It was interesting to read the item by Johanna Miller (Physics Today, May 2021, page 14) on the asymmetry between up and down antiquarks in the proton. It does indeed provide fascinating insight into the quark structure of the proton and especially the role of chiral symmetry, which requires that the proton be surrounded by a pion cloud.
But I feel that it is necessary to add a little to the incomplete discussion of the history of that asymmetry discovery. Using the cloudy bag model, which successfully incorporates chiral symmetry into the MIT bag model, I predicted the asymmetry1 in 1983, almost a decade before the violation of the Gottfried sum rule was experimentally confirmed.2
The mechanism is the dominance of the π+–neutron configuration when the proton emits a pion. The pion contribution to deep inelastic scattering was first mentioned by J. D. Sullivan and Richard Feynman and is often referred to as the Sullivan process. In 1983, however, almost no one in the high-energy-physics community took the idea of a contribution from the pion cloud seriously, as deep inelastic scattering was such a short-distance phenomenon; the constraints of chiral symmetry there were not understood. Certainly no one else, including Sullivan, had discussed the process as a source of flavor asymmetry.
In a November 2021 letter (page 11), Edward Shuryak describes an alternative explanation of the effect and suggests that lattice quantum chromodynamics can be used with the Δ baryon to test the mechanism. The idea of using lattice quantum chromodynamics calculations of the Δ+ to test the role of chiral symmetry in generating such an asymmetry was published several years ago.3 In particular, one can expect a significant enhancement of the asymmetry as the pion mass approaches the Δ–nucleon mass difference from above.
