Mounting evidence suggests neutrinos are key to why antimatter is rare

Tiny subatomic particles called neutrinos could help answer a really big question: why anything exists at all.

A new result reaffirms earlier hints that neutrinos behave differently than their antimatter counterparts, antineutrinos, physicists with the neutrino experiment T2K report. If confirmed, the particles’ divergence could help reveal how the universe avoided becoming an empty wasteland.

The cosmos is filled with matter. Its counterpart, antimatter, is much less common. But in the newborn cosmos, both existed in equal measure. Since matter and antimatter particles annihilate each other when they get together, that should have left the cosmos filled with nothing but energy.

For the universe to have formed as we know it, something must have tipped the balance toward matter. The new result, if reinforced by future measurements, would support a long-held hunch that neutrinos are key to explaining how matter got the upper hand.

“This is definitely very exciting and motivating,” says neutrino physicist Georgia Karagiorgi of Columbia University, who was not involved with the study. T2K scientists declined to comment on the paper, published October 9 at arXiv.org, as the result has not yet been peer reviewed.

Each known matter particle has an antimatter mirror image with the opposite electric charge. An electron’s antimatter analog, for example, is a positron. Typically, matter and antimatter behave similarly, aside from their opposite charges. But the two can sometimes diverge, an effect known as a CP violation (for “charge parity”). If neutrinos violate CP today, theories suggest, the universe’s first moments might have been beset by additional CP violation that would explain how matter prevailed.

To test for CP violation in neutrinos, T2K researchers sent beams composed of neutrinos or antineutrinos on a nearly 300-kilometer trek across Japan to an underground detector at the Kamioka Observatory in Hida. There was reason for the long journey: As they travel, neutrinos can oscillate, meaning they morph between three particle types — electron neutrinos, muon neutrinos and tau neutrinos. The same goes for antineutrinos.

T2K’s beams initially consist of muon neutrinos or muon antineutrinos. The researchers counted how often the particles converted into electron neutrinos or electron antineutrinos.Collected over nearly a decade, the data suggest that neutrinos oscillated more than expected, while antineutrinos oscillated less than expected — a sign of CP violation.

The result continues a “slow buildup” of evidence for CP violation in neutrinos, says neutrino physicist Jonathan Link of Virginia Tech in Blacksburg. Previous results from T2K had shown early signs of CP violation (SN: 8/8/17). But the new results constrain the amount of CP violation — measured by a quantity known as delta CP — better than ever before.

For the first time, the researchers are beginning to narrow down the potential values of delta CP, concluding with a significance of three sigma, or about a 99.7 percent confidence level, that certain values aren’t possible. The scientists still aren’t able to say whether CP violation occurs, however.

It’s an exciting time to be a neutrino physicist, says Patricia Vahle of William & Mary in Williamsburg, Va. “We are closing in on things that we have wanted to measure for a long, long time now, so every little step forward is pretty exciting,” she says.