Hydrogen’s nucleus is composed of a single proton, while deuterium contains a loosely bound proton and neutron. To examine the nature of these particles, Reimer led a second experiment at Fermilab which aimed a proton beam at targets of liquid hydrogen and liquid deuterium.
“The current experiment extended the kinematic range of the predecessor experiment, and we can confirm our results in the region where we have overlapping data,” Aidala said. Now, the current study is interested in examining these quark pairs in higher momentum ranges of the proton-those marbles carrying more momentum than the others. The earliest study, conducted at CERN, a particle physics laboratory in Switzerland, found just a single datapoint demonstrating this asymmetry.Ī study conducted at Fermilab, a particle physics laboratory and accelerator outside Chicago, confirmed the CERN study’s findings-with many more data points, says U-M physicist Christine Aidala, who has been a part of the current experiment since 2010. Two prior studies found evidence of this asymmetry, and that there are as many as 50% more anti-down quarks than anti-up quarks. “The question is, ‘Does this asymmetry exist out in this region where antiquarks are rare? And if it does, what can it tell us about how they are formed?'” Reimer said. “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.Īccording to previous models, the strong force shouldn’t care if it’s producing an up quark and anti-up quark pair, or a down-quark and anti-down quark pair-but it should be producing them in equal numbers, says Reimer. Image credit: Paul Reimer, Argonne National Laboratory Scientists detected these muons to gain insight into the quark asymmetry of the proton. Graphic of quarks annihilating (left green lines), producing a photon (middle line), and producing two muons (right magenta lines). 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. This means the up and down quarks in neutrons and protons have corresponding antimatter quarks, called anti-up and anti-down quarks. 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. Quarks are held together by one of the fundamental forces of physics, called the strong force.
The proton and neutron are composed of even smaller particles with their own positive and negative charges, called up quarks and down quarks. Rather than a tiny, impenetrable point, the atom is a collection of particles: each is composed of protons, neutrons and electrons. 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. By investigating the world at the smallest level, scientists are advancing technology we use every day. Understanding the properties of the proton helps physicists answer some of the most fundamental questions in all of science. But a research team that includes University of Michigan physicists has found the proton displays asymmetry in its makeup. Scientists originally thought protons, the positively charged particle at the center of every atom, displayed symmetry. Symmetry is an important underlying structure of nature, present not only in mathematics and art, but also in living organisms and galaxies. Image credit: Fermi National Accelerator Laboratory
The proton beams pass through each of the shown layers, with the iron wall at the end of the path in the upper right corner of the image. Image of the apparatus used in the experiment.