Future collider sensitivities to $\nu$SMEFT interactions

TL;DR

This paper evaluates the potential of future lepton colliders to probe heavy neutrino interactions within the $ u$SMEFT framework, providing exclusion limits in the high-mass regime and comparing sensitivity with other collider options.

Contribution

It presents the first prospective exclusion plots for heavy neutrino interactions at a future 0.5 TeV lepton collider within the $ u$SMEFT framework, focusing on high-mass, prompt decay scenarios.

Findings

Lepton colliders can set competitive bounds on heavy neutrino interactions.

Semi-leptonic final states with jets offer the highest sensitivity.

Bounds are comparable to those from displaced decay searches at LHC and future colliders.

Abstract

The discovery of neutrino oscillations and masses provides strong motivation to extend the Standard Model by including right-handed neutrinos, which lead to heavy neutrino states that could exist at the electroweak scale. These states may also be influenced by new high-scale, weakly interacting physics. Incorporating right-handed neutrinos into an effective field theory framework -- the $\nu$SMEFT -- offers a systematic approach to study the phenomenology of heavy neutrinos in current and upcoming experiments. In this work, we present the first prospective 95\% exclusion plots achievable at a future lepton collider operating at a center-of-mass energy of $\sqrt{s}=0.5 ~\rm{TeV}$ for what we term the agnostic $\nu$SMEFT scenario. This study focuses on the high-mass regime where the heavy neutrino $N$ decays promptly into leptons and jets. Specifically, we analyze the processes $e^+e^- \to \nu N \to \nu \mu^{-} \mu^{+} \nu$ and $e^+e^- \to \nu N \to \nu \mu^{-} \mathrm{j} \mathrm{j}$, deriving the exclusion regions in the $\frac{\alpha}{\Lambda^2}$ vs. $m_N$ parameter space. When compared to prospective limits for the LHeC, we find that the semi-leptonic process with final jets in a lepton collider offers the greatest sensitivity, even with a straightforward cut-based analysis. The expected bounds are as stringent as those considered in recent studies for the low-mass regime where the $N$ may be long-lived and detectable via displaced decay searches, both at the LHC and future colliders.