Cause of the baryon asymmetry of the universe

Determine the exact cause of the observed matter–antimatter (baryon) asymmetry in the universe and ascertain whether mechanisms such as baryogenesis via leptogenesis can account for it.

Background

The universe exhibits a pronounced asymmetry between matter and antimatter, commonly referred to as baryon asymmetry. Although several theoretical frameworks exist, including baryogenesis via leptogenesis with heavy neutral leptons, the definitive origin of this asymmetry is not established.

The paper discusses heavy neutral leptons as a potential ingredient that could generate the required asymmetry through CP-violating decays in the early universe, linking collider searches to a fundamental cosmological question.

References

The presence of matter in the universe today implies an asymmetry between matter and antimatter in the early universe, known as baryon asymmetry. The exact cause of this imbalance remains an open question in cosmology.

— Review of searches for vector-like quarks, vector-like leptons, and heavy neutral leptons in proton-proton collisions at $\sqrt{s}$ = 13 TeV at the CMS experiment  (2405.17605 - Collaboration, 2024) in Theoretical motivation for heavy neutral leptons, opening paragraphs

One of the most significant open problems is the baryon asymmetry of the Universe. Observations indicate that the present Universe is overwhelmingly dominated by matter, while antimatter is almost absent. However, within the standard cosmological picture, the Big Bang is expected to produce matter and antimatter in nearly equal amounts in the early Universe.

— Precise bubble wall velocity in a specific phase transition pattern in the CxSM and beyond  (2609.11747 - Tang et al., 10 Sep 2026) in Section 1, Introduction