First Observation of an Exotic Reggeon

We present new \compass high-statistics measurements of the peripheral production of $\eta {\pi }^{-}$ and ${\eta }^{\mathrm{\prime }}{\pi }^{-}$ pairs in the reactions ${\pi }^{-}p\to {\eta }^{(\mathrm{\prime })}{\pi }^{-}p$. For the first time, we perform an unbinned analysis of the high-mass region of the $\eta {\pi }^{-}$ and ${\eta }^{\mathrm{\prime }}{\pi }^{-}$ systems, which allows us to disentangle the exchange mechanisms governing their production. We report the first observation in high-energy scattering of an exotic Reggeon with high significance exceeding $5\sigma$ for both channels, which is, most likely, related to the exotic ${\pi }_1(1600)$.8