η Meson Production in Proton-Proton Collisions at Excess Energies of 40 and 72 MeV

Abstract: The production of η mesons in proton-proton collisions has been studied with the WASA detector using internal pellet targets in the CELSIUS storage ring at the The Svedberg Laboratory in Uppsala.Data were taken at two beam energies, 1360 MeV and 1445 MeV, corresponding to CM excess energies of 40 and 72 MeV, respectively. The η was detected via its 2γ decay in a near-4π electromagnetic calorimeter, whereas the protons were measured by a combination of straw chambers and plastic scintillator planes in the forward direction. The measurements were kinematically complete. The analysis yielded 69·103 events at 1360 MeV and 93·103 events at 1445 MeV, with a background contribution of less than 5%. Data were acceptance-corrected using a parametrization of a matrix element which includes all states up to two units of total angular momentum. The final state interaction between protons in the 1S0 state was included by a momentum-dependent enhancement factor in the relevant amplitudes. Angular distributions of the final state, invariant mass spectra and Dalitz plots are presented. The cos θη-distribution is found to be anisotropic with its maximum at 90° at both energies. From the parametrization it is inferred that this is due to interference between the Ss and Sd final states. A significant contribution from the Pp final state is also needed to describe data.