A Japanese dinner party in a Kyoto restaurant in 1956, with (left to right) Hideki Yukawa, Cecil Powell, Mrs Powell, & Mrs Yukawa. Yukawa (1907- 1981) was a Japanese theoretical physicist; he developed a theory of the nuclear binding force (the strong force) in 1935. The theory required the existence of a new particle, later called the pion, which Cecil Powell discovered in 1947. Powell (1903-1969) was an English experimental physicist who pioneered the use of special photographic emulsions as a means of recording the tracks of subatomic particles.