Abstract
Dominance of type-II seesaw mechanism for neutrino masses has attracted considerable attention because of a number of advantages. We show a novel approach to achieve Type-II seesaw dominance in non-supersymmetric $SO(10)$ grand unification where a low mass $Z^{\prime}$ boson and specific patterns of right-handed neutrino masses are predicted within the accessible energy range of the Large Hadron Collider. In spite of the high value of the seesaw scale, $M_{\Delta_L} \simeq 10^8-10^9$ GeV, the model predicts new dominant contributions to neutrino-less double beta decay in the $W_L-W_L$ channel close to the current experimental limits via exchanges of heavier singlet fermions used as essential ingredients of this model even when the light active neutrino masses are normally hierarchical or invertedly hierarchical. We obtain upper bounds on the lightest sterile neutrino mass $m_s\lesssim 3.0$ GeV, $2.0$ GeV, and $0.7$ GeV for normally hierarchical, invertedly hierarchical, and quasi-degenerate patterns of light neutrino masses, respectively. The underlying non-unitarity effects lead to lepton flavor violating decay branching ratios within the reach of ongoing or planned experiments and the leptonic CP-violation parameter nearly two orders larger than the quark sector. Some of the predicted values on proton lifetime for $p\to e^+\pi^0$ are found to be within the currently accessible search limits. Other aspects of model applications including leptogenesis etc. are briefly indicated.
Highlights
Mν = m I ν I + mνI, (1) −MD MDT, (2) f vL (3)
Scale vacuum expectation value (VEV) of VR, the Type-II seesaw mechanism predicts RH neutrino masses which can be testified at the Large Hadron Collider (LHC) or future high energy accelerators
We derive an analytic formula for the half-life of 0νββ decay as a function of the singlet fermion masses, predicting a lower bound on the lightest sterile neutrino mass eigenvalue from the current experimental data on the lower bounds
Summary
Where MD(MN ) is the Dirac (RH-Majorana) neutrino mass, vL is the induced vacuum expectation value (VEV) of the left-handed (LH) triplet L , and f is the Yukawa coupling of the triplet. Two-step breakings of LR gauge theory was embedded earlier in non-SUSY GUTs with low-mass Z (for earlier work on, Z boson in GUTs embedding two-step breaking of left-right gauge symmetry, see [33,34,35]), its successful compliance with neutrino oscillation data has been possible in the context of an inverse seesaw mechanism [36]. Compared to earlier existing SO(10)-based Type-II seesaw dominant models whose RH neutrino masses are in the inaccessible range and new gauge bosons are in the mass range 1015–1017 GeV, the present model predictions on LHC scale Z , light and heavy Majorana type sterile neutrinos, RH Majorana neutrino masses in the range 100–10, 000 GeV accessible to LHC in the WL − WL channel through dilepton production, the LFV branching ratios closer to experimental limits, and dominant 0νββ decay amplitudes caused by sterile neutrino exchanges provide a rich testing ground for new physics signatures.
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