Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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 $\pi\pi$ elastic scattering via the
Fermi-Watson theorem. We use also the values of the modulus at several higher
timelike energies, where the data from $e^+e^-$-annihilation and $\tau$-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.

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The categories of this paper are: cond-mat
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The categories of this paper are: math-ph
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The categories of this paper are: bayes-an
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The categories of this paper are: patt-sol
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The categories of this paper are: supr-con
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The categories of this paper are: hep-lat
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The categories of this paper are: nlin
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The categories of this paper are: astro-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: chao-dyn
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The categories of this paper are: physics
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The categories of this paper are: gr-qc
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The categories of this paper are: chem-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: econ
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The categories of this paper are: q-bio