Indirect probes of heavy neutral leptons through the dipole portal at FCC-ee

Abstract

We study indirect probes of the gauge-invariant dipole portal of Majorana heavy neutral leptons (HNLs) at FCC-ee. Virtual HNLs modify electroweak gauge-boson propagation and lepton couplings through loops with two dipole interactions. We calculate the resulting finite matching corrections and their effects on $Z$-pole observables. For real, flavour-aligned couplings, the local response is specified by four combinations of coupling shifts. Independent local ultraviolet interactions can reproduce all four, making the dipole interpretation dependent on assumptions about those interactions. With improved Standard Model predictions, FCC-ee can probe equal, same-sign hypercharge and weak dipole coefficients as small as $0.14\,\mathrm{TeV}^{-2}$ at an HNL mass of 1 TeV. This projection assumes dipole dominance, with independent ultraviolet coefficients set to zero at twice the HNL mass. We quantify how additional ultraviolet contributions can weaken or strengthen the sensitivity. At 1 TeV, the projected reach is competitive with HL-LHC same-sign dilepton searches for Majorana HNLs, while larger masses allow smaller dipole coefficients to be probed within the effective theory's domain of validity. Radiative Higgs decays offer a modest further improvement. The FeynRules, FeynArts and UFO models used in this analysis are publicly available at \href{https://doi.org/10.5281/zenodo.23667622}{doi:10.5281/zenodo.23667622}.