum spectral function, we compute the
decay constants of the pseudoscalar mesons B and B_s. In the computation, we
employ the recent three loop calculation of the pseudoscalar two-point function
expanded in powers of the running bottom quark mass. The sum rules show
remarkable stability over a wide range of the upper limit of the finite energy
integration. We obtain the following results for the pseudoscalar decay
constants: f_B=178 \pm 14 MeV and f_{B_s}=200 \pm 14 MeV. The results are
somewhat lower than recent predictions based on Borel transform, lattice
computations or HQET. Our sum rule approach of exploiting QCD quark hadron
duality differs significantly from the usual ones, and we believe that the
errors due to theoretical uncertainties are smaller
First Results with a new Method for calculating 2-loop Box-Functions
We describe a first attempt to calculate scalar 2-loop box-functions with
arbitrary internal masses, applying a novel method proposed in hep-ph/9407234.
Four of the eight integrals are accessible to integration by means of the
residue theorem, leaving a rational function in the remaining variables. The
result of the procedure is a three- or sometimes two-dimensional integral
representation over a finite volume that can be further evaluated using
numerical methods.Comment: 10 pages, LaTeX2e, 11 eps-figures, needs epsfig.st
QCD determination of the leading order hadronic contribution to the muon g-2
The leading order hadronic contribution to the muon magnetic moment anomaly,
, is determined entirely in the framework of QCD. The result in
the light-quark sector, in units of , is , and in the heavy-quark sector , and
, resulting in . The
main uncertainty is due to the current lattice QCD value of the first and
second derivative of the electromagnetic current correlator at the origin.
Expected improvement in the precision of these derivatives may render this
approach the most accurate and trustworthy determination of the leading order
.Comment: Invited talk at "Les Rencontres de Physique de la Vallee d'Aosta",
March 2017. Speaker: C. A. Dominguez. To be published in Nuovo Cimento
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