Shears mechanism and particle-vibration coupling

The shears mechanism, underlying the rotational-like behavior observed in the $M1\mathrm{}$ bands in neutron-deficient Pb isotopes, is interpreted as a consequence of a residual interaction between the proton and neutron blades. By deriving the angle θ between the proton and neutron spin vectors $j_{\pi }$ and $j_{\nu }$ in the ${}^{198,199}\mathrm{Pb}$ bands, it is shown that the main ingredient of this effective force can be described by a $P_2\left(\theta \right)$ term with a strength of 400–600 keV. Such an interaction can be mediated through the core by particle-vibration coupling, and qualitative agreement with experimental estimates may indicate the possible role of this coupling in the appearance of shears bands.