Observation of exotic meson production in the reaction $ π^{-} p \to η^{\prime} π^- p$ at 18 GeV/c

Abstract

An amplitude analysis of an exclusive sample of 5765 events from the reaction $π^{-} p \to η^{\prime} π^- p$ at 18 GeV/c is described. The $η^{\prime} π^-$ production is dominated by natural parity exchange and by three partial waves: those with $J^{PC} = 1^{-+}, 2^{++},$ and $4^{++}$. A mass-dependent analysis of the partial-wave amplitudes indicates the production of the $a_2(1320)$ meson as well as the $a_4(2040)$ meson, observed for the first time decaying to $η^{\prime}π^-$. The dominant, exotic (non-$q\bar{q}) $ $1^{-+}$ partial wave is shown to be resonant with a mass of $1.597 \pm 0.010^{+0.045}_{-0.010}$ GeV/c^2 and a width of $0.340 \pm 0.040 \pm 0.050$ GeV/c^2 . This exotic state, the $π_1(1600)$, is produced with a $t$ dependence which is different from that of the $a_2(1320)$ meson, indicating differences between the production mechanisms for the two states.