The saga of proton sea asymmetry

Wen-Chen Chang^1*

¹Institute of Physics, Academia Sinica, Taipei, Taiwan

* Presenter:Wen-Chen Chang, email:changwc@phys.sinica.edu.tw

Proton is arguably the most important QCD bound state. It is composed of the valence u and d quarks together with the strongly interacting gluons. The antiquark component can be understood due to gluon splitting. Contrary to a naive expectation from the perturbative QCD, a remarkable asymmetry between the anti-d and anti-u quarks has been observed through the Drell-Yan process by NA10 and E866 experiments. E866 experiment observed an intriguing signal of swapping of the antiquark asymmetry at large x. However, the turn-over of antiquark asymmetry is not compatible with any theoretical models, which can account for the observed large antiquark asymmetry.

Starting from 2008, a joint team from the US, Japan, and Taiwan have worked on SeaQuest/E906 Drell-Yan experiment in Fermilab, U.S., aiming at measuring the large-x antiquark distributions. In this talk, I will highlight the recent results, published in Nature 590, 561 (2021). Our accurate measurements clearly show that the distributions of antiquarks are considerably different, with more abundant anti-d than anti-u over a wide range of x, up to 0.45. Our results will improve the accuracy of the antiquark 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 for new physics.

Keywords: sea quark, quantum chromodynamics, hadron physics, Drell-Yan process