Exploring the Universe through a Discovery with Neutrinos
A Dialogue between Prof Henry Tye and Prof Kam-Biu Luk on High Energy Physics
Prof Kam-Biu Luk IAS Senior Visiting Fellow
Graduated with a BSc in Physics from the University of Hong Kong
Obtained his PhD from Rutgers University
Started postdoctoral research at the University of Washington
Received an R.R. Wilson Fellowship from Fermilab
Joined the University of California at Berkeley and the Lawrence Berkeley National Laboratory as a faculty member
Initiated the Daya Bay Reactor Neutrino Experiment
Awarded the 2014 W.K.H. Panofsky Prize in Experimental Particle Physics by the American Physical Society
Won the 2016 Breakthrough Prize in Fundamental Physics as the co-leader of the Daya Bay Reactor Neutrino Experiment
The Daya Bay Collaboration team is co-led by Prof Luk (front row, 9th from left) and Prof Yifang Wang from IHEP (front row, 8th from left).
Kam-Biu, I am very happy that you have been in Hong Kong for the past five months. We have had many discussions about neutrino physics and colliders and we have benefited a great deal from your presence. We all know that the Daya Bay Reactor Neutrino Experiment that you have been leading for more than 10 years is a major accomplishment in neutrino physics. Could you share with us how you initiated the project and the vision of the experiment?
Thanks for hosting my visit here at IAS, Henry. I have really enjoyed the visit. In particular, having so many outstanding staff supporting the Institute is a big plus. I have also enjoyed talking with the young researchers at HKUST and other folks outside the university.
At the beginning of this millennium, I was a member of the KamLAND experiment (the neutrino experiment using the Kamioka Liquid Scintillator Antineutrino Detector at the Kamioka Observatory in Japan). Toward the end of 2001, my Japanese colleagues invited me to join what is now called the T2K experiment (another neutrino oscillation* experiment using the Super-Kamiokande detector and a neutrino beam sent from Tokai to Kamioka in Japan). Because of this invitation, we formed a working group at Berkeley to look into various neutrino projects. In 2003, KamLAND announced the first discovery of reactor anti-neutrino disappearance, which inspired me to further explore the neutrino oscillation parameters. We believed that the neutrino mixing angle, θ13, should be smaller than the other two angles and could be zero. Knowing the value of θ13 is critical for defining the direction of the search for charge-parity violation (CP violation) in neutrino oscillation. This was so important that I started to think of initiating a project on θ13 measurement.
Regarding the Daya Bay experiment, the idea was conceived over the Christmas in 2002. Because the experiment would involve nuclear power reactors, I looked at all of the reactors worldwide and eventually narrowed the choice down to a few places, preferably the Daya Bay Nuclear Power Plant. With the help of my friends, Prof Kwong Sang Cheng and Prof John Leung from the Department of Physics at the University of Hong Kong (HKU), I managed to get some attention from the China Light and Power Company Limited regarding the possibility of setting up the experiment at Daya Bay. However, due to SARS, the first serious discussion with the Hong Kong colleagues about the proposal in person was delayed until July 2003. In November 2003, we held the first meeting at HKU with Chinese colleagues from the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences and other institutions, which led to the birth of the collaboration and the experiment. By a stroke of luck, the US Department of Energy also became interested and was willing to collaborate as an equal partner. The multinational collaboration was finally shaped with researchers from mainland China, the US, Hong Kong, Taiwan, Russia, and the Czech Republic getting involved.
The experiment began data taking with six detectors on the Christmas Eve of 2011. By early 2012, after we had collected the first batch of data, we found to our great surprise that θ13 is indeed smaller than the two other mixing angles, but is still much bigger than we had anticipated. This discovery completely changed our direction on neutrino oscillation. With a non-zero value, we can now use reactors to measure the neutrino mass hierarchy, an important element for understanding other neutrino properties such as neutrinoless double beta-decays and the detail of supernova explosion, and we can study the charge-conjugation parity symmetry of neutrino oscillation in the future. The current goal of the Daya Bay experiment is to push the precision of θ13 to the limit. We are trying our best to provide the best measurement in the world.
* Neutrino oscillation is how one type of neutrino morphs into another type and back.
That’s a great story! So the team will continue measuring θ13, and we look forward to hearing more good news. Apart from the mass hierarchy problem, CP violation is another remaining question in neutrino observation. We know that the US and Japan are each launching related
experiments. Which experiment do you think will be the most promising in obtaining relevant information?
At this stage, the experiment in the US, the Deep Underground Neutrino Experiment (DUNE) operated by Sanford Underground Research Facility (SURF), seems to have the best chance to make advancements in this area. Yet, the Japanese Hyper-Kamiokande experiment (Hyper-K) may be able to get ahead of DUNE in a few years. The future of Hyper-K may depend on Japan’s decision on whether to host the International Linear Collider (ILC) or not. I can’t tell for sure. However, if we look at the scientific potential of the experiments, I think DUNE is on pretty good ground.
The Daya Bay Reactor Neutrino Experiment is carried out at the Daya Bay Nuclear Power Plant with eight antineutrino detectors installed in three underground experimental halls.
Just now you mentioned ILC. What is your take on the possibility of building the collider in mainland China? How do you think Hong Kong can play a role in high energy physics? Especially that we have just learned that such a collider may be within 100km from Hong Kong.
I think that is a very natural outcome because the current Beijing Electron–Positron Collider (BEPC) cannot last forever. To continue the research, China needs to move forward with a new machine. My concern is more about the global coordination between countries. As most countries have already been involved in other international programs, I think they will try to be neutral about China’s. Our Chinese colleagues will have to push the project by themselves until there is a better idea about collaboration.
As an international city with excellent infrastructure, Hong Kong could serve as a lively hub for research activities in high energy physics. Obviously, the programs and workshops organized by IAS provide a good platform for bringing theorists, experimentalists and accelerator physicists together to interact. Should the Chinese future collider be located close to Hong Kong, since there is already a highway and a high-speed train line running in the proximity of the site, it would be quite convenient to access the site from here. Thus, I can imagine some of the foreign physicists would station in Hong Kong and commute to the facility as needed. They could even affiliate with IAS or the other local universities as visiting scientists. In the long run, it would benefit Hong Kong immensely in many dimensions, not just in basic research.
You grew up in Hong Kong and obtained your undergraduate degree in physics at HKU. How did you become interested in science, in particular physics? What excites you about research and drives you to pursue research excellence?
I liked to build models and telescopes when I was small so I discovered that I liked science. I settled on physics because I am a practical person. And, to be honest, I am not good at memorizing names and vocabulary so I eliminated biology and chemistry.
I ruled out mathematics because it is a bit too abstract to me. So in the end I chose physics. I love to get my hands dirty and take things apart and then put them back together. Research is just a good fit to my personality. To me, research is like solving puzzles. Also, the eureka moment is just fantastic. Both the process and the end result excite me and make me look forward to different challenges.
After undergraduate study, Prof Luk pursued his PhD at the Rutgers University in the US.
As a researcher myself, I completely agree with you that it feels amazing when you have finally solved a puzzle. Still, being a researcher is not without its frustrations and may not be suitable for just anyone. What advice would you give to Hong Kong students who want to devote themselves to research?
First of all it is important to have a dream or a goal. Then you need to have the passion and the persistence to pursue it. I believe success is tied to determination and endurance. Knowing your own learning style can help to find the right place to grow. Some students prefer self-exploration while others like to work with peers, so it is very important to know what you want to do and how you want to do it.
© Photo by Steve Jennings/Getty Images for Breakthrough Prize
The Daya Bay Collaboration won the 2016 Breakthrough Prize in Fundamental Physics together with four other research teams “for the fundamental discovery and exploration of neutrino oscillations, revealing a new frontier beyond, and possibly far beyond, the standard model of particle physics.” The recipients, including Prof Luk and Prof Yifang Wang, were at the prize presentation ceremony held on 8 November 2015.
Undoubtedly, 2015 and 2016 have been remarkable years for neutrino science, with physicists in the field being awarded the 2015 Nobel Prize and the neutrino experiments including the Daya Bay team receiving the 2016 Breakthrough Prize in Fundamental Physics. This is a major achievement and very well deserved. I am happy that neutrinos are very well recognized and look forward to more breakthroughs.
Prof Henry Tye