Phenomenological consequences of singlet neutrinos

Abstract

In this paper, we study the phenomenology of right-handed neutrino isosinglets. We consider the general situation where the neutrino masses are not necessarily given by m 2 D /M , where m D and M are the Dirac and Majorana mass terms respectively. The consequent mixing between the light and heavy neutrinos is then not suppressed, and we treat it as an independent parameter in the analysis. It turns out that µ − e conversion is an important experiment in placing limits on the heavy mass scale (M ) and the mixing. Mixings among light neutrinos are constrained by neutrinoless double beta decay, as well as by solar and atmospheric neutrino experiments. Detailed one-loop calculations for lepton number violating vertices are provided.

Muon conversion to electron in nuclei in type-I seesaw models

“…Refs. [11][12][13] also exhibit differences in the relative size and/or sign of the crossed and not crossed box contributions, that is, in the amplitudes resulting from the last two diagrams in Fig. 1.…”

“…Nevertheless, below 100 MeV the absence of zero distance effects in short baseline oscillation experiments (such 10 In the low mass regime, the amplitude for µ → eγ is analogous to that for b → sγ, while those for µ → e conversion and µ → 3e exhibit only a GIM cancelation quadratic in the light masses instead of the logarithmic one for quark transitions such as b → se + e − , which are proportional to fermion electric charges inside the loop. 11 For small sterile masses above the eV , the seesaw approximation UνN ∝ Y † N v/mN ( 1) in Eq. (2.13) still holds, and an extra x −2 N factor has to be added in Eqs.…”

“…The µ → e conversion rate R µ→e has been calculated in the literature for the various possible types of seesaw, with right-handed neutrinos [10][11][12][13][14][15][16], scalar triplet(s) [17][18][19] and fermion triplets [20]. For the right-handed neutrino case ("type-I" seesaw), a comparison of the various calculations shows that there is no agreement on what is actually the result.…”

Phenomenology of neutral heavy leptons

“…To show that the differences between theories with massive neutrinos and our model should not be taken too seriously, we note that there is a variant of our model where the lepton number symmetry is slightly broken and the neutrinos are given a small mass [17,18] and there are variants of see-saw models where the restriction from the ratio D 2 M N is avoided (assuming certain symmetries in the neutrino mass matrix) and possibly large mixings of NHL's arise as a result [19,20].…”

“…2.30 become Dirac and Majorana n × n mass matrices) that avoid this suppression [19,20]. For instance, Pilaftsis [19] finds a relation among the elements of D and M matrices that leads to massless neutrinos at the tree-level and small Majorana masses are generated radiatively.…”