The see-saw portal at future Higgs Factories

TL;DR

This paper explores how future Higgs Factories can detect new physics involving right-handed neutrinos and higher-dimensional operators, potentially probing energy scales up to 1000 TeV beyond current indirect measurements.

Contribution

It analyzes the sensitivity of future lepton colliders to effective operators involving right-handed neutrinos and assesses their potential to explore new physics at very high energy scales.

Findings

Future colliders can test the theory up to 500-1000 TeV.

They can surpass indirect measurements of Higgs and Z widths.

Potential to identify underlying flavor structure of new physics.

Abstract

We consider an extension of the Standard Model with two right-handed singlet fermions with mass at the electroweak scale that induce neutrino masses, plus a generic new physics sector at a higher scale $\Lambda$. We focus on the effective operators of lowest dimension $d=5$, which induce new production and decay modes for the singlet fermions. We assess the sensitivity of future Higgs Factories, such as FCC-ee, CLIC-380, ILC and CEPC, to the coefficients of these operators for various center of mass energies. We show that future lepton colliders can test the cut-off of the theory up to $\Lambda \simeq 500 - 1000\;$TeV, surpassing the reach of future indirect measurements of the Higgs and $Z$ boson widths. We also comment on the possibility of determining the underlying model flavor structure should a New Physics signal be observed, and on the impact of higher dimensional $d=6$ operators on the experimental signatures.