Neutrinos and their antimatter counterparts, antineutrinos, come in three 鈥渇lavours鈥� 鈥� muon, electron, and tau. They can spontaneously switch between these flavours in a phenomenon called neutrino oscillation. But they鈥檙e hard to catch in the act, because neutrinos only rarely interact with normal matter.
Now, the NOvA collaboration at Fermi National Accelerator Laboratory in Illinois has confirmed that antineutrinos oscillate, detecting muon antineutrinos morphing into electron antineutrinos with more certainty than we鈥檝e ever had before.
If we could pin down how often neutrinos morph between these three types, and how that may differ from the rate at which antineutrinos do the same thing, it could help us understand why the universe is mostly made of normal matter and not antimatter.
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Mining the depths
Set 105 metres below ground, the NOvA experiment used a beam of muon antineutrinos, which passed through one detector in Illinois and another 810 kilometres away at ground level in Minnesota. As the muon antineutrinos travel through the earth, a few become electron antineutrinos, while some turn into tau antineutrinos, which the detectors can鈥檛 see.
By comparing the data from the two detectors, the researchers can determine how many of the antineutrinos went from muon to electron. If these particles didn鈥檛 oscillate at all, the team would have expected to find about 5 electron antineutrino candidates, coming from sources outside the muon beam. But they saw 18, indicating that muon antineutrinos actually do oscillate into electron antineutrinos, which supports prevailing theories.
The hope is that by comparing these identity switches in neutrinos and antineutrinos, we鈥檒l be able to learn something about the masses of the three types of neutrinos. We know that there are three different neutrino masses. Two of these are similar, but we鈥檙e not sure whether the third mass is much larger or much smaller.
The laws of physics should apply in the same way to a particle and its antiparticle, so the rate at which neutrinos and antineutrinos oscillate should be the same. But we know that this rule is broken in some way 鈥� if it wasn鈥檛, the big bang should have produced equal amounts of matter and antimatter.
Using Earth as a laboratory lets us probe whether electron neutrinos break that rule. 鈥淓arth has lots of electrons in it, which means that electron neutrinos feel the Earth in a way that the electron antineutrinos do not, because there鈥檚 not antielectrons along the path for them to interact with,鈥� says NOvA team member Alex Himmel.
Matter over antimatter
NoVA can also measure muon neutrino beams, and so can compare how the particles and their antiparticles oscillate, which will help sort out the neutrino mass hierarchy. If the third neutrino mass outweighs the others, we should see more electron neutrinos and fewer electron antineutrinos at the Minnesota detector. If it鈥檚 far lighter, the final detector should record the opposite ratio.
Previous results have leaned slightly towards the third neutrino mass being larger, but this new antineutrino experiment doesn鈥檛 push the argument one way or the other. Himmel says it will take years before we have enough data to make a statistically valid conclusion. Still, it鈥檚 a first step.
Understanding neutrino masses could give us a better picture of what the early universe was like, says Shanahan, because there are so many of them and they may have played a bigger role when the universe was smaller. It could also prove key to our understanding of why the universe is mostly made of matter and not antimatter, says Xin Qian at Brookhaven National Laboratory in New York.
Read more: LHC sees matter and antimatter misbehaving in alternate particle
