The Search for Missing Resonances: the Neutron Anomaly

Abstract

We report the observation of a narrow structure in the invariant mass spectrum of the η‐neutron pairs produced in the reaction γd→ηn(p) which is absent in the η‐proton pairs produced in the reaction γd→ηp(n). These results are obtained from a liquid deuterium target, the data sample was collected with the Crystal Barrel and TAPS detectors at the electron accelerator ELSA. This structure is also seen in preliminary analyses of the data samples taken by the Crystal Ball and TAPS detectors at the electron accelerator MAMI with a liquid deuterium target and a liquid helium 3 target ruling out a nuclear effect. We report the measurements of the position and width of this narrow structure in the invariant mass spectrum at W≈1665 MeV and with a FWHM of Γ = 25 MeV.