Charged lepton flavor violating Higgs decays at future $e^+e^-$ colliders

TL;DR

This paper assesses the potential of future electron-positron colliders to measure rare charged lepton flavor violating Higgs decays, providing improved bounds and implications for new physics models.

Contribution

It evaluates the sensitivity of future colliders to rare Higgs decays and discusses the resulting constraints on new physics parameters.

Findings

Projected upper bounds on branching ratios are 1-2 orders of magnitude better than current limits.

Future colliders can significantly constrain lepton flavor violating couplings.

Results impact theories like 2HDM, SMEFT, RS models, and heavy neutrino scenarios.

Abstract

After the discovery of the Higgs boson, several future experiments have been proposed to study the Higgs boson properties, including two circular lepton colliders, the CEPC and the FCC-ee, and one linear lepton collider, the ILC. We evaluate the precision reach of these colliders in measuring the branching ratios of the charged lepton flavor violating Higgs decays $H\to e^\pm\mu^\mp$, $e^\pm\tau^\mp$ and $\mu^\pm\tau^\mp$. The expected upper bounds on the branching ratios given by the circular (linear) colliders are found to be $\mathcal{B}(H\to e^\pm\mu^\mp) < 1.2\ (2.1) \times 10^{-5}$, $\mathcal{B}(H\to e^\pm\tau^\mp) < 1.6\ (2.4) \times 10^{-4}$ and $\mathcal{B}(H\to \mu^\pm\tau^\mp) < 1.4\ (2.3) \times 10^{-4}$ at 95\% CL, which are improved by one to two orders compared to the current experimental bounds. We also discuss the constraints that these upper bounds set on certain theory parameters, including the charged lepton flavor violating Higgs couplings, the corresponding parameters in the type-III 2HDM, and the new physics cut-off scales in the SMEFT, in RS models and in models with heavy neutrinos.