Testing sterile neutrino extensions of the Standard Model at future lepton colliders

TL;DR

This paper evaluates how future lepton colliders can detect sterile neutrinos, which are extensions of the Standard Model that explain neutrino masses, through various direct and indirect experimental methods.

Contribution

It provides a comprehensive analysis of the sensitivity of upcoming lepton colliders to sterile neutrino models, especially symmetry protected seesaw scenarios.

Findings

Future colliders can probe sterile neutrino masses around the electroweak scale.

Direct searches at colliders can detect sterile neutrino decays and Higgs boson decays involving sterile neutrinos.

Current bounds from LEP and LHC are summarized and compared with future sensitivities.

Abstract

Extending the Standard Model (SM) with sterile ('right-handed') neutrinos is one of the best motivated ways to account for the observed neutrino masses. We discuss the expected sensitivity of future lepton collider experiments for probing such extensions. An interesting testable scenario is given by 'symmetry protected seesaw models', which theoretically allow for sterile neutrino masses around the electroweak scale with up to order one mixings with the light (SM) neutrinos. In addition to indirect tests, e.g. via electroweak precision observables, sterile neutrinos with masses around the electroweak scale can also be probed by direct searches, e.g. via sterile neutrino decays at the Z pole, deviations from the SM cross section for four lepton final states at and beyond the WW threshold and via Higgs boson decays. We study the present bounds on sterile neutrino properties from LEP and LHC as well as the expected sensitivities of possible future lepton colliders such as ILC, CEPC and FCC-ee (TLEP).