of the setup would not be so different for $pn$ and $\pi^+n$ pairs. That seems to be exactly what we see in our experiment.

![Pion-exchange mechanism of $NN\to NN\eta$. Isospin factors, which accompany the $\pi NN$ coupling $g$ and the $\pi N\to\eta N$ amplitude $T$, and the Fermi-statistics signs (both shown in this Figure) jointly determine the big difference between the cross sections of $pp\to pp\eta$ and $pn\to pn\eta$ (plus $pn\to d\eta$). Antisymmetrization of the initial state and initial/final state interactions are not shown.[]{data-label="diagrams-NNeta"}](fig_pp.ps "fig:"){width="50.00000%" height="13ex"} ![Pion-exchange mechanism of $NN\to NN\eta$. Isospin factors, which accompany the $\pi NN$ coupling $g$ and the $\pi N\to\eta N$ amplitude $T$, and the Fermi-statistics signs (both shown in this Figure) jointly determine the big difference between the cross sections of $pp\to pp\eta$ and $pn\to pn\eta$ (plus $pn\to d\eta$). Antisymmetrization of the initial state and initial/final state interactions are not shown.[]{data-label="diagrams-NNeta"}](fig_pn.ps "fig:"){width="50.00000%" height="13ex"}

![Simulation of $NN$ pairs emitted in decays of $\eta$-mesic nuclei. Shown are distributions over the kinetic energy and velocity of one of the nucleons, the total energy of the pair and the relative angle.[]{data-label="simulation-NN"}](figNN-TN.ps "fig:"){width="36.00000%"} ![Simulation of $NN$ pairs emitted in decays of $\eta$-mesic nuclei. Shown are distributions over the kinetic energy 