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In May 2024, Dr. Kazuki Kojima joined the Energy Frontier Group at IPNS as a postdoctoral Fellow.

We interviewed him about what sparked his interest in the world of particle physics, the research he has conducted, and how he refreshes his mind when faced with challenges in his work.

My first encounter with particle physics happened when I was 15 years old. I came across an article in the science magazine Newton titled “The Physics of Nothing,” which introduced me to the Higgs particle. I was fascinated by the idea that even in a vacuum, removing all matter, Higgs particles still exist there, implying that “nothingness” is not truly empty. Although this conclusion sounds paradoxical and puzzled me, it also captured me and deepened my interest in particle physics.

I belonged to Nagoya University and participated in the Belle II experiment, focusing on B meson decays to search for new physics beyond the Standard Model of particle physics. B mesons are short-lived particles that decay into more stable particles, accompanying leptons in about 25% of these decays. Leptons come in three types with different masses, and the decay rates are predicted by the Standard Model depending solely on these mass differences. However, several experiments have reported that the decay rate into the tau which is the heaviest lepton, is unexpectedly higher than the theoretical prediction.
To verify this deviation, we analyzed data collected between 2019 and 2022, newly introducing machine-learning techniques to reconstruct decay particles. Our measurement achieved a precision comparable to previous measurements, despite using only about one-fourth of the data. The results were consistent with the Standard Model predictions, unfortunately showing no significant deviations to support new physics.

At KEK, I will contribute to the ATLAS experiment at CERN, aiming to study the nature of the vacuum characterized by the Higgs particle. The Large Hadron Collider (LHC), providing collisions for the ATLAS experiment, is the only facility in the world capable of directly producing Higgs particles. My goal is to reveal “the physics of nothingness” through Higgs physics. In particular, I will focus on searching for events where two Higgs particles are produced simultaneously, which are yet to be observed. This research could provide crucial insights into whether the vacuum of our universe is stable, as predicted by the Standard Model, or whether it possesses different properties due to new physics.

I find playing the horn an excellent way to refresh my mind. Practicing the horn allows me to focus on the music, providing a break from research. Additionally, playing the horn is a good exercise that requires deep breathing. Orchestra rehearsals are often held on weekends in Tokyo, and the short trip from Tsukuba also contributes to refreshment.

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We look forward to your future activities at the IPNS!