Mechanism of Near-Threshold Production of the η Meson (original) (raw)

2007, Physical Review Letters

Measurements of the analysing power for the pp → ppη reaction have been performed in the close-to-threshold energy region at beam momenta of p beam = 2.010 and 2.085 GeV/c, corresponding to excess energies of Q = 10 and 36 MeV, respectively. The determined analysing power is essentially consistent with zero implying that the η meson is produced predominantly in the swave at both excess energies. The angular dependence of the analysing power, combined with the hitherto determined isospin dependence of the total cross section for the η meson production in nucleon-nucleon collisions, reveal a statistically significant indication that the excitation of the nucleon to the S11(1535) resonance, the process which intermediates the production of the η meson, is predominantly due to the exchange of the π meson between the colliding nucleons.