Hide & Seek: Sterile Neutrinos Remain Elusive

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 new results, appearing in the journal Physical Review Letters, show no evidence for sterile neutrinos in a previously unexplored mass range.

There is strong theoretical motivation for sterile neutrinos.  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 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 December 24, 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.

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.  While scientists don't know the exact values of the neutrino masses, they are able to measure the differences between them, or “mass splittings.” They also know that these particles are dramatically less massive than the well-known electron, though both are members of the family of particles called “leptons.” 

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 new Daya Bay paper describes the search for such a light neutral particle, the “sterile neutrino,” by looking for evidence that it mixes with the three known neutrino types—electron, muon, and tau. If, like the known flavors, the sterile neutrino also exists as a mixture of different masses, it would lead to mixing of neutrinos from known flavors to the sterile flavor, thus giving scientists proof of its existence. That proof would show up as a disappearance of neutrinos of known flavors.

The rate at which they transform is the basis for measuring the mixing angles (for example, θ13), and the mass splitting is determined by how the rate of transformation depends on the neutrino energy and the distance between the reactor and the detector. 

That distance is also referred to as the “baseline.” With six detectors strategically positioned at three separate locations to catch antineutrinos generated from the three pairs of reactors, Daya Bay provides a unique opportunity to search for a light sterile neutrino with baselines ranging from 360 meters to 1.8 kilometers.

Daya Bay performed its first search for a light sterile neutrino using the energy dependence of detected electron antineutrinos from the reactors. Within the searched mass range for a fourth possible mass state, 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.  

Jun Cao, co-spokesperson, IHEP, +86-10-88235808, caoj@ihep.ac.cn

Kam-Biu Luk, co-spokesperson, Lawrence Berkeley National Laboratory and UC Berkeley, 510-486-7054, 510-642-8162, k_luk@lbl.gov

The collaborating institutions of the Daya Bay Reactor Neutrino Experiment are Beijing Normal University, the U.S. Department of Energy's (DOE) Brookhaven National Laboratory, California Institute of Technology, Charles University in Prague, Chengdu University of Technology, China General Nuclear Power Group, China Institute of Atomic Energy, Chinese University of Hong Kong, Dongguan University of Technology, East China University of Science and Technology, Joint Institute for Nuclear Research, University of Hong Kong, Institute of High Energy Physics, Illinois Institute of Technology, Iowa State University, DOE's Lawrence Berkeley National Laboratory, Nanjing University, Nankai University, National Chiao-Tung University, National Taiwan University, National United University, National University of Defense Technology, North China Electric Power University, Princeton University, Pontifical Catholic University of Chile, Rensselaer Polytechnic Institute, Shandong University, Shanghai Jiao Tong University, Shenzhen University, Siena College, Temple University, Tsinghua University, University of California at Berkeley, University of Cincinnati, University of Houston, University of Defense Technology, University of Illinois at Urbana-Champaign, University of Science and Technology of China, Virginia Polytechnic Institute and State University, University of Wisconsin-Madison, College of William and Mary, Xi'an Jiao Tong University, Yale University, and Sun Yat-Sen (Zhongshan) University.

For more information, visit http://dayabay.ihep.ac.cn/ 

Brookhaven Lab's role in the Daya Bay experiment is supported by the DOE Office of Science (HEP). A complete list of funding agencies for the experiment can be found in the published paper.

Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.  For more information, please visit science.energy.gov.

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