Evidence for exotic $J^{\mathrm{PC}}{=1\mathrm{}}^{-+}$ meson production in the reaction ${\pi }^{-}\overrightarrow{p}\eta {\pi }^{-}p$ at $18\mathrm{GeV}/c$

Details of the analysis of the $\eta {\pi }^{-}$ system studied in the reaction ${\pi }^{-}\overrightarrow{p}\eta {\pi }^{-}p$ at $18\mathrm{GeV}/c$ are given. Separate analyses for the 2γ and ${\pi }^{+}{\pi }^{-}{\pi }^0$ decay modes of the η are presented. An amplitude analysis of the data indicates the presence of interference between the $a_2^{-}\mathrm{}\left(1320\right)\mathrm{}$ and a $J^{\mathrm{PC}}{=1\mathrm{}}^{-+}$ wave between 1.2 and $1.6\hspace{0.5em}\mathrm{GeV}{/c}^2.$ The phase difference between these waves shows a phase motion not attributable solely to the $a_2^{-}\mathrm{}\left(1320\right).$ The data can be fitted by interference between the $a_2^{-}\mathrm{}\left(1320\right)\mathrm{}$ and an exotic $1^{-+}$ resonance with $M=(1370\mathrm{}\pm {16}_{-30}^{+50\mathrm{}})\mathrm{MeV}{/c}^2$ and $\Gamma =(385\mathrm{}\pm {40}_{-105}^{+65\mathrm{}})\mathrm{MeV}{/c}^2.$ Our results are compared with those of other experiments.