Measurements of the meson-photon transition form factors of light pseudoscalar mesons at large momentum transfer

Production of even C-parity hadronic matter in ee scattering provides a unique opportunity to study the properties of strong interactions. To leading order in quantum electrodynamics (QED) these processes are described as the interaction between two photons emitted by the scattered electrons. Although in ee scattering the probe and the target are both represented by photons that are carriers of the electromagnetic force, these space-like photons can produce a pair of quarks that interact strongly and are observed in the form of hadrons. Therefore, by measuring the four-momenta of the scattered electrons we can study the dynamics of strong interactions. The quantities of interest in these studies are the form factors associated with the transitions between the photons and the hadrons. In this paper we briefly describe the final results of our measurements 1,2 of the differential cross sections for the production of a single pseudoscalar meson in ee scattering: ee → eeR, where R is a π, η or η. We measure these cross sections in a “single-tagged” experimental mode where one of the scattered electrons is detected (“tagged”), while the other electron is scattered at a very small angle and therefore remains undetected (“untagged”). The mesons produced in ee scattering are observed through their decays to various fully reconstructed final states. The tagged electron emits a highly off-shell photon (γ), whereas the untagged electron emits a nearly on-shell photon (γ). We measure the dependence of the meson production rate on the squared momentum transfer Q carried by the highly off-shell photon. This momentum transfer is determined by energymomentum conservation as applied to the tag: Q ≡ −(pb−pt) 2 = 2EbEt(1−cosθt), where pb and pt are the four-momenta of the incident beam-energy electron and the tag, Eb and Et are corresponding energies, and θt is the scattering angle. From the measurements of the differential rates dσ(ee → eeR)/dQ we obtain the