Evidence for Exotic Meson Production in the Reaction ${\pi }^{-}\mathit{p}\to \mathit{\eta }{\pi }^{-}\mathit{p}$ at $18\mathrm{GeV}/\mathit{c}$

The $\eta {\pi }^{-}$ system has been studied in the reaction ${\pi }^{-}p\to \eta {\pi }^{-}p$ at $18\mathrm{GeV}/c$. A large asymmetry in the angular distribution is observed indicating interference between $L$-even and $L$-odd partial waves. The $a_2\left(1320\right)$ is observed in the $J^{\mathrm{PC}}{=2\mathrm{}}^{++}$ wave, as is a broad enhancement between 1.2 and $1.6\mathrm{GeV}/c^2$ in the $1^{-+}$ wave. The observed phase difference between these waves shows that there is phase motion in addition to that due to $a_2\left(1320\right)$ decay. The data can be fitted by interference between the $a_2\left(1320\right)$ and an exotic $1^{-+}$ resonance with $M=(1370\pm 16\{+50\mathrm{}\}\{-30\mathrm{}\})\mathrm{MeV}/c^2$ and $\gamma =(385\pm 40{}_{-105\mathrm{}}^{+65\mathrm{}})\mathrm{MeV}{/c}^2$.