Why early universe didn't self-annihilate decoded

Neutrinos are responsible for matter dominating its counterpart antimatter in the early universe, according to a study which sheds light on the role played by the neutral fundamental subatomic particles in the evolution of the cosmos. The scientists, including those from Imperial College London in the UK, said the current laws of physics do not explain why matter persists over antimatter even though equal amounts of the two were created at the beginning of the universe.

They said the two entities should have wiped each other out, annihilating the universe as it began.

However, according to the study, published in the journal Nature, there must be differences in the way matter and antimatter behave that explain why matter persisted and now dominates the universe.

The researchers explained that each particle of matter has an antimatter equivalent, with the ones for neutrinos called antineutrinos.

The two should be exact opposites in their properties and behaviour, which is what makes them annihilate each other on contact, the scientists said.

Contrary to this prediction, the scientists found the strongest evidence yet that neutrinos and antineutrinos behave differently, and therefore may not wipe each other out.

"This result brings us closer than ever before to answering the fundamental question of why the matter in our universe exists," said study co-author Patrick Dunne, from the Department of Physics at Imperial College London.

If confirmed - at the moment we're over 95 per cent sure - it will have profound implications for physics and should point the way to a better understanding of how our universe evolved," Dunne said.

While scientists had earlier found some differences in behaviour between matter and antimatter versions of subatomic particles called quarks, the dissimilarities were not large enough to account for the dominance of matter in the universe, the study noted.

But the current study indicates that the differences in the behaviour of neutrinos and antineutrinos appear to be quite large.

Neutrinos and antineutrinos can come in three 'flavours', known as muon, electron and tau, and they can 'oscillate' -- changing into a different flavour -- as they travel, the researchers said.

In the study, the scientists fired beams of muon neutrinos and antineutrinos from the J-PARC facility at Tokai, Japan, and detected how many electron neutrinos and antineutrinos arrived at the Super-Kamiokande detector 295km away.

When they looked for differences in how the neutrinos or antineutrinos changed flavour, they found that neutrinos appeared to be much more likely to change than antineutrinos.

Based on the data, the scientists strongly discount the possibility that neutrinos and antineutrinos are as just likely as each other to change flavour.

"What our result shows is that we're more than 95 per cent sure that matter neutrinos and antineutrinos behave differently," Dunne said.

"This is big news in itself, however we do already know of other particles that have matter-antimatter differences that are too small to explain our matter-dominated universe," he added.

According to the researchers, measuring the size of the difference is significant for determining whether neutrinos can answer this fundamental question.

"Our result today finds that unlike for other particles, the result in neutrinos is compatible with many of the theories explaining the origin of the universe's matter dominance," Dunne said.

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The researchers hope to reduce any uncertainties with further studies, and gather more data by upgrading the detectors and beamlines.

"When we started, we knew that seeing signs of differences between neutrinos and antineutrinos in this way was something that could take decades, if they could ever be seen at all, so it is almost like a dream to have our result be celebrated on the cover of Nature this week," said study co-author Yoshi Uchida from the Imperial College London.