Narrow resonances studies with the radiative return method

Abstract

Using the radiative return method, experiments at high luminosity electron-positron colliders allow one to explore the kaon and the pion form factors in the timelike region up to fairly high energies. This opens the possibility to study kaon and pion pair production at and around the narrow resonances $J/\ensuremath{\psi}$ and $\ensuremath{\psi}(2S)$ and to explore the interference between electromagnetic and hadronic amplitudes. Parametrizations of charged and neutral kaon as well as pion form factors are derived, which lead to an improved description of the data in the region of large invariant masses of the meson pair. These form factors are combined with the hadronic couplings of charged and neutral kaons to $J/\ensuremath{\psi}$ and $\ensuremath{\psi}(2S)$ and implemented into the Monte Carlo generator PHOKHARA, which is now, for the first time, able to simulate the production of narrow resonances and their decay into kaon, pion, and muon pairs.