The Smallest Neutrino Mass Revisited.

As is well known, the smallest neutrino mass turns out to be vanishing in the minimal seesaw model, since the effective neutrino mass matrix $M^{}_\nu$ is of rank two due to the fact that only two heavy Majorana neutrinos are introduced. In this paper, we point out that the one-loop matching condition for the effective dimension-five neutrino mass operator can make an important contribution to the smallest neutrino mass. By using the available one-loop matching condition and two-loop renormalization group equations in the supersymmetric version of the minimal seesaw model, we explicitly calculate the smallest neutrino mass in the case of normal neutrino mass ordering and obtain $m^{}_1 \approx 6.7\times 10^{-10}~{\rm eV}$ at the Fermi scale $\Lambda^{}_{\rm F} = 91.2~{\rm GeV}$ with some typical input at the seesaw scale $\Lambda^{}_{\rm SS} = 10^{14}~{\rm GeV}$.