$\\pi^-p\\to\\eta^{(\\prime)}\\, \\pi^- p$ in the double-Regge region

The production of $\\eta^{(\\prime)}\\pi$ pairs constitutes one of the golden channels to search for hybrid exotics, with explicit gluonic degrees of freedom. Understanding the dynamics and backgrounds associated to $\\eta^{(\\prime)}\\pi$ production above the resonance region is required to impose additional constraints to the resonance extraction. We consider the reaction $\\pi^-p\\to \\eta^{(\\prime)} \\pi^- \\,p$ measured by COMPASS. We show that the data in $2.4 < m_{\\eta^{(\\prime)}\\pi} < 3.0$ GeV can be described by amplitudes based on double-Regge exchanges. The angular distribution of the meson pairs, in particular in the $\\eta' \\pi$ channel, can be attributed to flavor singlet exchanges, suggesting the presence of a large gluon content that couples strongly to the produced mesons.