Constraining the SMEFT Extended with Sterile Neutrinos at FCC-ee

TL;DR

This paper explores how the FCC-ee collider can detect and constrain heavy neutral leptons within the SMEFT framework, focusing on new interactions and their effects on HNL production and decay.

Contribution

It introduces a comprehensive EFT approach to study HNLs at FCC-ee, including vector, scalar, and tensor operators, and assesses sensitivities through monophoton and displaced vertex signatures.

Findings

FCC-ee can set competitive bounds on active-sterile mixing angles.

Constraints on Wilson coefficients translate into lower bounds on new physics scale.

Analysis covers both Majorana and Dirac HNL scenarios.

Abstract

We investigate how extensions of the Standard Model (SM) involving heavy neutral leptons (HNLs) can be probed at FCC-ee, the proposed high-energy circular $e^+e^-$ collider. Using the effective field theory (EFT) approach, we determine the impact of new interactions on the production and decay of HNLs at FCC-ee. In particular, we consider $d\leq 7$ $\nu$SMEFT operators which induce vector, scalar and tensor four-fermion and effective charged- and neutral-current interactions of HNLs, that may also mix with the active neutrinos of the SM. We consider sensitivities to the active-sterile mixing and EFT Wilson coefficients from monophoton searches and displaced vertex decay signatures. In both analyses, we consider the scenarios where HNLs are Majorana or Dirac fermions. We translate the upper bounds on the Wilson coefficients to lower limits on the scale of new physics.