Difference between stable and exotic nuclei : medium polarization effects

The 1S0 phase shift is large and positive at low densities (relative momenta), while it vanishes and eventually becomes negative at densities of the order of the saturation nuclear density. The bare NN-potential, parametrized so as to reproduce these phase shifts, leads to a sizable Cooper pair binding energy in nuclei along the stability valley. It is a much debated matter whether this value accounts for the 'empirical' value of the pairing gap or whether a similarly important contribution arises from the exchange of collective vibrations between Cooper pair partners. In keeping with the fact that two-particle transfer reactions are the specific probe of pairing in nuclei, and that exotic halo nuclei like 11Li are extremely polarizable (representing, as far as this property is concerned, almost a caricature of stable nuclei), we find that the recently studied reaction, namely 11Li+p → 9Li+t, provides, for the first time, direct evidence of phonon-mediated pairing in nuclei.