Dialogue@ZJU: With Dr. XIAO Meng, a particle physicist

2020-09-28 Global Communications

[Message from the Editor: Recently we interviewed Dr. XIAO Meng from Department of Physics about her work on God Particle, LHC in Europe and international collaborations.]

Q1: This recent work helped to further develop our understanding of the Higgs boson’s properties. Why is it so important for elementary particle physics?

XIAO: To describe the elementary particles and their interactions in a unified way, physicists have proposed a lot of theories. Among them, the Standard Model (SM) is the most successful so far. The Higgs boson is a very special particle, it gives the mass to all the other particles. The SM predicted the existence of a Higgs boson with very unique properties. Despite the success so far, the SM theory is incomplete. There is more matter than antimatter in the universe. This is something the SM could not explain. One of the important ingredients to make the matter-antimatter asymmetry is charge-parity (CP) violation. New sources of CP violation would point to the origin of the asymmetry. As the last discovered elementary particle, the Higgs boson opens a new door to the search for CP violation. In the SM, there is no CP violation in the interactions between the Higgs boson and the top quark—the heaviest fermion. Measuring the CP violation in this interaction for the first time, not only helps understanding the property of the Higgs boson, but also provides a new way to look for the answer to the matter-antimatter asymmetry.

Q2: Valuable data of your work was derived from Large Hadron Collider (LHC) in Europe. What does the LHC do?

XIAO: The LHC is the world’s largest and most powerful particle accelerator. Inside the accelerator, two high-energy proton beams travel at close to the speed of light before they are made to collide. The energy of the proton reaches 6.5 TeV. The beams inside the LHC are made to collide at four locations around the accelerator ring, corresponding to the positions of four particle detectors: ATLAS, CMS, ALICE and LHCb. The collisions create abundant physics processes, and the particles produced in the processes are recorded by the detectors. By studying these processes, we hope to answer the fundamental open questions in physics. For example, before the LHC, we don’t know if the Higgs mechanism is the origin of the elementary particles’mass. The discovery of the Higgs boson at the LHC in 2012 definitely answers this question. Other questions include: what is the nature of the dark matter, that seems to account for 27% of the mass-energy in the universe? What makes the universe nowadays mostly matter instead of antimatter? Could the beyond SM theories, like supersymmetry or extra dimensions be seem by the LHC?

Q3: As a participant of LHC, a world project for science, in your opinion, how does international cooperation contribute to this project?

XIAO: I would say everything about the LHC is international cooperation. It is the largest international scientific collaboration in history. The collider was built by the European Organization for Nuclear Research (CERN) in collaboration with over 10,000 scientists and hundreds of universities and laboratories from more than 100 countries. The CMS experiment I’m working on has 5000 particle physicists, engineers, technicians, students from around 200 institutes and 50 countries. Every little aspect of the experiment involves the efforts of people from different backgrounds. Taking the hardware upgrade project I’m involved in as an example: a detection module has complicated components, teams in France, CERN, UK, the US, Pakistan and many other countries including our university are responsible for the design of different pieces. We talk to each other to make sure the pieces are compatible. Eventually the components will be produced and assembled, which will also happen in several places in the world, including IHEP in China.

Q4: The COVID-19 outbreak may go down in History as one of the most severe crisis for international cooperation. As nations are closing their borders, interrupting global value chains, and trying to protect their domestic economy and employment, the pandemic is jeopardizing regional integration across different continents. Could you share with us your perspectives on the future of international cooperation on research?

XIAO: As a physicist working at the LHC, I definitely value international cooperation a lot. Many things at the LHC simply won’t happen if there weren’t cooperation. The COVID-19 has affected everyone’s life, but in my view its damage to the international cooperation at the LHC is mild. As many people in this collaboration are from different continents, the experiment has established an effective way for multi-continent collaboration. The intellectual communication mostly happened online, which has been proved quite effective. COVID-19 did not changed the situation much. Of course, some projects require on-site operation, and this is inevitably restricted. To many people, COVID-19 changed the usual way of international cooperation. It forced people to do things online as much as possible. While everyone is trying to adapt to the new situation, I believe the most efficient way could be figured out eventually. I would also say that in the future online cooperation will be more and more popular. On this aspect, the LHC has set a good example. However, with all the workaround being said, I think the most important thing is the recognition of international collaboration. CERN and all the participating countries have made huge effort to make the collaboration seem effortless. If international cooperation is not valued, nothing would work out, whether or not there is COVID-19.

Q5: What got you into physics?

XIAO: Some people are lucky, they know what they’re interested in and they’re determined to do it for the rest of their lives. Unfortunately, I am not one of them. In college I’m free to choose major in fundamental science, and I landed on physics simply because I’m not bad at it.At the time I decided to start graduate school, the LHC is about to taking data, and I told myself why not give it a try. Then it was several intense and exciting years. I did physics research in the big international collaboration ATLAS of the LHC, where thousands of people worked together. I was fascinated by the wide expertise people had, and found it so satisfying to learn new things everyday. The physics at the LHC was quite rich, even now I could get lost when listening to seminars in CMS. During my PhD, I participated in the discovery of the Higgs boson, and it makes me decide to continue the research in the filed, because we know so little about this new particle. So I spent the time after that studying the property of the Higgs boson. Up to now, there are still a lot of unknowns about the Higgs boson, and I’m happy that my research could help.

Q6: What big problem would you like to see solved in your lifetime?

XIAO: The LHC has been running for quite some time, and all the data so far seems to agree with the SM well. At the same time, we know the SM is not a theory of all. There has to be something else to explain the matter-antimatter asymmetry. As a physicist who looked for the hint from the Higgs sector for a long time, I would be really thrilled if someday an evidence showed up somewhere. Maybe it will be at the LHC, maybe in some future experiment. I am curious in which form the nature will reveal itself to us. It would be a big puzzle solved in the community.