An international group of researchers have published the results of the study that is an important step in exploring differences between matter and antimatter. Experts from the University of Wrocław are among those engaged in the Japanese experiment ‘T2K.’
T2K (Eng. Tokai to Kamioka) is an experiment within particle physics that studies neutrino oscillation. The study is being conducted in Japan and it involves around 500 physicists and engineers from more than 60 research institution in Europe, Asia and North America. It is also a renowned CERN experiment (RE13).
In the prestigious journal ‘Nature,’ there were published new results on the strongest constraint yet for the parameter that governs breaking the symmetry between matter and antimatter in combination with the mirror symmetry of the neutrinos oscillation. Using the beam of muon neutrinos and antineutrinos, T2K studied how these particles and antiparticles transform into electron neutrinos and antineutrinos. The parameter that describes breaking the matter/antimatter symmetry in the neutrinos oscillations, called ‘δCP phase,’ may take the values from the interval -180º to 180º. For the first time and with a high probability (99,7%), T2K rejected nearly half of the possible values of δCP, showing thereby a primary feature of neutrinos that had never been measured before. These results, based on the data collected by T2K in 2018, was published on 15 April in Nature, an interdisciplinary scientific journal.
– Our role is to work on the neutrino event generator Monte Carlo NuWro that was built solely in Wrocław. My associate, Dr Cezary Juszczak, played a crucial role in its construction. NuWro is a ‘laboratory’ for testing new theoretical models and for developing new methods of using them in numerical calculations, says Prof. Dr hab. Jan Sobczyk from the Neutrino Physics Division at the University of Wrocław.
Sobczyk’s multiannual collaboration with the Japanese started with breaking his leg.
– I went to Japan for the first time in 2001, he mentions. It was a NuInt conference, which was very important for me because I was changing the research area after my habilitation. A few days prior to the departure, I had broken my leg when walking on the pavement. My determination was so strong, though, that I flew out in pain and with my leg in a cast. Later, it turned out that the doctor had misjudged the injury and I shouldn’t have walked at all! The result: my leg was immobilized for two months… But, thanks to this mistake, I went to Japan and established new contacts, which I maintain to this day.
The outcomes that were published in Nature by the international team, including Jan Sobczyk, is an important step in the research on the difference between matter and antimatter. Most often, laws of physics provide symmetric, i.e. the same, description of conservation of matter and antimatter. However, this symmetry is not preserved in an universal way. Asymmetry effect between matter and antimatter is most apparent in observations of the Universe, which is mostly composed of matter and only a tiny amount of antimatter. Usually, it is assumed that, in the beginning of the Universe, matter and antimatter were created in equal amounts. In order for the Universe to reach the state of dominance of matter over antimatter, it was necessary to break the Charge-Parity Symmetry (CP). So far, the CP violation has been observed only for subatomic particles (quarks). However, the size of this effect is not sufficient enough in order to explain the dominance of matter over antimatter in the Universe. The T2K experiment is out for a new source of CP violation in neutrino oscillations that would manifest themselves as a difference in the probabilities of oscillation for neutrinos and antineutrinos.
In the T2K experiment, there is used a beam consisting mainly of muon neutrinos or antineutrinos, which is created with the use of proton beam at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai, Japan. A small fraction of these neutrinos or antineutrinos are detected in a distance of 295km at the Super-Kamiokande detector, which is situated under a mountain in Kamioka, close to the West-Coast of Japan. While muon neutrinos and antineutrinos transverse from Tokai to Kamioka (thus ‘T2K’), a certain amount starts to oscillate and change their type to electron neutrinos and antineutrinos. Neutrinos are detected in the Super-Kamiokande detector thanks to Cherenkov radiation that is emitted by particles broken in the neutrinos reaction. Switching the beam into the neutrino or antineutrino mode, it is possible to study their oscillation separately.
T2K team published the results of the analysis of the data gathered for the neutrino and antineutrino beam corresponding to 1,49×10²¹ and 1,64×10²¹ protons from the accelerator that are colliding with the shield. As a result of these collisions, there are created new particles that form a beam of neutrinos and antineutrinos. If the δCP equalled 0º or 180º, neutrinos and antineutrinos would change their type (from muon to electron) in the same way during the oscillation. However, the δCP parameter may have high values that strengthen the oscillation of neutrinos or antineutrinos, breaking thereby the CP symmetry. Still, notwithstanding a lack of CP violation, the number of cases of neutrinos and antineutrinos reactions will not be the same due to the fact that the detector and the beam are made of matter, not antimatter. In order to separate δCP from the disrupting effects of creating the beam and reacting with matter, there are applied adjustments in the analysis that are based on the data gathered from the ND280 near detector, placed 280m from the shield.
T2K observed 90 cases – candidates for electron neutrinos and 15 for electron antineutrinos. With the maximum amplification of neutrinos (δCP = -90º), it was expected to get 82 candidates for electron neutrinos and 17 for electron antineutrinos, whereas with the maximum amplification of antineutrinos (δCP = +90º): 56 electron neutrinos and 22 electron antineutrinos. Data from T2K best suits the value of δCP close to -90º, which significantly increases the probability of the oscillation neutrinos. Using this data, T2K set the confidence interval for the δCP. A range from -2º to 165º was excluded at the confidence level 3σ (99,7%). So far, this result is the strongest constraint for the δCP. Values 0º and 180º are rejected at 95% confidence level, much like the previous T2K results published in 2017. It suggests that the CP symmetry might be broken during the neutrinos oscillation.
In order to enhance sensitivity of the experiment to the effects of CP violation, J-PARC will increase the intensity of the proton beam, and T2K cooperation will modernise the ND280 near detector. Both modifications will not only allow us to gather more data but, also, it will increase the measurement accuracy.
The T2K experiment is supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and it uses the research facilities of the High Energy Accelerator Research Organization (KEK) and the Institute for Cosmic Ray Research (ICRR), being part of the University of Tokyo. The experiment has been designed and built by the collaboration of 500 researchers from 68 institutions in 12 countries (France, Spain, Japan, Canada, Germany, Poland, Russia, Switzerland, UK, Vietnam, Italy and USA.)
More information about the experiment is to be found on the T2K website.
 hab. – Polish equivalent of postdoctoral academic degree.