New Insights into the Coupling of $\eta $ and $f_1$ Mesons to the Nucleon

We show that the contact couplings of neutral pseudoscalar and axial mesons to the nucleon are proportional to $\Delta s$, the fraction of nucleon spin carried by the strange quark sea, and thus are strongly suppressed relative to the couplings of charged mesons to the nucleon. On the other side, recent high accuracy data on $\eta $ photoproduction at threshold reveal the need for non--negligibile $\eta NN$ vertices, while fitting $\bar N N$ phase shifts by means of effective meson exchange potentials requires a substantial presence of $f_1$ mesons there. We here advocate the idea to attribute the couplings of neutral pseudoscalar and axial vector mesons to effective triangular diagrams con…