A = 4 0/sup +/ - 1/sup +/ binding-energy difference

The A = 4 ..lambda..-hypernuclei provide a rich source of information about the s-wave properties of the fundamental hyperon-nucleon (YN) force as well as offer a unique opportunity to investigate the complications that arise in calculations of the properties of bound systems in which one baryon (here the ..lambda..) with a given isospin couples strongly to another (the ..sigma..) with a different isospin. The ..lambda../sup 4/H - ..lambda../sup 4/He isodoublet ground-state energies are not consistent with a charge symmetry hypothesis for the YN interaction. The (spin-flip) excitation energies are quite sensitive to the ..lambda..N - ..sigma..N coupling of the YN interaction. In particular, when one represents the free YN interaction in terms of one-channel effective ..lambda..N potentials, the resulting 0/sup +/ (ground) state and 1/sup +/ (excited) spin-flip state are inversely ordered in terms of binding energies, the 1/sup +/ state being more bound. It is the ..sigma.. suppression that results from the reduced strength of the ..lambda..N - ..sigma..N off-diagonal coupling potential when the trinucleon core is restricted to isospin-1/2 which we study here. We find this spin-isospin suppression of the ..lambda..-..sigma.. conversion, which is due to the composite nature of the nuclear cores of the ..lambda../sup 4/H and ..lambda../sup 4/He hypernuclei, to be a significant factor in understanding the 0/sup +/ - 1 /sup +/ binding energy relationship