This is the first part of the Open Questions Deep Dives series. Each part takes one of the unresolved questions in modern neutrino physics — why it matters, what experiments are testing it, and when we should expect an answer. We start with the question that drives the largest current experimental investment: CP violation in the lepton sector.
What CP violation is
In particle physics, C (charge conjugation) swaps particles for antiparticles. P (parity) reflects spatial coordinates as in a mirror. CP does both at once. If the laws of physics were perfectly CP-symmetric, the universe would behave identically with matter replaced by antimatter and mirrored.
For most of physics, CP symmetry holds. The two known exceptions: weak interactions in the quark sector (observed by Cronin and Fitch in 1964 in kaon decays, Nobel Prize 1980) and possibly weak interactions in the lepton sector — which is what neutrino oscillation experiments are now testing.
In the quark sector, CP violation is parametrized by a single complex phase in the CKM matrix. The size of the effect is small but well-measured. It cannot explain the matter-antimatter asymmetry of the universe — too small by ten orders of magnitude.
In the lepton sector, CP violation would be parametrized by an analogous phase in the PMNS matrix. This phase, conventionally called $\delta_{CP}$, is what experiments are trying to measure.
Why it matters: leptogenesis
The universe today is dominated by matter — the antimatter version of every atom is essentially absent. But the Big Bang produced equal numbers of particles and antiparticles. Something must have generated the asymmetry before they could annihilate.
Andrei Sakharov in 1967 identified three necessary conditions for baryogenesis (the generation of matter-antimatter asymmetry):
- Baryon-number violation.
- C and CP violation.
- Out-of-equilibrium dynamics.
The Standard Model has all three at some level, but in quantitative detail it fails by ~10 orders of magnitude.
Leptogenesis offers an alternative. If heavy right-handed neutrinos exist (as predicted by the seesaw mechanism), their CP-violating decays in the early universe would generate a lepton asymmetry, which sphaleron processes would partially convert to a baryon asymmetry. The required size of CP violation is typically much larger than the quark sector can deliver — but well within the range that lepton-sector CP violation could produce.
Observing $\delta_{CP} \neq 0$ or $\pi$ in the lepton sector would not directly confirm leptogenesis. But it would establish that the lepton sector is the right place to look for the explanation of why we exist.
How experiments measure it
The signature is the difference between two oscillation probabilities:
- $P(\nu_\mu \to \nu_e)$ — muon neutrino oscillating to electron neutrino.
- $P(\bar\nu_\mu \to \bar\nu_e)$ — antineutrino version.
If CP is conserved ($\delta_{CP} = 0$ or $\pi$), these two are equal. If $\delta_{CP}$ is something else, they differ.
The experimental challenge is to measure both with the same detector, by running the same beam alternately in neutrino-mode and antineutrino-mode. The detection rates are subject to a complication: Earth-matter effects (the MSW effect along the long baseline) also produce different oscillation probabilities for neutrinos vs antineutrinos. The asymmetry from matter effects depends on the mass ordering, which adds a degeneracy that experiments must break.
Long baselines (DUNE’s 1,300 km) have larger matter effects than short baselines (T2K’s 295 km). Combining T2K + NOvA + DUNE + Hyper-K helps untangle the matter-effect and CP contributions.
Current state
T2K (Japan, 295 km) has run since 2010 in both ν and ν̄ modes. Latest fits favor $\delta_{CP}$ near $3\pi/2$ — close to maximal violation. The exclusion of CP-conserving values ($\delta = 0, \pi$) is at about 2-3σ.
NOvA (USA, 810 km) provides complementary measurements. The combined T2K+NOvA fit favors substantial CP violation but with tension between the two experiments’ preferred regions in $\delta_{CP}$ vs mass-ordering space.
Current overall picture: CP violation in the lepton sector is disfavored from zero at moderate significance, with $\delta_{CP}$ best-fit near maximal violation. Not yet discovery-level.
DUNE and Hyper-K
The 2030s flagships should produce definitive answers:
DUNE: 1,300 km baseline, broad-spectrum on-axis beam, liquid-argon TPC. The long baseline means large matter effects, enabling simultaneous determination of $\delta_{CP}$ and mass ordering. Expected first 5σ CP-violation discovery for a substantial fraction of the possible $\delta_{CP}$ range after ~10 years of running.
Hyper-Kamiokande: 295 km baseline (same as T2K), off-axis narrow-band beam, 260-kiloton water Cherenkov. Higher statistics than T2K, lower matter effects. First operations 2027. Independent and complementary measurement.
The combination of DUNE and Hyper-K — measuring CP violation at very different baselines and with very different detector systematics — should produce a robust, redundant discovery (or robust null result) by the late 2030s.
What if CP violation is small?
If $\delta_{CP}$ turns out to be near 0 or $\pi$, lepton-sector CP violation alone won’t explain the baryon asymmetry. Other mechanisms — possibly heavy right-handed neutrinos at much higher energies than current accelerators can produce — would have to do the work. The lepton sector would still be the right place to start looking, but the smoking gun for leptogenesis would have to come from elsewhere.
If $\delta_{CP}$ is near maximal violation (the current best fit), that’s strong circumstantial evidence pointing at leptogenesis as the mechanism behind the matter universe we live in.
When will we know?
- 2027-2030: T2K and NOvA accumulate enough statistics for combined ~3σ confidence on CP violation.
- 2030-2035: Hyper-K starts; DUNE’s first beam arrives; first DUNE results.
- 2035-2040: Combined DUNE + Hyper-K + JUNO mass-ordering should produce definitive 5σ CP-violation answer.
The next part of this series turns to the question that has to be settled first before CP violation can be definitively pinned down: the mass ordering.
Frequently asked
What does CP violation mean?
CP — Charge-Parity — is the symmetry that says the laws of physics should be the same if you simultaneously swap every particle for its antiparticle and reflect spatial coordinates. CP violation means the laws are slightly different for matter than for antimatter. It has been observed in the quark sector since 1964. The question is whether it exists in the lepton sector — and how big it is.
Why does CP violation in neutrinos matter?
The universe is mostly matter, not antimatter, even though the Big Bang should have produced equal amounts. Something must have introduced an asymmetry. CP violation is one of the three Sakharov conditions for this asymmetry. Quark-sector CP violation is too small to do the job. CP violation in the lepton sector — especially through leptogenesis — could provide the missing piece.
Have we measured CP violation in neutrinos yet?
Not at discovery significance. T2K and NOvA have both produced measurements consistent with substantial CP violation, but neither alone reaches 3σ. Combined fits suggest the CP-violating phase δ_CP is near maximal violation but with large uncertainty. DUNE and Hyper-Kamiokande should reach discovery sensitivity in the 2030s.
What is δ_CP?
The CP-violating phase in the PMNS mixing matrix — a single complex phase that determines how much CP violation neutrinos exhibit. δ_CP can range from 0 to 2π. δ_CP = 0 or π means no CP violation; any other value means CP is violated. Current best fits put δ_CP near 3π/2, consistent with maximal violation, but with uncertainty still about ±50°.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2026, March 2). Open Questions — Part 1: CP violation in the lepton sector. Neutrino Times. https://neutrino-times.com/articles/open-questions-part-1-cp-violation/
Chicago
Neutrino Times Editorial Team. "Open Questions — Part 1: CP violation in the lepton sector." Neutrino Times, March 2, 2026. https://neutrino-times.com/articles/open-questions-part-1-cp-violation/.
MLA
Neutrino Times Editorial Team. "Open Questions — Part 1: CP violation in the lepton sector." Neutrino Times, 2 Mar. 2026, https://neutrino-times.com/articles/open-questions-part-1-cp-violation/.
BibTeX
@misc{neutrino-times-open-questions-part-1-cp-violation,
author = {Neutrino Times Editorial Team},
title = {Open Questions — Part 1: CP violation in the lepton sector},
howpublished = {Neutrino Times},
year = {2026},
month = {mar},
url = {https://neutrino-times.com/articles/open-questions-part-1-cp-violation/},
note = {Accessed: 2026-03-02}
} RIS
TY - GEN TI - Open Questions — Part 1: CP violation in the lepton sector AU - Neutrino Times Editorial Team PY - 2026 DA - 2026-03-02 PB - Neutrino Times UR - https://neutrino-times.com/articles/open-questions-part-1-cp-violation/ ER -