"The weak force gives rise to reactions... These reactions involve a change of flavor... one version involving the exchange of a positively charged quantum and the other the exchange of a negatively charged quantum. The existence of such quanta was first discussed by some of us in the late 1950s, and they were discovered at CERN twenty-five years later, in experiments that procured a Nobel prize for Carlo Rubbia and Simon van der Meer. The quanta are usually called W + and W -, as they were designated in a celebrated paper by T. D. Lee and C. N. Yang..."