Lepton collider probes for Majorana neutrino effective interactions

TL;DR

This paper explores how future lepton colliders can detect heavy Majorana neutrinos through angular asymmetries, providing a new method for probing neutrino interactions at high energies.

Contribution

It introduces a novel approach using forward-backward asymmetries in lepton collider experiments to identify heavy Majorana neutrinos via effective four-fermion operators.

Findings

Forward-backward asymmetry can reach 12σ sensitivity for 100 GeV neutrinos.

Effective couplings of 0.2 at 1 TeV scale are detectable.

Method is an alternative to displaced vertex and jet-based searches.

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{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$\sigma$ 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.