Study of η’ formation in photon-photon collisions

Two-photon formation of the η’ in the reaction e+e-→e+e-ηm’→e+e-π+π-γ has been studied for nearly real and virtual photons at an e+e- 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 GeV2 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’→π+π-γ decay mode.