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

TL;DR

This paper reports the observation of an exotic meson, the (1600), produced in  p    at 18 GeV/c, with detailed amplitude analysis revealing its resonant nature and production characteristics.

Contribution

First observation of the exotic (1600) meson decaying to  , with detailed amplitude analysis confirming its resonant properties.

Findings

Resonant (1600) with mass .00 GeV/c^2 and width .40 GeV/c^2

First decay observation of the (2040) meson to 

Different production mechanisms for (1600) and a_2(1320) mesons.

Abstract

An amplitude analysis of an exclusive sample of 5765 events from the reaction $\pi^{-} p \to \eta^{\prime} \pi^- p$ at 18 GeV/c is described. The $\eta^{\prime} \pi^-$ 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 $\eta^{\prime}\pi^-$. 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 $\pi_1(1600)$, 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.