Double Seesaw Mechanism Research Articles

3+2neutrino scheme from a singular double seesaw mechanism

We obtain a $3+2$ neutrino spectrum within a left-right symmetric framework by invoking a singular double seesaw mechanism. Higgs doublets are employed to break $S{U}_{R}(2)$ and three additional fermions, singlets under the left-right symmetric gauge group, are included. The introduction of a singularity into the singlet fermion Majorana mass matrix results in a light neutrino sector of three neutrinos containing predominantly ${\ensuremath{\nu}}_{\ensuremath{\alpha}L}$, $\ensuremath{\alpha}=e,\ensuremath{\mu},\ensuremath{\tau}$, separated from two neutrinos containing a small ${\ensuremath{\nu}}_{\ensuremath{\alpha}L}$ component. The resulting active-sterile mixing in the $5\ifmmode\times\else\texttimes\fi{}5$ mixing matrix is specified once the mass eigenvalues and the $3\ifmmode\times\else\texttimes\fi{}3$ submatrix corresponding to the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixing matrix are known.

Left-right model of Quark and Lepton masses without a scalar bidoublet.

We propose a left-right model of quarks and leptons based on the gauge group SU(3)(C)xSU(2)(L)xSU(2)(R)xU(1)(B-L), where the scalar sector consists of only two doublets: (1,2,1,1) and (1,1,2,1). As a result, any fermion mass, whether it be Majorana or Dirac, must come from dimension-five operators. This allows us to have a common view of quark and lepton masses, including the smallness of Majorana neutrino masses as the consequence of a double seesaw mechanism.

Gauge model for maximal neutrino mixing

The recently announced Super-Kamiokande data on atmospheric neutrino oscillation seems to require a maximal mixing between the ν μ and ν τ within the conventional three neutrino picture. It is then tempting to suggest as has been done in literature, that the solar neutrino deficit be also understood as resulting from a maximal mixing between ν e and ν μ . In this letter, we propose a left-right symmetric extension of the standard model where permutation symmetry leads to one of the maximal mixing patterns in a technically natural manner. The double seesaw mechanism gives small Majorana masses for neutrinos needed to understand the atmospheric as well as the solar neutrino puzzles.

Three neutrinoΔm2scales and the singular seesaw mechanism

It is shown that the singular seesaw mechanism can simultaneously explain all the existing data supporting nonzero neutrino masses and mixing. The three mass-squared differences that are needed to accommodate the atmospheric neutrino data (through $\nu_\mu - \nu_s$ oscillation), the solar neutrino data via MSW mechanism (through $\nu_e - \nu_\tau$ oscillation), and the positive result of $\nu_\mu - \nu_e$ oscillation from LSND can be generated by this mechanism, whereas the vacuum oscillation solution to the solar neutrino problem is disfavored. We find that the electron and tau neutrino masses are of order $10^{-3}$ eV, and the muon neutrino and a sterile neutrino are almost maximally mixed to give a mass of order 1 eV. Two heavy sterile neutrinos have a mass of order 1 keV which can be obtained by the double seesaw mechanism with an intermediate mass scale $\sim 10^5$ GeV. A possible origin of such a scale is discussed.