Long ago, antimatter all but vanished from existence, allowing matter to predominate and form the stars and planets of the universe. Exactly why this happened has been a mystery, but a particle accelerator in Japan may have found a new clue, and one that does not seem to fit the standard model of particle physics.
Earlier particle experiments had already seen a slight preference for matter over antimatter. For example, particles called kaons are less apt to self-destruct than their antimatter counterparts, antikaons. This preference can even be explained theoretically within the standard model of particle physics, as a lopsidedness of the weak nuclear force (technically called CP violation).
But that is far too slight to account for the predominance of matter in the universe. With only the standard model effect at work, in the dense early universe there would have been almost as much antimatter as matter.
Most of the antimatter would have collided with matter particles, destroying both in a burst of radiation. The universe today would be filled with light and other electromagnetic radiation, plus just a scattering of protons and electrons far too sparse to have formed stars or solid stuff.
So some other process must have favoured matter. Now an international collaboration working at the KEK accelerator in Tsukuba, Japan, may have seen a sign of it.
KEK collides electrons and their antiparticles, positrons, to create unstable heavy particles called B mesons. A detector called Belle then monitors how these B mesons break up.
What the team saw was a difference in the way various types of B mesons decay. Uncharged B mesons fell apart more often than their antiparticles, while charged B mesons fell apart less often than their antiparticles.
According to Belle spokesman Tom Browder of the University of Hawaii, that is a sign of new physics. "The standard-model expectation is that the asymmetries in the two decay modes should be the same. We find that they are different."
More evidence of a fundamental preference for matter comes from a European group, which analysed data from several experiments on a similar particle, the BS meson. Their research also shows a non-standard preference for matter.
Luca Silvestrini of the Italian National Institute of Nuclear Physics in Rome, a member of the European group, thinks their results are more certain than those from Belle, which he says might be accounted for within the standard model.
But he adds that the results might support one another. "The most exciting possibility is that the two effects are connected," Silvestrini told New Scientist.
It is still unclear whether the odd behaviour of B mesons actually reveals an effect powerful enough to create a preponderance of matter in the universe.
"Making the connection from the low-energy world of quarks or neutrinos to the extremely high-energy scale of the early universe is very difficult," says Browder. "Since we cannot directly probe the extreme energies of the early universe, we take what we can get."
Cosmologist Peter Coles of the University of Cardiff in the UK is cautiously positive. "I don't think there is a compelling theoretical understanding of the new CP-violation results. They don't fit naturally within standard model physics and they may therefore point to a resolution of the issue. I don't think you could say that they solve this problem though."
As with so many of today's questions in particle physics, the Large Hadron Collider could soon provide a clearer answer.
Due to begin operation at CERN in Geneva within a few months, this giant particle accelerator includes a detector called LHC-b that is specifically designed to investigate B mesons and their matter-antimatter biases. "It is unlikely that the puzzle will be solved and a full picture emerge before the LHC starts taking data this year," says LHC-b team member Val Gibson of Cambridge University in the UK.