An Imbalance Among Tiny Particles Offers a Big Cosmology Clue

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Physicists have detected the strongest evidence yet of a behavioral difference between elementary particles called neutrinos and their mirror-image twins, antineutrinos. The asymmetry could be the key to why so much more matter than antimatter arose during the Big Bang—further explaining why anything at all exists today, since matter and antimatter in equal portions would have mutually annihilated.

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research develop­ments and trends in mathe­matics and the physical and life sciences.

“This is a hint that there is a large asymmetry between neutrinos and antineutrinos,” said Deborah Harris, a neutrino physicist at the Fermi National Accelerator Laboratory in Illinois and York University in Canada who was not involved in the work. “It’s a big deal,” she said, since “we’re trying to figure out what process could have tipped the balance in favor of matter over antimatter.”

“I am excited because this is the first time we have solid indications,” said Federico Sanchez Nieto of the University of Geneva, a co-spokesperson for the T2K experiment in Japan, which reported the results in Nature.

The T2K team started seeing signs of a discrepancy in the behavior of neutrinos and antineutrinos in 2016. Their new result, following years of additional data collection and improvements to the data-analysis techniques, rises to a statistical level that physicists regard as official evidence of a physical effect. “The significance of the effect increases with the collected data, which is what one expects when the result is correct,” said Werner Rodejohann, a neutrino physicist at the Max Planck Institute for Nuclear Physics in Germany who was not involved in the experiment.

And while next-generation experiments will be needed to gather enough data to definitively claim a discovery, the evidence is piling up years faster than experimenters expected, because neutrinos and antineutrinos seem to differ as much as they possibly could. “Nature seems to be very kind to us,” Rodejohann said.

Neutrinos are omnipresent but mysterious, easy to create but hard to catch. They spew from nuclear reactions in the sun and stars and stream through our bodies by the trillions each second. The super lightweight particles are so elusive that their properties are still being explored.

Experiments since the 1990s show that as neutrinos and antineutrinos fly along, they change between three types, or “flavors,” labeled electron, muon and tau.

Since 2010, the T2K scientists have been generating muon-flavored neutrinos and antineutrinos in Tokai, Japan, and beaming them 295 kilometers to Kamioka—the location of the Super-Kamiokande neutrino observatory, an underground, sensor-lined tank of 50,000 metric tons of pure water. Occasionally, upon arrival, one of the elusive particles interacts with an atom inside the tank and generates a telltale flash of radiation. The scientists fish for the neutrinos and antineutrinos that have oscillated from muon flavor into electron flavor during their cross-country journey.

Illustration: Lucy Reading-Ikkanda/Quanta Magazine

The data implies that neutrinos have a higher probability of oscillating than antineutrinos, a distinction expressed by a quantity called the CP-violating phase. If this phase were zero and neutrinos and antineutrinos behaved the same, the experiment would have detected roughly 68 electron neutrinos and 20 electron antineutrinos. Instead, it found 90 electron neutrinos and only 15 electron antineutrinos—highly skewed results indicating that the CP-violating phase could be as large as theoretically possible.

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