Every particle physicist knows the standard picture: every particle has an antiparticle. The electron has the positron. The proton has the antiproton. Each particle-antiparticle pair has opposite electric charge but identical mass and lifetime. When they meet, they annihilate.
Neutrinos are the one corner of the Standard Model where this picture might not actually hold. Because a neutrino has no electric charge, the distinction between “neutrino” and “antineutrino” becomes considerably less obvious. The question of whether they are genuinely separate particles, or whether each neutrino is its own antiparticle, has been open for almost a century. It is arguably the single most important unresolved question in particle physics.
The two possibilities are usually called Dirac neutrinos and Majorana neutrinos.
The two scenarios in plain language
A Dirac neutrino is the conventional picture. The neutrino and the antineutrino are genuinely distinct particles. They have, in some sense, opposite lepton number: the neutrino carries +1, the antineutrino carries -1. When the two meet, they annihilate, just like an electron and a positron.
A Majorana neutrino is the other possibility. The neutrino is its own antiparticle. There is no separate “antineutrino” — what we have been calling neutrinos and antineutrinos are just the same particle in two different spin states (left-handed and right-handed). This idea was proposed by the Italian physicist Ettore Majorana in 1937, shortly before he disappeared at sea, never to be seen again.
The difference sounds philosophical, but it has profound experimental consequences. The most striking is a process called neutrinoless double-beta decay, which can happen if and only if neutrinos are Majorana particles.
Why the question matters
Three answers, from most pragmatic to most cosmic.
It would explain why neutrinos are so light. In the Majorana scenario, the small neutrino mass arises naturally from the see-saw mechanism: very heavy right-handed neutrino partners (perhaps at energies far beyond anything we can build) push the masses of the ordinary neutrinos down to the tiny values we observe. The math is elegant. The Dirac scenario, by contrast, requires the neutrino’s Yukawa coupling to the Higgs field to be a million times smaller than that of any other particle, with no clear reason why.
It would tell us whether lepton number is fundamental. In the Standard Model, total lepton number — counting electrons, muons, taus, and their neutrino partners — is a conserved quantity. Majorana neutrinos would violate it. A whole class of processes forbidden in the Standard Model would become possible, with implications for grand unified theories that go far beyond neutrino physics.
It might explain why anything exists. The matter-antimatter asymmetry of the universe is one of the great unsolved problems in physics. Leptogenesis — a class of early-universe scenarios in which heavy Majorana neutrinos decay slightly more often to leptons than to antileptons — is a leading candidate explanation. Without Majorana neutrinos, leptogenesis is impossible.
How to tell experimentally
The cleanest signature is neutrinoless double-beta decay (0νββ).
Ordinary double-beta decay is a process in which two neutrons in a nucleus turn simultaneously into two protons, emitting two electrons and two antineutrinos. It is rare but well measured in several nuclides — germanium-76, xenon-136, tellurium-130, and others.
Neutrinoless double-beta decay would look almost the same, except no neutrinos come out. The two electrons carry away the full available energy, producing a sharp spectral line at the decay endpoint. If you see such a line, the only physics that can produce it is a Majorana neutrino mass.
Several experiments are hunting for it. GERDA and LEGEND use germanium-76. KamLAND-Zen uses xenon-136 dissolved in liquid scintillator. CUORE uses tellurium-130. None has yet seen a confirmed signal. Current lower limits on the half-life of 0νββ in xenon-136 are around 10²⁶ years — that is, ten thousand trillion trillion years, vastly longer than the age of the universe.
That is the level of sensitivity required to find a process that might happen, on average, once per nucleus per 10²⁶ years.
What we already know
Even without finding 0νββ, oscillation experiments have given us a partial answer. The fact that neutrinos oscillate tells us they have mass. The fact that we don’t see exotic decays like μ → eγ at observable rates tells us lepton flavor mixing is small in the charged sector. None of this distinguishes Majorana from Dirac.
The mass measurements from KATRIN provide an upper bound on the effective Majorana mass that experiments like GERDA and LEGEND must reach to claim a discovery — and that bound has been steadily tightening.
What’s next
The next generation of 0νββ experiments — LEGEND-1000, nEXO, CUPID — aim to push the half-life sensitivity to 10²⁸ years or beyond. If neutrinos are Majorana with masses in the inverted ordering, these experiments should find the decay. If they don’t, the inverted ordering will be in serious tension with the Majorana hypothesis, and the Dirac picture will gain weight.
Either way, a definitive answer should arrive in the next decade or two. The result will tell us whether the universe runs on a small surplus of one kind of lepton over another — and if so, whether the surplus came from heavy Majorana neutrinos decaying asymmetrically in the first second after the Big Bang.
Ettore Majorana would presumably approve.
For the conceptual basics, see What is a neutrino, anyway?. For oscillation, the phenomenon that proved neutrinos have mass in the first place, see How neutrino oscillation works.
Further reading
Primary sources
- E. Majorana, “Teoria simmetrica dell’elettrone e del positrone”, Nuovo Cimento 14:171 (1937) — the original Majorana fermion paper (Italian)
- P. A. M. Dirac, “The quantum theory of the electron”, Proc. Roy. Soc. A 117:610 (1928) — the Dirac equation
Background and context
- F. F. Deppisch, M. Hirsch, H. Päs, “Neutrinoless Double Beta Decay and Physics Beyond the Standard Model”, J. Phys. G 39:124007 (2012) — modern review of the Majorana/Dirac question
- S. M. Bilenky, “Majorana and Dirac mass terms,” in Introduction to the Physics of Massive and Mixed Neutrinos (Lecture Notes in Physics, Springer 2018)
- Wikipedia: Majorana fermion
- Quanta Magazine — “What is a Majorana fermion?” — accessible explainer
Frequently asked
What is the Majorana vs Dirac question?
It asks whether the neutrino is its own antiparticle. A Dirac neutrino is distinct from its antineutrino, like the electron and positron. A Majorana neutrino is identical to its antineutrino — the same particle in different helicity states. Only electrically neutral particles can be Majorana; all charged particles must be Dirac.
Why is it still open?
Because in ordinary neutrino interactions, neutrinos and antineutrinos behave identically enough that no laboratory experiment has yet distinguished the two possibilities. The cleanest test is neutrinoless double-beta decay — a process that can only occur if neutrinos are Majorana. Despite decades of searching, no signal has been seen.
Who was Ettore Majorana?
An Italian theoretical physicist (1906-1938 disappeared) who published the symmetric theory of fermions in his last paper, showing that the Dirac equation has a special real-valued solution describing a neutral particle that is its own antiparticle. He vanished under mysterious circumstances in 1938. The particle concept now bears his name.
Which experiments are trying to settle it?
The major neutrinoless double-beta decay programs — LEGEND (germanium), KamLAND-Zen (xenon), nEXO (xenon), CUORE/CUPID (tellurium, molybdenum), NEXT-100 (gas xenon). A signal in any of these would prove the Majorana nature. The current generation is reaching the sensitivity where, if neutrinos are Majorana with inverted mass ordering, a signal should appear.
What if neutrinos turn out to be Dirac?
Then neutrinoless double-beta decay would never be observed at any sensitivity. The simplest see-saw scenarios for explaining small neutrino masses would need modification. The connection between neutrino physics and the matter-antimatter asymmetry through leptogenesis would weaken. Each of these implications would reshape particle-physics theory in specific ways.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2025, June 19). Majorana or Dirac? Why the neutrino's identity is still an open question. Neutrino Times. https://neutrino-times.com/articles/majorana-or-dirac-neutrino-identity/
Chicago
Neutrino Times Editorial Team. "Majorana or Dirac? Why the neutrino's identity is still an open question." Neutrino Times, June 19, 2025. https://neutrino-times.com/articles/majorana-or-dirac-neutrino-identity/.
MLA
Neutrino Times Editorial Team. "Majorana or Dirac? Why the neutrino's identity is still an open question." Neutrino Times, 19 Jun. 2025, https://neutrino-times.com/articles/majorana-or-dirac-neutrino-identity/.
BibTeX
@misc{neutrino-times-majorana-or-dirac-neutrino-identity,
author = {Neutrino Times Editorial Team},
title = {Majorana or Dirac? Why the neutrino's identity is still an open question},
howpublished = {Neutrino Times},
year = {2025},
month = {jun},
url = {https://neutrino-times.com/articles/majorana-or-dirac-neutrino-identity/},
note = {Accessed: 2025-06-19}
} RIS
TY - GEN TI - Majorana or Dirac? Why the neutrino's identity is still an open question AU - Neutrino Times Editorial Team PY - 2025 DA - 2025-06-19 PB - Neutrino Times UR - https://neutrino-times.com/articles/majorana-or-dirac-neutrino-identity/ ER -