An amplitude analysis of an exclusive sample of 5765 events from the reaction ${\pi }^{-}p\to {\eta }^{\prime }{\pi }^{-}p$ at $18\mathrm{GeV}/c$ is described. The ${\eta }^{\prime }{\pi }^{-}$ production is dominated by natural parity exchange and by three partial waves: those with $J^{\mathrm{PC}}{=1\mathrm{}}^{-+}$, $2^{++}$, and $4^{++}$. A mass-dependent analysis of the partial-wave amplitudes indicates the production of the $a_2\left(1320\right)$ meson as well as the $a_4\left(2040\right)$ meson, observed for the first time decaying to ${\eta }^{\prime }{\pi }^{-}$. The dominant, exotic (non- $q\overline{q}$) $1^{-+}$ partial wave is shown to be resonant with a mass of $1.597\pm {0.010}_{-0.010\mathrm{}}^{+0.045\mathrm{}}\mathrm{GeV}/c^2$ and a width of $0.340\pm 0.040\pm 0.050\mathrm{GeV}/c^2$. This exotic state, the ${\pi }_1\left(1600\right)$, is produced with a $t$ dependence which is different from that of the $a_2\left(1320\right)$ meson, indicating differences between the production mechanisms for the two states.

• Received 2 February 2001

DOI:https://doi.org/10.1103/PhysRevLett.86.3977