The dd -> 4He eta Reaction Near Threshold

The cross section for the reaction $dd\rightarrow \,^4\!H\!e\,\eta$ close to threshold is estimated in a two-step model, similar to that which successfully describes near-threshold $pd\rightarrow \,^3\!H\!e\,\eta$ production. A $\pi$-meson, created in a $dp\rightarrow \,^3\!H\!e\,\pi^o$ process on one nucleon in the target deuteron, is converted into an $\eta$-meson by a secondary $\pi^o n \to n\,\eta$ reaction on the other nucleon. The kinematics are such that both processes are nearly physical so that only small Fermi momenta are required. The predictions of the model are in good agreement with recent Saclay experimental data. An analogous model, when applied to the near-threshold $n\,^3\!H\!e\rightarrow \,^4\!H\!e\,\eta$ reaction, predicts a cross section of a rather similar size, but experimental difficulties render this a doubtful means of studying the $\eta\, ^4\!H\!e$ interaction.