Low energy effects of neutrino masses

TL;DR

The paper establishes that while all Majorana neutrino masses share a common dimension-5 Weinberg operator, distinct Seesaw realizations imprint characteristic dimension-6 operators at low energy. It derives the complete set of d=6 operators for Type I (singlet fermions), Type II (scalar triplets), and Type III (fermionic triplets) and shows that their coefficients scale as Y^†(1/M^2)Y, enabling model discrimination through precision electroweak tests, LFV processes, and collider signatures. A key insight is the decoupling pattern where d=5 masses are suppressed by a small lepton-number-violating parameter μ, while d=6 effects can remain sizable at TeV scales, yielding potentially observable non-unitarity and LFV signals in upcoming experiments. The work further quantifies current bounds on the new Yukawa couplings across the three Seesaw types and discusses prospects for TeV-scale discovery at the LHC/ILC, highlighting the direct lepton-violation paradigm as a natural route to observable low-energy consequences. Overall, the study provides a unified framework to test the origin of neutrino masses via low-energy effective operators tied directly to the high-energy Seesaw physics.

Abstract

While all models of Majorana neutrino masses lead to the same dimension five effective operator, which does not conserve lepton number, the dimension six operators induced at low energies conserve lepton number and differ depending on the high energy model of new physics. We derive the low-energy dimension six operators which are characteristic of generic Seesaw models, in which neutrino masses result from the exchange of heavy fields which may be either fermionic singlets, fermionic triplets or scalar triplets. The resulting operators may lead to effects observable in the near future, if the coefficients of the dimension five and six operators are decoupled along a certain pattern, which turns out to be common to all models. The phenomenological consequences are explored as well, including their contributions to $μ\to e γ$ and new bounds on the Yukawa couplings for each model.

Figures (2)

• Figure 1: The three generic realizations of the Seesaw mechanism, depending on the nature of the heavy fields exchanged: SM singlet fermions (type I Seesaw) on the left, SM triplet scalars (type II Seesaw) and SM triplet fermions (type III Seesaw) on the right.
• Figure 2: Diagrams contributing to $\mu\rightarrow e \gamma$. $\phi^-$ is the Goldstone boson associated with the $W^-$ boson, $\eta$ is the Goldstone boson associated with the Z boson and $H$ stands for the physical Higgs boson.