Supersymmetric lepton flavor violation in low-scale seesaw models

We study a new supersymmetric mechanism for lepton flavor violation in $\mu$ and $\tau$ decays and $\mu \to e$ conversion in nuclei, within a minimal extension of the minimal supersymmetric standard model with low-mass heavy singlet neutrinos and sneutrinos. We find that the decays $\mu \to e\gamma$, $\tau \to e\gamma$ and $\tau \to \mu \gamma$ are forbidden in the supersymmetric limit of the theory, whereas other processes, such as $\mu \to eee$, $\mu \to e$ conversion, $\tau \to eee$ and $\tau \to e\mu \mu$, are allowed and can be dramatically enhanced several orders of magnitude above the observable level by potentially large neutrino Yukawa coupling effects. The profound implications of supersymmetric lepton flavor violation for present and future experiments are discussed.

Figure 1

Feynman graphs giving rise to leading SLFV effects in the lowest order of an expansion in $⟨H_u^0⟩$ and $m_N^{-1}$. Not shown are diagrams obtained by replacing the tilted SUSY states ${\tilde{H}}_u^{-}$, ${\tilde{W}}^{-}$, ${\tilde{N}}_{\alpha }$ and ${\tilde{\nu }}_l$ with their untilted counterparts.

Figure 2

Exclusion contours of ${\mathbf{\Omega }}_{e\mu }$ versus $m_N$ derived from experimental limits on $B({\mu }^{-}\to e^{-}\gamma )$ (horizontal solid lines), $B({\mu }^{-}\to e^{-}{e}^{-}{e}^{+})$ (dashed line) and $\mu \to e$ conversion in titanium (dash-dotted lines) and Gold (dash-double-dotted line), assuming ${\mathbf{\Omega }}_{ee}={\mathbf{\Omega }}_{\mu \mu }={\mathbf{\Omega }}_{e\mu }$ and ${\mathbf{\Omega }}_{\tau \tau }=0$. In the lower panel, the quantum effects due to ${\tilde{N}}_{1,2,3}$ have been ignored. The diagonal solid and dotted lines are defined by the conditions $\mathrm{Tr}({\mathbf{h}}_{\nu }^{†}{\mathbf{h}}_{\nu })=4\pi$ and $\mathrm{Tr}({\mathbf{h}}_{\nu }^{†}{\mathbf{h}}_{\nu })=g_w^2$, respectively. The areas that lie above or within the contours are excluded; see the text for more details.

Figure 3

Exclusion contours of ${\mathbf{\Omega }}_{e\tau }$ versus $m_N$ derived from present experimental upper limits on $B({\tau }^{-}\to e^{-}\gamma )$ (horizontal solid lines), $B({\tau }^{-}\to e^{-}{e}^{-}{e}^{+})$ (dashed lines) and $B({\tau }^{-}\to e^{-}{\mu }^{-}{\mu }^{+})$ (dash-dotted lines), assuming that ${\mathbf{\Omega }}_{ee}={\mathbf{\Omega }}_{\tau \tau }={\mathbf{\Omega }}_{e\tau }$ and ${\mathbf{\Omega }}_{\mu \mu }=0$. The diagonal solid and dotted lines are defined by the conditions $\mathrm{Tr}({\mathbf{h}}_{\nu }^{†}{\mathbf{h}}_{\nu })=4\pi$ and $\mathrm{Tr}({\mathbf{h}}_{\nu }^{†}{\mathbf{h}}_{\nu })=g_w^2$, respectively. More details are given in the text.