Daya Bay neutrino experiment publishes a new result on its first search for a “sterile” neutrino

The Daya Bay Collaboration, an international group of scientists studying the subtle transformations of subatomic particles called neutrinos, is publishing its first results on the search for a so-called sterile neutrino, a possible new type of neutrino beyond the three known neutrino “flavors,” or types. The existence of this elusive particle, if proven, would have a profound impact on our understanding of the universe, and could impact the design of future neutrino experiments.

The Daya Bay Experiment is situated close to the Daya Bay and Ling Ao nuclear power plants in China, 55 kilometers northeast of Hong Kong. These reactors produce a steady flux of antineutrinos that the Daya Bay Collaboration scientists use for research at detectors located at varying distances from the reactors. The collaboration includes more than 200 scientists from six regions and countries.

The Daya Bay experiment began its operation on 24 December 2011. Soon after, in March 2012, the collaboration announced its first results: the observation of a new type of neutrino oscillation—evidence that these particles mix and change flavors from one type to others—and a precise determination of a neutrino “mixing angle,” called θ₁₃, which is a definitive measure of the mixing of at least three mass states of neutrinos [1-3].

The fact that neutrinos have mass at all is a relatively new discovery, as is the observation at Daya Bay that the electron neutrino is a mixture of at least three mass states. And while scientists don’t know the exact values of the neutrino masses, they do know these particles are dramatically less massive than the well-known electron, a member of the same particle family.

These unexpected observations have led to the possibility that the electrically neutral, almost undetectable neutrino could be a special type of matter and a very important component of the mass of the universe. Given that the nature of matter and in particular the property of mass is one of the fundamental questions in science, these new revelations about the neutrino make it clear that it is important to search for other light neutral particles that might be partners of the active neutrinos, and may contribute to the dark matter of the universe.

The first search for sterile neutrinos was held in the experiment on the basis of the energy spectrum of detected reactor electron antineutrinos. But no evidence of the existence of the fourth mass neutrino state was obtained in the frames of the mass range under study [4].

The search was conducted by three independent groups. One of them is a group of physicists from the Dzhelepov Laboratory of Nuclear Problems of the Joint Institute for Nuclear Research (the “sterile” group included I.Butorov, M.Gonchar, D.Naumov). The scientists in the group developed their own software and methods of data analysis of the Daya Bay experiment and took an active part in the data analysis and the preparation of the paper of the collaboration. For over a year the joint work was very active. As a result, the findings of all three groups turned out to be in good agreement.

Thus, the result of the Daya Bay experiment is the best world limit on sterile neutrino parameters in a wide range of mass is in the agreement with the existing standard three-flavour picture of neutrino oscillations. It is one of the most important tasks in neutrino physics to check the existence of sterile neutrinos and determine their parameters. A big number of experiments are aimed at solving this task. The new result of the Daya Bay experiment considerably shortens the unexplored region.

D. Naumov

References

1. Daya Bay Collaboration (An F. P. et al.). Spectral Measurement of Electron Antineutrino Oscillation Amplitude and Frequency at Daya Bay // Phys. Rev. Lett. 2014. V. 112. P. 061801. arXiv:1310.6732.
2. Daya Bay Collaboration (An F. P. et al.). Improved Measurement of Electron Antineutrino Disappearance at Daya Bay // Chin. Phys. C. 2013. V. 37. P. 011001. arXiv:1210.6327.
3. Daya Bay Collaboration (An F. P. et al.). Observation of Electron-Antineutrino Disappearance at Daya Bay // Phys. Rev. Lett. 2012. V. 108. P. 171803. arXiv:1203.1669.
4. Daya Bay Collaboration (An F. P. et al.). Search for a Light Sterile Neutrino at Daya Bay // Phys. Rev. Lett. 2014. V. 113. P. 141802. arXiv:1407.7259.