Effects of virtual Majorana neutrinos on charged Lepton Flavor Violation decays from a seesaw variant with radiatively induced light neutrino masses

TL;DR

This work analyzes a radiative seesaw variant in which light neutrino masses arise at loop level and heavy Majorana neutrinos at the TeV scale induce non-unitarity in the light-neutrino sector. The authors derive complete one-loop amplitudes for lα→lβγ, showing that the branching ratios depend on the non-unitarity parameter η through the heavy-minus-light neutrino loop difference, G(x_i,y_i). Through a detailed numerical study of four ξ textures, they find μ→eγ is the most sensitive probe, with upper bounds on |ηαβ|∼10^{-5}–10^{-6} set by current data and projected future sensitivities, while τ decays remain highly suppressed. The results demonstrate that radiative cLFV in this framework can be within reach of upcoming experiments like MEG II and establish a clear link between non-unitarity and observable cLFV, with distinct texture-dependent predictions and a plan to extend to μ→e conversion and μ→3e in future work.

Abstract

Lepton flavor violating decays $\ell_α \to \ell_β γ$, being forbidden in the Standard Model framework, provide a sensitive probe for new physics. We study these processes in a seesaw variant in which small neutrino masses are generated radiatively. By analyzing the parameter space constrained by electroweak precision data, we investigate the correlation between these decays and non-unitary effects from TeV-scale heavy neutrinos. According to our results, $μ\to e γ$ is the most promising channel for new physics searches, with the bound $|η_{μe}| \lesssim 10^{-6}$ obtained for non-unitary effects in this radiative seesaw variant. Our estimations of $\mathcal{BR} \left( μ\to e γ\right)$, which depends on the mass of the heavy neutrinos, shows that both current and future experimental facilities might be sensitive to these effects.

Figures (11)

• Figure 1: Conventions for momenta used to carry out the calculation of the $\ell_{\alpha}\to\ell_{\beta}\gamma$ amplitude.
• Figure 2: Generic Feynman diagrams contributing to the $\ell_\alpha\to\ell_\beta\gamma$ decay amplitudes. Here, $n_{i}$ denotes a neutrino field, either light or heavy, where $n_1=\nu_1$, $n_2=\nu_2$, $n_3=\nu_3$, $n_4=N_1$, $n_5=N_2$ and $n_6=N_3$.
• Figure 3: Branching ratio $\mathcal{BR}(\mu \to e \gamma)$ in the $\rho_1$--$\rho_2$ parameter space. The current experimental upper limit $1.5 \times 10^{-13}$ is represented by the blue solid curve; projected sensitivity, $6 \times 10^{-14}$, is represented by the black dashed curve (see Table \ref{['tab:limi']}).
• Figure 4: Branching ratio $\mathcal{BR}(\mu \to e \gamma)$ depending on the mass of the heavy neutrino $M_N$ parameter with randomly sampled points. Current experimental upper limit ($1.5 \times 10^{-13}$) is shown as red dashed line; projected sensitivity ($6 \times 10^{-14}$) is given by the black solid line.
• Figure 5: Branching ratio $\mathcal{BR}(\tau \to e \gamma)$ and $\mathcal{BR}(\tau \to \mu \gamma)$ depending on the mass of the heavy neutrino $M_N$ parameter with randomly sampled points. The current experimental upper limits are shown as red dashed lines and the projected sensitivities as a black solid lines.