From Higgs physics to lepton flavour violation: current bounds and future prospects for vector-like lepton models

Abstract

We present a comprehensive phenomenological study of a class of six vector-like lepton models with seesaw-like mass contributions and strong modifications to Higgs and lepton phenomenology. We focus on lepton flavour conserving and violating observables across all lepton generations and collider observables like $Z$ and Higgs decays. In light of the recent progress at the LHC and precision measurements such as muon $g-2$, as well as in anticipation of upcoming experiments like MEGII, Mu2e/COMET, Mu3e, Belle II and at the HL-LHC, we systematically survey the viable parameter space and identify patterns and correlations. The considered class of models gives rise to a rich and testable phenomenology with robust and complementary probes that allow to distinguish between models in the coming experimental era.

Figures (12)

• Figure 1: Example of diagrams in the $L\oplus E$ model inducing the mass suppressed corrections to the SM couplings after EWSB. (a) tree-level correction to Higgs coupling and lepton mass. (b) tree-level correction to left- and right-handed lepton--$Z$ coupling. (c) one-loop (leading order) chirally enhanced correction to the lepton form factors.
• Figure 2: (a) 2$\sigma$$\chi^2$ exclusion limits from the measured lepton--$Z$ couplings assuming equal couplings to all 3 generations. (b) 2$\sigma$ bounds on the VLL Yukawa couplings $\overline\lambda=\lambda$ from the $S,T$ and $U$ parameters as a function of the Dirac masses (for $\xi_i=0$).
• Figure 3: Current constraint on the lepton dipole moments from Eq. \ref{['eq:Rii-correlation']} for the different values of $\mathcal{Q}=1$ (blue), $5$ (yellow) and $9$ (red). The green hatched regions show the current constraint from dipole measurements, the dashed lines show the future sensitivity of the muon EDM experiment at PSI. The gray hatched region shows the old bounds from Eq.\ref{['eq:WP20']}.
• Figure 4: Contours of $\mathcal{D}_{3e}^{21}$ (coloured) and $\mathcal{R}_{\chi}^{21}$ (black lines) for $\mu$--$e$ flavour violation in the $L\oplus E$ model for different planes of the $\xi^E_1/\xi^L_1$ -- $\xi^E_2/\xi^L_2$ -- $\overline\lambda$ parameter space relevant to the CLFV ratios (assuming real couplings). Lighter colours correspond to regions with larger dipole contributions. Note that the $\xi^E/\xi^L$ axes are log-scaled, but include both signs.
• Figure 5: $95\%$ CL exclusion lines for several observables, for $\overline\lambda=0$ (solid), $\overline\lambda=-0.5$ (dashed) and $\overline\lambda=-0.5i$ (dot-dashed). The allowed regions are always to the bottom/left. The Dirac mass parameters are fixed at $2$ TeV and other parameters not shown are set to $0$. In the third plot, $\rho_i = \sqrt{m_\tau / m_i}$. The grey region shows points with large precision loss in the SVD inversion (i.e. relative error $> 10^{-5}$ compared to the input masses).