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Photoproduction of the $f_1(1285)$ Meson

The $f_1(1285)$ meson with mass $1281.0 \pm 0.8$ MeV/$c^2$ and width $18.4 \pm 1.4$ MeV (FWHM) was measured for the first time in photoproduction from a proton target using CLAS at Jefferson Lab. Differential cross sections were obtained via the $ηπ^{+}π^{-}$, $K^+\bar{K}^0π^-$, and $K^-K^0π^+$ decay channels from threshold up to a center-of-mass energy of 2.8 GeV. The mass, width, and an amplitude analysis of the $ηπ^{+}π^{-}$ final-state Dalitz distribution are consistent with the axial-vector $J^P=1^+$ $f_1(1285)$ identity, rather than the pseudoscalar $0^-$ $η(1295)$. The production mechanism is more consistent with $s$-channel decay of a high-mass $N^*$ state, and not with $t$-channel meson exchange. Decays to $ηππ$ go dominantly via the intermediate $a_0^\pm(980)π^\mp$ states, with the branching ratio $Γ(a_0π\text{ (no} \bar{K} K\text{)}) / Γ(ηππ\text{(all)}) = 0.74\pm0.09$. The branching ratios $Γ(K \bar{K} π)/Γ(ηππ) = 0.216\pm0.033$ and $Γ(γρ^0)/Γ(ηππ) = 0.047\pm0.018$ were also obtained. The first is in agreement with previous data for the $f_1(1285)$, while the latter is lower than the world average.