[Paper Review] Singlet Fermion Assisted Dominant Seesaw with Lepton Flavor and Number Violations and Leptogenesis

M. K. Parida, Bidyut Prava Nayak|arXiv (Cornell University)|Jul 25, 2016

Particle physics theoretical and experimental studies112 references4 citations

TL;DR

This paper proposes a novel seesaw mechanism in left-right symmetric and SO(10) grand unified theories where gauge singlet fermions suppress the type-I seesaw contribution, leading to dominant extended or inverse seesaw-like neutrino masses. The mechanism enables TeV-scale $W_R$ and $Z_R$ bosons, predicts observable charged lepton flavor violation and dominant $W_L-W_L$ double beta decay via light sterile neutrinos, and supports resonant leptogenesis with quasi-degenerate sterile neutrinos.

ABSTRACT

In a recent review Mohapatra has discussed how type-I seesaw mechanism suppressed by fine tuning of Yukawa couplings, or specific textures of associated fermion mass matrices, can form the basis of neutrino masses in TeV scale $W_R$ boson models. In this paper we review recent works in another class of theories where the added presence of fermion singlets manifesting as sterile neutrinos render the type-I seesaw contribution vanishing but extended seesaw dominant where the light neutrino mass formula is same as the classic inverse seesaw but all massive neutrinos are Majorana fermions. We also show domunance of linear seesaw, or double seesaw, or type-II seesaw in due to cancellation of type-I seesaw. Embeddings of this mechnism in supersymmetric as well as non-supersymmetric SO(10) with low or intermediate masses of $W_R$ or $Z_R$ bosons are discussed. We also discuss how this cancellation criteria has led to a new mechanism of type-II seesaw dominance which permits $U(1)_{B-L}$ breaking scale much smaller than the left-handed triplet mass. Out of a number of new observable predictions, the most visible ones are the dominant contribution to LFV decays and neutrinoless double beta decay mediated by light sterile neutrinos in the $W_L-W_L$ channel. These seesaw dominance mechanisms are applicable in the extensions of the SM and high, intermediate, or low scale left-right gauge theories with or without their SO(10) origin. Other recent works on the applications of this mechanism covering dark matter and leptogenesis are noted..

Motivation & Objective

• To address the hierarchy problem in seesaw mechanisms by suppressing the canonical type-I seesaw contribution via gauge singlet fermions.
• To enable TeV-scale $W_R$ and $Z_R$ bosons in left-right and SO(10) models while preserving neutrino mass generation and consistency with oscillation data.
• To explore the implications of this mechanism for charged lepton flavor violation (LFV), double beta decay, and leptogenesis.
• To demonstrate that the $U(1)_{B-L}$ breaking scale can be significantly lowered compared to conventional type-II seesaw models.
• To provide a natural framework for sterile neutrinos with masses between a few GeV and 1 TeV, protected by lepton number symmetry.

Proposed method

• Introduce gauge singlet fermions $S_i$ that mix with right-handed neutrinos $N_i$, forming a mixed state that suppresses the type-I seesaw contribution via cancellation criteria.
• Apply a two-step block diagonalization procedure to derive a light neutrino mass matrix dominated by the inverse seesaw form, even when the type-I seesaw would otherwise dominate.
• Use the condition $M_N > M >> M_D, u_S$ to ensure the seesaw mechanism is dominated by the extended seesaw formula, with $M_N$ and $M$ being the RH Majorana and singlet fermion masses.
• Derive the light neutrino mass as $M_ u = -M_D^T M_S^{-1} M_D$, where $M_S$ is the singlet fermion mass matrix, analogous to the inverse seesaw but with a distinct origin.
• Implement the mechanism in both non-supersymmetric and supersymmetric SO(10) models, ensuring gauge coupling unification and proton lifetime constraints are satisfied.
• Analyze the phenomenological consequences: LFV decays via $W_L$ and $W_R$ exchange, $W_L-W_L$ double beta decay mediated by light sterile neutrinos, and resonant leptogenesis from quasi-degenerate sterile neutrinos.

Experimental results

Research questions

• RQ1 Can the type-I seesaw contribution be suppressed in left-right and SO(10) models through the introduction of gauge singlet fermions, enabling a dominant extended seesaw mechanism?
• RQ2 What are the phenomenological signatures of this mechanism, particularly in charged lepton flavor violation and double beta decay?
• RQ3 Can this mechanism allow for a $U(1)_{B-L}$ breaking scale much lower than the left-handed triplet mass, enabling TeV-scale $W_R$ and $Z_R$ bosons?
• RQ4 How does the presence of light sterile neutrinos from the singlet sector affect the dominant contribution to double beta decay in the $W_L-W_L$ channel?
• RQ5 Can resonant leptogenesis be achieved in this framework with quasi-degenerate sterile neutrinos at the TeV scale?

Key findings

• The type-I seesaw contribution is suppressed via cancellation criteria, leading to a dominant extended seesaw mechanism with light neutrino masses governed by the inverse seesaw formula.
• Charged lepton flavor violating decays such as $ au o e au$, $ au o uar{ u}$, and $ u o e au$ are predicted to be accessible to ongoing and future experiments due to the light sterile neutrino exchange.
• Double beta decay is dominated by the $W_L-W_L$ channel mediated by light sterile neutrinos, with rates saturating current experimental limits.
• The $U(1)_{B-L}$ breaking scale can be brought down to the TeV scale, enabling $W_R$ and $Z_R$ bosons to be accessible at the LHC, contrary to the conventional requirement of GUT/Planck-scale $W_R$ masses.
• Resonant leptogenesis is achieved via quasi-degenerate sterile neutrinos of the other two generations, with masses in the few GeV to 1 TeV range, enabling successful baryogenesis at low scales.
• The gauge singlet fermions are protected by global lepton number symmetry, ensuring their masses remain light (few GeV to ~1 TeV), consistent with 't Hooft's naturalness principle.