A photo shows two researchers in a rowboat inside the Super Kamiokande detector, which detects neutrinos as they slam into water molecules.
Inside a cavern, buried beneath a mountain in Japan, there’s a giant tank of water that has been very still for many years. And usually nothing happens.
Every once in a while though, a ring of light flickers around the edges of the tank — the signature of an electron or a similar, but heavier particle known as a muon passing through the water. Those electrons and muons are remnants of tiny, ghostly particles known as neutrinos that slammed into the tank’s water molecules in a rare interaction.
For years, the physicists of the T2K Collaboration have counted those rings of light, the only sign of a powerful neutrino beam fired through the Earth’s crust into the cavern from another subterranean facility 183 miles (295 kilometers) away. As the physicists of T2K count the rings, they separate out the clearly-defined ones, produced by heavier muons charging through the water, from the fuzzy rings, which are the signatures of lightweight electrons.
Over time, the physicists have noticed a discrepancy in their count. That discrepancy, they believe, could help explain the existence of matter in the universe.
Matter and antimatter should mirror each other, but they don’t
Just after the Big Bang, equal amounts of matter and antimatter existed in the universe, two substances that mirror each other and destroy each other if they ever touch. Hydrogen’s antimatter twin is antihydrogen. An electron’s antimatter twin is the positively charged positron. Muons have antimuons and neutrinos have antineutrinos and so on.
Antimatter and matter are so similar, in fact, that it’s a mystery why they didn’t simply cancel each other out in the beginning, leaving nothing behind but a burst of bright light. That suggests that there must be some fundamental differences between the particles, asymmetries that would explain why matter came to dominate antimatter. And we’ve already found one of those asymmetries.
“One of them is in the quarks, the particles that make up protons and neutrons,” said Mark Hartz, a physicist at the University of Pittsburgh and a member of the T2K Collaboration.
Back in 1964, physicists discovered smaller differences between how quarks and antiquarks, the subatomic particles that make up protons, neutrons and other particles, interact through the weak force — one of the four fundamental forces alongside the strong force, electromagnetism and gravity. But the quark asymmetry is too slight to explain the existence of the universe. There has to be some other discrepancy out there.
There are theories about another discrepancy, involving a class of particles called leptons, said Silvia Pascoli, a physicist at Durham University in England who was not involved in the T2K Collaboration.
Leptons are particles like neutrinos, muons, and electrons. And if there were an asymmetry between leptons and their antimatter counterparts, she told Live Science, that could lead over time to not only an excess of matter leptons but matter baryons — the class of particles that make up most of an atom’s mass.
The T2K Collaboration studies that tank of water looking for evidence of that lepton asymmetry, which physicists believe would become visible when neutrinos “oscillate” from one flavor to another.
Neutrinos could hold the key
There are three types of neutrino (that we know of): electron, muon, and tau. And each of those flavors has its own antineutrino. And all of those particles — neutrinos and antineutrinos — oscillate, meaning they change from one flavor to another. A muon neutrino can turn into a tau neutrino or an electron neutrino. A muon antineutrino can oscillate into tau or electron antineutrinos
Those oscillations take time, however. That’s why the T2K collaboration separated their neutrino beam generator and their water tank — known as the Super Kamiokande detector — by hundreds of miles. That gives the muon neutrinos the beam produces time as they travel to oscillate into electron neutrinos — the oscillation the collaboration studies.
Even when that happens though, the electron neutrinos are difficult to detect. Only rarely will an electron neutrino passing through Super Kamiokande smack into a water molecule and turn into an electron with its characteristic ring of faint, fuzzy light.
Still, Hartz said, with years of effort, firing their neutrino beam in short burst after short burst, Super Kamiokande’s submerged photon detectors have now seen hundreds of oscillations in the beam’s neutrino and antineutrino modes — enough to draw some real conclusions.
In a paper published today (April 15) in the journal Nature, the collaboration reported with 95% confidence a discrepancy between the neutrino and antineutrino beams — strong evidence that part of the matter-antimatter asymmetry comes from neutrinos.
The information here is limited, Hartz said. All that the collaboration directly measured is an asymmetry between the behaviors of faint, low-energy neutrinos. To fully understand the asymmetry and how it might have shaped the universe, he said, theorists will have to take their data and extrapolate it to higher-energy neutrinos and understand its implications for other leptons.
As for the T2K Collaboration, he said, the next step is to collect a lot more data and get the confidence level of their result up over 95%. Other, related efforts to build a bigger “Hyper Kamiokande” in that Japanese cavern, and a related US-based physics experiment known as the Deep Underground Neutrino Experiment (DUNE), could also accelerate the pace of research.
But this result has opened a first crack in a new door that could help explain this asymmetry from the beginning of time.
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