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Proton antimatter is even more uneven than we thought

The antimatter of the proton is out of place. A new measurement indicates that an imbalance between two types of antiparticles boiling inside the proton is even more severe than previously thought.

Protons are constructed from three quarks: two "up" quarks and one "down" quark. But they also contain a moving sea of ​​transient quarks and antiquarks that flow into existence before quickly annihilating. Within that sea, antiquarks are inferior to antiquarks, measurements revealed in the 1990s. And that disaster persists in a realm of unexplored quark moments, researchers from the SeaQuest experiment at Fermilab in Batavia, Ill., Report on February 24 in Nature.

Typically, each antiquark carries only a small portion of the total momentum of a proton. But sometimes a single antiquark can represent a big part of the momentum. Previous measurements have suggested that similar amounts of anticuarks could be found up and down with a considerable amount of momentum. But new tests, done by hitting protons against targets made of hydrogen and deuterium (hydrogen with an extra neutron in its nucleus), contradicted that idea. SeaQuest researchers found that low antiquarks were about 50 percent more prevalent than high antiquarks, even when a single antiquark carried almost half the total proton momentum.

The measurements are important for studies conducted at CERN's Large Hadron Collider in Geneva, which gathers protons to look for new phenomena. To fully understand collisions, physicists need a complete accounting of the proton components. “They need to know what they collide with,” says study co-author Paul Reimer of Argonne National Laboratory in Lemont, Ill.

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