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

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.