The weak interaction has been theoretically united with the electromagnetic force, a major victory for physics that occurred rather recently. It was in the 1960s that Glashow, Salam, and Weinberg came up with their unification scheme based on QED. Initially, their work predicted that there should be a total of four massless bosons to mediate the combined force, two being neutral and two having electric charge. However, it was already known that the weak interaction had a very short range, unlike the electromagnetic force. Thus the experimental evidence strongly suggested that the three new bosons (to add to the neutral photon) had to have fairly large masses. The W bosons and Z bosons were eventually found, as previously discussed.
But theorists still wondered why the masses of the exchange particles for the same fundamental force should be so different. British theorist Peter Higgs (among others) suggested a mechanism whereby the underlying symmetry between the exchange bosons could be broken, giving three of them (two Ws and a Z) their observed masses.
This theoretical mechanism postulates a new, invisible energy field that pervades the whole universe. This field exerts a kind of drag force on particles that we interpret as mass, and may even apply to other particles besides the W and Z bosons. As in any good quantum field theory, there should be at least one boson associated with this field, and so the Higgs boson was postulated to exist.
It was difficult to predict the mass of this particle with much certainty, but the masses of the W and Z bosons helped set the scale. The W and Z bosons already had very large masses compared to other elementary particles, and the Higgs was expected to be even more massive. For this reason, many years passed between the prediction of its existence and the experimental observation of the Higgs boson.
Finally, on July 4, 2012, scientists announced that the Higgs boson had definitely been produced and detected at the Large Hadron Collider, currently the world’s most powerful particle accelerator, located on the border between France and Switzerland. It had a mass in about the range expected, and seems to be the first elementary particle ever discovered with zero spin. It also has no electric charge and no color charge. It embodies a new field (called the Higgs field) that permeates all of space and opens the door for fundamental particles to have mass.
Without the Higgs mechanism, the quantum field theories predicted that all elementary particles would have zero mass, quite contrary to what we actually see. This discovery is therefore just the most recent triumph for the collection of theories known as the “standard model” of particle physics.
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