Physicists Find Suspected Flaw in Cosmic Symmetry
In general, symmetry is the rule in the universe—the world makes perfect sense if seen in a mirror, for instance—but in April, physicist Edward Stephenson at Indiana University found a flaw in the balance of nature that researchers have been seeking for decades. Scientists have assumed this imbalance, called charge-symmetry breaking, had to exist because without it there would be no hydrogen, and hence no galaxies, planets, or people.
“There was a point about one second after the Big Bang when neutrons and protons condensed out of the underlying mixture of particles,” Stephenson says. “The neutrons decayed into protons, but the protons remained stable. After 10 or 20 minutes, there was an enormous amount of the subatomic materials needed to form hydrogen, which is the building block of stars and galaxies. It is all a consequence of charge symmetry breaking down.” The effects of charge-symmetry breaking are still evident today. Neutrons are measurably more massive than protons, which have an electric charge but are otherwise identical, because of a bias built into the laws of physics.
Until recently, all of this remained theory. Stephenson put it to the test at the Indiana University Cyclotron Facility. He and his colleagues slammed a beam of heavy hydrogen atoms into a cloud composed of more heavy hydrogen. Most of the time, the encounter obliterated the atoms. One time in 10 billion, however, two heavy hydrogen nuclei fused to make a helium atom and a particle called a pion, which helps bind an atomic nucleus together. That reaction can occur only by breaking charge symmetry. Physicists at Ohio University observed similar evidence of symmetry violation by colliding neutrons and protons to form heavy hydrogen and pions. They also announced their results in April.
The big question now is why particles can occasionally evade laws that apply the rest of the time. Stephenson plans further experiments to measure the rate of symmetry violation, which may help piece together this puzzle.
—Kathy A. Svitil
Particles and Theory Collide
Courtesy of Peter Ginter Physicist Mike Kelsey checks equipment in the BaBar Detector, where a team of 600 scientists study subatomic particle collisions 24 hours a day, nine months each year. |
The new particle is thought to be a short-lived union between a charm quark and a strange antiquark. Quarks are ethereal particles that make up protons and neutrons—the building blocks of atoms—and other bits of subatomic matter. They come in six varieties: up, down, top, bottom, strange, and charm. Each has an antimatter counterpart. Although particle accelerators routinely produce unusual configurations of quarks and antiquarks, Ds(2317) was peculiar because its mass is at least 9 percent lower than expected.
In the world of subatomic particles, finding a 9 percent mass discrepancy is like seeing an elephant do a disappearing act. Surprisingly, some researchers suggest that the low mass might be because Ds(2317) is actually not a charm-antistrange composite but a quark “molecule,” built out of four quarks. No such particle has ever been seen; however, a five-part pentaquark was discovered in July (see “New Matter Detected at Japanese Accelerator,” page 45).
BaBar team leader Marcello Giorgi, a physicist from the University of Pisa in Italy, thinks Ds(2317) may be a harbinger of a paradigm shift in the world of subatomic physics. Mass and energy are equivalent at these small scales, so Giorgi and his colleagues reason that they can get the mass of Ds(2317) to fall within the right range by tinkering with the strength of the strong nuclear force that binds the charm quarks and the strange antiquarks. If experiments now in the works prove them right, it means that previous calculations of the strong nuclear force, one of the most fundamental forces in the universe, may be wrong. “We would have to revisit all the knowledge that we have about the force that binds elementary quarks to produce matter,” Giorgi says. “That would be a very big deal.”
—Kathy A. Svitil
