Unexpected Substructures in the Fundamental Components of All Matter

Quarks in Deuterium

Artistic rendering of quarks in deuterium. Credit: Ran Shneor

Quarks and Antiquarks at High Momentum Shake the Foundations of Visible Matter

Jefferson Lab and Fermilab experiments present new results on nucleon structure.

Two independent studies have illuminated unexpected substructures in the fundamental components of all matter. Preliminary results using a novel tagging method could explain the origin of the longstanding nuclear paradox known as the EMC effect. Meanwhile, authors will share next steps after the recent observation of asymmetrical antimatter in the proton.

Both groups will discuss their experiments at DOE’s Thomas Jefferson National Accelerator Facility and Fermilab during the 2021 Fall Meeting of the APS Division of Nuclear Physics.

One study presents new evidence on the EMC effect, identified nearly 40 years ago when researchers at published earlier this year in Nature. New analysis indicates that in the most extreme case, a single antiquark can be responsible for almost half the momentum of a proton.

“This surprising result clearly shows that even at high momentum fractions, antimatter is an important part of the proton,” said Shivangi Prasad, a researcher at Argonne National Laboratory. “It demonstrates the importance of nonperturbative approaches to the structure of the basic building block of matter, the proton.”

Prasad will discuss the SeaQuest experiment that found more “down” antiquarks than “up” antiquarks within the proton. She will also share preliminary research on sea-quark and gluon distributions.

“The SeaQuest Collaboration looked inside the proton by slamming a high-energy beam of protons into targets made of hydrogen (essentially protons) and deuterium (nuclei containing single protons and neutrons),” said Prasad.

“Within the proton, quarks and antiquarks are held together by extremely strong nuclear forces—so great that they can create antimatter-matter quark pairs out of empty space!” she explained. But the subatomic pairings only exist for a fleeting moment before they annihilate.

The antiquark results have renewed interest in several earlier explanations for antimatter asymmetry in the proton. Prasad plans to discuss future measurements that could test the proposed mechanisms.

Meeting: 2021 Fall Meeting of the APS Division of Nuclear Physics