A Big Discovery in the Study of Neutrinos, Tiny Particles that Have a Big Role in the Universe

An international team of physicists has determined a key parameter, which governs how neutrinos behave. This discovery measures a critical linchpin in the study of the tiny particles and in advancing the understanding of how these building blocks of all things, from galaxies to tea cups, came to be.

The Daya Bay Reactor Neutrino Experiment, a multinational collaboration including a team from Virginia Tech, discovered a new type of neutrino oscillation in which the particles appear to vanish as they travel. The researchers found that the rate of oscillations was much larger than many scientists had expected. This surprising result could open the gateway to a new understanding of fundamental physics and may eventually solve the riddle of why the universe today is dominated by matter as opposed to antimatter.

Neutrinos can be one of three types, which physicists call flavors. Owing to their bizarre physical - quantum mechanical - nature neutrinos can mix or oscillate between flavors. The rate of oscillation is controlled by parameters known as mixing angles.

The Daya Bay researchers gathered data that allowed them to measure the mixing angle theta one-three (θ13) with unmatched precision. Theta one-three, the last of three mixing angles to be measured, controls the rate at which electron neutrinos mix.

"This is the first time that any experiment has been able to definitively say that this mixing angle, theta one-three, is not zero," said Jonathan Link, associate professor of physics and director of Virginia Tech's Center for Neutrino Physics, home of the university's Daya Bay experiment team.

The Daya Bay collaboration's first results, which measured the mixing angle as part of the expression sin2 2 θ13, and found it to be equal to 0.092 plus or minus 0.017.

Neutrinos, the wispy particles that flooded the universe in the earliest moments after the big bang, are continually produced in the cores of stars and other nuclear reactions. Untouched by electromagnetism, they respond only to weak nuclear force and even weaker gravitational force, passing mostly unhindered through everything from planets to people. The challenge of capturing these elusive particles in the act of mixing inspired the Daya Bay collaboration in the design and precise placement of its detectors.

Traveling at close to the speed of light, the three basic neutrino "flavors" - electron, muon, and tau, as well as their corresponding antineutrinos - mix together in a process scientists refer to as oscillations but this process is extremely difficult to detect.

Collecting data from Dec. 24, 2011, until Feb. 17, 2012, scientists in the Daya Bay collaboration observed tens of thousands of interactions of electron antineutrinos in six massive detectors buried in the mountains adjacent to the powerful nuclear reactors of the Daya Bay Nuclear Power Plant in south China. These reactors produce millions of quadrillions of the elusive electron antineutrinos every second.

"Although we're still two detectors shy of the complete experimental design, we've had extraordinary success in determining the number of electron antineutrinos that disappear as they travel from the reactors to the detectors two kilometers away," said Kam-Biu Luk of the U.S. Department of Energy's Lawrence Berkeley National Laboratory and the University of California at Berkeley. Luk is co-spokesperson of the project and heads U.S. participation. "What we didn't expect was the sizable disappearance, equal to about 6 percent. Although vanishing has been observed in other reactor experiments over large distances, this is a new kind of disappearance for the reactor electron antineutrino."

The Daya Bay experiment counts the number of electron antineutrinos observed in detectors placed near to the reactors and calculates how many would reach the detectors placed further away if there were no oscillations. The number of antineutrinos that appear to vanish on the way due to their oscillation into other flavors determines the value of theta one-three.

"Even with only the six detectors already operating, we have more target mass than any similar experiment, plus as much or more reactor power," said William Edwards of Berkeley Lab and UC Berkeley is the U.S. project and operations manager for the Daya Bay experiment. Since Daya Bay will continue to have an interaction rate higher than any other experiment, Edwards said, "It is the leading theta one-three experiment in the world."

In the future, the initial results will be honed by collecting extensive additional data and reducing statistical and systematic errors.

"The large value of theta one-three opens up the opportunity for the scientific community to learn a great deal about the universe through neutrinos," said Deb Mohapatra, a Virginia Tech research scientist in the Center for Neutrino Physics.

The consortium researchers will be expanding the Daya Bay facilities for further experiments aimed at learning more about how neutrinos behave.

"The Daya Bay experiment plans to stop the current data-taking this summer to install a second detector in the Ling Ao Near Hall, and a fourth detector in the Far Hall, completing the experimental design," said Yifang Wang of China's Institute of High Energy Physics and co-spokesperson of the Daya Bay experiment.

Refined results will open the door to further investigations and influence the design of future neutrino experiments - including how to determine which neutrino flavors are the most massive, whether there is a difference between neutrino and antineutrino oscillations, and, eventually, why there is more matter than antimatter in the universe. Matter and antimatter presumably were created in equal amounts in the big bang and should have completely annihilated one another. So, the real question is, why there is any matter in the universe at all.

"Exemplary teamwork among the partners has led to this outstanding performance," said James Siegrist, associate director for high energy physics at the U.S. Department of Energy's Office of Science. "These notable first results are just the beginning for the world's foremost reactor neutrino experiment."

Source: Virginia Tech