Non-Standard Interactions in Radiative Neutrino Mass Models

Abstract

Models of radiative Majorana neutrino masses require new scalars and/or fermions to induce lepton number violating interactions. We show that these new particles also generate observable neutrino nonstandard interactions (NSI) with matter. We classify radiative models as type-I or II, with type-I models containing at least one Standard Model (SM) particle inside the loop diagram generating neutrino mass, and type-II models having no SM particle inside the loop. While type-II radiative models do not generate tree-level NSI, popular models which fall under the type-I category are shown, somewhat surprisingly, to generate observable NSI at tree-level, while being consistent with direct and indirect constraints from colliders, electroweak precision data and charged-lepton flavor violation (cLFV). We survey such models where neutrino masses arise at one, two and three loops. In the prototypical Zee model which generates neutrino masses via one-loop diagrams involving charged scalars, we find that diagonal NSI can be as large as ($8\%, 3.8\%, 9.3\%$) for ($\varepsilon_{ee},\varepsilon_{\mu \mu}, \varepsilon_{\tau \tau}$), while off-diagonal NSI can be at most ($10^{-3}\%, 0.56\%, 0.34\%$) for ($\varepsilon_{e\mu},\varepsilon_{e \tau}, \varepsilon_{\mu \tau}$). In one-loop neutrino mass models using leptoquarks (LQs), $(\varepsilon_{\mu\mu},\, \varepsilon_{\tau\tau})$ can be as large as $(21.6\%,\,51.7\%)$, while $\varepsilon_{ee}$ and $(\varepsilon_{e\mu},\, \varepsilon_{e\tau},\,\varepsilon_{\mu\tau})$ can at most be 0.6\%. The most stringent constraints on the diagonal NSI are found to come from neutrino oscillation and scattering experiments, while the off-diagonal NSI are mostly constrained by low-energy processes, such as atomic parity violation and cLFV. While our analysis is focused on radiative neutrino mass models, it essentially covers all NSI possibilities with heavy mediators.