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⁻, K⁰_sK^±π^∓, K⁺K^-π⁰, π⁺π⁻η, π⁺π⁻η', and ρ⁺ρ⁻ using an integrated luminosity of 140 pb⁻¹ at √s ≃ 91 GeV and of 52 pb⁻¹ at √s ≃ 183 GeV. The result is Γ_γγη_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.