Z-Boson Decays into Majorana or Dirac (Heavy) Neutrinos

Abstract

We computed the kinematics of Z-boson decay into a heavy-light neutrino pair when the Z-boson is produced at rest in electron-positron collisions, including the subsequent decay of the heavy neutrino into a visible final state containing a charged-lepton. We concentrated on heavy-neutrino masses of order dozens of GeV and the issue of addressing the nature of the neutrinos - Dirac fermions or Majorana fermions. We find that while it is not possible to tell the nature of the heavy and light neutrinos on an event-by-event basis, the nature of the neutrinos can nonetheless be inferred given a large-enough sample of heavy-light neutrino pairs. We identify two observables sensitive to the nature of neutrinos. One is the forward-backward asymmetry of the daughter-charged-leptons. This asymmetry is exactly zero if the neutrinos are Majorana fermions and is non-zero (and opposite) for positively- and negatively-charged daughter-leptons if the neutrinos are Dirac fermions. The other observable is the polarization of the heavy neutrino, imprinted in the laboratory-frame energy distribution of the daughter-charged-leptons. Dirac neutrinos and antineutrinos produced in electron-positron collisions at the Z-pole are strongly polarized while Majorana neutrinos are at most as polarized as the $Z$-bosons.

12 pages, 7 figures, references added in v2, results and conclusions unchanged

References in corpus (6)

• How to find neutral leptons of the nuMSM?
• Double Bangs from New Physics in IceCube
• Constraints on Sterile Neutrinos in the MeV to GeV Mass Range
• Extended Black Box Theorem for Lepton Number and Flavor Violating processes
• Dark Matter and Neutrino Mass from the Smallest Non-Abelian Chiral Dark Sector
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• Searches for Long-Lived Particles at the Future FCC-ee
• New Benchmark Models for Heavy Neutral Lepton Searches
• Distinguishing Dirac and Majorana Heavy Neutrinos at Lepton Colliders
• Monojet search for heavy neutrinos at future $Z$-factories
• New Physics Search at the CEPC: a General Perspective
• New Particles at the Z-Pole: Tera-Z factories as discovery and precision machines
• Constraints on Heavy Neutral Leptons interacting with a singlet scalar
• Characterizing Heavy Neutral Fermions via their Decays