Particle physicists have released the results of a decade-long search, taking us a crucial step closer toward understanding where all of the universe’s antimatter has gone.
The universe’s matter is can be divided into two classes—matter and antimatter—where each matter particle has an antimatter partner with the same mass and opposite electrical charge. But given the similarity between the two, physicists still don’t understand why the universe is dominated by matter. Experiments are working to find places where matter and antimatter behave differently, as part of an ongoing quest to understand this mystery. A project called the T2K collaboration in Japan has now published their findings. Their paper doesn’t confirm whether neutrinos differ from antineutrinos, but it does give us some important clues.
“This paper represents a very significant step,” Ed Blucher, co-spokesperson of the DUNE neutrino experiment from the University of Chicago who was not involved in this work, told Gizmodo. “It shows that the experiment has presented enough data to start making important constraints on this parameter. But it’s a first significant step on what’s likely a long road to establish definitively whether CP-symmetry is violated or not.” By CP-symmetry, he means whether or not neutrinos behave differently from antineutrinos.
If matter and antimatter were exactly the same and followed the laws of physics in exactly the same way, then there would be only photons in the universe, since matter and antimatter annihilate each other on contact. Decades ago, physicist Andrei Sakharov proposed three conditions that a process must meet in order to explain the excess of matter over antimatter, or, put simply, why stuff exists (more those conditions later). Perhaps the easiest of the conditions to hunt for is CP-symmetry violation, or physical processes that differ between a particle and the same particle’s mirror image (that’s the P, for parity) with the opposite charge (that’s the C, for charge). Basically, CP-violating processes are those that work differently between particles and their antiparticles.
Scientists have discovered some CP violation in the class of subatomic particles called quarks that make up protons and neutrons, but it’s still not enough to explain why there’s so much more matter in the universe. So they’re also hunting for CP violation is in leptons—the class of particles that includes electrons and neutrinos. Today, the T2K collaboration in Japan is releasing the results of data taken since 2010, hunting for evidence of a CP-violating process in neutrinos, the difficult-to-detect but most abundant matter particle in the universe. The experiment doesn’t confirm or deny whether neutrinos undergo a CP-violating process, but it gives scientists hope that an answer is coming soon and demonstrates that neutrinos likely do violate CP-symmetry.
T2K consists of a particle accelerator on Japan’s east coast, north of Tokyo, which creates a beam of neutrinos by colliding protons with a target. This then creates a beam of other particles that decay into a flavour of the neutrino called the muon neutrino. This beam passes into a detector that measures the neutrinos, then travels through the Earth and nearly 300 kilometres away into a detector called Super-Kamiokande, a tank containing 50,000 tons of water and sensitive tubes to detect the water-neutrino interactions.
Here, the team measures how many of the neutrinos have switched their flavour into electron neutrinos through a process called neutrino oscillation. Then, the T2K team switches the accelerator’s magnetic field, creating muon antineutrinos instead of muon neutrinos, allowing them to hunt for electron antineutrinos instead of electron neutrinos. Finally, they compare the results of the two measurements.
The complex physics and analysis required means that the result of this experiment isn’t a cut-and-dry answer. Instead, the outcome is an angle measurement, called the CP phase. If the CP phase measures zero, 180, or -180, then the neutrino does not violate CP-symmetry (i.e., things are the same between the neutrino and the antineutrino). If the angle measures anything else, it does violate CP symmetry—the laws of physics differ between neutrinos and their antiparticles. This new study strongly disfavors a wide swath of angles, including zero, but does not rule out 180. It also seems to imply that the best angle to explain the data is somewhere around -90, the maximum amount of CP symmetry violation, according to the paper published today in Nature. All this leans toward the conclusion that neutrinos and antineutrinos differ in some very important ways, but again, it’s not enough to know for sure.
“This result, for the first time, puts a strong constraint on the CP phase of the leptons by measuring neutrino oscillations, that is, by measuring muon neutrino to electron neutrino oscillations and muon antineutrino to electron antineutrino oscillations,” T2K spokesperson Atsuko Ichikawa told Gizmodo.
The statistics favour a scenario where neutrinos do violate CP-symmetry, but the experimental data isn’t conclusive yet. Things are getting closer. Perhaps more importantly, it shows that upcoming experiments like the LBNF/DUNE and Super-Kamiokande’s successor, Hyper-Kamiokande, will be able to give a more solid answer in the coming decade. These experiments will produce more powerful neutrino beams and will be coupled with more sensitive detectors, allowing scientists to take data and produce results much faster, Federico Sánchez, T2K international co-spokesperson, told Gizmodo.
With these projects, researchers will be able to gather in just one year the same amount of data that T2K would gather in 20 years. As is always the case in physics, more data will get scientists closer to the rigorous statistical threshold required to declare a discovery. Scientists will also need to better model the physical theory of how neutrinos interact with matter and with their detector, Sánchez said.
But even if neutrinos do violate CP, that won’t be the end of the story—it’s just one of Sakharov’s three conditions for explaining the matter-antimatter asymmetry mystery I mentioned before. Scientists must find other yet-to-be-discovered processes, such as lepton or baryon number violation—essentially, processes where core numbers describing neutrinos and protons change in yet-to-be-observed ways, like protons decaying or neutrinos annihilating themselves. And even then, theorists must find the right model in which these deviations actually lead to the differences between matter and antimatter observed in our universe. Observing CP violation in leptons as well as lepton number violation would provide circumstantial evidence that leptons were the culprit behind the universe’s matter-antimatter asymmetry, physics professor Silvia Pascoli at Durham University in the United Kingdom told Gizmodo.
And so, the search continues and likely will for decades to come. “You have to see this result as one small stone toward building a huge building,” Sánchez told Gizmodo. “It goes in the right direction, but it doesn’t uncover the mystery.”