η–Helium Quasi-Bound States

The cross section and tensor analysing power t20 of the ~ d d → He η reaction have been measured at six c.m. momenta, 10 ≤ pη ≤ 90 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 → He η case, suggests strongly the existence of a quasi-bound state in the η He system and optical model fits indicate that this probably also the case for η He. PACS: 13.60.Le, 25.10.+s, 25.45.-z