About heavy neutrinos: lepton-flavor violation in decays of charged leptons

Abstract

The fundamental description of nature, beyond the Standard Model (SM), may include heavy neutrinos that mix and thus allow processes in which lepton flavor is not preserved. We investigate the impact of charged currents that couple heavy gauge bosons to heavy neutrinos and SM leptons on lepton-flavor-violating decays of SM leptons into three charged leptons, with no final-state neutrinos. We implement our expressions for the leading contributions to Br(lα → l β lσ lσ), which hold for either Dirac or Majorana neutrinos, to the trilepton decay µ → 3e, of the muon, and so determine sets of masses of heavy neutrinos and the heavy gauge boson, within GeVs to few TeVs, that are consistent with the upper bounds provided by the SINDRUM Collaboration. We find, however, that constraints dictated by the upper bound on Br(µ → eγ), from the MEG Collaboration, are more stringent. We utilize such parameters to find that the contributions to tau decays are ∼ 10 −15 − 10 −13 , well below bounds from B factories. The mixing of heavy and SM charged bosons is also investigated. We find that current experimental data from MEG and SINDRUM would allow mixing angles as large as ∼ 10 −2 , for a relatively light new charged boson, but the expected sensitivity of the Mu3e experiment would be capable of setting an upper bound on this angle as small as ∼ 10 −4 if the mass of this boson is within the range of few TeVs.

Radiative seesaw corrections and charged-lepton decays in a model with soft flavour violation

“…The predictions for the lepton-flavour-violating charged-lepton decays may be used to discriminate among theoretical models. For instance, it has been found [43] that, in a model with a heavy charged gauge boson, the present bounds on µ − → e − γ and µ − → e − e + e − restrict the parameters of the model in such a way that the decays τ − → − is proportional to |X 1 2 | 2 , where the quantities X 1 2 defined in (2.3) depend on m 4,5,6 and U R .…”

νe → νe scattering with massive Dirac or Majorana neutrinos and general interactions

“…There are a number of proposals to probe the specific nature of the neutrino, being the most promising the neutrino-less double beta decay (0νββ) of nuclei [4][5][6]. 1 There are alternative possibilities, such as coherent scattering of neutrinos on nucleus with bremsstrahlung radiation [7], lepton number violating processes [8][9][10][11][12][13][14], lepton flavor changing decays [15], electromagnetic properties of charged leptons [16], and triple gauge boson couplings [17,18], most of them motivated by the propagation of massive heavy neutral leptons. Moreover, many searches for the neutrino's nature are also conducted within the framework of cosmology, including cosmic neutrino capture via tritium [19], the evaporation of primordial black holes [20], the cosmological effective number of neutrino species [21,22] and more [23][24][25][26].…”