The [equation] transition form factor in the decay [equation] and in the crossed channels [equation] and [equation]

摘要

The Dalitz distribution and the effective mass spectrum of the lepton pair produced in the decay (eta_c o gamma ell^+ ell^-), as well as the differential cross sections for the crossed channels (gamma e^- o eta_c e^- ) and (e^+ e^- o eta_c gamma), are calculated in terms of the sole electromagnetic transition form factor for the vertex (eta_c ogamma gamma^*), where (gamma^*) is a virtual photon with a space-like (for (gamma e^- o eta_c e^- )) or a time-like (for (e^+ e^- o eta_c gamma) and (eta_c ogamma 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 o gamma ell^+ ell^-), with zero relative three-momentum of the annihilating quarks.