Matter–antimatter asymmetry

Matter–antimatter asymmetry is the observed dominance of matter over antimatter in the universe. Its origin remains unexplained by the known imbalance between particles and antiparticles.

Baryogenesis: The family of processes that could have generated an excess of baryons over antibaryons in the early universe. It names the broad class of proposed processes that could produce the observed imbalance.
Baryon: A composite particle made of three quarks, including protons and neutrons. The cosmic matter excess is often quantified as a net excess of baryons over antibaryons.
Standard Model of particle physics: The quantum field theory describing known elementary particles and three of the four fundamental interactions. Its known sources of CP violation do not account for the observed cosmic matter excess.
CP symmetry: The symmetry under combined charge conjugation and parity transformations. Exact CP symmetry would make particle and antiparticle processes behave alike, obstructing the required asymmetry.
Sakharov conditions: Three necessary conditions for generating a baryon asymmetry from an initially symmetric state. They specify the symmetry violations and out-of-equilibrium dynamics a baryogenesis mechanism needs.
Antimatter: Matter composed of antiparticles, which have the opposite charges and quantum numbers of corresponding particles. The asymmetry compares the abundance of matter with that of its antiparticle counterpart.
Neutrino: An electrically neutral elementary particle that interacts through the weak force and gravity. Neutrino properties may reveal whether leptogenesis can explain the asymmetry.
CPT symmetry: The combined symmetry of charge conjugation, parity reflection, and time reversal in local Lorentz-invariant quantum field theories. CPT symmetry constrains how matter and antimatter properties can differ despite cosmic abundance imbalance.
CP violation: A difference between the behavior of physical processes and their charge-conjugated, parity-reflected counterparts. Known CP violation distinguishes matter from antimatter, but appears insufficient to explain the cosmic excess.
Big Bang nucleosynthesis: The formation of light atomic nuclei during the first minutes of the universe. Its successful predictions constrain how many baryons the early universe contained.

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