Measurement ofγγ*→π0transition form factor at Belle
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Abstract
We report a measurement of the process $\ensuremath{\gamma}{\ensuremath{\gamma}}^{*}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}$ with a $759\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ data sample recorded with the Belle detector at the KEKB asymmetric-energy ${e}^{+}{e}^{\ensuremath{-}}$ collider. The pion transition form factor, $F({Q}^{2})$, is measured for the kinematical region $4\text{ }\text{ }{\mathrm{GeV}}^{2}\ensuremath{\lesssim}{Q}^{2}\ensuremath{\lesssim}40\text{ }\text{ }{\mathrm{GeV}}^{2}$, where $\ensuremath{-}{Q}^{2}$ is the invariant-mass squared of a virtual photon. The measured values of ${Q}^{2}|F({Q}^{2})|$ agree well with the previous measurements below ${Q}^{2}\ensuremath{\simeq}9\text{ }\text{ }{\mathrm{GeV}}^{2}$ but do not exhibit the rapid growth in the higher ${Q}^{2}$ region seen in another recent measurement, which exceeds the asymptotic QCD expectation by as much as 50%.