Probing the fermionic Higgs portal at lepton colliders

TL;DR

This paper investigates how future lepton colliders can detect new fermionic particles linked to the Higgs boson through precision measurements, extending the search for new physics beyond current collider capabilities.

Contribution

It provides a detailed analysis of the fermionic Higgs portal's UV completions, especially the singlet-doublet fermion model, using effective field theory and one-loop calculations to assess collider sensitivities.

Findings

Future colliders can probe TeV-scale fermionic states.

Precision electroweak measurements are sensitive to new physics.

The EFT approach effectively captures the relevant contributions.

Abstract

We study the sensitivity of future electron-positron colliders to UV completions of the fermionic Higgs portal operator $H^\dagger H \bar \chi \chi$. Measurements of precision electroweak $S$ and $T$ parameters and the $e^+e^- \to Zh$ cross section at the CEPC, FCC-ee, and ILC are considered. The scalar completion of the fermionic Higgs portal is closely related to the scalar Higgs portal, and we summarize existing results. We devote the bulk of our analysis to a singlet-doublet fermion completion. Assuming the doublet is sufficiently heavy, we construct the effective field theory (EFT) at dimension-6 in order to compute contributions to the observables. We also provide full one-loop results for $S$ and $T$ in the general mass parameter space. In both completions, future precision measurements can probe the new states at the (multi-)TeV scale, beyond the direct reach of the LHC.