Formation of the η_c in two-photon collisions at LEP

Abstract

The two-photon width Γ_γγ of the η_c meson has been measured with the L3 detector at LEP. The η_c is studied in the decay modes π+π−π+π−, π+π−K+K−, K0sK±π∓, K+K−π0, π+π−η, π+π−η′, and ρ+ρ− using an integrated luminosity of 140 pb⁻¹ at √<math><mtext>s</mtext><mtext>≃91</mtext></math> GeV and of 52 pb⁻¹ at √s ≃ 183 GeV. The result is Γ_γγ (η_c)=6.9±1.7 stat ± 0.8 sys ± 2.0(BR) keV. The Q² dependence of the η_c cross section is studied for Q² < 9 GeV2. It is found to be better described by a Vector Meson Dominance model form factor with a J-pole than with a ρ-pole. In addition, a signal of 29±11 events is observed at the χ_c0 mass. Upper limits for the two-photon widths of the χ_c0, χ_c2, and η_′c are also given.