Neutrino masses from new generations

Abstract

We reconsider the possibility that Majorana masses for the three known neutrinos are generated radiatively by the presence of a fourth generation and one right-handed neutrino with Yukawa couplings and a Majorana mass term. We find that the observed light neutrino mass hierarchy is not compatible with low energy universality bounds in this minimal scenario, but all present data can be accommodated with five generations and two right-handed neutrinos. Within this framework, we explore the parameter space regions which are currently allowed and could lead to observable effects in neutrinoless double beta decay, $\mu - e$ conversion in nuclei and $\mu \rightarrow e \gamma$ experiments. We also discuss the detection prospects at LHC.Comment: 28 pages, 4 figures. Version to be published. Some typos corrected. Improved figures 3 and

Fourth generation Majorana neutrinos

“…However, as cosmological bounds suffer from large systematic uncertainties, we restrict ourselves here to the discussion of neutrinoless double beta decay. For a discussion of fourth generation induced loop effects on light neutrinos, we refer to [72,73] and the recent papers [65,74]. However, the findings of this article in the context of neutrinoless double beta decay remain valid on their own.…”

From the Trees to the Forest: A Review of Radiative Neutrino Mass Models

“…9f. This idea has been recently revived and experimentally excluded in the context of extra chiral generations [311], but the mechanism can still work in the case of vector-like leptons. Lepton number is violated by the SM singlet Majorana fermion N in the center of the diagram and thus there is a tree-level contribution in addition to the 2-loop contribution to neutrino mass.…”

On the nature of the fourth generation neutrino and its implications

“…To give a conservative estimate we consider only the contribution of the first three generations because we expect their Yukawa couplings to the fourth right-handed neutrino to be somewhat suppressed due to universality and LFV constraints [16,17,23,53] (say, Y k4 ∼ 10 −2 ). Once we fix the neutrino masses and the Yukawa couplings between the fourth-generation neutrino and the first three, m R4 grows quadratically with m Rk .…”

“…In this work we aim to discuss the naturality of the various scenarios arising when new generations and right-handed neutrinos are brought together. Several previous works have considered such association, either explicitly, in order to provide a mechanism for mass generation, or implicitly, when assuming Dirac neutrinos in their analyses [16][17][18][19][20][21][22][23][24][25]. We argue that unless a symmetry is invoked which separates the new family from the first three, the coexistence of both Dirac and Majorana neutrinos is not stable under radiative corrections and doesn't seem natural [25,26].…”

“…We argue that unless a symmetry is invoked which separates the new family from the first three, the coexistence of both Dirac and Majorana neutrinos is not stable under radiative corrections and doesn't seem natural [25,26]. Furthermore, the presence of a fourth family plus a right-handed Majorana neutrino triggers the generation of Majorana masses for the light species through a well-known mechanism [16,17,23,25,[27][28][29]; the upper bounds on the light neutrino masses can thus be translated into bounds on the mixings with the new, heavy generations. This paper is structured as follows.…”