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

TL;DR

This study provides evidence for an exotic meson resonance in the $ ext{ηπ}^{-}$ system produced in $ ext{π}^{-} p$ collisions at 18 GeV/c, revealing phase motion and interference effects indicative of non-conventional meson states.

Contribution

The paper reports the observation of an exotic $1^{-+}$ meson resonance with specific mass and width, demonstrating evidence for non-ordinary meson states in high-energy pion-proton interactions.

Findings

Detection of a broad enhancement in the $J^{PC} = 1^{-+}$ wave between 1.2 and 1.6 GeV/c^2.

Observation of phase motion indicating interference between conventional and exotic mesons.

Evidence supporting the existence of an exotic meson resonance at approximately 1370 MeV/c^2.

Abstract

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