Fermi's theory of weak interactions survived nearly unaltered over the years. Its basic structure was slightly modified by the addition of Gamow–Teller terms and finally by the determination of the V-A form, but its essence as a four fermion interaction remained. Fermi's original insight was based on the analogy with electromagnetism; from the start it was clear that there might be vector particles transmitting the weak force the way the photon transmits the electromagnetic force. Since the weak interaction was of short range, the vector particle would have to be heavy, and since beta decay changed nuclear charge, the particle would have to carry charge. The weak (or W) boson was the object of many searches. No evidence of the W boson was found in the mass region up to 20 GeV.

The V-A theory, which was equivalent to a theory with a very heavy W, was a satisfactory description of all weak interaction data. Nevertheless, it was clear that the theory was not complete. As described in Chapter 6, it predicted cross sections at very high energies that violated unitarity, the basic principle that says that the probability for an individual process to occur must be less than or equal to unity.