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The Daya Bay Collaboration, an international group of scientists studying the subtle transformations of subatomic particles called neutrinos, announced new results today (2 October) about the search for sterile neutrinos, a possible new type of neutrinos 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 new results, appeared in the journal Physical Review Letters, show no evidence for sterile neutrinos in a previously unexplored mass range. Because of its importance, the paper has been selected to be a featured article in Editors’ Suggestion. The Daya Bay Collaboration includes more than 200 scientists from six regions and countries. A research team from The Chinese University of Hong Kong (CUHK) and The University of Hong Kong (HKU) has been an active member of the Daya Bay Collaboration since its formation in 2004, and Prof. Chu Ming-chung of CUHK is the principal investigator of the Hong Kong team. 

There are strong theoretical motivations for the existence of sterile neutrinos.  Unlike the three known types – electron, muon, and tau, the sterile neutrino does not have the conventional weak interaction.  Interacting only through gravity, the sterile neutrino is a popular candidate of dark matter – the dominant form of matter in the universe. Yet, the experimental landscape is unsettled—several experiments have hinted that sterile neutrinos may exist, but the others yielded null results.  Having amassed one of the largest samples of neutrinos in the world, the Daya Bay Experiment is poised to shed light on the existence of sterile neutrinos.

The new Daya Bay paper describes the search for a hypothetical fourth type of neutrinos, sterile neutrinos, by looking for the disappearance of electron antineutrinos produced from the reactors.  If, like the known flavors, the sterile neutrino also exists as a mixture of different masses, it would lead to mixing of neutrinos from the electron flavor to the sterile flavor, thus giving scientists proof of its existence. Within the searched mass range, Daya Bay found no evidence for the existence of a sterile neutrino. 

This data represents the best world limit on sterile neutrinos over a wide range of masses and so far supports the standard three-flavor neutrino picture. Given the importance of clarifying the existence of the sterile neutrino, there are continuous quests by many scientists and experiments. The Daya Bay’s new result remarkably narrowed down the unexplored area.