Antimatter loses again.

Physicists have added another piece to the puzzle of why so little antimatter exists in today's universe.

In theory, the explosive birth of the universe produced equal amounts of matter and antimatter. Those incompatible substances didn't completely annihilate one another. Enough matter survived to form galaxies and other features of the universe. However, scientists looking for naturally occurring antimatter have found only a smidgeon of it, for instance, in cosmic rays.

Researchers explain matter's dominance as a consequence of differences in the way a fundamental force of nature, called the weak force, affects subatomic particles and their antiparticles. Since the 1960s, experimenters have found such differences, or charge-parity (CP) violations, among particles known as kaons (SN: 3/6/99,p. 148). In 2001, researchers in California and Japan uncovered a type of CP violation among B mesons (SN: 3/3/01, p. 143).

Now, the U.S.-based team has discovered another kind of CP violation, called direct CP violation, among B mesons of the [B.sup.0] type. Studying 227 million pairs of [B.sup.0] and anti-[B.sup.0] mesons created in a particle collider at the Stanford Linear Accelerator Center, the researchers found that 13 percent more [B.sup.0] mesons than anti-[B.sup.0] mesons decayed into kaons and pions.

That's a much larger difference than was seen among kaons. All these examples of CP violations could contribute to the observed imbalance between matter and antimatter. To fully account for the universe's shortage of antimatter, however, even larger violations must be found, says team member David G. Hitlin of the California Institute of Technology in Pasadena.

The new findings appear in a report posted Aug. 2 at a physics Web site (www. arXiv.org/pdf/hep-ex/0407057). Last month, the Japan-based team unveiled similar findings at www.arXiv.org/pdf/hepex/0407025.--P.W.