Experimental study of ppη dynamics with WASA‐at‐COSY

To investigate the interaction of η‐meson with the nucleons, its production, near the kinematical threshold, in proton‐proton collisions has been studied with the WASA detector at COSY storage ring in Juelich, Germany. The data has been taken at beam energy 1400 MeV (corresponding to excess energy $(Q = 57 MeV).$ The η‐meson was detected via its $3π0$ decay in nearly 4π detector and two protons were measured in forward direction. The determination of four vectors of both protons and the η‐meson in the final state allowed to derive complete kinematical information of the ppη‐system. The analysis resulted in $9×106$ events of $η→3π0$ giving total production cross‐section $(8.87±0.03stat±2.57sys)μb.$ The angular distribution of η‐meson in the center of mass frame is anisotropic and squared invariant mass distributions for proton‐proton and proton‐η shows deviation from pure phase space.