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The \(\eta_c\gamma \gamma ^*\) transition form factor in the decay \(\eta_c\to \gamma \ell^+\ell^-\) and in the crossed channels \(\gamma e^-\to \eta_c e^- \) and \(e^+e^-\to\eta_c\gamma\)

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Abstract.

The Dalitz distribution and the effective mass spectrum of the lepton pair produced in the decay \(\eta_c\to \gamma \ell^+ \ell^-\), as well as the differential cross sections for the crossed channels \(\gamma e^-\to \eta_c e^- \) and \(e^+ e^-\to \eta_c \gamma\), are calculated in terms of the sole electromagnetic transition form factor for the vertex \(\eta_c\to\gamma \gamma^*\), where \(\gamma^*\) is a virtual photon with a space-like (for \(\gamma e^- \to \eta_c e^- \)) or a time-like (for \(e^+ e^-\to \eta_c \gamma\) and \(\eta_c\to\gamma \ell^+\ell^-\)) four-momentum. Explicit parameterizations of the transition form factor are found in two different approaches, namely in the vector dominance model and in a QCD-inspired consideration on the basis of the process \(c \overline c\to \gamma \ell^+ \ell^-\), with zero relative three-momentum of the annihilating quarks.

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Received: 22 February 2000 / Revised version: 12 April 2000 / Published online: 8 June 2000

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Di Salvo, E., Rekalo, M. & Tomasi-Gustafsson, E. The \(\eta_c\gamma \gamma ^*\) transition form factor in the decay \(\eta_c\to \gamma \ell^+\ell^-\) and in the crossed channels \(\gamma e^-\to \eta_c e^- \) and \(e^+e^-\to\eta_c\gamma\) . Eur. Phys. J. C 16, 295–302 (2000). https://doi.org/10.1007/s100520050022

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