Search

The fundamental building blocks of the proton--quarks and gluons--have been known for decades. However, we still have an incomplete theoretical and experimental understanding of how these particles and their dynamics give rise to the quantum bound state of the proton and its physical properties, such as its spin.sup.1. The two up quarks and the single down quark that comprise the proton in the simplest picture account only for a few per cent of the proton mass, the bulk of which is in the form of quark kinetic and potential energy and gluon energy from the strong force.sup.2. An essential feature of this force, as described by quantum chromodynamics, is its ability to create matter-antimatter quark pairs inside the proton that exist only for a very short time. Their fleeting existence makes the antimatter quarks within protons difficult to study, but their existence is discernible in reactions in which a matter-antimatter quark pair annihilates. In this picture of quark-antiquark creation by the strong force, the probability distributions as a function of momentum for the presence of up and down antimatter quarks should be nearly identical, given that their masses are very similar and small compared to the mass of the proton.sup.3. Here we provide evidence from muon pair production measurements that these distributions are considerably different, with more abundant down antimatter quarks than up antimatter quarks over a wide range of momenta. These results are expected to revive interest in several proposed mechanisms for the origin of this antimatter asymmetry in the proton that had been disfavoured by previous results.sup.4, and point to future measurements that can distinguish between these mechanisms. Quark-antiquark annihilation measurements provide a precise determination of the ratio of down and up antiquarks within protons as a function of momentum, which confirms the asymmetry between the abundance of down and up antiquarks., Author(s): J. Dove [sup.1] , B. Kerns [sup.1] , R. E. McClellan [sup.1] [sup.18] , S. Miyasaka [sup.2] , D. H. Morton [sup.3] , K. Nagai [sup.2] [sup.4] , S. [...]

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 [...]