Glashow submitted "Partial-Symmetries Of Weak Interactions"#OTD in 1960, an important step towards electroweak unification. It answered the question: How can there be a symmetry between electromagnetism and the weak interaction if one conserves parity but the other does not? 
Electromagnetism is based on the exchange of the photon, a massless and electrically neutral vector boson. Schwinger had proposed in the late 1950s that weak interactions were mediated by the exchange of two charged and massive vector bosons called the W^{+} and W^{-}.
Electromagnetic interactions (as well as gravity and the strong interaction, as far as we know) conserve parity. That means the rules work the same way and produce the same results if we replace everything with its mirror reflection.
But the weak interaction *does not* work this way, a possibility that was first proposed by Yang and Lee in 1956 and then famously confirmed by Chien Shiung Wu's experiment the following year.
Physicists were stumped trying to cobble together a framework in which the photon and the W bosons had such different properties. Glashow's paper, which appeared in 1961, offered a resolution to the puzzle of how one interaction would conserve parity and the other would not.
Glashow suggested that a model with a bigger symmetry group – SU(2) x U(1) – and a *fourth* vector boson – the "Z" – was needed. He showed how one of the bosons could be responsible for a parity-conserving interaction, while the others mediated an interaction that violates it.
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