The proton is a positively charged particle that exists at the center of every atom. It is a confined complex system of strongly interacting fundamental particles, quarks, and the nuclear force carriers, gluons. Its properties like charge are dominated by an excess of three quarks — two “up” quarks and one “down” quark, called valence quarks. However, take a closer look, and the proton “sea” becomes a turbulent and enigmatic mix of quarks and their antimatter counterparts, anti quarks, that pop in and out of existence before quickly annihilating each other. Scientists call these quarks the sea quarks. The Sea Quest Collaboration studied the antimatter makeup of the proton sea for a wide range of quark momenta with higher precision than ever before. They found that protons have, on average, 1.4 down anti quarks for every up antiquark.
Protons, subatomic particles with a positive charge, are present in every atom. Understanding the structure of the proton can provide insight into the forces that hold protons together. These insights help physicists answer some of the most fundamental questions in all of science. The Sea Quest data agree with two of the many competing models of the proton, demonstrating the importance of the proton’s sea quarks. The Sea Quest findings also will help scientists parse through data from particle collisions at the Large Hadron Collider in search of new physics.
Scientists with the Sea Quest research group, a national laboratory-university joint effort, investigated an asymmetry between the up and down flavors of anti quarks in the proton sea. Their data show that in the proton, down anti quarks outnumber up anti quarks over a wide range of quark momenta. To probe the quarks and anti quarks in the proton, the scientists shot high-energy beams of protons at targets of liquid hydrogen and deuterium. They studied the aftermath of collisions between protons from the beam and nuclei in the targets. When protons collide, many things can happen. In one process, called the Drell-Yan process, the quarks and anti quarks in the colliding protons annihilate, and two new fundamental particles called muons come out of the annihilation, acting as the interaction’s signature. By studying the signatures from the collisions, the scientists determined that down anti quarks are about 40 percent more abundant in the proton than up anti quarks, even in the rare case that a single antiquark carries more than a third of the proton’s total momentum. The origin of antimatter within the proton and the observed asymmetry can be explained by models in which some of the anti quarks are carried within a cloud of virtual particles forming a field around the proton. This class of models make additional predictions about the spin of the antimatter in the proton that can be tested with future measurements.
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