Clyde Cowan and Frederick Reines, nuclear-weapons physicists, referred to the neutrino as “the smallest amount of material reality ever envisaged by man.” That was said in a commentary for Nature in 1956, which was released a short time after a study announcing the experimental discovery of neutrinos was published in Science. These subatomic particles have relatively little interaction with other types of matter; therefore, they lack an electrical charge and are very difficult to detect. They pondered the connection between neutrinos and antineutrinos, their opposites. With the advantage of hindsight, the question ended up being quite significant. Researchers in this week’s issue of Nature make the direct successor to Cowan and Reines’s suggestion that one of the greatest mysteries of the Universe might be explained by differences between neutrinos and antineutrinos.
Every particle of matter in the early Universe should have been generated paired with an antimatter counterpart around 13.8 billion years ago, at the moment of the Big Bang. The antithesis of matter in terms of physical properties, such as electrical charge, describes antimatter, which is identical to matter. At least, it is what the theories of today suggest. Why there appears to be so much more matter than antimatter in the present Universe is the big unsolved puzzle for physicists. The Universe would have been completely made up of photons and dark matter if there had been equal amounts of both particles, thus it’s a good thing there aren’t equal amounts of either.
The Russian physicist and human rights activist Andrei Sakharov postulated a mechanism for how the equilibrium, or symmetry, between matter and antimatter may have come to be broken, ten years after Cowan and Reines discovered the neutrino. One of Sakharov’s proposed explanations was that they each had somewhat distinct characteristics and had imperfect symmetry. This distinction may have caused an excess of matter during the cooling that followed the Big Bang. Was Sakharov accurate, though? He may have been, according to a particle physics experiment called Tokai to Kamioka, or T2K, which is being conducted by a large multinational team of researchers.
The Japan Proton Accelerator Research Complex (J-PARC) near Tokai, Japan’s east coast, produces neutrinos for the T2K experiment. From there, they are launched underground and travel 295 kilometers to the west coast, where the Super-Kamiokande neutrino observatory is located. The focal point of the observatory is a massive water tank surrounded by tens of thousands of detectors ready to record the light produced when neutrinos interact with the water. These kinds of experiments take years to complete since neutrinos have a very slim chance of interacting, making it difficult for researchers to make meaningful findings. Just 90 neutrinos and 15 antineutrinos, out of the about 1020 possible neutrino-generating collisions at J-PARC, were detected by T2K in a decade.
The T2K collaboration calculated the likelihood that a neutrino will alternate between several physical characteristics, or “flavours,” while traveling. After that, the researchers repeated the experiment using antineutrinos and compared the results. The odds should be the same if matter and antimatter are fully symmetrical. However, the findings imply that they are not. In contrast to what would be predicted if they behaved identically, T2K observed a larger probability for neutrinos to change flavor throughout their 300-km journey and a correspondingly lower probability for antineutrinos.
If this discovery is validated, it would support Sakharov’s theory from 1967 that matter and antimatter have unique properties. The new discovery, however, does not meet the 5-sigma (5) level of confidence that particle physicists normally require to classify the outcome as a discovery. If matter-antimatter symmetry is to be completely ruled out, the statistical significance of the current T2K results drops from 3 to 2. Nevertheless, it’s critical to publish such foundational work as it develops. Results that are not yet at the 5 significance play a critical role in guiding the community’s choices on future investments because particle physics experiments can take decades to plan and construct.
The researchers might have had more time to wait. But even if they did, it’s unclear that the T2K experiment would have offered the extra information needed to cross the 5-meter finish line. Scientists will require information from the following generation of neutrino detectors to reach 5. Fortunately, three such detectors are scheduled to go online: JUNO in China, which intends to be the first of the three to go live, in 2022; Hyper-Kamiokande, which is close to Super-Kamiokande and is anticipated to start in 2027; and DUNE in the United States. Time will tell if these initial findings are accurate. The necessity of continuing to look for solutions to some of the Universe’s greatest riddles is underscored by this result, which comes at a time when significant investments in high-energy physics are coming under heightened scrutiny.