2019 Impact factor 4.389
Particles and Fields

Eur. Phys. J. C 16, 295-302
DOI 10.1007/s100520000423

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 $e^+e^-\to\eta_c\gamma$

E. Di Salvo1 - M.P. Rekalo2 - E. Tomasi-Gustafsson3

1 Dipartimento di Fisica dell' Università di Genova and I.N.F.N. - Sez. Genova, Via Dodecaneso 33, 16146 Genova, Italy
2 Middle East Technical University, Physics Department, Ankara 06531, Turkey
3 DAPNIA/SPhN, CEA/Saclay, 91191 Gif-sur-Yvette Cedex, France

Received: 22 February 2000 / Revised version: 12 April 2000 /
Published online: 8 June 2000 - © Springer-Verlag 2000

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.


Copyright Società Italiana di Fisica, Springer-Verlag 2000