$\pi^-p\to\eta^{(\prime)}\, \pi^- p$ 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 $\pi ^-p\rightarrow \eta ^{(\prime )} \pi ^- \,p$ measured by COMPASS. We show that the data in $2.4< m_{\eta ^{(\prime )}\pi } < 3.0{\mathrm {\,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.