Formation of the ηc 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 − , K 0 s K ± π ∓ , K + K − π 0 , 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 γγ (η c )=6.9±1.7 stat ±0.8 sys ±2.0 (BR) keV. The Q 2 dependence of the η c cross section is studied for Q 2 2 . 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.