Lepton collider probes for Majorana neutrino effective interactions

Abstract

AbstractThe 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 $$e^{+}e^{-} \rightarrow N \nu $$ e + e - → N ν , followed by a purely leptonic decay $$N \rightarrow \mu ^{-} \mu ^{+} \nu $$ N → μ - μ + ν or a semi-leptonic decay $$N \rightarrow \mu ^{-} \text {j} \text {j} $$ N → μ - j , for masses $$m_N > 50$$ m N > 50  GeV. In this regime, we consider the N production and decays to be dominated by scalar and vectorial four-fermion $$d=6$$ d = 6 single $$N_R$$ 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$$\sigma $$ σ for $$m_N=100$$ m N = 100  GeV, for effective couplings $$\alpha =0.2$$ α = 0.2 and new physics scale $$\Lambda =1$$ Λ = 1  TeV. In the case of the semi-leptonic decay, we can compare the final muon and higher $$p_T$$ p T jet flight directions, again finding up to 12$$\sigma $$ σ sensitivity to the effective signals.