Daya Bay Experiment: Sterile neutrinos remain elusive

Antineutrino detectors in Daya Bay Hall #3. 

The detectors rest in a pool being filled with ultra-pure water. 

Credit: University of California - Lawrence Berkeley National Laboratory

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.

Daya Bay and Ling Ao Power Plants

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 flavours from one type to others-and a precise determination of a neutrino "mixing angle," called ?13, which is a definitive measure of the mixing of at least three mass states of neutrinos.

Shown here are the photomultiplier tubes in the Daya Bay detectors. 

Photo by Roy Kaltschmidt, Berkeley Lab

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.

Search for a light sterile neutrino
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 flavours, the sterile neutrino also exists as a mixture of different masses, it would lead to mixing of neutrinos from known flavours to the sterile flavour, thus giving scientists proof of its existence.

That proof would show up as a disappearance of neutrinos of known flavours.

Measuring disappearing neutrinos isn't as strange as it seems. In fact that's how Daya Bay scientists detect neutrino oscillations.

The scientists count how many of the millions of quadrillions of electron antineutrinos produced every second by the six China General Nuclear Power Group reactors are captured by the detectors located in three experimental halls built at varying distances from the reactors.

The detectors are only sensitive to electron antineutrinos. Calculations based on the number that disappear along the way to the farthest reactor give them information about how many have changed flavours.

More information: "Search for a Light Sterile Neutrino at Daya Bay" Physical Review Letters, journals.aps.org/prl/abstract/… ysRevLett.113.141802

Mysterious electron Cluster found hidden among Van Allen belts

This NASA rendering depicts Earth's Van Allen radiation belts and the path of the Van Allen Probe spacecraft, which were launched in August 2012. 

Data from the spacecraft have confirmed a never-before-seen phenomenon-a long-lived zone of high-energy electrons residing between the inner and outer radiation belts. (Credit: NASA illustration)

U.S. researchers, including a trio from Los Alamos National Laboratory, have witnessed the mysterious appearance of a relatively long-lived zone of high-energy electrons stored between Earth's Van Allen radiation belts.

The surprising findings, discovered by NASA's Van Allen Probes (formerly known as the Radiation Belt Storm Probes), were outlined Thursday in Science Express and during a press conference at NASA headquarters in Washington, D.C.

The research was led by Dan Baker of the University of Colorado, Boulder, Laboratory for Atmospheric and Space Physics.

"Nature keeps on surprising us by producing long-lived harsh environments in space in regions not previously considered," said Los Alamos plasma physicist Reiner Friedel of LANL's Intelligence and Space Research Division. "This finding may impact the planning of future space missions."

The Van Allen radiation belts - named in honor James Van Allen, who discovered them nearly 50 years ago - are a pair of donut shaped zones of charged particles that surround Earth and occupy the inner region of our planet's Magnetosphere.

The outer belt contains extremely high-energy electrons, while the inner belt is comprised of energetic protons and electrons.

The belts have been studied extensively since the dawn of the Space Age, because the high-energy particles in the outer ring can cripple or disrupt spacecraft. Long-term observation of the belts have hinted that the belts can act as efficient and powerful particle accelerators; the recent observations by the Van Allen Probes-a pair of spacecraft launched in August 2012-now seem to confirm this.

Shortly after launch, the spacecraft activated their Relativistic Electron-Proton Telescope (REPT) instruments to measure particles within the belts and their immediate environs.

The instrument immediately detected on September 1, 2012, the presence of a stable zone of high-energy electrons residing between the belts. This donut-shaped third ring nestled between the belts existed for nearly a month before being obliterated by a powerful shockwave of particles emanating from center of the solar system.

Such a distinct, long-lasting ring of high-energy electrons had never before been seen by any prior instrument in space or on Earth.

The findings suggest that the Van Allen Belts somehow capture and store energetic electrons in a circular path around our home planet, perhaps in much the same way as a cyclotron can capture and store charged particles here on Earth.

"One of the main reasons the Van Allen Probe instruments are seeing these new features are their unprecedented sensitivity and rejection of backgrounds," Friedel said.

"As the mission proceeds, we expect further surprises that will challenge our conventional wisdom on the transport, loss and energization processes in these highly energetic electron radiation regions."