First joint data analysis on measuring neutrino oscillations at the NOvA and T2K experiments published in Nature
The result of the multi-year work of the NOvA and T2K collaboration presented in Nature is a wonderful example of fruitful cooperation, an important milestone and solid basis for further joint work. This analysis is one of the most precise results of measuring parameters of neutrino oscillations in the world.
“These results are an outcome of a cooperation and mutual understanding of two unique collaborations, both involving many experts in neutrino physics, detection technologies and analysis techniques, working in very different environments, using different methods and tools,” says T2K collaborator Tomáš Nosek.
Negotiations between the NovA and T2K collaborations about joint analysis began almost right after obtaining first results. The first meeting of specialists of both collaborations took place in 2017. Seven years passed from the idea of the joint analysis to the announcement of its first results. This duration is explained not only by the complexity of the analysis but also by necessity of thorough discussion of all details in the collaborations that is connected with the initiative being unprecedented. The main technical difficulty is to differ approaches to simulating and data analysis that was exacerbated by large amount of correlating systematic uncertainties. It is still the most difficult element of this joint analysis. In case of fundamentally different facilities, uncertainties of experiments are independent and this kind of a collaboration would not bring any difficulties.
The objectives of the NOvA and T2K accelerator experiments are measuring parameters of neutrino oscillations, and, especially, CP-violation and mass neutrino hierarchy. Mass hierarchy plays a significant role in simulating neutrino fluxes at supernova explosions, and it is important also for evaluation of sensitivity of many experiment on searching for neutrinoless double beta decay. Mass hierarchy is an input parameter for experiments on direct measurement of neutrino masses and search for relic neutrinos. The phase of the CP-invariance violation in lepton sector in some models is associated with a very important phenomenon – the emergence of asymmetry of matter and antimatter in the Universe.
Both the design of the NOvA and T2K experiments and their goals are very similar. However experiments do not only compete but also complement each other: due to longer basis of oscillations, the NOvA experiment is more sensitive to neutrino mass hierarchy, and, taking into account known ordering of neutrino masses, turns out to be sensitive to $\delta_{CP}$ in a certain range of its values. Both NOvA and T2K neutrino accelerator experiment work with (anti)neutrino beam acquired as a result of interaction of accelerated proton beam and motionless target. Two detectors are used in both experiments: near and far detectors. The NOvA experiment is located in the Fermi National Accelerator Laboratory (USA). The near detector is 1km away from the target and is used for measuring the initial flavour neutrino beam composition. Next, the beam travels a distance of 809 km, and the final neutrino flavor composition is detected in the far detector. The T2K experiment is located on the site of the J-PARC proton accelerator complex (Japan). The neutrino flux is detected in two detector complexes, separated by a distance of 295 km.
The statistics of neutrino events used in the joint analysis of data from the two experiments, as well as the state of each individual data analysis approach (event selection, data approximation methods, etc.), are current as of 2020. The individual experimental results show a discrepancy with a small (< 2σ) statistical significance. Specifically, for the normal neutrino mass ordering, the NOvA experiment does not show an asymmetry between the appearance of electron neutrinos and antineutrinos, while in the T2K experiment, this asymmetry is observed. As a result, the values of $\delta_{CP}$ obtained in the two experiments are different. This fact has stimulated a number of explanations involving physics beyond the Standard Model, for example, by allowing for non-standard neutrino interactions. Remarkably, however, the experiments are in agreement under the assumption of the inverted mass ordering.
The results of the joint analysis are more precise compared to the results of each experiment individually. For example, the obtained combined value of $\Delta m_{32}^{2}$ is currently the most precise. For the still unknown parameters of the neutrino mass ordering and CP violation in the lepton sector, the joint analysis yielded a slight preference for the inverted neutrino mass ordering. In this case, a range of $\delta_{CP}$ values, including those corresponding to the absence of CP violation, are excluded at the > $3\sigma$ level. "The joint analysis allows us to obtain more precise results than each experiment separately," says Liudmila Kolupaeva, a member of the NOvA collaboration, candidate of sciences (physics and mathematics), deputy head of the Department of Particle Physics, "As a rule, high-energy physics experiments have different designs, even if they pursue the same scientific goal. The joint analysis allows us to use their complementary features."
For reference:
JINR has been participating in the NOvA experiment since 2014. During this time, staff of the Department of particle Physics at DLNP have made a substantial and multifaceted contribution to it. Methodological studies were used to refine a number of systematic uncertainties and to improve detector simulation. Members of the group conduct research on the study of atmospheric muon fluxes, the search for magnetic monopoles and neutrinos from supernova explosions, and the detection of atmospheric neutrinos. For many years, physicists from JINR have played an important role in the main analysis of the NOvA experiment aimed at studying three-flavor neutrino oscillations. The experience gained has also been applied in the joint analysis of the NOvA and T2K experiments for measuring neutrino oscillation parameters.
JINR has been participating in the T2K experiment since 2020. Physicists from Department of Multiple Hadron Processes and the Department of Research and Innovation conduct methodological research, participate in simulating neutrino fluxes using measurements made in the SHINE experiment (CERN), and have already contributed to the assembly of the experiment's new detector target.
Based on the article by L. D. Kolupaeva and A. G. Olshevsky:
https://www.jinr.ru/posts/pervyj-sovmestnyj-analiz-dannyh-eksperimentov-nova-i-t2k/
The article by L. D. Kolupaeva about publication in Nature can be found on the JINR website.
