Guang-Juan: My name is Guang-Juan Wang. Wang is my family name, Guang-Juan is given name by my parents. I graduated from Peking University, and obtained my Ph.D. degree in 2018. After that, I stayed at Peking University for two years working as a post-doctor, and then I moved to JAEA as a JSPS postdoc.

Shoji: What was the subject of your research?

Guang-Juan: OK, when I was studying as a PhD student I worked on the exotic hadrons. We all know that the quark model is very popular. It has successfully described the conventional hadrons including mesons and baryons: the mesons are composed of a quark and an anti-quark, and the baryons are made of three quarks. However, since 2003, there have been a lot of new exotic particles observed in various experiments. For example, the LHCb and the Belle here in Japan, the BESIII in China, and others. All these states cannot directly be understood in the quark model. To distinguish them from the conventional hadrons we call them exotic hadrons. They may have very exotic quantum numbers. The quantum numbers of the mesons and baryons in the quark model are constrained because of the Fermi statistics of inner quarks. Some of the exotic states have the quantum number that are not allowed in the quark model. Even for some exotic states with the allowed quantum numbers, if you look at its observation channel, you will find they may contain more than three quarks, such as four quarks. 

For some of the exotic states, their masses are quite different from the theoretical predictions. Above all, there are many different kinds of exotic states. 

For the states which cannot be simply described by the conventional quark model, we call them exotic hadrons. 

Shoji: I see. So, the quark model describes the quarks and anti-quarks bound together as a kind of hydrogen-like atom, for instance, and that makes the state. According to quantum mechanics, you can tell what is the ground state, what are excited states and so on. That is a classical way of interpreting the hadrons. The mesons are interpreted in that way. Baryons are probably more complicated. But basically, you can use the same technique. But for the exotic states, you cannot understand in that way, andsome new way of understanding is necessary.  And that is what you are doing.

Guang-Juan: Yes.

Shoji: For instance, the four-quark state means that quark and anti-quark plus quark and anti-quark. They combine together.

Guang-Juan: Yes.

Shoji: I see.

Guang-Juan: There are also other exotic states such as compact five-quark states, or even the color-singlet states consisting of just gluons instead of quarks, these are called glueballs. There are some exotic states that contain both the gluon and the quark which are called hybrid. QCD allows their existence while in the quark model, such kinds of structures do not exist. The quark model actually is a phenomenological model of QCD. The exotic state will provide some information that is absent in the conventional model but exists in QCD.

Shoji: I see. 

Guang-Juan: So, an exotic state contains pieces of the less known information of QCD and becomes mysterious. You know, QCD has confinement and asymptotic freedom. We can hardly understand the physics in the low energy range, particularly at the hadron mass scale, for example, 1GeV. The perturbative calculation is invalid because the coupling constant becomes very large. We need to find some phenomenological models or effective field theories to understand the non-perturbative QCD. The quark model can only explain parts of the hadrons. We need to understand their inner structures and inner dynamics to extract information about the QCD in the nonpertubative region.

Shoji: I see. The traditional theoretical framework doesn’t work perfectly in this case, so you have to invent something new, and that is what you’re working on.

Guang-Juan: Yeah.

Shoji: Maybe it’s very difficult but can you explain it in simple words about what is the theoretical framework that you are working on? 

Guang-Juan: Yes. What we want to study is exotic hadrons. We want to study their inner structures. When there are more than three quarks inside, for example, four or five quarks, there are more ways for them to form the color singlet states. We want to understand what kind of interactions govern the formation and how will the quarks perform inside. We need to use some theoretical methods. I mainly use phenomenological models and effective field theories. There are different methods to form the color singlet states, and it is better to choose different models to describe them. For example, for the four-quark states, one quark and one antiquark may first form a meson. The meson and the other meson can interact with each other to form a hadron. Another choice is the four quarks just directly compactly bounded together. In the previous case, it is simpler to understand the interaction at the hadron level. I just consider a light meson exchanging between the two singular mesons. For the latter case, it is better to start at a quark degrees of freedom. There is an important challenge. For the theoretical methods, there will be a lot of undetermined parameters due to the lack of experimental data. They cannot be determined in the two or three-quark system. Recently I would like to work with researchers working on lattice QCD. Lattice QCD can do the non-participative calculation. They will provide very reliable results but at the same time we want to know how the interactions govern this, so we will use different phenomenological models based on symmetry.

We recently used lattice QCD calculations to determine parameters in the phenomenological model. As far as I know, lattice calculation now is in a very fast developing period. For exotic states, there are some unsettled problems, and it may cost a lot of time to calculate the heavy flavor system. On the other side, the phenomenal model is more flexible, so I think the connection between them can combine the flexible and the reliable property of lattice QCD together to understand the whole system.

Shoji: Right. I see. It sounds like there’s no single perfect theoretical framework that explains everything at once. So, you have to introduce this method and that method, and combine them together. And, they use a lot of experimental data and they tried to start struggling to explain what has been found so far.

Guang-Juan: Yeah, the situation of exotic states is just unclear. Nobody knows what will happen in this region and there are no consistent results even though there have been some calculations from different theoretical groups. This region is chaotic but also it provides an opportunity to understand what is happening in the non-perturbative region.

Shoji: Interesting. Our real world is made of atoms. Atoms make molecules and there are lots of larger molecules and so on. And already inside the hadrons, there might be a very complicated structure like you mentioned, two quark and anti-quark pair mainly interact with each other but not only that. You could think about the four quarks bound together like a single ball.

Guang-Juan: I have mentioned the first configuration is the two quark-anti-quark pair first form two color single mesons then they interact with each other by exchanging the light mesons. We know that the light mesons have a small mass, so the effective interaction range will be large. The exotic state looks like a molecule similar to deuteron. On the other side, all quarks are bounded together compactly by exchanging gluons, they are like a ball in some sense. But we don’t know the inner structure. We just call them compact tetraquark or pentaquark states, because it’s bounded compactly.

Shoji: Wow, I see. By the way, how many exotic particles have so far been confirmed or observed?

Guang-Juan: So far, I think at least 60. I remember that some people give the number in some talks. Actually, because there are too many exotic states, so few people will count them one by one. And actually, every year there have been several new exotics states found.

Shoji: Still?

Guang-Juan: Now still. For example, Last two years, and fully heavy tetraquarks have been reported.  We have also invited a very famous researcher from the CMS to give a talk here, so people know the new exotic states have been observed.

Shoji: Wow.

Guang-Juan: Yeah, so this field tends to have more experimental progresses. We have a lot of things to understand.

Shoji: So, it’s still growing, a lot of new data and new puzzles are coming.  You may have to understand lots of different things, that is what you are doing.

Guang-Juan: Yeah, so in the future, the number of exotic states is growing. At the same time, the theoretical interpretations are still in a very early stage. I mean a lot of work needs to be done now to understand what happened in this field.

Shoji: I see, very interesting.

Guang-Juan: Thank you. (18:00)

Shoji: And finally let me ask this question. What do you think about or feel about the research environment of KEK or Japan, compared to your graduate school days?

Guang-Juan: Okay, and you mean compared with Peking University?

Shoji: Yeah, for instance.

Guang-Juan: I think both sides of the people have some similar things, for example, people working here and in China have a great passion for research. Before I come here, I heard people abroad will have a rest during the weekend but when I come here I know some people work so hard. Secondly, I think both Peking University and KEK can give a very good research environment for researchers. When I came here, I was a little afraid because I cannot speak Japanese but all people here are so kind to me. They will choose to speak English in some meetings just to make me understand what’s talking about. I feel very comfortable here. The difference between Peking University and here maybe, first of all, Peking University has so many students, so for each group, we will have some group meetings, and we will have a lot of seminars from outside. And here we have a lot of seminars outside but we don’t have so many students. But at the same time, we also have some benefits because I have more time to discuss with some professional professors. I think both of the two places are very good for research. In the life part, I’m very lucky. I thank people who really helped me a lot when I moved here. I really enjoyed the peaceful life here, and I can concentrate on my research.

Shoji: I see. That’s good. Okay, so you are probably trying to go back and forth between China and Japan in the future to construct their research collaboration or whatever cooperation. Do you have any plans for working together?

Guang-Juan: Actually, what I have done in the past is to connect the researchers in Japan and in China and also in other countries. We have set up a collaboration group and we have published two important papers in our field about the heavy strange mesons. I think the collaboration between the two sides will have diversity. Different people from different countries have different interviews and viewpoints on one problem and I want to get them together. Our small collaboration group is now really good. We welcome the new people coming in. And we’re working on some programs very efficiently.  I think maybe in the future we will include more people inside, especially young students. And also including more different regions.

Shoji: Yes. Great.

Guang-Juan: Thank you. I hope in the future if our group did a very good job, we can have an even larger collaboration group for deeper collaboration and apply for some grant funding, like what they have done between China and Germany in our field. I hope in the future we can do that. 

Shoji: Great. Okay, I wish you success in your research and also you enjoy your life in Japan.

Guang-Juan: Thank you. Now I enjoyed it.