The photoproduction of $\eta$-mesic ${}^3\mathrm{He}$ has been investigated using the TAPS calorimeter at the Mainz Microtron accelerator facility MAMI. The total inclusive cross section for the reaction $\gamma {}^3\mathrm{He}\to \eta X$ has been measured for photon energies from threshold to 820 MeV. The total and angular differential coherent $\eta$ cross sections have been extracted up to energies of 745 MeV. A resonancelike structure just above the $\eta$ production threshold with an isotropic angular distribution suggests the existence of a resonant quasibound state. This is supported by studies of a competing decay channel of such a quasibound $\eta$-mesic nucleus into ${\pi }^{0}pX$. A binding energy of $(-4.4\pm 4.2)\mathrm{MeV}$ and a width of ($25.6\pm 6.1)\mathrm{MeV}$ is deduced for the quasibound $\eta$-mesic state in ${}^3\mathrm{He}$.

• Received 11 December 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.252001