Nature may have handed scientists a new clue in a longstanding mystery: how matter beat out antimatter for dominance of the universe. Early data from twin experiments at the Tevatron, the world's reigning particle accelerator at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., suggest an unexpected chink in the hugely successful standard model of particle physics.

The twist comes from odd behavior in a particle called the B S (pronounced "B-sub-S"), which flips back and forth between its matter and antimatter forms three trillions times per second. Researchers believe that such a breakdown, known as CP violation, is required to explain why matter is so abundant.

Researchers say the finding is well worth following up to make sure it is not a random clump in the data, as frequently happens in particle physics experiments. "This is exciting, definitely," says physicist Jacobo Konigsberg of the University of Florida in Gainesville, cospokesperson for CDF, one of two detectors that may have glimpsed the effect.

Antimatter is well-known to science fiction fans as the stuff that explodes on contact with regular particles such as protons and electrons, which have the same mass as their antiparticles but the opposite charge. The hot, early universe contained equal parts matter and antimatter. Yet somehow, as the cosmos cooled, matter was not completely annihilated.

Researchers strongly suspect that the key to this riddle lies in the weak nuclear force, which governs radioactive decay, along with more exotic reactions created in particle accelerators. In nearly all cases, matter obeys something called CP symmetry, which states that a particle ought to behave identically to the mirror image of its antiparticle. Not so when acted on by the weak nuclear force.

The amount of CP violation observed in experiments (and enshrined in the standard model), however, is far too little to explain why matter should have prevailed in its ancient war with antimatter. To get a clean look at CP symmetry, DZero and its sibling detector, CDF, focus on the B S , which consists of a bottom quark and a strange antiquark. (Quarks are components of protons and neutrons.) Working independently, the two detectors both found an extra dose of CP violation beyond what the standard model predicts.

Neither result on its own was very convincing, so a team of European researchers combined the data, similar to the way medical researchers cull information from independent clinical trials, to look for rare side effects. Together, the data make it 99.7 percent likely that the discrepancy is real, not due to chance, says physicist Luca Silvestrini of the National Institute for Nuclear Physics in Rome, who took part in the study submitted to Physical Review Letters.

Such analyses require making judgment calls, but Silvestrini says he is confident in the finding. "Everything points in the same direction, and so I think it's rather unlikely this is a statistical fluke," he says. Konigsberg says that if it is a fluke, that should become clear by the end of the summer as the Fermilab teams analyze more data.

Whether the hypothetical CP violation would fully explain matter's dominance over antimatter depends on the new physics that gave rise to it. According to theoretical physicist Robert Fleischer of CERN, the European Organization for Nuclear Research in Geneva, Switzerland, the simplest explanation would be a massive, photonlike particle similar to known members of the standard model and capable of interacting directly with bottom quarks and strange antiquarks.

Another possibility is supersymmetry, a proposed standard model extension that gives each known particle a heavier doppelganger, or super-partner. In that case, the B S oscillations could feel indirect effects from different combinations of super-partners, Fleischer says.

He notes that if the effect is real, the Large Hadron Collider, set to become the world's top dog in particle smashers after it goes on line later this year near Geneva, should be able to quickly confirm it and then probe for the underlying particles. "By 2010," he says, "I'm sure we will know the final answer."