Lepton collider probes for Majorana neutrino effective interactions

Abstract

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 $e^{+}{e}^{-}\to N\nu$, followed by a purely leptonic decay $N\to {\mu }^{-}{\mu }^{+}\nu$ or a semi-leptonic decay $N\to {\mu }^{-}\mathrm{jj}$, 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$\sigma$ for $m_N=100$ GeV, for effective couplings $\alpha =0.2$ and new physics scale $\Lambda =1$ 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$\sigma$ sensitivity to the effective signals.

Comments: 25 pages, 12 figures, 3 tables. v3 References updated, new organization and plots