Tsung-Dao Lee
At Columbia, Wu knew the Chinese-born theoretical physicist Tsung-Dao Lee personally. In the mid-1950s, Lee and another Chinese theoretical physicist, Chen Ning Yang, grew to question a hypothetical law of elementary particle physics, the "law of conservation of parity". One example highlighting the problem was the puzzle of the theta and tau particles, two apparently differently charged, strange mesons. They were so similar that they would ordinarily be considered to be the same particle, but different decay modes resulting in two different parity states were observed, suggesting that and were different particles, if parity is conserved: