Observation of the $ϕ(1680)$ and the Y(2175) in $e^+ e^- \to ϕπ^+π^-$

The cross sections for $e^+ e^- \to ϕπ^+π^-$ and $e^+ e^- \to ϕ\fzero$ are measured from threshold to $\sqrt{s}=3.0$ $\hbox{GeV}$ using initial state radiation. The analysis is based on a data sample of 673 fb$^{-1}$ collected on and below the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. First measurements are reported for the resonance parameters of the $ϕ(1680)$ in the $ϕπ^+π^-$ mode: $m=(1689\pm 7\pm 10)$ MeV/$c^2$ and $Γ=(211\pm 14\pm 19)$ MeV/$c^2$. A structure at $\sqrt{s}=2.1 \hbox{GeV}/c^2$, corresponding to the so called Y(2175), is observed; its mass and width are determined to be $2079\pm13^{+79}_{-28}$ MeV/$c^2$ and $192\pm23^{+25}_{-61} \hbox{MeV}/c^2$, respectively.

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