The Royal Swedish Academy of Sciences (RSAS) announced today that the 2013 Nobel Prize in Physics is to be awarded to François Englert of the Free University of Brussels in Belgium and Peter Higgs of the University of Edinburgh in Scotland.
Higgs and, independently, Englert and his late colleague Robert Brout discovered a theoretical mechanism in 1964 that yields massive, force-carrying particles. Without it, the weak nuclear force, which is mediated by massive bosons, could not be brought into the same theoretical fold as the electromagnetic force, which is mediated by massless photons.
François Englert (left) and Peter Higgs (right) at CERN on 4 July 2012 when the discovery of the Higgs boson was announced. CREDIT: CERN
The Brout-Englert-Higgs mechanism entails the introduction of a complex scalar field, a manifestation of which is a massive neutral particle later dubbed the Higgs boson. Compelling evidence of the boson’s existence arrived last summer, when two independent particle detectors, ATLAS and CMS, at the Large Hadron Collider each reported a telltale decay peak at the same mass, 125–126 GeV/c2.
Given the near-half-century gap between theory and experiment, the RSAS physics committee fittingly titled its press conference on the morning of the prize announcement “Here at last!”
Early steps
When Brout, Englert, and Higgs formulated their theories in 1964, what’s now known as the standard model of particle physics had yet to be conceived. Quarks were a recent and unverified theoretical proposal and the electromagnetic and weak nuclear forces awaited unification. It would take another decade before the theory of quark–quark interactions, quantum chromodynamics, attained its final form.
Even so, the standard model’s basic mathematical framework, a class of non-abelian gauge theories that had been expounded in 1954 by C. N. Yang and Robert Mills, was already in place. “Gauge” refers to redundant degrees of freedom that render a field theory invariant under transformations of the defining fields. Non-abelian transformations, like putting on socks and shoes, depend on the order in which they are performed.
But the otherwise promising Yang–Mills framework seemed to require that all interparticle forces be mediated by massless bosons. That requirement was met by the electromagnetic force, but manifestly not met by the weak nuclear force. If a non-abelian gauge-invariant theory is to encompass all subatomic particles, it needs a way to allow some force-carrying bosons to have mass.
A way around that impasse was presaged by the work of Yoichiro Nambu in 1960. Nambu demonstrated that the same spontaneous symmetry breaking behind the superconducting transition might endow force-carrying bosons with mass if one included an additional scalar field. But a year later, Jeffrey Goldstone seemed to prove that the approach would inevitably yield massless, spinless bosons that evidently did not exist.
In 1962 Philip Anderson found a loophole in Goldstone’s no-go theorem that killed off the massless bosons while sustaining massive, force-carrying bosons and preserving gauge invariance in the nonrelativistic regime. Anderson presciently declared in his paper, “The Goldstone zero-mass difficulty is not a serious one, because we can probably cancel it off against an equal Yang–Mills zero-mass problem.” The stage was set.
Acquiring mass
Brout and Englert were the first to publish a relativistic solution. Submitted to Physical Review Letters on 26 June 1964, their paper coupled an abelian gauge theory to a complex scalar field. Their approach did indeed “cancel off” the two problems. The two theorists later proved that their approach worked for non-abelian gauge theories too.
Higgs submitted his first paper to Physical Review Letters a month later, on 27 July. He tackled the same combination of fields as Brout and Englert and also introduced an expression for the mass of the scalar particle that Brout and Englert had found. Like Brout and Englert, he later proved that the massive particles also arise in non-abelian gauge theories.
Other theorists had been working on the same approach at the same time. On 12 October 1964 Gerald Guralnik, Carl Hagen, and Tom Kibble published a more comprehensive exposition of the mechanism. A paper by Alexander Migdal and Alexander Polyakov of the Soviet Union eventually overcame official resistance and was published in 1965.
It now became possible to further unify the forces of nature. In 1967 Steven Weinberg formulated a gauge-invariant theory that encompassed the electromagnetic and the weak nuclear force. To obtain an expression for the masses of the W+, W−, and Z bosons that mediate the interaction, he invoked the Brout-Englert-Higgs mechanism.
Final vindication
By itself, the Brout-Englert-Higgs mechanism does not yield a value for the mass of the Higgs boson. But the mass can’t be too low, lest the theory become metastable. And it can’t be too high, lest it threaten the integrity of the standard model. When the CERN Council approved the construction of the Large Hadron Collider and its detectors in 1994, an accelerator of a few TeV was deemed necessary to find the Higgs at the few-hundred-GeV/c2 mass anticipated by theorists.
The first attempt at CERN to create and measure the Higgs began in March 2010 at a beam energy of 3.5 TeV. No evidence of the Higgs emerged. A second, longer run in 2010 collected more data at the same beam energy—and saw a suggestive yet not formally significant bump at a mass of around 120 GeV/c2.
The third run in 2012 boosted the beam energy to 4 TeV and nearly doubled the luminosity. Both the ATLAS and CMS detectors registered significant detections at a level of five standard deviations above background and at statistically identical energies. The Higgs boson had been found at last.
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