to secure her special visas just to enter the country. This eventually led to her decision to stay in the United States. With the help of Columbia chairman Charles H. Townes, Wu would become a US citizen in 1954. Establishing beta decay In her post-war research, Wu, now an established physicist, continued to investigate beta decay. Enrico Fermi had published his theory of beta decay in 1934, but an experiment by Luis Walter Alvarez had produced results at variance with the theory. Wu set out to repeat the experiment and verify the result. Wu was already heavily invested in working on beta decay as she took on the subject at UC Berkeley. In the year 1949, Wu completely established Fermi's theory and showed how beta decay worked, especially in creating electrons, neutrinos, and positrons. Supposedly, most of the electrons should come out of the nucleus at high speeds. After careful research, Wu suspected that the problem was that a thick and uneven film of copper(II) sulfate () was being used as a copper-64 beta ray source, which was causing the emitted electrons to lose energy. To get around this, she adapted an older form of the spectrometer, a solenoidal spectrometer. She added detergent to the copper sulfate to produce a thin, even film. She then demonstrated that the discrepancies observed were the result of experimental error; her results were consistent with Fermi's theory. The speeds of the electrons that were commonly produced in experiments were now shown to be significantly slower. Thus by analyzing radioactive materials used by previous researchers, she proved that this was the cause of the problem and not from theoretical flaws. Wu thus established herself as the leading physicist on beta decay. Her work on beta decay became hugely beneficial to her later research and to modern physics in general. Parity experiment 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