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Junpei Fujimoto

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Junpei Fujimoto

I was born in Japan in a town located between Tokyo and Kyoto, called Nagoya. It is the same town where Professor Kobayashi and Professor Maskawa were born.

In Japan, each town has its own type of soybean paste soup, called miso. The taste of the soup is so distinct in each town that its citizens recognize the different kinds very easily. In Nagoya, people are quite famous for cooking with a special dark-red miso. I personally love it too. My institute, KEK, is located east of Tokyo, very far from Nagoya. Their miso soup is very different, and I am still not used to it in the cafeteria at KEK.

My first graduate research was to be involved to construct the TOPAZ detector for the TRISTAN experiments at KEK. Simultaneously, the professor of my research group suggested me to calculate the radiative corrections (RC) to the processes in electron-positron collisions. At that time, the TRISTAN-Theory group made these calculations at KEK. I joined that group and learned how to make these calculations. Therefore, my doctoral thesis was dedicated to the one-loop corrections to top-quark pair production in the electro-weak theory.

In those days, one did not know the mass of the top quark, and there was a chance that TRISTAN could discover it. Unfortunately, however, the top quark was not in the energy region of the TRISTAN experiments. But I was able pass my defense because my thesis was one of the first calculations to get the analytical expressions for the RC to the top-quark pair process. It was good, but I was so exhausted to get the Feynman amplitudes and the analytic formulas of the Feynman integrals with massive particles.

When I became a staff member of KEK, I joined the GRACE project, which made automatic calculations of Feynman diagrams. When people declare the initial state, the final state and the order of the processes, GRACE provides the possible Feynman diagrams and writes the FORTRAN source codes to evaluate the cross section. I had the responsibility to develope a part of the one-loop corrections. At last, GRACE can calculate the process of my thesis only in a few minutes. When I wrote my thesis, I did not have GRACE and had to spend a year and a half to finish calculations by hand! I am so satisfied with this revenge, and so far, we have calculated lots of the cross sections by means of the GRACE system.

In the future, I would like to expand the ability of the GRACE to treat the multi-loop diagrams. In order to do it, we must first have to acconplish the Feynman integrals. Unfortunately at this moment, we do not have the general formulas with masses, even beyond two-loop. I am now studying the structure of the multi-loop Feynman integrals.

I also collaborate with the Frenches on LAPTH and the Russians from Moscow State University on these automatic calculations. Every year we exchange collaborators. It is a lot of fun to stay abroad with good friends.