Wednesday, February 27, 2013

1302.6373 (B. Ananthanarayan et al.)

Parametrization-free determination of the shape parameters for the pion
electromagnetic form factor

B. Ananthanarayan, I. Caprini, Diganta Das, I. Sentitemsu Imsong
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 $t=0$. 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 $\pi\pi$ 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 in a region of stability, taken as $(0.65, 0.70)\,{\rm GeV}$, we obtain, with no specific parametrization, the prediction $\langle r_\pi^2 \rangle \in\, (0.42,\,0.44)\,\fmsq $ for the charge radius. The same formalism leads also to very narrow allowed ranges for the next higher-order shape parameters at $t=0$, with a strong correlation among them.
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