HKU Bulletin October 2012 (Vol. 14 No.1)

Cover Story Particle physics is the study of the building blocks of the universe, but an important part of the story has been missing. Anti-matter should, according to Einstein’s theory of relativity, be released at the same time in the same amount as matter when there is a burst of energy like the Big Bang. But when physicists look around the universe, the anti-matter doesn’t seem to be there. Now, an experiment at Daya Bay involving HKU and nearly 300 scientists from 39 institutes in China, the US, Taiwan and the Czech Republic, is helping to bring them closer to an answer. The scientists have been working to detect the anti-matter of the electron neutrino, also known as the ‘ghost particle’ for its elusiveness. Neutrinos and anti-neutrinos are released in nuclear reactions such as those of the sun and nuclear power plants, and Daya Bay is considered an ideal site because it has one of the largest nuclear power clusters in the world. Early results from their experiments, which started in the summer of 2011, have shown they are on the right path. In fact, they have been able to detect electron anti-neutrinos (the anti-matter form of electron neutrinos) in bigger quantities and in a much shorter period of time than originally planned, showing the eight years of preparation and construction behind the experiment have been well worth the effort. Elusive neutrinos “The neutrino is one of the most mysterious kinds of particles,” says Dr Jason Pun, Teaching Consultant in the Department of Physics. “It takes a lot of effort to detect even though it’s all around us. The sun gives off lots of neutrinos and billions of them pass through us every second without us noticing.” The scientists have had to screen the electron anti-neutrinos from all other types of particles, harvesting only 200 to 300 electron anti- neutrinos from the trillions of neutrinos that pass through the anti-neutrino detectors every day. A genuine signal is generated when an electron anti-neutrino enters the detector and generates, through nuclear reaction and at a very low rate, a neutron and a positron. The scientists measured the electron anti-neutrinos at three locations under a nearby hillside, two about 500 metres away from the reactors and one about 1.6 kilometres away. There are three kinds of neutrinos and they can ‘oscillate’ into each other so the aim was to see if the quantity of electron anti-neutrinos ‘disappeared’ at a further distance from the nuclear reactors. The disappearance rate turned out to be six per cent, which was the upper limit of what the scientists had expected. “Our initial plan was to take data for two or three years before we would see something,” says Associate Professor Dr John Leung. “But very excitingly, we started to see something after half a year. It had to undergo a lot of verification but we concluded that we had seen what we were looking for.” HKU as the starting point Hong Kong’s involvement, which includes Dr Leung, Dr Pun, and Professor Chu Ming-chung from The Chinese University of Hong Kong, has entailed designing and building part of the monitoring system for the detectors, and doing background measurements of cosmic rays at Aberdeen Tunnel. HKU was also where the project was first discussed during a workshop here in 2003. The project has also benefited HKU physics students, who have helped out at both the Daya Bay and Aberdeen Tunnel sites, offering a rare opportunity for them to be involved in a large- scale research project. Further such opportunities in the field of particle physics are expected after the Faculty of Science signed an Expression of Interest in July with the European Organization for Nuclear Research (CERN) that allows for exchanges and research opportunities for both students and staff. “Trying to answer the question of why there is more matter than anti-matter in nature is ultimately telling us why we are here,” Dr Pun says. “If so much matter and anti-matter existed, they would annihilate each other. There must be something, some physics interaction at the energy scale for particular kinds of particles, that breaks that beautiful symmetry.” The next stage of their investigations will continue to pursue an answer to what that might be. M z p r l l r l r j v p x m r p o r j k y k y r y p j p m k y k s p o r { p r j k k q m y v m r p o r r r j m l r y p q o q q r s k j r r p r p k j m y v p q q p m k y © r j m l r y p q r p r p k j m y v p q q r m y v m r p o r r j r r y x y y r q ª k j p k j ~ j « v k y u x y p o m j s j k l p o r q r s p } ~ j « k o y ª r x y n v m p o s j k l p o r q r s p y v p x r y p v p p o r r y p j y r p k p o r r j r r y ª k j p k j q k p r m y v m r k y r k s p o r j k v v ¨ q m y | v m y v m r p o r r j r r y x y y r q o r p o j r r y p m ¨ y r x p j m y k r p r p k j v } ¬ l p q q ¬ l m l r p r j r o } v x l r j n r m y x j r { p r j m y v m r p o r ~ § i q q q q Theremust be something, some physics interaction at the energy scale for particular kinds of particles, that breaks that beautiful symmetry. The Unexplained Fallout from the Big Bang The Big Bang theory may explain the beginning of the universe, but it also raises an important question. The explosion should have released the same amount of matter and anti-matter, so what’s happened to the anti-matter? ® ¯ ¢ ¡ ¢ 13 The University of Hong Kong Bulletin October 2012