Why does the universe contain so much matter and so little antimatter? Particle physicists have puzzled over that question for 40 years. Now, new measurements may point to a hole in the current explanation for the subtle differences between matter and antimatter and could provide a better understanding of how the universe came to be chock-a-block with matter.
The key lies in a slight flaw in the mirrorlike relationship between matter (common particles such as protons and electrons) and antimatter (particles with identical masses but opposite charges). Dubbed charge-parity (CP) violation, the asymmetry was first seen in 1964 in the decays of fleeting particles called K mesons and their antimatter partners. In 1967, Russian theoretical physicist Andrei Sakharov suggested that CP violation might explain how, shortly after it sprang into existence, the infant universe produced so much matter and essentially no antimatter. Unfortunately, that scenario doesn't quite hang together. Through decades of experimentation, physicists have hammered together a "standard model" of the known particles, and it provides far too little CP violation to explain the imbalance.
Now, researchers have spotted an anomaly that might help square the accounts. It appeared in results from the Belle particle detector at the KEK laboratory in Tsukuba, Japan, where physicists measure the decays of a family of particles known as B mesons. The Belle team studied how a B ° meson decays into a K meson and a particle called a pion and compared it with how the B ° meson's antimatter partner decays into the corresponding antiparticles. As a result of CP violation, the decay rates were asymmetric. The researchers also studied the decay of a related particle called a B+ meson into a K meson and a pion and compared it with its antimatter counterpart. They observed a second asymmetry. Previous theoretical work suggested that the two asymmetries ought to be the same. They were not.
That discrepancy could indicate that undiscovered particles lurk inside B mesons in addition to the known mix of quarks and gluons, says team member Wei-Shu Hou, a physicist at National Taiwan University in Taipei. The interactions of such new particles could provide a bigger source of CP violation, which might ultimately help explain the preponderance of matter in the universe. Someday, "people may look back on the Belle measurement as the tip of the tip of the iceberg," Hou says.
Other researchers, however, say the results, published today in Nature, should be interpreted cautiously. It could all be an effect produced by run-of-the-mill particles, says Hassan Jawahery, a physicist at the University of Maryland, College Park and spokesperson for a group performing similar experiments at the Stanford Linear Accelerator Center in Menlo Park, California. Jawahery notes that his team has seen the same discrepancy, albeit with less statistical significance, and that the Belle measurements have been circulating for a while. "The result has been out for some time, and no one has been jumping to the conclusion that this is new physics," he says. Still, the finding raises that tantalizing possibility.