Discovering heavy neutrino-antineutrino oscillations at the $Z$-pole

TL;DR

This paper explores how heavy neutrino-antineutrino oscillations could be observed at collider experiments like FCC-ee and CEPC, providing a new way to detect lepton number violation despite approximate symmetry protections.

Contribution

It introduces the concept of observing neutrino-antineutrino oscillations as signatures of heavy neutral leptons at colliders, even when lepton number violation is not directly observable.

Findings

Oscillating signatures can be detected in collider final state distributions.

Monte Carlo simulations identify parameter space for resolving NNOs.

NNOs provide a novel probe of heavy neutrino properties at colliders.

Abstract

Collider-testable type I seesaw extensions of the Standard Model are generally protected by an approximate lepton number (LN) symmetry. Consequently, they predict pseudo-Dirac heavy neutral leptons (HNLs) composed of two nearly degenerate Majorana fields. The interference between the two mass eigenstates can induce heavy neutrino-antineutrino oscillations (NNOs) leading to observable lepton number violation (LNV), even though the LN symmetry is approximately conserved. These NNOs could be resolved in long-lived HNL searches at collider experiments, such as the proposed Future Circular $e^+e^-$ Collider (FCC-$ee$) or Circular Electron Positron Collider (CEPC). However, during their $Z$-pole runs, the LN carried away by the light (anti)neutrinos produced alongside the HNLs prevents LNV from being observed directly. Nevertheless, NNOs materialise as oscillating signatures in final state distributions. We discuss and compare a selection of such oscillating observables, and perform a Monte Carlo simulation to assess the parameter space in which NNOs could be resolved.