Direct constraints on the agnostic $\nu$SMEFT from heavy-neutrino searches at the LHC
The observation of neutrino oscillations and masses motivates extensions of the Standard Model containing right-handed neutrinos. If additional new physics exists at scales beyond the direct reach of present experiments, its effects can be systematically described within the neutrino Standard Model Effective Field Theory ($\nu$SMEFT). In this framework, heavy neutral leptons provide a promising target for collider searches. In this work, we reinterpret a CMS search for heavy Majorana neutrinos in the same-sign dimuon plus two jets final state to constrain the parameter space of the dimension-six $\nu$SMEFT. Using a detailed recast of the experimental analysis, we present results for both a pure-agnostic benchmark and a benchmark incorporating neutrinoless double-beta-decay bounds. For heavy-neutrino masses in the range $200~\mathrm{GeV} \le m_N \le 15~\mathrm{TeV}$, the observed upper limits on the effective coupling strength range from $5.8\times10^{-7}\,\mathrm{GeV}^{-2}$ to $2.3\times10^{-6}\,\mathrm{GeV}^{-2}$. These results constitute the first experimental exclusion limits on the agnostic $\nu$SMEFT parameter space obtained from a dedicated recast of an LHC heavy-neutrino search.