ETA-HELIUM QUASI-BOUND STATES

The cross section and tensor analysing power t(20) of the (d) over rig ht arrow d --> He-4 eta reaction have been measured at six c.m. moment a, 10 less than or equal to p eta less than or equal to 90 MeV/c. The threshold value of t(20) is consistent with 1/root 2, which follows fr om parity conservation and Bose symmetry. The much slower momentum var iation observed for the reaction amplitude, as compared to that for th e analogous p d --> He-3 eta case, strongly suggests the existence of a quasi-bound state in the eta He-4 system and simple optical model fi ts tentatively indicate that this might also be the case for eta He-3. (C) 1997 Elsevier Science B.V.