The Slightly Twisted World of Protons

Ping pong ping. These mirrors were used to detect electrons bouncing off hydrogen or deuterium targets.

When Alice went through the looking-glass, she found a world far stranger than her own. The differences between our world and its mirror image are much more subtle. Now a team has announced a new source of asymmetry that helps make our world unlike its mirror image: Protons contain an unexpected amount of internal asymmetry, a finding that may give clues to the particle's subatomic structure.

The idea that the universe isn't identical to its mirror image is nothing new. In 1956, physicists discovered that the direction in which a nucleus of radioactive cobalt-60 is most likely to emit radiation depends on its direction of spin. If the spin of the nucleus curves around in the same direction as the fingers of someone's left hand, then the nucleus is likely to emit beta rays in the direction of the left thumb--an asymmetry that can be loosely described as "left-handedness." So far, all cases of mirror asymmetry in physics, including the left-handedness of the cobalt atom, have been ascribed to the weak nuclear force.

One such asymmetry can be found in electrons, whose polarity determines whether they are right-handed or left-handed. Like loaded dice, left-handed electrons are slightly more likely to bounce back from a proton than right-handed electrons are. Physicists expected this effect to result from a weak interaction between electrons and protons called "strange magnetism."

To determine the strength of this strange magnetism, a team of 35 researchers designed an experiment at the Massachusetts Institute of Technology that they called SAMPLE. The researchers had to count more than a trillion electrons over 3 months. "In some ways, it's a boring experiment," says Douglas Beck of the University of Illinois, Urbana-Champaign, who helped design it, "like rolling dice a trillion times." The researchers bombarded either hydrogen (which has a nucleus of 1 proton) or deuterium (which has a nucleus of 1 proton and 1 neutron) with right- and left-handed electrons. The team reports in the 15 December issue of Science that 1 extra left-handed electron bounced back out of every million collisions with either deuterium or hydrogen nuclei.

The team expected to see extra bounce-backs from the proton in the hydrogen nuclei, but they were surprised to see about the same number of bounce-backs from deuterium. They thought the strange magnetism of the neutron would roughly cancel the strange magnetism from the proton, so they should have seen very little asymmetry when they bounced electrons off deuterium nuclei. But the asymmetry persisted, which led the researchers to conclude that it comes not from strange magnetism but from an asymmetry within the proton itself--probably from the weak interactions between quarks inside the proton.

The results will provide important benchmarks for theorists trying to understand the structure of the proton, because the strength of weak interactions at low energies is very difficult to compute. "The [interactions within the proton] are the most striking and interesting feature" of the new SAMPLE findings, says particle physicist Frank Maas, who leads a similar experiment at the University of Mainz in Germany.

Related sites

The SAMPLE Web site

More about mirror asymmetry