Antimatter in the proton is more down than up

Nature, Published online: 24 February 2021; doi:10.1038/d41586-021-00430-3

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.