The see-saw portal at future Higgs factories: the role of dimension six operators

TL;DR

This paper explores how future Higgs factories can probe new physics beyond the Standard Model through effective operators up to dimension six, focusing on right-handed neutrinos and their phenomenology.

Contribution

It introduces a comprehensive analysis of the $ u$SMEFT framework at future colliders, highlighting the potential to explore new physics scales up to 60 TeV.

Findings

Future Higgs factories can probe new physics scales up to 60 TeV.

Additional decay modes for right-handed neutrinos are significant for detection.

Sensitivity varies depending on whether new states decay promptly or are displaced.

Abstract

We study an extension of the Standard Model with electroweak scale right-handed singlet fermions $N$ that induces neutrino masses, plus a generic new physics sector at a higher scale $\Lambda$. The latter is parametrized in terms of effective operators in the language of the $\nu$SMEFT. We study its phenomenology considering operators up to $d=6$, where additional production and decay modes for $N$ are present in addition to those arising from the mixing with the active neutrinos. We focus on the production with four-Fermi operators and we identify the most relevant additional decay modes to be $N\to \nu \gamma$ and $N\to 3f$. We assess the sensitivity of future Higgs factories on the $\nu$SMEFT in regions of the parameter space where the new states decay promptly, displaced or are stable on detector lengths. We show that new physics scale up to $5-60\;$TeV can be explored, depending on the collider considered.