# Review of phenomenological analyses of $\eta^{(\prime)} \pi$ resonances

## TL;DR

This paper analyzes the $ ext{η}^{( ext{prime})} ext{π}$ resonance system using COMPASS data, employing a coupled-channel formalism to extract resonance parameters and investigate exotic states, finding only one exotic resonance consistent with lattice QCD.

## Contribution

It provides a robust coupled-channel analysis of $ ext{η}^{( ext{prime})} ext{π}$ resonances, including the first extraction of the exotic $ ext{π}_1(1600)$ parameters from experimental data.

## Key findings

- Confirmed the existence of the $ ext{π}_1(1600)$ resonance.
- No evidence found for a second exotic state.
- Resonance parameters of $a_2(1320)$ and $a'_2(1700)$ were determined.

## Abstract

We present a robust analysis of the $\eta^{(\prime)} \pi$ system in COMPASS data. We fit the extracted relative phases and intensities with a coupled-channel formalism enforcing both unitarity and analyticity. We provide a robust extraction of a single exotic $\pi_1(1600)$ decaying to both $\eta^{(\prime)} \pi$ final states, and the resonance parameters of the $a_2(1320)$ and $a'_2(1700)$. We find no evidence for a second exotic state, which is compatible with recent Lattice QCD estimates.