What's the matter? Particles fly from an electron-positron smash. Analysis of these events reveals a tantalizing difference in the properties of matter and antimatter.

Physicists Confront a 'Number From Hell'

SAN FRANCISCO--Physicists are tantalizingly close to answering one of the most fundamental questions imaginable: "Why is there matter in the universe?" After all, theories maintain that the big bang forged equal amounts of matter and antimatter, which should have annihilated each other. New data from two teams of particle physicists suggest that matter had a tiny but measurable competitive edge in the earliest moments of the universe. Observers' interpretations of the tentative results range from "interesting" to "depressing."

When matter and antimatter meet, they vanish as flashes of energy. That was the blazing fate for almost all particles and antiparticles created in the big bang. However, the fierce interactions among particles somehow skewed slightly in favor of matter, by a mere one extra particle per billion. That leftover matter then formed galaxies, stars, planets, and people. Theorists believe the surplus arose because particles of matter don't decay in precisely the same way as their oppositely charged mirror images, a trait called "CP violation."

Physicists discovered a simple form of CP violation in 1964 within the decays of K mesons, which are unstable mixtures of matter and antimatter. For the last 2 years, teams at the Stanford Linear Accelerator Center (SLAC) in California and the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan, have probed for a deeper signal of CP violation in B mesons, the heavy brothers of K mesons. Special machines create tens of millions of B mesons by smashing electrons into their antimatter counterparts, positrons. However, only a fraction of the collisions are "golden events"--pairs of B's and anti-B's that might show the clearest signature of CP violation. As of January, physicists had seen 630 such events at SLAC and 260 at KEK.

That's enough for a preliminary analysis, reported SLAC physicist Patricia Burchat here on 16 February at a meeting of the American Association for the Advancement of Science, which publishes ScienceNOW. Physicists represent matter-antimatter asymmetry in this experiment as a dimensionless number with possible values from -1 to 1, with 0 representing no CP violation. The Standard Model, the long-standing theoretical edifice of particle physics, predicts a value of 0.72, Burchat notes.

SLAC's value to date is 0.34, but the error range is large: +/- 0.20. That means there's a 5% chance (twice the error bar, or two standard deviations) that the real value could match the prediction of the Standard Model, or it could be 0. For that reason, says SLAC physicist Stewart Smith, "It's the number from hell." KEK's preliminary number is closer to the Standard Model value but with an even larger error (0.58 +/- 0.33). Both teams will present their results in more detail this week at a conference in Ise-Shima, Japan.

Despite the uncertainties, other physicists applaud the rapid progress. "It's starting to get interesting," says theorist Joseph Lykken of the Fermi National Accelerator Laboratory in Batavia, Illinois. "We're almost at the point of challenging the Standard Model and its explanation of CP violation." Theorist Michael Dine of the University of California, Santa Cruz, has a more visceral reaction: "It's depressing. I desperately wanted it to be 0." That result, far out of whack with the Standard Model, would have supported a sweeping but untested theory of particles and forces called supersymmetry, Dine explains.

Related sites
SLAC news announcement
Background information on B meson physics
Technical paper from KEK team
Technical paper from SLAC team