EPJ

2021 Impact factor 4.991
EPJ C - Particles and Fields
Particles and Fields
Eur. Phys. J. C 5, 327-335
DOI 10.1007/s100529800848

Flavor symmetry breaking and mixing effects
in the $\eta \gamma$ and $\eta'\gamma$ transition form factors

T. Feldmann - P. Kroll

Department of Theoretical Physics, University of Wuppertal, D-42097 Wuppertal, Germany
(e-mail: kroll@theorie.physik.uni-wuppertal.de)

Received: 13 November 1997 / Revised version: 13 January 1998 / Published online: 10 March 1998

Abstract
We reanalyze the $\eta \gamma$ and $\eta'\gamma$transition form factors within the modified hard scattering approach on the basis of new experimental data from CLEO [CLEO97] and L3 [L397]. Our approach perfectly describes the experimental data over a wide range of the virtuality of the probing photon, 1 GeV$^2\leq Q^2 \leq 15$ GeV2. The analysis provides hints that the conventional flavor octet-singlet scheme for the $\eta$-$\eta'$mixing is too simple. A more general mixing scheme on the other hand, involving two mixing angles, leads to a very good description of the transition form factors and also accounts for the two-photon decay widths of the $\eta$ and $\eta'$ mesons as well as for the ratio of the widths for the $J/\psi \to \eta'\gamma$ and $J/\psi \to \eta\gamma$decays. We also investigate the questions of possible deviations of the $\eta$ and $\eta'$ distribution amplitudes from the asymptotic form and of eventual intrinsic charm in the $\eta$ and $\eta'$ mesons. We estimate the charm decay constant of the $\eta'$ meson to lie within the range -65 MeV${}\leq f_{\eta'}^c\leq 15$ MeV.

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