Abstract
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.
Highlights
Lepton number and baryon number are, at the classical level, accidental global symmetries of the renormalizable Standard Model (SM) Lagrangian.1 If one allows for generic nonrenormalizable operators consistent with the SM gauge symmetries and particle content, lepton number and baryon number will no longer be conserved
According to [19], the contribution to Majorana neutrino masses from the physics that leads to Eq (1.4) at the tree level saturates the upper bound on neutrino masses for Λ ∼ 1 GeV
This means that, for Λ ≫ 1 GeV, the physics responsible for Eq (1.4) will lead to neutrino masses that are too small to be significant while the rates of other Lepton-number violation (LNV) phenomena, including μ− → eþ-conversion in nuclei, may be within reach of next-generation experiments
Summary
Lepton number and baryon number are, at the classical level, accidental global symmetries of the renormalizable Standard Model (SM) Lagrangian. If one allows for generic nonrenormalizable operators consistent with the SM gauge symmetries and particle content, lepton number and baryon number will no longer be conserved. It is possible that even though baryon number or lepton number are not conserved and the new degrees-offreedom are neither weakly coupled nor very heavy, only a subset of baryon-number-violating or lepton-numberviolating phenomena are within reach of particle physics experiments. New-physics scenarios that violate lepton-number conservation at the tree level in a way that LNV low-energy phenomena are captured by the “all-singlets” dimensionnine operator: L. According to [19], the contribution to Majorana neutrino masses from the physics that leads to Eq (1.4) at the tree level saturates the upper bound on neutrino masses for Λ ∼ 1 GeV This means that, for Λ ≫ 1 GeV, the physics responsible for Eq (1.4) will lead to neutrino masses that are too small to be significant while the rates of other LNV phenomena, including μ− → eþ-conversion in nuclei, may be within reach of next-generation experiments. VI, we briefly comment on possible extensions of these scenarios which can account for the observed neutrino masses, summarize our results, and conclude
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