Study ofη’formation in photon-photon collisions

Two-photon formation of the η’ in the reaction ${\mathit{e}}^{+}$ ${\mathrm{e}}^{\mathrm{-}}$→${\mathrm{e}}^{+}$ ${\mathrm{e}}^{\mathrm{-}}$ηm’→${\mathrm{e}}^{+}$ ${\mathrm{e}}^{\mathrm{-}}$ ${\mathrm{\pi }}^{+}$ ${\mathrm{\pi }}^{\mathrm{-}}$γ has been studied for nearly real and virtual photons at an $e^{+}$ $e^{\mathrm{-}}$ center-of-mass energy of 29 GeV. The γγ width of the η’ is found to be 4.5±0.3(stat.)±0.7(syst.) keV. The measured dependence of this γγ width upon the photon four-momentum squared in the range 0–5 ${\mathrm{GeV}}^2$ agrees with predictions both from QCD and from ρ dominance. A spin analysis results in a strong preference for a spin-zero, relative to a spin-two, assignment for the η’. No evidence for C-parity violation is found in this η’→${\pi }^{+}$ ${\pi }^{\mathrm{-}}$γ decay mode.