Neutrinoless double beta decay in a left-right symmetric model with a double seesaw mechanism

Abstract

We discuss a left-right (L-R) symmetric model with the double seesaw mechanism at the TeV scale generating Majorana masses for the active left-handed (LH) flavor neutrinos ${\ensuremath{\nu}}_{\ensuremath{\alpha}L}$ and the heavy right-handed (RH) neutrinos ${N}_{\ensuremath{\beta}R},\ensuremath{\alpha},\ensuremath{\beta}=e,\ensuremath{\mu},\ensuremath{\tau}$, which in turn mediate lepton number violating processes, including neutrinoless double beta decay. The Higgs sector is composed of two Higgs doublets ${H}_{L}$, ${H}_{R}$, and a bidoublet $\mathrm{\ensuremath{\Phi}}$. The fermion sector has the usual for the L-R symmetric models quarks and leptons, along with three $SU(2)$ singlet fermion ${S}_{\ensuremath{\gamma}L}$. The choice of bare Majorana mass term for these sterile fermions induces large Majorana masses for the heavy RH neutrinos leading to two sets of heavy Majorana particles ${N}_{j}$ and ${S}_{k},j,k=1,2,3$, with masses ${m}_{{N}_{j}}\ensuremath{\ll}{m}_{{S}_{k}}$. Working with a specific version of the model in which the ${\ensuremath{\nu}}_{\ensuremath{\alpha}L}\ensuremath{-}{N}_{\ensuremath{\beta}R}$ and the ${N}_{\ensuremath{\beta}R}\ensuremath{-}{S}_{\ensuremath{\gamma}L}$ Dirac mass terms are diagonal, and assuming that ${m}_{{N}_{j}}\ensuremath{\sim}(1--1000)\text{ }\text{ }\mathrm{GeV}$ and $\mathrm{max}({m}_{{S}_{k}})\ensuremath{\sim}(1--10)\text{ }\text{ }\mathrm{TeV}$, ${m}_{{N}_{j}}\ensuremath{\ll}{m}_{{S}_{k}}$, we study in detail the new ``nonstandard'' contributions to the $0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$ decay amplitude and half-life arising due to the exchange of virtual ${N}_{j}$ and ${S}_{k}$. We find that in both cases of NO and IO light neutrino mass spectra, these contributions are strongly enhanced and are dominant at relatively small values of the lightest neutrino mass ${m}_{1(3)}\ensuremath{\sim}({10}^{\ensuremath{-}4}--{10}^{\ensuremath{-}2})\text{ }\text{ }\mathrm{eV}$ over the light Majorana neutrino exchange contribution. In large part of the parameter space, the predictions of the model for the $0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$ decay generalized effective Majorana mass and half-life are within the sensitivity range of the planned next generation of neutrinoless double beta decay experiments LEGEND-200 (LEGEND-1000), nEXO, KamlAND-Zen-II, CUPID, and NEXT-HD.

Topics & Concepts