Disappearing particles and the search for sterile neutrinos at Daya Bay: a PRL Editors' suggestion

The Daya Bay Collaboration has announced the first results in its search for the “sterile” neutrino.  Unlike the three known neutrino “flavors” or types, electron, muon, and tau, which are responsible for mediating the weak interaction, this conjectured fourth flavor cannot participate in the weak interaction.

While there is strong theoretical motivation for the existence of this fourth flavor, experimental evidence remains elusive: scientists on the Daya Bay Collaboration found no evidence for the existence of sterile neutrinos within a significant, previously unexplored mass range.

The Daya Bay Collaboration, a multinational group of about 230 scientists from 41 institutions and four continents, is situated at the powerful Daya Bay nuclear reactors near Hong Kong, China. Having accumulated one of the largest sample datasets of neutrinos in the world, the Daya Bay experiment is the world’s most sensitive reactor neutrino experiments, designed to study the most subtle transformations of the neutrino.

The Daya Bay team at the University of Illinois at Urbana Champaign is led by Professor Jen-Chieh Peng and includes Dr. Jiajie Ling, and graduate students En-Chuan Huang and Jason Dove. Two recent physics doctoral program graduates, Ry Ely and Daniel Ngai, also contributed to the experiment.

Ling, a postdoctoral researcher at Physics Illinois, led the data analysis effort on the current study and is the corresponding author of the article, “Search for a Light Sterile Neutrino at Daya Bay”, which will appear in an upcoming issue of Physical Review Letters and has been designated a “PRL Editors’ suggestion.” (published online on October 1, 2014).

Neutrinos and their antiparticles are weakly interacting elementary particles that lack electric charge; while their exact masses remains a mystery, they are much lighter than electrons. And, they are everywhere—travelling at nearly the speed of light. Neutrinos and antineutrinos are produced in high-energy particle collisions at nuclear power stations, at particle accelerators, in the explosion of nuclear bombs, and in stars.

The importance of neutrinos to our understanding of the history of the universe cannot be understated: among all subatomic particles, only neutrinos can directly convey astronomical information over vast cosmological distances, because they are weakly interacting and so are unabsorbed as they travel across the universe. This same property makes neutrinos very challenging to detect and study.

In 2012, the collaboration announced the observation 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 experiment uses six electron antineutrino detectors located at distances ranging from a few hundred to almost two thousand meters from six nuclear reactors. It generates huge quantities of data through the detection of millions of quadrillions of electron antineutrinos. When antineutrinos transform into a different flavor before they reach one of the detectors, the observed deficit indicates neutrino oscillations.

Ling explains, “The Daya Bay experiment was originally designed for a most sensitive search for the mixing angle θ₁₃ for active neutrinos. After measuring θ₁₃ with high precision, we then realized that the same data could also be analyzed to search for sterile neutrinos.”

“It is hard enough to detect ordinary neutrinos which hardly interact, and it is much more challenging to search for sterile neutrinos which might not even exist,” adds Peng. “This latest Daya Bay result hopefully would guide us toward even more sensitive searches in the future.”