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Why the Higgs?

Maximilien Brice/CERN
Part of CERN’s Large Hadron Collider under construction, Cessy, France, 2007

The following is part of an introduction to James Baggott’s new book Higgs: The Invention and Discovery of the “God Particle,” which will be published in August by Oxford University Press. Baggott wrote his book anticipating the recent announcement of the discovery at CERN near Geneva—with some corroboration from Fermilab—of a new particle that seems to be the long-sought Higgs particle. Much further research on its exact identity is to come.

It is often said that what was at stake in the search for the Higgs particle was the origin of mass. True enough, but this explanation needs some sharpening.

By the 1980s we had a good comprehensive theory of all observed elementary particles and the forces (other than gravitation) that they exert on one another. One of the essential elements of this theory is a symmetry, like a family relationship, between two of these forces, the electromagnetic force and the weak nuclear force. Electromagnetism is responsible for light; the weak nuclear force allows particles inside atomic nuclei to change their identity through processes of radioactive decay. The symmetry between the two forces brings them together in a single “electroweak” structure. The general features of the electroweak theory have been well tested; their validity is not what has been at stake in the recent experiments at CERN and Fermilab, and would not be seriously in doubt even if no Higgs particle had been discovered.

But one of the consequences of the electroweak symmetry is that, if nothing new is added to the theory, all elementary particles, including electrons and quarks, would be massless, which of course they are not. So, something has to be added to the electroweak theory, some new kind of matter or field, not yet observed in nature or in our laboratories. The search for the Higgs particle has been a search for the answer to the question: What is this new stuff we need?

The search for this new stuff has not been just a matter of noodling around at high-energy accelerators, waiting to see what turns up. Somehow the electroweak symmetry, an exact property of the underlying equations of elementary particle physics, must be broken; if we are to account for mass, the electroweak symmetry must not apply directly to the particles and forces we actually observe.1 It has been known since the work of Yoichiro Nambu and Jeffrey Goldstone in 1960–1961 that symmetry-breaking of this sort is possible in various theories, but it had seemed that it would, as a matter of theory, necessarily entail new massless particles, which we knew did not exist in fact.

It was the independent work of Robert Brout and François Englert; Peter Higgs; and Gerald Guralnik, Carl Hagen, and Tom Kibble, all in 1964, that showed that in some kinds of theories…

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