This is the sixth and final part of the Open Questions Deep Dives series. We conclude with the question that ties everything together: why does the universe exist?
The matter-antimatter mystery
The universe today is made of matter — atoms, stars, galaxies, us — with essentially no antimatter present in bulk. Anti-protons exist in cosmic rays in tiny numbers, but only as secondaries produced in cosmic-ray collisions. There are no anti-galaxies, no anti-planets.
This is strange. The Big Bang should have produced equal numbers of particles and antiparticles. As the universe cooled, almost all of them should have annihilated, leaving only photons.
What we observe is a small excess of matter over antimatter — about one extra baryon per billion photons. This excess survived annihilation. Everything we see is the relic.
Some mechanism in the early universe must have generated the asymmetry. This mechanism is the subject of baryogenesis — and the leading candidate explanation, especially in models involving neutrinos, is leptogenesis.
Sakharov’s conditions
In 1967, Andrei Sakharov identified three necessary conditions for any mechanism to generate a matter-antimatter asymmetry from an initially symmetric state:
1. Baryon-number violation. The total baryon number must change. Something must turn the symmetric state into one with net baryon number.
2. C and CP violation. The dynamics must distinguish particles from antiparticles, otherwise both would be produced equally.
3. Non-equilibrium dynamics. The process must occur out of thermal equilibrium, otherwise CPT symmetry would force inverse processes to undo the asymmetry.
The Standard Model has all three at some level. Sphaleron processes at high temperatures violate baryon and lepton number. The CKM matrix has CP-violating phases. The electroweak phase transition could in principle be out-of-equilibrium.
But quantitatively, the Standard Model fails:
- CP violation in the quark sector is too small by ~10 orders of magnitude.
- The electroweak phase transition is a smooth crossover rather than the strongly first-order transition needed.
Standard Model alone cannot explain why we’re here. New physics is required.
Leptogenesis: the basic idea
Proposed by Fukugita and Yanagida in 1986, leptogenesis exploits the seesaw mechanism that explains why ordinary neutrinos are so light.
The seesaw posits heavy right-handed Majorana neutrinos $N_i$ at a high mass scale — typically $M_N \sim 10^9$ to $10^{15}$ GeV. Their CP-violating decays in the early universe (when the universe’s temperature was at this energy scale) produce a small asymmetry between leptons and antileptons.
The mechanism in three steps:
1. Heavy Majorana neutrinos decay. As the universe expands and cools through $T \sim M_N$, the heavy $N$ particles decay into Standard Model leptons + Higgs (and anti-leptons + Higgs). Because the $N$‘s have CP-violating phases in their couplings to the Higgs, the decay rate to leptons vs anti-leptons differs slightly. A small lepton asymmetry develops.
2. Sphaleron processes convert lepton to baryon asymmetry. Standard Model electroweak sphalerons — non-perturbative processes that violate both B and L while conserving B-L — operate efficiently in the hot early universe. They convert about 1/3 of the lepton asymmetry into a baryon asymmetry.
3. The asymmetry survives. As the universe cools below the electroweak scale, sphaleron processes turn off. The surviving baryon asymmetry is locked in. Subsequent annihilation between baryons and antibaryons leaves behind the small surplus we observe today.
What the experimental connection looks like
Leptogenesis works at energies far above current accelerator reach. We cannot directly produce or observe the heavy Majorana neutrinos involved. But the framework constrains observable parameters in ways that experiments are now starting to probe.
Required: Majorana nature. The heavy neutrinos must be Majorana. If light neutrinos are Dirac (the question targeted by 0νββ — see Part 4 of this series), the simplest seesaw and leptogenesis scenarios are ruled out.
Required: lepton-sector CP violation. The mechanism needs CP-violating couplings of the heavy neutrinos. While not directly observable, the existence of CP violation at the light-neutrino scale (Part 1) is suggestive evidence that CP violation operates throughout the leptonic sector.
Required: appropriate mass-ordering and absolute mass. Different choices of ordering and absolute mass favor different leptogenesis scenarios. Constraining these parameters tightens the viable parameter space.
Possible direct probe: resonant leptogenesis. If the heavy Majorana neutrinos happen to be nearly degenerate in mass at the GeV-to-TeV scale (rather than the typical $10^9-10^{15}$ GeV), they could be produced and detected in laboratory experiments. Heavy Neutral Lepton (HNL) searches at LHC, FASER, SHiP, and future colliders are looking for exactly this possibility.
The case so far
Current evidence is suggestive but not yet conclusive:
- Atmospheric and solar oscillation parameters are consistent with seesaw expectations.
- CP violation in the lepton sector is preferred at moderate significance.
- The favored absolute mass scale is consistent with seesaw.
- Majorana nature is being tested at ton-scale 0νββ.
The most that would ever be experimentally established is strong circumstantial evidence for leptogenesis. The direct mechanism — heavy Majorana decay at high temperature in the early universe — is forever out of reach unless resonant variants happen to exist at accessible energies.
What the answer might look like
A complete leptogenesis picture in 2040 might look like this:
- 0νββ observed at $m_{\beta\beta} \approx 20$ meV. Majorana nature confirmed.
- $\delta_{CP}$ measured at $\sim 3\pi/2$. CP violation maximal in the lepton sector.
- Mass ordering determined to be normal at 5σ.
- Absolute mass scale measured via cosmology at $\Sigma m_\nu \approx 60$ meV.
- HNL searches set strong limits ruling out low-scale resonant leptogenesis.
Such a picture would establish: lepton number is violated, the lepton sector has substantial CP violation, the mass structure is consistent with a seesaw mechanism at high scale. The framework of leptogenesis would be well-supported circumstantially — and the alternative explanations for the matter-antimatter asymmetry would be correspondingly weakened.
We won’t have proven leptogenesis. But the conclusion that the universe exists because of neutrinos becoming heavy Majorana states in the first nanoseconds after the Big Bang would have moved from speculation to well-supported physical hypothesis.
This concludes the Open Questions Deep Dives series. For chronological context, see the Neutrino History series. For the experiments doing the measurements, see Detector Deep Dives. For where these neutrinos come from, see Sources of Neutrinos. For the beginner-friendly overview, see Neutrinos 101.
Frequently asked
What is leptogenesis?
A mechanism that generates the universe's matter-antimatter asymmetry through CP-violating decays of heavy right-handed Majorana neutrinos in the very early universe. The lepton asymmetry produced this way is then partially converted to a baryon asymmetry by Standard Model sphaleron processes. The result: more matter than antimatter, eventually leading to atoms, stars, planets, and us.
Why can't the Standard Model alone explain the matter asymmetry?
Sakharov's three conditions for baryogenesis (baryon-number violation, CP violation, non-equilibrium dynamics) are formally satisfied in the Standard Model, but quantitatively all three are too weak. CP violation in the quark sector is ten orders of magnitude smaller than required. The electroweak phase transition is a smooth crossover rather than the strongly first-order transition needed for non-equilibrium dynamics. New physics is required.
Can we test leptogenesis directly?
Not in the simplest scenarios — the heavy Majorana neutrinos involved typically have masses around 10⁹-10¹⁵ GeV, far above any conceivable accelerator energy. But circumstantial evidence can come from confirming Majorana nature (0νββ), measuring large lepton-sector CP violation (DUNE, Hyper-K), determining the absolute mass scale and ordering, and searching for resonant leptogenesis at lower scales (heavy neutral lepton searches).
What would confirm leptogenesis?
No single observation. A compelling case would emerge from multiple converging measurements: (1) Majorana nature confirmed via 0νββ, (2) large δ_CP confirmed in long-baseline experiments, (3) consistent absolute mass scale and ordering, (4) seesaw-mechanism-favored mass structure. The chain doesn't directly observe the heavy Majorana decays themselves but constrains the framework that makes them plausible.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2026, March 11). Open Questions — Part 6: Leptogenesis and the matter mystery. Neutrino Times. https://neutrino-times.com/articles/open-questions-part-6-leptogenesis/
Chicago
Neutrino Times Editorial Team. "Open Questions — Part 6: Leptogenesis and the matter mystery." Neutrino Times, March 11, 2026. https://neutrino-times.com/articles/open-questions-part-6-leptogenesis/.
MLA
Neutrino Times Editorial Team. "Open Questions — Part 6: Leptogenesis and the matter mystery." Neutrino Times, 11 Mar. 2026, https://neutrino-times.com/articles/open-questions-part-6-leptogenesis/.
BibTeX
@misc{neutrino-times-open-questions-part-6-leptogenesis,
author = {Neutrino Times Editorial Team},
title = {Open Questions — Part 6: Leptogenesis and the matter mystery},
howpublished = {Neutrino Times},
year = {2026},
month = {mar},
url = {https://neutrino-times.com/articles/open-questions-part-6-leptogenesis/},
note = {Accessed: 2026-03-11}
} RIS
TY - GEN TI - Open Questions — Part 6: Leptogenesis and the matter mystery AU - Neutrino Times Editorial Team PY - 2026 DA - 2026-03-11 PB - Neutrino Times UR - https://neutrino-times.com/articles/open-questions-part-6-leptogenesis/ ER -