With the recent successful results of the T2K collaboration, we asked Professor Walter Toki to share the story behind his work on the T2K experiment, as well as reflect on his broader career.

#### When and why did you start working on T2K project? What sparked your interest?

After finishing my work on the BaBar experiment in which the collaboration observed CP violation in B meson oscillations, I participated in writing a proposal to build a neutrino detector in the Henderson mine with Professor Bob Wilson. During this time, I started looking into joining a new experiment. In 2006, the T2K experiment in Japan was recently approved. This experiment sought to search for muon type neutrino oscillations into electron type neutrinos, $\nu _\mu \rightarrow \nu _e$. After working with grad students Adam Pearlstein and Michael Gordon on a study of neutrino oscillation physics, it was clear that this was a very important fundamental measurement. However, at that time, there were upper limits from previous experimental searches and I expected in the T2K experiment we would spend years setting an upper limit. Fortunately, this was not the case.

#### What’s been the biggest challenge with this project? And the biggest reward?

The biggest challenge was building the US P0D detector parts at CSU and assembling the P0D detectors at Stony Brook University during my sabbatical. The CSU group took a major part in the design, by Dave Warner, and the construction of the P0D detector parts at CSU, where Bob Wilson, Norm Buchanan and Bruce Berger led important construction projects. Our work was funded by DOE and it was completed within budget and on schedule.

The biggest reward was that the detectors worked as designed and the T2K experiment observed this new oscillation of $\nu _\mu \rightarrow \nu _e$ about a year before the tragic 2011 Tohoku earthquake in Japan. During data taking, before the earthquake, it was an extraordinary time when every few weeks at a T2K phone conference call, a new event candidate would be presented as more data was accumulated and everyone in the T2K collaboration began to realize that a major discovery was unfolding.

#### What do you see emerging in the next 5 and 10 years of the research?

In the next five years, the T2K experiment has a modest upgrade and in the next ten years, there are major new neutrino projects that are starting up. This includes the Hyper-K experiment, which recently has been approved by the Japanese Diet and the US DUNE project. Both propose to do a precision neutrino and antineutrino oscillation measurements to test CP violation.

#### Tells us more about the recent T2K discoveries?

After the discovery of $\nu _\mu \rightarrow \nu _e$ oscillations, the T2K experiment made a number of engineering measurements of neutrino cross sections, improved the precision of the $\nu _\mu \rightarrow \nu _e$ oscillation measurements and very recently T2K completed important measurements of both neutrino and antineutrino oscillations where probabilities, P($\nu _\mu \rightarrow \nu _e$) is observed to be greater than $P(anti- \nu _\mu \rightarrow anti- \nu _e)$ by a 3 standard deviation significance.  This measurement indicates that CP is violated in neutrino oscillations. This is now being published in NATURE in April 2020.