Origin of the matter–antimatter asymmetry

Determine the physical mechanism that generated the observed baryon-to-photon ratio ηB ≈ 6.1 × 10^−10 in the early universe, specifying whether electroweak baryogenesis, leptogenesis, or an alternative process accounts for the matter–antimatter asymmetry consistent with BBN and CMB measurements.

Background

Observations from Big Bang nucleosynthesis and the cosmic microwave background consistently measure a nonzero baryon-to-photon ratio across widely separated energy scales, indicating a robust matter–antimatter asymmetry.

The chapter reviews electroweak baryogenesis and leptogenesis as leading candidates, highlighting that Standard Model physics is insufficient and extensions (e.g., beyond-SM Higgs sectors or heavy Majorana neutrinos) are required, yet the definitive origin remains to be established.

References

The origin of the observed matter-antimatter asymmetry in the universe remains largely unknown.

— The First Particles  (2501.15543 - Huang, 26 Jan 2025) in Conclusion

Open questions of the Standard Model (SM) --- the matter-antimatter asymmetry of the universe, the nature of dark matter, and the origin of neutrino masses --- motivate searches for physics beyond the SM.

— Search for rare charm decays at BESIII  (2610.07773 - Li et al., 6 Oct 2026) in Section 1, Introduction