Antimatter in the proton is more down than up

Protons are found in all atoms, so it might be surprising to learn that they contain antimatter. It now emerges that there is an imbalance in the types of antimatter in the proton -- a finding for which there is no agreed theoretical explanation. Flavour symmetry of antiquarks in the proton is broken.
Author(s): Haiyan Gao Author Affiliations: Antimatter in the proton is more down than up Protons and neutrons, which are collectively known as nucleons, are the building blocks of atomic nuclei [...]