Proton-helium elastic scattering: a possible high-energy polarimeter at RHIC-BNL

We examine a suggestion to use ${p-} ^4{He}$ elastic scattering, as an absolute polarimeter for high-energy polarized proton beams, by means of a Coulomb-Nuclear Interference effect for the single-spin asymmetry $A_N(t)$, around the diffractive minimum of the differential cross section $|t|\sim 0.21 GeV^2$. Although this reaction has a fairly simple dynamical structure, our theoretical uncertainties and the present experimental inaccuracy of the differential cross section in this $t$ region, allows one to generate dramatic effects for $A_N(t)$, which will be discussed.