measurements reported earlier [@sokol99; @sokol00; @sokol08]. It can be interpreted as an evidence for one-nucleon annihilation of produced low-energy $\eta$-mesons (bound or unbound).

Electron/positron peaks shown in Fig. \[bC\] originate from calibration runs with the lead plate inserted. They were not included into simulations made.

![Velocity distribution of charged particles selected according to the criterion $\Delta E_i > 0$ (for all $i=1,2,3$) at $E_e=650$ and 850 MeV. A well visible excess of events over the INC simulation is seen at the right panel — in the case of the beam energy exceeding the $\eta$-photoproduction threshold — in both velocity regions corresponding to the expected velocities of the $\pi N$ and $NN$ decay products of $\eta$-mesic nuclei.[]{data-label="bC"}](fig-bC-650.eps){width="45.00000%"}

The observed proton peak in the $\beta_C$ distribution is very unusual because it corresponds to $pn$ pairs with very high kinetic energies $T_p\sim T_n\sim 200{-}300$ MeV and transverse momenta $p_p\sim
p_n\sim 400{-}800$ MeV/c. One should keep in mind that photons which produce such pairs have quite a modest energy $650~{\rm MeV} < E_\gamma
< 850$ MeV. Ordinary photoproduction reactions do not give nucleons with such a high energy and momentum. Creation and annihilation of intermediate low-energy $\eta$-mesons seems to be the only explanation to 