Neutrinos hint at why antimatter didn't blow up the universe
Physics 4 July 2016
By Lisa Grossman
Super-Kamiokande: a huge detector looking out for tiny particles
Kamioka Observatory/ICRR(Institute for Cosmic Ray Research)/The University of Tokyo
It could all have been so different. When matter first formed in the universe, our current theories suggest that it should have been accompanied by an equal amount of antimatter – a conclusion we know must be wrong, because we wouldn't be here if it were true. Now the latest results from a pair of experiments designed to study the behaviour of neutrinos – particles that barely interact with the rest of the universe – could mean we're starting to understand why.
Neutrinos and their antimatter counterparts, antineutrinos, each come in three types, or flavours: electron, muon and tau. Several experiments have found that neutrinos can spontaneously switch between these flavours, a phenomenon called oscillating.
The T2K experiment in Japan watches for these oscillations as neutrinos travel between the J-PARC accelerator in Tokai and the Super-Kamiokande neutrino detector in Kamioka, 295 kilometres away. It began operating in February 2010, but had to shut down for several years after Japan was rocked by a magnitude-9 earthquake in 2011.
Puff of radiation
In 2013, the team announced that 28 of the muon neutrinos that took off from J-PARC had become electron neutrinos by the time they reached Super-Kamiokande, the first true confirmation that the metamorphosis was happening.
They then ran the experiment with muon antineutrinos, to see if there was a difference between how the ordinary particles and their antimatter counterparts oscillate. An idea called charge-parity (CP) symmetry holds that these rates should be the same.
CP symmetry is the notion that physics would remain basically unchanged if you replaced all particles with their respective antiparticles. It appears to hold true for nearly all particle interactions, and implies that the universe should have produced the same amount of matter and antimatter in the big bang.
Matter and antimatter destroy one another, so if CP symmetry holds, both should have mostly vanished in a puff of radiation early on in the universe's history, well before matter was able to congeal into solid stuff. That's clearly not what happened, but we don't know why. Any deviation from CP symmetry we observe could help explain this discrepancy.
"We know in order to create more matter than antimatter in the universe, you need a process that violates CP symmetry," says Patricia Vahle, who works on NoVA, a similar experiment to T2K that sends neutrinos between Illinois and Minnesota. "So we're going out and looking for any process that can violate this CP symmetry."
Flavour changers
We already know of one: the interactions of different kinds of quarks, the constituents of protons and neutrons in atoms. But their difference is not great enough to explain why matter dominated so completely in the modern universe. Neutrino oscillations are another promising place to look for deviations.
This morning at the Neutrino conference in London, UK, we got our first signs of such deviations. Hirohisa Tanaka of the University of Toronto, Canada, reported the latest results from T2K. They have now seen 32 muon neutrinos morphing into the electron flavour, compared to just 4 muon antineutrinos becoming the anti-electron variety.
This is more matter and less antimatter than they expected to see, assuming CP symmetry holds. Although the number of detections in each experiment is small, the difference is enough to rule out CP symmetry holding at the 2 sigma level – in other words, there is only around a 5 per cent chance that T2K would see such differences if CP symmetry is preserved in this process.
Particle physicists normally wait until things reach the 3 sigma level before getting excited, and won't consider it a discovery until 5 sigma, so it's early days for neutrinos breaking CP symmetry. But at the same conference, Vahle presented the latest results from NoVA that revealed the two experiments were in broad agreement about the possibility.
The extent of CP violation rests on a key parameter called delta-CP, which ranges from 0 to 2π. Both teams found that their results were best explained by setting the value equal to 1.5π. "Their data really does prefer the same value that T2K does," says Asher Kaboth, who works on T2K. "All of the preferences for the delta-CP stuff are pointing in the same direction."
NoVA plans to run its own antineutrino experiments next year, which will help firm up the results, and both teams are continuing to gather more data. It's too soon to say definitively, but one of the mysteries of why we are here could be on the road to getting solved.
Additional reporting by Jacob Aron
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This article appeared in print under the title: Neutrinos hint at why antimatter didn't blow up the universe
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