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) flavour neutrinos$ u_{alpha L}$ and the heavy right-handed (RH) neutrinos $N_{beta R}$, $alpha,beta = e,mu,tau$, 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 bi-doublet $Phi$. The fermion sector has the usual for the L-R symmetric models quarks and leptons, along with three $SU(2)$ singlet fermion $S_{gamma L}$. 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 $m_{N_j} ll m_{S_k}$. Working with a specific version of the model in which the $ u_{alpha L} – N_{beta R}$ and the $N_{beta R} – S_{gamma L}$ Dirac mass terms are diagonal, and assuming that $m_{N_j} sim (1 – 1000)$ GeV and ${rm max}(m_{S_k}) sim (1 – 10)$ TeV, $m_{N_j} ll m_{S_k}$, we study in detail the new “non-standard” contributions to the $0 ubetabeta$ 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 $m_{1(3)} sim (10^{-4} – 10^{-2})$ eV over the light Majorana neutrino exchange contribution. In large part of the parameter space, the predictions of the model for the $0 ubetabeta$ decay generalised 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, NEXT-HD.
https://indico.ijs.si/event/1692/