Neutrino find may uncover matter secrets

Chinese and foreign physicists have made a pivotal breakthrough in the study of neutrinos, which may explain the predominance of matter over antimatter in the universe.

The research on the subatomic particles, which was conducted at a nuclear power plant in south China, is expected to define the future of particle physics.

The findings come from the Daya Bay Reactor Neutrino Experiment, which was conducted close to the Daya Bay Nuclear Power Station in Guangdong Province.

Based on data collected from two powerful nuclear reactors, multinational scientists have been able to confirm and measure a third type of neutrino oscillation, Wang Yifang, a co-spokesperson for the experiment and head of the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS), said at a press conference Thursday in Beijing.

Neutrinos, the wispy particles that flooded the universe in the earliest moments after the Big Bang, are continually produced in the hearts of stars and and other nuclear reactions.

Traveling at close to the speed of light, the three basic neutrino "flavors" -- electron, muon, and tau neutrinos, as well as their corresponding antineutrinos -- mix together and oscillate. This activity, however, is extremely difficult to detect.

Two types of oscillation, solar or atmospheric neutrino oscillation, were confirmed in experiments conducted in the 1960s and 1990s, while the third type of oscillation had not been detected prior to the Daya Bay experiment.

Nobel laureate George Smoot III congratulated the Daya Bay experiment progress, saying it is a difficult measurement requiring very large detectors and a large neutrino flux.

The first positive detection of a value for the least-known mixing angle denoted by theta one-three "completes the basic interconnection of neutrino species physics," said Smoot, winner of the 2006 Nobel Prize in Physics and a professor at the University of California, Berkeley.

"It bears on charge parity violation which is relevant to why our universe is made of matter and not antimatter," Smoot told Xinhua.

From last December, the scientists in the Daya Bay experiment observed tens of thousands of interactions of electron antineutrinos caught by six detectors installed in the mountains adjacent to the powerful nuclear reactors, Wang said.

The data revealed for the first time the strong signal of the effect that the scientists were searching for -- a so-called "mixing angle" named theta one-three, a new type for neutrino oscillation, he said.

"It is surprisingly large," Wang said. "Our precise measurement will complete the understanding of the neutrino oscillation and pave the way for the future understanding of matter-antimatter asymmetry in the universe."

Scientists believe the intense heat of the Big Bang should have forged equal amounts of matter and its "mirror image" antimatter. But as we live in a universe composed overwhelmingly of matter, physicists have been puzzled by the apparent "disappearance" of antimatter.

The findings have been submitted to the Physical Review Letters for publication, Wang said.

Chinese physicists proposed to look for the third type of neutrino oscillation by studying neutrinos produced by nuclear reactors and measure the amplitude of such oscillation, said Zhao Guangda, a CAS academician and director general of the China Society of High Energy Physics.

Zhao said the value of theta one-three would "determine the future of particle physics."

"The mystery of why antimatter disappears can be solved," Zhao said.

Co-spokesperson Kam-Biu Luk, a lead physicist at the U.S. Department of Energy Lawrence Berkeley National Laboratory and UC Berkeley, said the study results would also represent a major contribution to understanding the role of neutrinos in the evolution of basic kinds of matter in the earliest moments after the Big Bang.

"We've had extraordinary success in detecting the number of electron antineutrinos that disappear as they travel from the reactors to the detectors 2 km away," Luk said.

"It is the leading theta one-three experiment in the world," said William Edwards of Berkeley Lab and UC Berkeley, the U.S. project and operations manager for the experiment.

"The Daya Bay experiment is of crucial scientific significance, leading to a bright future for particle physics," said CAS academician Zhan Wenlong, who is also vice CAS president and president of the Chinese Physical Society.

IDEAL NUCLEAR PLANT

Neutrinos are electrically neutral particles produced in nuclear reactions, such as in the sun, by cosmic rays, and in nuclear power plants. Scientists say the latter are ideal for research as neutrino detectors can be placed close to the source of radioactivity.

China and the United States launched the Daya Bay experiment in 2006. It involves collaboration of 250 researchers from 39 global institutes including the IHEP, Lawrence Berkeley National Laboratory, Brookhaven National Laboratory, California Institute of Technology, Tsinghua University and Charles University in Prague.

Initially, the scientists proposed eight nuclear plants around the world for the experiment to confirm the third oscillation. Only three were included in the experimental phase: Daya Bay, France's Double Chooz and Reno of the Republic of Korea.

Daya Bay stood out for its powerful reactors and its ideal location -- in a coastal area shielded by a row of hills that provide a natural barrier to the interference of cosmic rays, Wang said. The experiment site is located about 50 km from downtown Shenzhen, bordering Hong Kong.

Chinese investment in the experiment topped 160 million yuan (25.6 million U.S. dollars), while the U.S. chipped in equipment worth 80 million yuan, making it the second largest offshore investment by the Department of Energy after that to the European Organization for Nuclear Research (CERN) in Geneva, Wang said.

Construction of facilities was completed by the middle of last year, followed by data collection.

Eight large cylinder detectors, weighing 110 tons each, were installed in 10-meter-deep water pools built in three underground experimental halls set in the form of a triangle and linked by underground tunnels about 100 meters beneath the mountains.

Two of the halls were close to Daya Bay and Ling'ao nuclear power reactors while the third was located 2 km away. The detectors in the first two halls measured the raw flux of electron antineutrinos from the reactors, while the detectors in the far hall looked for a depletion in the expected antineutrino flux.

The experiment recorded the precise difference in flux and energy distribution between the near and far detectors to measure the neutrino oscillation, the scientists said.

FASTER-THAN-LIGHT CONTROVERSY

Neutrinos grabbed the world's attention recently as a European experiment claimed last year that the subatomic particles might travel faster than light, threatening to overthrow one of Albert Einstein's fundamental theories upon which modern physics developed.

Einstein's Special Theory of Relativity, published in 1905, states that nothing moves faster than light, and if it does, it would be like traveling back in time.

But the CERN experiment clocked neutrinos traveling at a speed of 300,006 kilometers per second, 60 nanoseconds faster than the velocity of light.

The findings sparked widespread skepticism. In late February, the CERN announced scientists were still checking a cable connection that might have caused a flaw in the results.

Since their first detection, neutrinos have been a topic of intense study in the fields of particle physics, astrophysics and cosmology. The Nobel Prize in Physics was awarded for neutrino-related research three times over the last two decades -- in 1988, 1995, and 2002.

Neutrinos travel through space and matter -- including humans, buildings, and planets -- with almost no interaction at all. This feature of weak interaction interests scientists because it means neutrinos can be used to probe environments that other radiation, such as light or radio waves, cannot penetrate.

TIGHT BUDGET

The Daya Bay experiment has been hailed as a milestone in China's basic science research, which has long suffered underinvestment.

"I often hear people say 'why are you doing basic science research if it has no practical use and has little potential of being awarded a Nobel Prize'? " Wang said wrote in a paper.

"This snobbish idea is popular in society, among government officials, intellectuals, project managers, and even some scientists," Wang commented.

Chinese scientists participating in the experiment said they spent eight years working on the tough project. Due to the tight budget, the scientists were used to roughing in in farmers' residences, instead of hotels, frequently harassed by problems like broken air-conditioning, leaky ceilings, and disruptions in tap water supply.

Li Xiaonan, a project manager, said everyday more than 100 scientists woke up before 7 a.m., had breakfast and marched to the project site about 10 km away like battalion soldiers.

But unlike soldiers, these scientists did not have a uniform sleeping time -- many worked into the wee hours of the morning to communicate with overseas researchers by email.

"People could not even believe we built such advanced facilities with so little money," Li told Xinhua. "Many of us joined the project out of enthusiasm to advance basic research."

Physicist Cao Jun, like Li, was a research fellow at the Fermi National Accelerator Laboratory in the United States. He said he jumped at the invitation of Wang who called him in 2003 to join the Daya Bay experiment.

"It sounded like a perfect opportunity to me. I wanted to return to China but there had been no suitable basic science research before that," Cao said.

"Particle physics is a small community. Everybody knows what you are doing," he said. "As the experiment progressed, we felt that more and more foreign researchers started to pay attention to China."

Wang said the experiment put China's particle physics programs back into the international spotlight after the completion of the upgrade for the Beijing Electron Positron Collider (BEPCII).

The BEPCII signals the IHEP as one of the most advanced institutions in the world for accelerator and detector technologies, Wang said.