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HKU Physicists Participate in the Daya Bay Reactor Neutrino Experiment Which Helps Record Anti-neutrinos

15 Aug 2011

Combining the efforts of researchers all over the world, the Daya Bay Reactor Neutrino Experiment has begun its quest to answer some of the most puzzling questions about elusive elementary particles known as neutrinos. The experiment's first completed set of twin detectors is now recording interactions of anti-neutrinos (anti-partners of neutrinos) as they travel away from the powerful reactors of the China Guangdong nuclear power cluster in southern China. The Hong Kong team, comprising Dr John K C Leung and Dr Jason C S Pun of the Department of Physics, The University of Hong Kong and Professor Ming-chung Chu of the Department of Physics, The Chinese University of Hong Kong, has been an active member of the Daya Bay Collaboration since its formation, and the designed and built subsystems for detector monitoring and data acquisition have also been put into service. ‘It took us four years to design and another four years to construct the detectors. I'm glad that the operation of this significant project is now going at full steam,' said Dr John K C Leung, HKU. Dr Jason C S Pun also echoed Dr Leung, saying: ‘Yes, it's exciting to witness the growth of this experiment. After all, it is a rare opportunity for Hong Kong to participate in such a major and large-scale research project in elementary particle physics. Our vicinity to the Daya Bay Nuclear Plant has really strengthened our role in the project, particularly in aspects of manpower and logistics management.'

Neutrinos are uncharged particles produced in nuclear reactions, such as in the sun, by cosmic rays, and in nuclear power plants. They come in three types or "flavours" - electron, muon, and tau neutrinos - that morph, or oscillate, from one form to another, interacting hardly at all as they travel through space and matter, including people, buildings, and planets like the Earth.

The start-up of the Daya Bay experiment marks the first step in the international effort of the Daya Bay Collaboration to measure a crucial quantity related to the third type of oscillation, in which the electron-flavoured neutrinos morph into the other two flavoured neutrinos. This transformation is due to the least-known neutrino "mixing angle," denoted by θ13 (theta one-three), and could reveal clues leading to an understanding of why matter predominates over antimatter in the universe, a key condition for our existence.

The Daya Bay Collaboration includes over 200 scientists from 39 institutes in China, USA, Hong Kong, Taiwan, Czech Republic and Russia. This is the first time in history that researchers from Hong Kong have joined such a major international research project in elementary particle physics. With funding support of HK$17M from the Research Grants Council of Hong Kong, the Hong Kong team is primarily responsible for designing and building a mineral oil monitoring system and a Nitrogen cover gas system for the antineutrino detectors, as well as a continuous Radon monitoring system for the experiment. The team also contributes to the data acquisition and analysis works.

The Daya Bay Experiment is well positioned for a precise measurement of the poorly known value of θ13 because it is close to some of the world's most powerful nuclear reactors - the Daya Bay and Ling Ao nuclear power reactors, located some 55 kilometres from Hong Kong - and it will take data from a total of eight large, virtually identical detectors in three experimental halls deep under the adjacent mountains, which provide natural shielding from cosmic rays.

Experimental Hall #1, a third of a kilometre from the twin Daya Bay reactors, is the first to start operating. Hall #2, about a half kilometre from the Ling Ao reactors, will come online this fall. Hall #3, the farthest hall, about two kilometres from the reactors, will be ready to take data in the summer of 2012.

The Daya Bay experiment is a "disappearance" experiment, powered by the enormous quantities of electron antineutrinos produced in the nearby reactors. The detectors in the two closest halls will measure the raw flux of electron antineutrinos from the reactors. The detectors at the far hall will look for depletion in the expected antineutrino flux. After two to three years of collecting data with all eight detectors, the Daya Bay Experiment intends to meet the goal of measuring the oscillation amplitude of θ13 with a sensitivity of one percent.

In support of the Daya Bay Experiment, the Department of Physics, HKU - in collaboration with researchers from CUHK, U.C. Berkeley, Brookhaven National Laboratory, National Taiwan University, National Chiao-Tung University, National United University, and Institute of High Energy Physics, Chinese Academy of Sciences - has also developed a satellite laboratory inside Aberdeen Tunnel to study high energy cosmic rays that penetrate deep underground and their interactions. This allows for the experimental study of the background radiations in an underground environment similar to those in the Daya Bay experimental halls. Over 20 physics students from HKU have contributed to the Aberdeen Tunnel Experiment.

For more information, please visit http://dayawane.ihep.ac.cn/twiki/bin/view/Public/WebHome

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