Determination of the pole position of the lightest hybrid meson candidate

TL;DR

This study uses a coupled-channel amplitude analysis of eta(') pi systems to identify a single hybrid meson resonance at approximately 1564 MeV, clarifying previous experimental ambiguities and aligning with theoretical predictions.

Contribution

It provides the first robust extraction of a single exotic pi1 hybrid meson pole, resolving past controversies about multiple candidates and confirming its coupling to eta(') pi channels.

Findings

Identified a single pi1 hybrid meson resonance at 1564 MeV.

Found no evidence for a second exotic hybrid state.

Provided resonance parameters for a2(1320) and a2'(1700).

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, pi1(1400) and pi1(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 pi1 resonant pole, with mass and width 1564 +- 24 +- 86 MeV 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 a2(1320) and a2'(1700).