SCIENCE - Scientists decode why universe is dominated by matter

Scientists have solved the puzzle of the universe being dominated by matter rather than its close relative anti-matter.

Physicists at the University of Wisconsin-Madison made a precise measurement of elusive, nearly massless particles, and obtained a crucial hint as to why the universe is dominated by matter.

The particles, called anti-neutrinos, were detected at the underground Daya Bay experiment, located near a nuclear reactor in China.

Anti-particles are almost identical twins of sub-atomic particles (electrons, protons and neutrons) that make up our world. When an electron encounters an anti-electron, for example, both are annihilated in a burst of energy.

Failure to see these bursts in the universe tells physicists that anti-matter is vanishingly rare, and that matter rules the roost in today's universe.

"At the beginning of time, in the Big Bang, a soup of particles and anti-particles was created, but somehow an imbalance came about," says Karsten Heeger, a professor of physics at UW-Madison.

"All the studies that have been done have not found enough difference between particles and anti-particles to explain the dominance of matter over anti-matter.

"But the neutrino, an extremely abundant but almost massless particle, may have the right properties, and may even be its own anti-particle, Heeger said in a statement.

"And that's why physicists have put their last hope on the neutrino to explain the absence of anti-matter in the universe," he said.

Reactors, Heeger says, are a fertile source of anti-neutrinos, and measuring how they change during their short flights from the reactor to the detector, gives a basis for calculating a quantity called the "mixing angle", the probability of transformation from one flavour into another.

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