The T2K experiment in Japan and the NOvA experiment in the United States conducted a joint analysis of data on neutrino oscillations and published their initial findings in the prestigious journal Nature. Dr Tomáš Nosek associated with the National Centre for Nuclear Research (NCBJ) made a significant contribution to this work.

It is widely accepted that at the beginning of the Universe, the amount of matter and antimatter was equal. However, in this case, there should have been a complete annihilation of matter and antimatter. Considering our existence and that of the entire Universe around us, this did not happen – for an unknown reason, matter won over antimatter. One of the leads that scientists are currently pursuing is the peculiar behaviour of particles called neutrinos. They are abundant, but interact very rarely with other matter, which makes them difficult to detect. During their "journey" from the place where they were created to the place where we detect them, neutrinos can change their type (known as their "flavour"). Scientists suspect that this property may be related to the lack of symmetry between matter and antimatter in the Universe.

One of the still unknown properties of neutrinos is their mass hierarchy, i.e. determining which neutrino is the lightest. The masses of neutrinos themselves are extremely small (less than 10^-37 kg), and additionally, they are not directly related to the type (flavour) of neutrino. Each neutrino flavour consists of a mixture of three masses, and each mass can behave like any flavour with varying probabilities. There are two possible neutrino mass hierarchies – normal (in which two masses are significantly smaller than the third) and inverted (in which two masses are significantly larger than the third). Depending on the hierarchy adopted, the probabilities of neutrino oscillations change, but these differences may also result from the violation of charge-parity symmetry (CP symmetry), i.e. differences between particles and antiparticles. To study these phenomena, large experiments involving hundreds of scientists from around the world have been established.

The Japanese T2K (short for "Tokai-to-Kamioka") and the American NOvA (short for "the NuMI Off-axis ν_e Appearance") are two currently existing long-base neutrino experiments. They use an intense beam of neutrinos that pass through a detector placed close to the beam source and a detector located several hundred kilometres away, allowing for a comparison of data from both detectors, i.e. changes in the spectrum and composition of neutrino beams. The experiments differ in the length of the so-called base, i.e. the distance between the place where the neutrino beam is generated and the place where it is recorded, and they also study different neutrino energy ranges. As a result, combining data from these two locations has made it possible to determine the difference between neutrino masses with extreme precision – with an uncertainty of less than 2%. The research also shows that the degree of CP symmetry breaking may be severely limited. A paper presenting these results was published today in the journal Nature. - These results are the outcome of bilateral cooperation and studies by two teams involving many experts in the field of neutrino physics, detection techniques and data analysis, working on very different experiments and using different research methods and tools - says T2K experiment participant Dr Tomáš Nosek associated with the National Centre for Nuclear Research. 

The work of Dr. Nosek, employed at NCBJ as part of the NCN Sonata Bis project and still collaborating with the Institute, was crucial to the analysis published today. Polish scientists have made a significant contribution to the international T2K neutrino experiment. The Polish Neutrino Group, which includes researchers from the National Centre for Nuclear Research, the University of Warsaw, the Warsaw University of Technology, the University of Silesia, the University of Wrocław and the Institute of Nuclear Physics of the Polish Academy of Sciences, has been participating in this experiment since 2007. Our scientists, physicists and engineers participated in the construction of the T2K near detector, as well as in data collection, simulation, calibration and data quality testing. Polish scientists were also involved in analysing the collected data. The importance of this group is underlined by the significant roles that researchers from Poland have played and continue to play in the experiment's governing bodies (Executive Board, Analysis Steering Group, Publication Board) and as coordinators of working groups.

Although the new, combined results do not provide a clear answer as to which neutrino mass hierarchy is correct, they have expanded physicists' knowledge of these extraordinary particles. They have also demonstrated the value of collaboration between experiments that can complement each other. This work provides a basis for future experiments, such as Hyper-Kamiokande (the successor to Super-Kamiokande) currently under construction in Japan. - As today's analysis shows, there are no “competing” experiments; they all share a common scientific goal, which is to study the phenomenon of oscillation - says Dr. Nosek. - Cooperation is naturally important for the transfer of knowledge, methodology and experience, and the exchange of resources, ideas and tools. The collaboration between T2K and NOvA is not just the simple sum of the results of T2K and NOvA. It is much, much more than that. - If the next generation of large neutrino experiments collaborate, as NOvA and T2K have done, much greater progress will be possible in finding answers to questions about the properties of neutrinos, symmetry breaking and the origins of our Universe.

Further details on the experiments involved in the research are available in the press release: 

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