Representation of the proton. The large spheres represent the three valence quarks, the small spheres represent the other quarks that make up the proton, and the springs represent the nuclear force holding them together. Image credit: Brookhaven National Laboratory Graphical
Symmetry is an important underlying structure of nature, present not only in mathematics and art, but also in living organisms and galaxies.
Scientists originally thought protons, the positively charged particle at the center of every atom, displayed symmetry. But a research team that includes University of Michigan physicists has found the proton displays asymmetry in its makeup.
Understanding the properties of the proton helps physicists answer some of the most fundamental questions in all of science. By investigating the world at the smallest level, scientists are advancing technology we use every day. Studies of the proton have led to the development of proton therapy for cancer treatment, measurement of proton radiation during space travel, and even understanding of star formation and the early universe.
Rather than a tiny, impenetrable point, the atom is a collection of particles: each is composed of protons, neutrons and electrons. The proton and neutron are composed of even smaller particles with their own positive and negative charges, called up quarks and down quarks. Quarks are held together by one of the fundamental forces of physics, called the strong force.
Protons and neutrons are characterized by an excess of three quarks, but inside the proton, the strong force produces many short-lived matter-antimatter quark pairs. This means the up and down quarks in neutrons and protons have corresponding antimatter quarks, called anti-up and anti-down quarks. Scientists originally thought that anti-up and anti-down quarks were balanced in the proton, but the new study shows that there are more anti-down quarks than anti-up quarks, even into a momentum range where antimatter quarks are very rare in the proton.
“The fact that there are more anti-down quarks in the proton than anti-up quarks continues out as far as we could measure, and that’s interesting because, according to all existing models we have of the proton, there’s no real reason that this asymmetry should be there,” said Paul Reimer, spokesperson on the study and experimental physicist at the Argonne National Laboratory.
Find out how U-M physicists are involved by reading the article, "Study finds unexpected antimatter asymmetry in the proton" by U-M News contact Morgan Sherburne.