η-helium quasi-bound states

The cross section and tensor analysing power t₂₀ of the ->dd -> ⁴He η reaction have been measured at six c.m. momenta, 10 <= p_η <= MeV/c. The threshold value of t₂₀ is consistent with 1 / √2, which follows from parity conservation and Bose symmetry. The much slower momentum variation observed for the reaction amplitude, as compared to that for the analogous p d -> ³He η case, strongly suggests the existence of a quasi-bound state in the η ⁴He system and simple optical model fits tentatively indicate that this might also be the case for η ³He.