The sea of quarks and antiquarks in the nucleon

Abstract

The quark and gluon structure of the proton has been under intense experimental and theoretical investigation for five decades. Even for the distributions of the well-studied valence quarks, challenges such as the value of the down quark to up quark ratio at high fractional momenta remain. Much of the sea of quark-antiquark pairs emerges from the splitting of gluons and is well described by perturbative evolution in quantum chromodynamics. However, experiments confirm that there is a non-perturbative component to the sea that is not well understood and hitherto has been difficult to calculate with ab initio non-perturbative methods. This non-perturbative structure shows up, perhaps most directly, in the flavor dependence of the sea antiquark distributions. While some of the general trends can be reproduced by models, there are features of the data that do not seem to be well described. This article discusses the experimental situation, the status of calculations and models, and the directions where these studies will progress in the near future.

Probing the flavor dependence of proton's light-quark sea in the SeaQuest experiment

“…As x increases,d/ū rises linearly, reaching a maximum at x ≈ 0.18, then starting to drop as x further increases. The striking flavor asymmetry betweend andū has inspired many theoretical models, reviewed in several articles [9,10,11,12,13]. While these models are able to explain thed >ū asymmetry at low x, none of the models could reproduced <ū at the largest x.…”

Light-front wave functions of mesons, baryons, and pentaquarks with topology-induced local four-quark interaction

“…However, these processes are basically flavor and chirality independent, while the observed flavor and spin asymmetries of the sea indicate that there must also exist some nonperturbative mechanism of its formation. For a general recent review see [16].…”

“…The available experimental data, for the difference of the sea antiquark distributionsd −ū (from [16]), are shown in Fig. 11.…”

“…Upper: Momentum distribution for pion, rho and etaprime mesons, calculated in the model. Lower (from[16]): Comparison between the measured pion PDF (points) and the JV model (lines).…”

“…Upper: Our calculation of the d quark distribution in the nucleon times x, xdðxÞ (red, solid) and delta (black, dashed) states. For comparison, the lower plot shows empirical structure functions (copied from[16]), where the valence xdðxÞ is also shown in red.…”