Determination of the η ~3He threshold structure from the low energy pd → η ~3He reaction

摘要

We analyze the data on cross sections and asymmetries for the pd → η ~3He reaction close to threshold and look for bound states of the η ~3He system. Rather than parameterizing the scattering matrix, as is usually done, we develop a framework in which the η ~3He optical potential is the key ingredient, and its strength, together with some production parameters, are fitted to the available experimental data. The relationship of the scattering matrix to the optical potential is established using the Bethe-Salpeter equation and the η ~3He loop function incorporates the range of the interaction given by the empirical ~3He density. We find a local Breit-Wigner form of the η ~3He amplitude T below threshold with a clear peak in |T |~2, which corresponds to an η ~3He binding of about 0.3 MeV and a width of about 3 MeV. By fitting the potential we can also evaluate the η ~3He scattering length, including its sign, thus resolving the ambiguity in the former analyses.