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Lepton collider probes for Majorana neutrino effective interactions

The extension of the standard model with new high-scale weakly coupled physics involving right-handed neutrinos in an effective field theory framework (SMNEFT) allows for a systematic study of heavy neutrinos phenomenology in current and future experiments. We exploit the outstanding angular resolution in future lepton colliders to study the sensitivity of forward-backward asymmetries to discover the possible single production of heavy Majorana neutrinos via $e^{+}e^{-} \to N ν$, followed by a purely leptonic decay $N \to μ^{-} μ^{+} ν$ or a semi-leptonic decay $N \to μ^{-} \mathrm{j} \mathrm{j} $, for masses $m_N > 50$ GeV. In this regime, we consider the $N$ production and decays to be dominated by scalar and vectorial four-fermion $d=6$ single $N_R$ operators. This is an alternative analysis to searches using displaced vertices and fat jets, in a higher mass regime, where the $N$ is short-lived but can be found by the angular distribution of its decay products. We find that a forward-backward asymmetry between the final muons in the pure leptonic decay mode provides a sensitivity up to 12$σ$ for $m_N=100$ GeV, for effective couplings $α=0.2$ and new physics scale $Λ=1$ TeV. In the case of the semi-leptonic decay, we can compare the final muon and higher $p_T$ jet flight directions, again finding up to 12$σ$ sensitivity to the effective signals.