Another look at the proton sea
DOI: 10.1063/PT.3.4871
Johanna Miller nicely summarizes the current experimental situation with the puzzling asymmetry of the proton’s antiquark “sea” in the May 2021 issue of Physics Today (page 14
Miller’s report mentions two theoretical ideas proposed to explain the asymmetry. One is that the presence of two u valence quarks leads to “Pauli blocking” of sea u quarks, the twin brothers of
Unfortunately, Miller does not mention a third idea that has been put forth, which is more nontrivial and seems likelier to explain the puzzling asymmetry. It started with an observation by Alexander Dorokhov and Nikolai Kochelev
1
that the so-called ’t Hooft effective four-quark Lagrangian
2
is “flavor nondiagonal,” leading to processes
In a way, the effect is also due to the Pauli exclusion principle, but at a different level. Topological tunneling events, known as instantons, create fields so strong that they fix the color and spin states of participating quarks uniquely. Instead of six possibilities, there remains only one, thus a complete blocking. Since the proton has two valence u quarks and only one valence d quark, that mechanism would suggest
Recently I made the first attempt to evaluate that effect quantitatively, by calculating the wavefunction of the five-quark
How can one test that idea further? If that explanation is true, the sea of Δ++ baryons, which have three up quarks, would have only
A second test is related to the other explanation Miller mentions, the pion cloud. While pions can indeed generate asymmetry in the isospin of the sea, they will not do so for the spin, since pions have spin zero. The ’t Hooft Lagrangian, on the other hand, leads to strict predictions for the quark polarizations. For example, a left-handed up quark
References
1. A. E. Dorokhov, N. I. Kochelev, Phys. Lett. B 304, 167 (1993). https://doi.org/10.1016/0370-2693(93)91417-L
2. G. ’t Hooft, Phys. Rev. D 14, 3432 (1976). https://doi.org/10.1103/PhysRevD.14.3432
3. E. Shuryak, Phys. Rev. D 100, 114018 (2019). https://doi.org/10.1103/PhysRevD.100.114018
More about the Authors
Edward Shuryak. (edward.shuryak@stonybrook.edu) Stony Brook University, Stony Brook, New York.