Physicists have succeeded in increasing the accuracy of measurements of the difference in the oscillation speed of a neutrino and its antiparticle, an antineutrino, discovered about 10 years ago. Now the probability that this difference is due to random factors is below 1%. This difference, or as scientists call it the asymmetry of neutrinos and antineutrinos, may become a key fact explaining why there is much more matter in our Universe than antimatter. The measurements were carried out on the T2K setup and published in the latest issue of the journal Nature.
Neutrinos are the lightest and, at the same time, extremely weakly interacting with the surrounding matter, particles in abundance are born in nuclear reactions on the Sun and in nuclear reactors. Trillions of these particles pass through our body every second, nevertheless, their registration requires the construction of gigantic detectors filled with tons of water.
At the moment, the existence of three types of neutrinos, also called flavors, is known: electron, muonic and tau neutrinos. An amazing feature of these particles is the possibility of spontaneous transformation of one type of neutrino into another type of neutrinos. This process is called neutrino oscillations.
As part of the T2K (Tokai-to-Kamioka) experiment, muon neutrinos and antineutrinos are produced at the proton accelerator at J-PARC near the city of Tokai and are directed towards the Super-Kamiokande neutrino detector located at a distance of 295 km near the city of Kamioka. This detector is an underground tank filled with 50,000 tons of clean water, surrounded by thousands of detectors. When neutrinos pass through water, there is a small probability that a nuclear reaction will occur with the emission of gamma photons, which are recorded by the detectors.
The detector is capable of registering electron neutrinos and antineutrinos, and since there are none in the initial beam, it can measure how high the probability of oscillations for the transition between muonic and electron neutrinos is. However, due to the small distance between the neutrino source and the detector, this probability is low. Therefore, over the period from 2009 to 2018, only a few dozen such events were detected.
Nevertheless, this was enough to notice a significant difference in the oscillation rates of neutrinos and antineutrinos. If it were absent, the detector should have recorded about 68 electron neutrinos and about 20 antineutrinos. While his measured ratio was 90 to 15.
From a theoretical point of view, the difference between particles and antiparticles is due to the violation of the so-called charge parity, also called CP-symmetry. This symmetry states that particles and antiparticles should behave identically when mirroring the processes taking place. Its violation was discovered back in the 1960s in experiments with heavy particles.
And in 1967 A.D.Sakharov showed that a sufficiently strong violation of the CP-symmetry could naturally explain why there is practically no antimatter in the observable Universe. This problem of the asymmetry of matter and antimatter is one of the fundamental in modern cosmology. And it has not yet been resolved, since the CP asymmetry of heavy particles turned out to be too weak.
If neutrinos really have strong asymmetry, this, in principle, could explain the prevalence of matter in the Universe, but this requires confirmation of one more hypothesis. The fact is that neutrinos are still too light and interact too weakly with other particles, so the existence of their heavier analogue with similar properties is required. And such particles, it turns out, can exist.
Neutrinos have one more peculiarity: they are all “left-twisted”, that is, their own angular momentum, spin, is always directed along the direction of motion, while for antineutrinos it is vice versa. This allows us to assume that the neutrino has a partner particle with the opposite property. Such particles appear in some theories that extend the Standard Model of elementary particles, and must have a huge mass. Due to such a large mass, it has not yet been possible to create them in experiments, but they could exist in the dense and hot early Universe. In it, they would quickly decay into other particles, but due to the asymmetry, the particles and antiparticles would decay in different ways, which would lead to the emergence of asymmetry between matter and antimatter.
Nevertheless, despite the high statistical reliability of the results obtained, which exceeded the 3σ level, a 5σ level is required for unambiguous recognition of the discovery, and the T2K experiment will not be able to reach it. Even combining the results with another similar experiment NOvA, running in parallel in the United States, will not help. And all hope now is for the future, even larger DUNE and T2HK installations, which will not begin work until the end of the 2020s, and the required number of events will be registered only by the mid-2030s.
By the way, the possible violation of CP symmetry is not the only reason why neutrino physics can bring a breakthrough in our understanding of nature. I recently compiled a list of five more reasons to pay more attention to this area of science.
- Silvia Pascoli & Jessica Turner, Matter – antimatter symmetry violated // Nature News & Views
- Natalie Wolchover, Neutrino Asymmetry Passes Critical Threshold // Quanta Magazine