3-Neutrino Mass Spectrum from Combining Seesaw and Radiative Neutrino Mass Mechanisms

We extend the Standard Model by adding a second Higgs doublet and a right-handed neutrino singlet with a heavy Majorana mass term. In this model, there are one heavy and three light Majorana neutrinos with a mass hierarchy m_3 >> m_2 >> m_1 such that that only m_3 is non-zero at the tree level and light because of the seesaw mechanism, m_2 is generated at the one-loop and m_1 at the two-loop level. We show that the atmospheric neutrino oscillations and large mixing MSW solar neutrino transitions with $\Delta m^2_\mathrm{atm} \simeq m_3^2$ and $\Delta m^2_\mathrm{solar} \simeq m_2^2$, respectively, are naturally accommodated in this model without employing any symmetry.