Determination of the Pole Position of the Lightest Hybrid Meson Candidate

Abstract

Mapping states with explicit gluonic degrees of freedom in the light sector is a challenge, and has led to controversies in the past. In particular, the experiments have reported two different hybrid candidates with spin-exotic signature, $\pi_1$ ( 1400 ) and $\pi_1$ ( 1600 ) , which couple separately to $\eta\pi$ and $\eta'\pi$. This picture is not compatible with recent Lattice QCD estimates for hybrid states, nor with most phenomenological models. We consider the recent partial wave analysis of the $\eta(')\pi$ system by the COMPASS Collaboration. We fit the extracted intensities and phases with a coupled-channel amplitude that enforces the unitarity and analyticity of the S matrix. We provide a robust extraction of a single exotic $\pi_1$ resonant pole, with mass and width 1564 ± 24 ± 86 and 492 ± 54 ± 102 MeV , which couples to both $\eta(')\pi$ channels. We find no evidence for a second exotic state. We also provide the resonance parameters of the $\alpha_2$( 1320 ) and $\alpha_2'$( 1700 ) .