Lepton flavor violating signals driven by CP symmetry of order 4

TL;DR

This work extends the CP4 3HDM to the lepton sector and studies whether lepton-Higgs couplings can remain under control while yielding observable LFV signals. A two-step scanning strategy is developed to explore scalar and lepton sectors under current constraints, revealing that the nontrivial texture B1 is the most viable CP4-invariant leptonic scenario and can accommodate the CMS 146 GeV $h\to e\mu$ hint. By applying restricted Yukawa scans, LFV in $\mu \rightarrow e \gamma$ is suppressed, allowing compatibility with experimental bounds, while Cases B2 and B3 are more constrained. The analysis also highlights distinctive leptonic signatures from additional Higgs bosons, notably potential sizable $h_{146}\to ee$ signals, offering concrete tests for the CP4 3HDM at the HL-LHC or future $e^+e^-$ colliders.

Abstract

CP4 3HDM is a curious version of the three-Higgs-doublet model built upon a CP symmetry of order 4 (dubbed CP4). When extended to fermions, CP4 leads to unusually tight correlations between the scalar and Yukawa sectors and induces tree-level flavor changing neutral couplings. Still, viable scenarios exist, in which quark flavor changing signals remain within experimental limits. In this work, we extend CP4 to the lepton sector and investigate whether the lepton-Higgs couplings and lepton flavor violating (LFV) signals can also be kept under control. We consider two classes of LFV processes: tree-level lepton decays of the 125 GeV Higgs boson and one-loop radiative decay $μ\rightarrow eγ$. For each CP4-invariant lepton Yukawa scenario, we perform a focused Yukawa sector scan that uses physical lepton properties as input and suppresses LFV effects. We identify a promising CP4 3HDM scenario compatible with the present-day experimental constraints, show that it can accommodate the recent CMS hint of a 146 GeV scalar decaying to $eμ$, and argue that this interpretation can be tested at future colliders.

Figures (8)

• Figure 1: One-loop Feynman diagrams for $\mu^+ \to e^+\gamma$ due to neutral Higgs bosons (left) and charged Higgs bosons (right).
• Figure 2: The impact of the misalignment angle $\epsilon$ on the LFC (top row) and LFV (bottom row) decays of $h_{\hbox{\tiny SM}}$. The dashed red box shows the experimental limits. The points which satisfy all six experimental limits are shown as black dots.
• Figure 3: The impact of $\tan\beta$ on the LFC and LFV decays of $h_{\hbox{\tiny SM}}$. The color conventions are the same as in Fig. \ref{['hsm-ce']}.
• Figure 4: A comprehensive numerical scan of $h_{\hbox{\tiny SM}}$ lepton decay in high-mass region.
• Figure 5: Left: the results of the full Yukawa scan in the plane $\mathcal{B}_{\mu\tau}^h$ vs. $\mathcal{B}_{e\gamma}^\mu$. Right: the distribution of the ratio $\mathcal{B}_{e\gamma}^\mu (\tau)/\mathcal{B}_{e\gamma}^\mu$, the contribution of $\tau$ propagator diagram to $\mu\rightarrow e\gamma$. The red dashed lines show the experimental upper limits.