Fundamentals · Part 2 of 6
Dirac or Majorana? The Nature of Neutrino Mass
Is the neutrino distinct from its antiparticle, or is it its own antiparticle? The answer decides where its mass comes from — and whether a conservation law of the Standard Model is broken.
Once we accept that neutrinos have mass, a sharp question follows: is a neutrino fundamentally different from an antineutrino, or are the two the same particle seen in different motions? The answer is not a detail — it determines the very origin of neutrino mass.
Dirac: a particle and a separate antiparticle
A Dirac neutrino works like every other matter particle. It is genuinely distinct from its antiparticle, and giving it mass means introducing the missing right-handed neutrino so the Higgs can couple the two handedness states, exactly as for the electron. This is the conservative option: nothing new in principle, only a new, very small coupling. Its cost is that smallness — the coupling must be extraordinarily tiny, with no explanation offered.
Majorana: a particle that is its own antiparticle
In 1937 Ettore Majorana showed that a neutral fermion can be its own antiparticle. Because the neutrino carries no electric charge, it is the only known elementary fermion that could be of this Majorana type. If it is, no new right-handed partner is needed; the mass term is built from the neutrino alone. As the explainer on Dirac vs. Majorana puts it, the two pictures look identical in ordinary experiments but differ in a deep way.
The crucial difference: lepton number
The distinction has an observable consequence through lepton number — a bookkeeping quantity that counts leptons minus antileptons, conserved in every Standard Model process. A Dirac mass respects it. A Majorana mass violates it by two units, because it effectively turns a neutrino into an antineutrino.
That is why the question is testable at all. A process that creates or destroys two units of lepton number would prove the neutrino is a Majorana particle. The cleanest such process is neutrinoless double beta decay, the subject of Part 4.
Why it matters beyond the neutrino
The Majorana option is more than economical. It naturally accommodates the seesaw mechanism that could explain why neutrino masses are so small, and it underpins leptogenesis, a leading idea for the origin of matter. The Dirac option keeps lepton number sacred but leaves the smallness unexplained. Deciding between them is one of the central goals of the field — and, remarkably, an answer may be within experimental reach.
Key takeaways
- A Dirac neutrino is distinct from its antiparticle; a Majorana neutrino is its own antiparticle.
- A Majorana mass violates lepton number by two units, which makes the question experimentally testable.
- Neutrinoless double beta decay is the cleanest test of whether the neutrino is Majorana.
References
- 1.E. Majorana, Teoria simmetrica dell'elettrone e del positrone, Il Nuovo Cimento 14, 171 (1937).
- 2.S. Navas et al. (Particle Data Group), Review of Particle Physics, Phys. Rev. D (2024). DOI: 10.1103/PhysRevD.110.030001.