Antimatter-Matter Mirror Shows Warp.

Amid the vast energies and ultrasmall dimensions of particle physics, a looking-glass world of antimatter shimmers into view. In 1964, surprised physicists discovered that this antimatter realm is not a perfect mirror image of the familiar matter-dominated surroundings. Now, for the first time since that revelation 35 years ago, scientists have detected another flaw in the mirror.

"What we have found is a new physical effect in nature," says Bruce Winstein of the University of Chicago. The discovery may help illuminate how the universe went from nearly equal antimatter and matter at its birth to the overwhelming preponderance of matter seen today, the researchers say. It also dashes one theory offered to explain nature's uneven way of dealing with matter and antimatter and bolsters the prevailing theory of particle physics known as the standard model.

Before 1964, physicists assumed that the outcomes of experiments would remain the same if two types of symmetrical particle characteristics were both changed. Under so-called charge, or C, symmetry, particles and antiparticles are interchanged; under parity, or P, symmetry, directions such as clockwise or right transform into their mirror images.

The landmark experiment by James W. Cronin of the University of Chicago and Val L. Fitch, now of Princeton University, and their colleagues revealed that nature--or at least particles called K mesons, or kaons--didn't behave as expected. Cronin and Fitch later shared a Nobel prize for their demonstration that the combined symmetry could break down into a CP violation.

The new finding from Winstein and scores of his colleagues working on the "Kaons at the Tevatron," or KTeV, experiment at Fermi National Accelerator Laboratory in Batavia, Ill., represents the first definitive evidence of another type of CP violation sought since 1964. Fermilab announced the finding on March 1.

"It's a marvelous result," Fitch says.

Researchers at the European Laboratory for Particle Physics (CERN) near Geneva published similar findings in 1988 and 1993, notes Konrad Kleinknecht of the University of Mainz in Germany. However, those results had too much uncertainty to be considered definitive, KTeV researchers say.

Scientists have also reported preliminary indications of a CP violation in experiments with a particle called a B meson (SN: 2/20/99, p. 118).

The new effect, like the 1964 finding, arises in kaons but from a different aspect of their behavior. These particles have an unusual property that enables matter and antimatter forms to intermingle by the rules of quantum mechanics to create blended particles called K-long ([K.sub.l]) and K-short ([K.sub.s]). The experimenters observe these particles rather than the pure matter and antimatter.

Theoretical arguments based on CP symmetry demand that [K.sub.l] should always decay into three other particles, but Cronin and Fitch found that occasionally it would decay into only two particles in what physicists call an "indirect" CP violation.

The Fermilab experiment started with those signature decays of [K.sub.l] to only two particles. The scientists examined which events produced offspring carrying electric charges and which produced neutral ones. The team then compared those outcomes to the decays of [K.sub.s] particles. Analysis revealed that about one of every 300 of those [K.sub.l] decays was a "direct" CP violation that resulted from a process other than the matter-antimatter mixing.

The finding challenges a theory of CP violation that postulates the existence of an extremely weak force of nature, known as superweak. "This throws that scheme out the window," Fitch says.

While the standard model of particle physics predicts direct CP violation, theorists and experimenters have more work to do to determine whether the details of the model agree with the new findings, says Edward C. Blucher, also of the University of Chicago.