Lepton Flavor Violation Induced by a Neutral Scalar at Future Lepton Colliders

Many new physics scenarios beyond standard model often necessitate the existence of a (light) neutral scalar $H$, which might couple to the charged leptons in a flavor violating way, while evading all existing constraints. We show that such scalars could be effectively produced at future lepton colliders, either on shell or off shell depending on their mass, and induce lepton flavor violating (LFV) signals, i.e., $e^{+}{e}^{-}\to {\ell }_{\alpha }^{\pm }{\ell }_{\beta }^{\mp }(+H)$ with $\alpha \ne \beta$. We find that a large parameter space of the scalar mass and the LFV couplings can be probed well beyond the current low-energy constraints in the lepton sector. In particular, a scalar-loop induced explanation of the long-standing muon $g-2$ anomaly can be directly tested in the on-shell mode.

Figure 1

Examples of LFV signal from on-shell, off-shell, and resonant neutral scalar production at $e^{+}{e}^{-}$ collider.

Figure 2

$m_{e\mu }$ invariant mass distributions for the SM background $e^{+}{e}^{-}\to Zh$ and the signal $e^{+}{e}^{-}\to e^{\pm }{\mu }^{\mp }H$ at CEPC (left) and ILC (right).

Figure 3

Prospects of probing LFV couplings $h_{\alpha \beta }$ ($\alpha \ne \beta$) from searches of $e^{+}{e}^{-}\to {\ell }_{\alpha }^{\pm }{\ell }_{\beta }^{\mp }H$ at CEPC (red, $\sqrt{s}=240\mathrm{GeV}$ and $\mathcal{L}=5{\mathrm{ab}}^{-1}$) and ILC (blue, 1 TeV and $1{\mathrm{ab}}^{-1}$). Here we have assumed 10 LFV signal events and a BR of 1% (long-dashed), 10% (short-dashed), or 100% (solid) from $H$ decay to be visible. In the left panel, the region shaded in orange, pink, and purple and, respectively, excluded by muonium oscillation, $(g-2{)}_e$ and $ee\to \mu \mu$ data; in the middle panel, the pink and purple regions are excluded by $(g-2{)}_e$ and $ee\to \tau \tau$ data; in the right panel the gray region is disfavored by the $(g-2{)}_{\mu }$ data at the $5\sigma$ confidence level. In the left and right panels, the brown line could fit the central value of $\mathrm{\Delta }{a}_{\mu }$, and the green and yellow bands cover the $1\sigma$ and $2\sigma$ ranges of $\mathrm{\Delta }{a}_{\mu }$.

Figure 4

Prospects of $|h_{ee}^{†}{h}_{e\tau }|$ (left), $|h_{ee}^{†}{h}_{\mu \tau }|$ (middle) and $|h_{e\mu }^{†}{h}_{e\tau }|$ (right) from searches of $e^{+}{e}^{-}\to e^{\pm }{\tau }^{\mp },{\mu }^{\pm }{\tau }^{\mp }$ at CEPC (red, $\sqrt{s}=240\mathrm{GeV}$, $\mathcal{L}=5{\mathrm{ab}}^{-1}$) and ILC (blue, 1 TeV and $1{\mathrm{ab}}^{-1}$). Here we have assumed 10 signal events. Also shown are the constraints from the rare lepton decays, $(g-2{)}_e$, and $e^{+}{e}^{-}\to {\ell }^{+}{\ell }^{-}$ LEP data (cf. Table 1).