Production of η Mesons Near Threshold

It is argued that the strong energy dependence of the p d → 3Heη cross section near threshold is due to a final state interaction between the η meson and the 3He nucleus. The large scattering length that this implies is in accord with optical potential predictions and is evidence for a nearby virtual ‘‘bound’’ state in the η3He system. This model suggests that sharp structures should also be seen close to production thresholds on other light nuclei. The p d → 3Heη and p d → 3Heπ0 cross sections show striking but very different energy dependences for proton energies within a few MeV of their respective production thresholds. To illustrate this, define average squared amplitudes through |fη(π)| 2 = pp pη(π) (