Web edition: July 13, 2012
Print edition: July 28, 2012; Vol.182 #2 (p. 26)
For decades, physicists have been promising the world that it is worth the money to build enormous machines, costing billions of dollars, to shock empty space into revealing an exotic particle called the Higgs boson. After years of false starts and frustration, hints and hopes, the Higgs has finally been found at the Large Hadron Collider outside Geneva. It’s a cause for celebration — and for explanation, of what the Higgs is and why it matters.
Ever since scientists figured out that the universe began with an explosive bang, some of them have wondered how the initial incendiary chaos cooled into a cosmic palace of intricate structure. Galaxies full of stars and planets built from complex atoms somehow congealed out of the Big Bang’s formless fireball. As physicists developed equations to describe the basic particles of matter and the forces governing them, one aspect of reality seemed missing: None of the particles would possess any mass.
In 1964, physicist Peter Higgs of the University of Edinburgh proposed that the infant universe (as in, perhaps a trillionth of a second old) experienced a cosmic hiccup — technically, a phase transition. In much the way an iron bar can suddenly become a magnet when cooled below a certain temperature, space itself acquired a new feature. Instead of a magnetic field, space was filled with a new forcelike field — since named for Higgs.
Other physicists worked out similar scenarios at about the same time, and later work showed how the Higgs phase transition could explain the distinct identities of two of nature’s basic forces: electromagnetism and the weak nuclear force.
Before the Higgs field appeared in the vacuum, those two forces were one and indivisible. And all particles of matter and force carriers within the mathematical apparatus known as the standard model (shown) were massless. Afterward, particles of light, or photons, remained massless and propagated the force of electromagnetism. Weak force particles, and matter particles such as electrons and quarks, became massive.
Scientists use various analogies to explain what happened. Basically, particles moving through space are impeded by the presence of the Higgs field to a greater or lesser degree. Some, like photons, are not held back at all and therefore have no mass. But other particles chug through the Higgs field like bowling balls through mud, meeting resistance to their motion. Such resistance to motion (or more precisely, change in motion) is the very definition of inertia, which in turn is the very definition of mass.
With the Higgs field, physicists completed the standard model, which accurately describes the behaviors of all known particles and forces (except gravity). But proof of the Higgs field’s existence was lacking. Only one surefire method could verify the validity of the standard model: discovery of a particle — the Higgs boson — created out of the stuff of the Higgs field.
In the standard model, all particles are something akin to knots in an underlying field that are generated by a sufficient concentration of energy. Various clues hinted that the Higgs boson’s mass was very large, meaning a lot of energy would be needed to make one. So the Large Hadron Collider was designed to collide protons with energies exceeding several trillion electron volts. A Higgs boson created in such collisions would exist too briefly to detect. But its decay would give birth to detectable particles, and that’s how the physicists at the LHC discovered it. Detectors at the LHC recorded products of various Higgs decay paths, including one (shown) creating Z bosons that produce four leptons (such as an electron, positron, muon and antimuon) and another path that ends up producing two photons. Analyzing this debris indicated that the mass of the Higgs boson itself is about 125 billion electron volts, equivalent to the mass of 133 protons.
While the Higgs fills out the standard model, the quest to understand matter and energy doesn’t now end. Gravity has yet to be incorporated into the picture, for one thing. And scientists know that the universe contains much more matter than the standard model can accommodate. Entirely new species of particles are needed to explain invisible “dark matter” in space, which exists in quantities vastly greater than ordinary matter. Such particles may also be discovered at the LHC. Theorists suggest that these particles may be described by a mathematical framework known as supersymmetry, which posits a shadow partner particle for every known particle (illustrated). If so, more than one Higgs field would permeate space, and the Higgs boson may turn out to have several relatives awaiting discovery.
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Coverage of the 2013 American Association for the Advancement of Science meeting
The Year in Science 2012
Three-part series on the scientific struggle to explain the conscious self
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