Since the [weak] field must have spin 1, you will not be surprised to learn that it wound up in the same family as the EM field, the only other field with spin 1. But once again, working out the details wasn’t easy, and once again it was Schwinger who made the first step, in the same paper where he introduced the V and A equation. Using the lepton family (electron, muon and neutrino) as a model, Schwinger suggested that the two charged weak fields (which he named Z+ and Z-) be joined with the neutral EM field (which he renamed ZO) to make a family of three fields with spin 1. Schwinger thus was the first person to suggest what is now called electroweak unification.
From the general suggestions of a family of bosons that is the analog of the leptons, and the identification of its neutral member as the photon, we have been led to a dynamics of a charged, unit-spin Z-particle field that is interpreted as the invisible instrument of the whole class of weak interactions. – J. Schwinger
In postulating only two charged weak fields, Schwinger made the same mistake that Yukawa had made about the strong field. It was Schwinger’s student, Sheldon Glashow, who added a neutral weak field. Ironically, Schwinger’s Z notation survived for the neutral field that he did not introduce, while the ones he did introduce were later renamed W.
As Schwinger’s doctoral student, Sheldon Glashow was given the task of developing Schwinger’s idea that the weak field was part of a family of three fields with spin 1 (known as vector bosons).
Julian was convinced of the existence of an ‘intermediate vector boson’ and of a fundamental connection between weak interactions and electromagnetism… My task was not precisely delineated. It was to seek and perhaps find such a relation, and to explore its observable consequences… In those days of yore, our understanding of the microworld was expanding at breakneck speed. A once theoretically ‘dictated’ and experimentally ‘established’ parity-conserving model of the weak force was bit by bit giving way to the correct parity-violating V-A picture… He convinced himself (and me!) that a triplet of vector bosons … could possibly offer a plausible, elegant, and unified explanation of all EM and weak phenomena. – S. Glashow
After finishing his thesis, Glashow continued his “assignment” at the Bohr Institute in Copenhagen. It was there that he finally realized that if weak interactions violate parity conservation while EM interactions do not, they cannot be as closely related as Schwinger thought. This led him to add a neutral weak field that he called Zo, following Schwinger’s Z-notation, while moving the photon to a more “cousinly” relationship. “It took me over a year to see this, since I no longer had direct access to Julian,” he said later. His article was published in 1961.
You might think that this would have ended the matter, but there still remained a problem. The mass of these fields had to be very large to explain the feebleness of the weak interactions (as indicated by the long half-lives for beta decay), and there was no explanation for such a large mass.
It was Steven Weinberg and Abdus Salam (1926-1996) who independently came up with a way to explain the large mass in 1967 and 1968. They did this by invoking what is known as the Higgs mechanism (first suggested by Schwinger in the same paper where he introduced the V and A equation). In the process Weinberg changed Schwinger’s Z notation for the charged weak fields to W (for weak – or possibly Weinberg?), but retained Z for the neutral field, resulting in the present hybrid notation. For their achievements, Glashow, Weinberg and Salam shared the 1979 Nobel Prize, while Schwinger’s contribution was, as usual, largely forgotten.
Just as with Pauli’s neutrino (“I have postulated a particle that cannot be detected”), it was clear that detecting the weak field quantum would pose a serious challenge.
The direct identification of this hypothetical particle will not be easy. Its linear couplings are neither so strong that it would be produced copiously, nor are they so weak that an appreciable lifetime would be anticipated – J. Schwinger
It wasn’t until 1983 that evidence for the weak field quantum was obtained at the giant CERN accelerator in Geneva, Switzerland. In fact, all three quanta were detected: Schwinger’s charged fields and the neutral particle that Glashow introduced. The masses of the new field turned out to be over 500 times greater than that of the strong field, making it the heaviest known quantum field – and its range therefore the shortest. For this achievement, Carlos Rubbia and Simon van der Meer were awarded the 1984 Nobel Prize in physics.
Posted By: Rodney Brooks