Predictive Dirac and Majorana neutrino mass textures from SU(6) grand unified theories

Abstract

We present simple and predictive realizations of neutrino masses in theories based on the SUð6Þ grand unifying group. At the level of the lowest-dimension operators, this class of models predicts a skew-symmetric flavor structure for the Dirac mass term of the neutrinos. In the case that neutrinos are Dirac particles, the lowest-order prediction of this construction is then one massless neutrino and two degenerate massive neutrinos. Higher-dimensional operators suppressed by the Planck scale perturb this spectrum, allowing a good fit to the observed neutrino mass matrix. A firm prediction of this construction is an inverted neutrino mass spectrum with the lightest neutrino hierarchically lighter than the other two, so that the sum of neutrino masses lies close to the lower bound for an inverted hierarchy. In the alternate case that neutrinos are Majorana particles, the mass spectrum can be either normal or inverted. However, the lightest neutrino is once again hierarchically lighter than the other two, so that the sum of neutrino masses is predicted to lie close to the corresponding lower bound for the normal or inverted hierarchy. Near future cosmological measurements will be able to test the predictions of this scenario for the sum of neutrino masses. In the case of Majorana neutrinos that exhibit an inverted hierarchy, future neutrinoless double beta experiments can provide a complementary probe.

Naturally light Dirac neutrinos from SU(6)

“…Note that all are left-handed and ν c sits in a different multiplet from ν, whereas both are in the same multiplet in the recent proposal of ref. [13]. The added U(1) X is a global symmetry imposed on the dimension-four couplings of the resulting Lagrangian, but softly broken by JHEP07(2021)024 scalar SU( 6) SU( 5) SU(3 , N E 0 proportional to v 3 .…”

Axion model with the ${\rm SU}(6)$ unification

“…The alternative symmetry breaking pattern of SU(6) → SU(5) ⊗ U(1) N → SU(3) c ⊗ SU(2) L ⊗ U(1) Y was previously studied in the SU( 6) literatures [65][66][67][76][77][78][79][80], while this can potentially lower the scale of the sequential SU(5) breaking and bring tensions to the current Super-Kamionkande (SK) constraint [81,82] to the proton lifetime.…”