Experiments at Daya Bay shed light on antimatter

PUBLISHED : Sunday, 28 August, 2011, 12:00am
UPDATED : Sunday, 28 August, 2011, 12:00am


In an attempt to understand the origins of matter, Chinese and US scientists have begun a multimillion-dollar collaboration at the Daya Bay nuclear facility, the power station northeast of Hong Kong that provides the city with most of its electricity.

After eight years of building and planning, the Daya Bay Reactor Neutrino Experiment this month began measuring the side-effects of the plant's nuclear reactions, in an attempt to understand why, in a universe full of antimatter, matter exists at all.

The US Department of Energy has provided US$68 million to finance half the cost of the Daya Bay experiment, most of which is being conducted deep underground. The mainland is paying the other half and the civil-engineering costs.

Hong Kong is helping too, with a HK$17 million grant - and by reopening a physics lab nestled between the lanes of one of the city's busiest highways to provide back-up experiments and support.

The experiment being conducted by 200 scientists, including physicists from Hong Kong, Taiwan, the Czech Republic and Russia.

'This is a remarkable achievement after eight years of effort - four years of planning and four years of construction - by hundreds of physicists and engineers from around the globe,' said Yifang Wang of the Institute of High Energy Physics of the Chinese Academy of Sciences, a spokesman for the project.

'The Daya Bay experiment represents a new alliance between China and the US and it opens up new opportunities in high-energy physics,' according to a news report in the US scientific media.

The experiment will involve measuring the antineutrinos that are produced when protons change into neutrons as Daya Bay's two reactors provide the energy that lights up much of Hong Kong and neighbouring Guangdong province.

Detectors more than 200 metres underground at the facility will collect data about the three 'flavours' of neutrinos - electron, muon and tau - in an attempt to discover how they were formed when the universe was created in the Big Bang more than 14 billion years ago.

Neutrinos are elementary particles - one of the fundamental building blocks of all matter.

According to the big-bang theory, the early universe was so hot that most of it was just energy. As the universe expanded and cooled, the energy converted into matter and anti-matter. The Daya Bay experiment will seek to understand this process by measuring a property of neutrinos called theta one-three.

One theory among physicists is that neutrino oscillations break the symmetry between matter and antimatter. There have been advances in describing these oscillations, which determines whether they will be electrons, muons or taus, but the value of a key parameter - theta-one-three - remains unknown.

The physicists say the Daya Bay experiment will give them the most accurate measurement of this quantity, thanks to their equipment's unprecedented accuracy.

This will shed light on how electrons and their cousins were born in the moments after the big bang, and help explain why there is more matter than antimatter in the universe.

'Among the current generation of reactor neutrino-oscillation experiments for measuring theta one-three, Daya Bay has the best sensitivity,' said Luk Kam-biu, spokesperson for the Lawrence Berkeley National Laboratory in California, one of the participating institutes.

According to the laboratory's website, the detectors are filled with a clear liquid which reveals antineutrino interactions by the faint flashes of light they emit.

Lining the walls are sensitive photomultiplier tubes that amplify and record the telltale flashes. Only a small number of these interactions - about 1,000 a day - from the millions of quadrillions of antineutrinos that are produced by the reactors every second, are recorded by the detectors.

Daya Bay's first two detectors began taking measurements on August 15, with two more being installed. The last four detectors will be installed in 2012 and all will take measurements for the following next eight years.

The Hong Kong team, led by physicists Dr John Leung Kon-chong and Dr Jason Pun Chun-shing, of the University of Hong Kong; and Dr Chu Ming-chung, of Chinese University, has been an active member of the project since its formation.

With the HK$17 million from Hong Kong's Research Grants Council, they are primarily responsible for designing and building a mineral oil monitoring system and a nitrogen cover gas system for the antineutrino detectors.

'The unprecedented opportunity provided by the Daya Bay experiment, to collaborate with top scientists from all over the world to build the most sensitive neutrino detector ever, is just too good to miss,' said Chu, one of the project's principal investigators.

Pun agreed, saying: 'It is a rare opportunity for Hong Kong to participate in such a major and large-scale research project in elementary particle physics.'

Chu said it was 'an outstanding question of great importance in the world of physics why the universe evolved to be matter-dominated - that is, why there is far more matter than antimatter'.

Yet, since neutrinos interact weakly it is difficult to study their properties accurately. That is why understanding those properties and interactions are of 'fundamental' importance to the world of physics, Chu said.

The experiment has also brought back into use Hong Kong's only underground particle physics laboratory, tucked between the two tubes of the Aberdeen Tunnel.

The laboratory was built by HKU in the 1980s to study cosmic rays, and it was revived in 2003 by HKU and Chinese University as a satellite laboratory for the Daya Bay experiment. With the help of more than 30 physics students it is now measuring high-energy cosmic rays that penetrate deep underground and their interactions.

Its location, beneath all that rock is meaningful because it is effective at creating shielding against background radiations.

The Aberdeen Tunnel has rock types and depths similar in composition to those of the experimental halls in the Daya Bay experiment, Chu said, making it suitable for testing materials and components to be used at the nuclear facility.

On safety, Chu, said the experiment does not deal with particles that harm the human body.

'We only measure the antineutrinos produced through radioactive decay of the fission [the splitting of the nucleus of an atom which results in the release of a large amount of energy] products,' he said.

Nonetheless, stringent safety rules were drawn up according to Chinese and the American standards, and all personnel undergo safety training and tests before they can work in the underground laboratories.

The Hong Kong team has also designed, developed, and implemented a monitoring system for radon, a gas produced by nuclear reactions, in all Daya Bay's experimental halls. This is not only important for the experiment per se, but also for testing background radiation safety of workers, Chu said.


The number of research institutes providing scientists for the neutrino experiment

- Daya Bay is 55km from Hong Kong

Daya Bay neutrino detectors

Neutrinos are the fundamental building blocks of all known matter. They are emitted during certain types of nuclear reactions, and are thought to have been produced during the big bang. The energy released by the big bang was converted into matter and antimatter, but matter came to dominate the universe. Scientists hope the neutrino detectors will provide clues as to why this imbalance between matter and antimatter exists.

How a detector works

The detectors will measure the neutrinos produced by nuclear reactions. When a neutrino strikes the liquid scintillator, a flash of light is produced which is picked up for measurement by the photomultiplier tubes.