Exploring antiquark asymmetry in protons

Wen-Chen Chang and Shin'ya Sawada

Protons and neutrons, the components of every nucleus, are the dominating mass carriers of our visible universe. Our understanding of protons and neutrons (i.e., nucleons) has been evolving from a simple 3-quark structure to the 1d longitudinal momentum fraction (x) distributions of quarks and gluons and the multi-dimensional distributions with additional dependency on transverse size or transverse momentum.

Inside the nucleons, there is the existence of anti-quarks, originated from the splitting of gluons, the strong force carrier. Utilizing the Drell-Yan process, the NuSea/E866 experiment identified a large asymmetry of anti-d and anti-u asymmetry over the range of x=0.02-0.35, with an intriguing signal of a "turning over" at large x. This turning-over trend is puzzling since it is not compatible with theoretical pictures such as the pion-cloud model, which can account for the observed large antiquark asymmetry.

In 2008, a joint team from the USA, Japan, and Taiwan began working on the Fermilab SeaQuest/E906 experiment, whose measuring capability was on the large-x region. Their results regarding large-x antiquark asymmetry have been published in Nature 590, 561 (2021) https://www.nature.com/articles/s41586-021-03282-z, along with commentary at https://www.nature.com/articles/d41586-021-00430-3. The results clearly show that these distributions of antiquarks are considerably different, with more abundant anti-d than anti-u over a wide range of x, up to 0.45. The measurements will improve the accuracy of the anti-quark distributions of protons and are important for the studies at the Large Hadron Collider at CERN in terms of establishing the standard model baseline in the search of new physics. In addition, the results should motivate further theoretical and experimental studies to understand nucleon structure, such as more sophisticated models and measurements in charactering the spin and mass decomposition of nucleons (Fig. 1).

Fig. 1: Ratios of ̅d(x) to ̅u(x) in the proton (red filled circles) from the SeaQuest/E906 data, based on next-to-leading-order calculations of the Drell-Yan cross sections. Results of the previous measurement by the NuSea experiment, the calculated ratios with two PDFs, and the predictions of two theoretical models are shown for comparison. The figure is taken from Nature 590, 561 (2021)