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 $\eta {\pi }^{+}{\pi }^{-}$, $K^{+}{\bar{K}}^0{\pi }^{-}$, and $K^{-}{K}^0{\pi }^{+}$ decay channels from threshold up to a center-of-mass energy of 2.8 GeV. The mass, width, and an amplitude analysis of the $\eta {\pi }^{+}{\pi }^{-}$ final-state Dalitz distribution are consistent with the axial-vector $J^P=1^{+}$ $f_1(1285)$ identity, rather than the pseudoscalar $0^{-}$ $\eta (1295)$. The production mechanism is more consistent with $s$-channel decay of a high-mass $N^{\ast }$ state, and not with $t$-channel meson exchange. Decays to $\eta \pi \pi$ go dominantly via the intermediate $a_0^{\pm }(980){\pi }^{\mp }$ states, with the branching ratio $\Gamma(a_0\pi $\text{ (no}$ \bar{K} K$\text{)}$) / \Gamma(\eta\pi\pi $\text{(all)}$) = 0.74\pm0.09$\mathrm{.}Thebranchingratios$\Gamma(K \bar{K} \pi)/\Gamma(\eta\pi\pi) = 0.216\pm0.033$and$\Gamma(\gamma\rho^0)/\Gamma(\eta\pi\pi) = 0.047\pm0.018$werealsoobtained\mathrm{.}Thefirstisinagreementwithpreviousdataforthe$f_1(1285)$, while the latter is lower than the world average.