Singlet Fermion Assisted Dominant Seesaw with Lepton Flavor and Number Violations and Leptogenesis

Abstract

In a recent review Mohapatra has discussed how type-I seesaw mechanism suppressed by fine tuning 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 gauge singlet fermions render the type-I seesaw contribution vanishing but extended seesaw dominant. In this case the light neutrino mass formula is the same as the classic inverse seesaw derived earlier but the singlet fermion masses are governed by a separate type-I seesaw like formula. Embeddings of this mechanism in supersymmetric as well as nonsupersymmetric SO(10) with low or intermediate masses of W R or Z R bosons are discussed. We also discuss how this cancellation criterion 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 one in both cases are charged lepton flavor violating decays accessible to ongoing searches and dominant contribution to double beta decay mediated by the light gauge singlet fermions 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. Recent applications of this mechanism covering dark matter and leptogenesis are reviewed. Emergence of other dominant seesaw mechanisms are also briefly pointed out.

High Scale Type-II Seesaw, Dominant Double Beta Decay within Cosmological Bound and LFV Decays in SU(‎5)

“…In non-SUSY SO(10) models of unification of three forces implementing the cancellation of type-I seesaw [84,123,127], the TeV scale RH neutrinos are gauged but the neutrino masses are controlled by inverse seesaw. But in [13,128] the RHνs are gauged and the neutrino mass formula is linear seesaw or type-II seesaw [13]. In all type-II seesaw dominated SO(10) models, the RHν masses have the same hierarchy as the left-handed neutrino masses:M N 1 : M N 2 : M N 3 :: m 1 : m 2 : m 3 .…”

“…Due to introduction of heavy RHνs in the present model which were absent in [129], it may be natural to presume apriori that besides type-II seesaw, type-I seesaw may also contribute substantially to light neutrino masses and mixings. But it has been noted that there is a natural mechanism to cancel out type-I seesaw contribution while maintaining dominance of inverse seesaw [84,122,123,[125][126][127] or type-II seesaw or even linear seesaw [13,128] as the case may be. Briefly we discuss below how this cancellation mechanism operates in the present extended model resulting in type-II seesaw dominance even in the presence of heavy RHνs.…”

“…All the generational mixings have been found to be much larger than the corresponding quark mixings. Theoretically [6][7][8][9][10][11][12][13] neutrino masses are predicted through various seesaw mechanisms [14][15][16][17][18][19][20][21][22][23][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. In a minimal left-right symmetric [44][45][46][47] grand unified theory (GUT) like SO(10) [48,49] where parity (P) violation in weak interaction is explained along with fermion masses [50][51][52][53][54][55], a number of these seesa...…”

“…Despite inverse seesaw, observable double beta decay in the W L − W L channel and the non-QD type neutrino masses remained mutually exclusive until both the RH neutrinos and singlet fermions (S i ) were brought into the arena of LFV and LNV conundrum through the much needed extension of the Higgs sector. The King-Kang [122] mechanism cancelled out the ruling supremacy of canonical seesaw which was profoundly exploited in SO(10) models with the introduction of both the SO(10) Higgs representations 16 H and 126 † H [13,84,[123][124][125][126][127][128] with successful prediction of observable double beta decay in the W L − W L channel [20,87]. Very interestingly, even though high scale type-II seesaw can govern light neutrino masses of any hierarchy, possibility of observable LFV and double beta decay prediction in the W L − W L channel irrespective of light neutrino mass hierarchies has been realized at least theoretically [13,128].…”

Standard coupling unification in SO(10), hybrid seesaw neutrino mass and leptogenesis, dark matter, and proton lifetime predictions

“…Extensive investigations in such models have been reported with high intermediate scales [69,[129][130][131][132][133][134][135][136][137][138][139][140][141] and also with TeV scale W R , Z R bosons and verifiable seesaw mechanisms [142][143][144][145][146][147][148][149][150][151][152][153]. Out of a large number of possible models that are predicted from non-SUSY SO(10) [132] fermion mass fit has been investigated only in one class of models with Pati-Salam intermediate symmetry [69,137,139,140] and also including additional vector-like fermions [138].…”