A couple of weeks ago, Kai Yan gave one of four series of lectures at the “SAGEX PhD School in Amplitudes”. The School preceded the AMPLITUDES 2021 conference, which is one of the most important events for researchers in this field. Kai talked to the students about her research in QCD phenomenology and scattering amplitudes. In October 2021, Kai will take her career to the next step when she will assume a faculty position at Shanghai Jiao Tong University. Here she describes her research for us and talks about the importance of energy – for scattering processes and for discovering your own path.

**Kai, what is your research mainly about?**

In my work I try to make connections between the formal quantum field theory pictures and the calculations of scattering amplitudes to real world physics, particularly in the context of high-energy collider physics. This happens by trying to understand the implication of observables in particle scattering experiments.

**What are these observables?**

One of the observable quantities that I have been working on over the past couple of years is the energy correlator which measures how the energies in particle scattering processes are distributed in the final states. With this you study the expectation values of the products of energies that flow in different directions when the particles collide. In the language of mathematics, you obtain a correlation function of the angles between the detectors.

**What can we learn from studying these quantities?**

The definition of such observables is simple. It fits well into the framework of quantum field theory, and it is a natural and fundamental object to study. First of all, we hope to give predictions for these quantities in particle experiments as precisely as possible, so that we can explore the rich experimental data and test whether there is any deviation of the standard model – the theory that explains a lot, but still not everything, in particle physics. In the meantime, such observable quantities are part of many different areas in quantum physics, some examples: conformal field theory, quantum information theory, cosmology. They connect the research ideas in different fields and help bridge the gaps between them.

**How did it come about that you chose theoretical particle physics as your career path?**

When I was young, I just enjoyed solving problems. I like solving mysteries in detective stories. Finding solutions makes me very happy. In college I took lectures in different subjects and found out that high energy physics was the most fascinating for me. By solving the problems that I was confronted with I could find answers with beautiful mathematical structures. At the same time these problems are closely related to the real world of physics, which is really fascinating.

**What events brought you here today?**

I graduated in Physics at Peking University, then I got an offer from Harvard University, where I earned my PhD. My first postdoc position was here with the ERC project “Novel structures in scattering amplitudes” at the MPP. And from October on I will hold a tenure track faculty position at Shanghai Jiao Tong University.

**Besides finding joy in solving problems, what do you need to succeed in theoretical physics?**

In this field everything develops gradually, your knowledge accumulates and your understanding deepens by doing the work on a daily basis. So, patience and perseverance are vital in order to succeed – and please, hold on to your curiosity!

**The truth is that there are very few women in your field. What advice would you give them?**

As women, we should remind ourselves that we have the freedom to do what truly makes us happy rather than what other people think we should do or what other people define as the important things for us. Follow your instincts, ask questions that you think matter. In research in general, I think that originality comes first. Nobody can tell you what the right path is, you have to find it out for yourself.

**Thank you, Kai, and all the best!**