Future collider sensitivities to <i>ν</i>SMEFT interactions

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 ν 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msqrt> <mml:mrow> <mml:mi>s</mml:mi> </mml:mrow> </mml:msqrt> <mml:mo>=</mml:mo> <mml:mn>0.5</mml:mn> <mml:mspace width="1em"/> <mml:mi mathvariant="normal">TeV</mml:mi> </mml:math> for what we term the agnostic ν SMEFT scenario. This study focuses on the high-mass regime where the heavy neutrino N decays promptly into leptons and jets. Specifically, we analyse the processes e + e − → νN → νμ − μ + ν and e + e − → νN → νμ − jj, deriving the exclusion regions in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfrac> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mo mathvariant="normal">Λ</mml:mo> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mfrac> </mml:math> versus 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.