62 research outputs found
Optimal renormalization and the extraction of the strange quark mass from moments of the -decay spectral function
We introduce an optimal renormalization group analysis pertinent to the
analysis of polarization functions associated with the -quark mass relevant
in -decay. The technique is based on the renormalization group invariance
constraints which lead to closed form summation of all the leading and
next-to-leading logarithms at each order in perturbation theory. The new
perturbation series exhibits reduced sensitivity to the renormalization scale
and improved behavior in the complex plane along the integration contour. Using
improved experimental and theory inputs, we have extracted the value of the
strange quark mass and from presently available ALEPH and OPAL data
respectively. These determinations are in agreement with the determinations in
other phenomenological methods and from the lattice.Comment: 12 pages, 4 tables, 7 figures, v2 corresponds to version to appear in
Physical Review
Electromagnetic charge radius of the pion at high precision
We present a determination of the pion charge radius from high precision data
on the pion vector form factor from both timelike and spacelike regions, using
a novel formalism based on analyticity and unitarity. At low energies, instead
of the poorly known modulus of the form factor, we use its phase, known with
high accuracy from Roy equations for elastic scattering via the
Fermi-Watson theorem. We use also the values of the modulus at several higher
timelike energies, where the data from -annihilation and -decay
are mutually consistent, as well as the most recent measurements at spacelike
momenta. The experimental uncertainties are implemented by Monte-Carlo
simulations. The results, which do not rely on a specific parametrization, are
optimal for the given input information and do not depend on the unknown phase
of the form factor above the first inelastic threshold. Our prediction for the
charge radius of the pion is r_\pi=(0.657 \pm 0.003) \fm , which amounts to
an increase in precision by a factor of about 2.7 compared to the PDG average.Comment: 6 pages, 2 figures, typos corrected, citations added, version
accepted for publication in Physical Review Letter
Parametrization-free determination of the shape parameters for the pion electromagnetic form factor
Recent data from high statistics experiments that have measured the modulus
of the pion electromagnetic form factor from threshold to relatively high
energies are used as input in a suitable mathematical framework of analytic
continuation to find stringent constraints on the shape parameters of the form
factor at . The method uses also as input a precise description of the
phase of the form factor in the elastic region based on Fermi-Watson theorem
and the analysis of the scattering amplitude with dispersive Roy
equations, and some information on the spacelike region coming from recent high
precision experiments. Our analysis confirms the inconsistencies of several
data on the modulus, especially from low energies, with analyticity and the
input phase, noted in our earlier work. Using the data on the modulus from
energies above , we obtain, with no specific parametrization,
the prediction for the charge
radius. The same formalism leads also to very narrow allowed ranges for the
higher-order shape parameters at , with a strong correlation among them.Comment: v2 is 11 pages long using EPJ style files, and has 8 figures;
Compared to v1, number of figures has been reduced, discussion has been
improved significantly, minor errors have been corrected, references have
added, and the manuscript has been significantly revised; this version has
been accepted for publication in EPJ
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