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Register a new userAccessible Lepton-Number-Violating Models and Negligible Neutrino Masses
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Lepton-number violation (LNV), in general, implies nonzero Majorana masses for the Standard Model neutrinos. Since neutrino masses are very small, for generic candidate models of the physics responsible for LNV, the rates for almost all experimentally accessible LNV observables -- except for neutrinoless double-beta decay -- are expected to be exceedingly small. Guided by effective-operator considerations of LNV phenomena, we identify a complete family of models where lepton number is violated but the generated Majorana neutrino masses are tiny, even if the new-physics scale is below 1 TeV. We explore the phenomenology of these models, including charged-lepton flavor-violating phenomena and baryon-number-violating phenomena, identifying scenarios where the allowed rates for $mu^-to e^+$-conversion in nuclei are potentially accessible to next-generation experiments.
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We discuss SUSY models in which renormalizable lepton number violating couplings hide the decay of the Higgs through h -> chi_1^0 + chi_1^0 followed by chi_1^0 -> tau + 2 jets or chi_1^0 -> u_tau + 2 jets and also explain neutrino masses. This mechanism can be made compatible with gauge mediated SUSY breaking.
The Schechter-Valle theorem states that a positive observation of neutrinoless double-beta ($0 u beta beta$) decays implies a finite Majorana mass term for neutrinos when any unlikely fine-tuning or cancellation is absent. In this note, we reexamine the quantitative impact of the Schechter-Valle theorem, and find that current experimental lower limits on the half-lives of $0 u beta beta$-decaying nuclei have placed a restrictive upper bound on the Majorana neutrino mass $|delta m^{ee}_ u| < 7.43 times 10^{-29}~{rm eV}$ radiatively generated at the four-loop level. Furthermore, we generalize this quantitative analysis of $0 u beta beta$ decays to that of the lepton-number-violating (LNV) meson decays $M^- to {M^prime}^+ + ell^-_alpha + ell^-_beta$ (for $alpha$, $beta$ = $e$ or $mu$). Given the present upper limits on these rare LNV decays, we have derived the loop-induced Majorana neutrino masses $|delta m^{ee}_ u| < 9.7 times 10^{-18}~{rm eV}$, $|delta m^{emu}_ u| < 1.6 times 10^{-15}~{rm eV}$ and $|delta m^{mu mu}_ u| < 1.0 times 10^{-12}~{rm eV}$ from $K^- to pi^+ + e^- + e^-$, $K^- to pi^+ + e^- + mu^-$ and $K^- to pi^+ + mu^- + mu^-$, respectively. A partial list of radiative neutrino masses from the LNV decays of $D$, $D_s^{}$ and $B$ mesons is also given.
It is shown how pure Dirac neutrino masses can naturally occur at low energies even in the presence of Planck scale lepton number violation. The geometrical picture in five dimensions assumes that the lepton number symmetry is explicitly broken on the Planck brane while the right-handed neutrino is localised on the TeV brane. This physical separation in the bulk causes the global lepton number to be preserved at low energies. A small wavefunction overlap between the left-handed and right-handed neutrinos then naturally leads to a small Dirac Yukawa coupling. By the AdS/CFT correspondence there exists a purely four-dimensional dual description in which the right-handed neutrino is a composite CFT bound state. The global lepton number is violated at the Planck scale in a fundamental sector whose mixing into the composite sector is highly suppressed by CFT operators with large anomalous dimensions. A similar small mixing is then also responsible for generating a naturally small Dirac Yukawa coupling between the fundamental left-handed neutrino and the composite right-handed neutrino.
In this paper, we study the viability of having a fermion Dark Matter particle below the TeV mass scale in connection to the neutrino mass generation mechanism. The simplest realization is achieved within the scotogenic model where neutrino masses are generated at the 1-loop level. Hence, we consider the case where the dark matter particle is the lightest $mathbb{Z}_2$-odd Majorana fermion running in the neutrino mass loop. We assume that lepton number is broken dynamically due to a lepton number carrier scalar singlet which acquires a non-zero vacuum expectation value. In the present scenario the Dark Matter particles can annihilate via $t$- and $s$-channels. The latter arises from the mixing between the new scalar singlet and the Higgs doublet. We identify three different Dark Matter mass regions below 1 TeV that can account for the right amount of dark matter abundance in agreement with current experimental constraints. We compute the Dark Matter-nucleon spin-independent scattering cross-section and find that the model predicts spin-independent cross-sections ``naturally dwelling below the current limit on direct detection searches of Dark Matter particles reported by XENON1T.
We study the phenomenology of d=7 1-loop neutrino mass models. All models in this particular class require the existence of several new $SU(2)_L$ multiplets, both scalar and fermionic, and thus predict a rich phenomenology at the LHC. The observed neutrino masses and mixings can easily be fitted in these models. Interestingly, despite the smallness of the observed neutrino masses, some particular lepton number violating (LNV) final states can arise with observable branching ratios. These LNV final states consists of leptons and gauge bosons with high multiplicities, such as 4l+4W, 6l+2W, etc. We study current constraints on these models from upper bounds on charged lepton flavour violating decays, existing lepton number conserving searches at the LHC and discuss possible future LNV searches.
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