Higgs Boson Discovered
In July 2012, scientists at CERN, the European Organization for Nuclear Research near Geneva, announced that two independent experiments at the Large Hadron Collider had observed a new particle consistent with the long-sought Higgs boson. The discovery filled the last missing piece of the Standard Model of particle physics, the theory that describes the known elementary particles and three of the four fundamental forces. It also offered evidence for a mechanism that explains how fundamental particles acquire mass.
The idea dates to the 1960s. Physicists realized that the equations describing the electromagnetic and weak forces seemed to require force-carrying particles with no mass, yet the weak force's carriers are clearly heavy. In 1964, several groups, including Peter Higgs, François Englert and Robert Brout, proposed that an invisible field filling all of space could give particles mass through their interaction with it. If the field existed, it should also produce a detectable particle, which came to be called the Higgs boson. Over the following decades the rest of the Standard Model was confirmed, but the Higgs stayed elusive, in part because it is heavy and decays almost instantly.
Finding it required a machine of extraordinary scale. The Large Hadron Collider, a 27-kilometre ring beneath the French-Swiss border, began operating in 2008 and was designed in part to settle the question. It collides protons at enormous energies, and the resulting debris is recorded by giant detectors. Two of them, ATLAS and CMS, were built by separate international collaborations involving thousands of researchers. Neither could see the Higgs directly; instead, they looked for statistical excesses of particular decay patterns, such as two high-energy photons, above what background processes would produce.
By mid-2012 both experiments saw a clear signal at a mass of roughly 125 times that of a proton. The announcement, made at a packed seminar at CERN, was met with applause and visible emotion from some of the physicists involved. Scientists were initially careful to call it a "Higgs-like" particle, and further measurements of its properties over the following months and years supported the conclusion that it matches the Standard Model's prediction. Higgs and Englert shared the 2013 Nobel Prize in Physics.
The discovery was a triumph of theory and international collaboration, but it did not end the story. The Standard Model does not account for gravity, dark matter or dark energy, and it leaves open why the Higgs has the mass it does. Physicists continue to measure the particle's behavior precisely, hoping to find small deviations that might point toward new physics beyond the model.