First observation of an exotic Reggeon

This newly submitted paper reports the first observation of an exotic Reggeon in high-energy scattering by the COMPASS experiment at CERN. ExoHad researchers César Fernández Ramírez (UNED, co-PI), Giorgio Foti (Messina U., ExoHad PhD student), Nadine Hammoud (Barcelona U., ExoHad postdoc), Vincent Mahtieu (Barcelona U., co-PI), Gloria Montaña (Barcelona U., ExoHad postdoc), Robert J. Perry (MIT, ExoHad postdoc), Alessandro Pilloni (Messina U. & INFN Catania, co-PI), Arkaitz Rodas (ODU & JLab, co-PI), Vanamali Shastry (Indiana U., ExoHad postdoc), Wyatt A. Smith (William & Mary, U. California Berkeley, ExoHad postdoc), Adam P. Szczepaniak (Indiana U. & JLab, co-PI) Daniel Winney (UNAM, co-PI) together with COMPASS collaboration studied reactions in which a high-energy negative pion collides with a proton and produces either an $\eta\pi^-$ or an $\eta'\pi^-$ pair. By analysing a large dataset with an unbinned statistical method, they identified an exotic exchange contribution in both final states with a significance greater than $5\sigma$, the standard threshold for claiming an observation in particle physics. In the Regge description of high-energy reactions, particles are produced through the exchange of families of states called Regge trajectories, rather than through the exchange of a single particle. Most known trajectories correspond to ordinary mesons made primarily from a quark and an antiquark. The exchange found in this study is exotic because its quantum numbers are incompatible with that simple structure. The analysis was able to separate this contribution from conventional exchange mechanisms by studying the mass and angular distributions of the $\eta\pi^-$ and $\eta'\pi^-$ systems, particularly in the previously challenging high-mass region. The observed exotic Reggeon is most likely related to the $\pi_1(1600)$, a candidate exotic meson with quantum numbers $J^{PC} = 1^{-+}$. Such quantum numbers cannot be obtained from a conventional quark-antiquark meson, making the $\pi_1(1600)$ an important probe of additional gluonic or multiquark degrees of freedom in quantum chromodynamics. This result strengthens the evidence the $\pi_1(1600)$ and shows it may belong to an exotic Regge trajectory and can govern particle-production processes at high energies.