Formation of the $η_c$ in Two-Photon Collisions at LEP

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$_s^0$K$^\pmπ^\mp$, K$^+$K$^-π^{0}$, $π^+π^-η$, $π^+π^-η'$, and $ρ^+ρ^-$ using an integrated luminosity of 140 pb$^{-1}$ at $\sqrt{s} \simeq 91$ GeV and of 52 pb$^{-1}$ at $\sqrt{s} \simeq 183$ GeV. The result is $Γ_{γγ}(η_c) = 6.9 \pm 1.7 (stat.) \pm 0.8 (sys.) \pm 2.0$(BR) keV. The $Q^2$ dependence of the $η_c$ cross section is studied for $Q^2 < 9$ GeV$^{2}$. 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 $29 \pm 11$ 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.

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