When physicists at CERN's Large Hadron Collider announced the discovery of a new particle on July 4, they did not call it “the Higgs boson.” This was not just the typical caution of scientists. It also signified that the announcement comes at a profound moment. We are at the end of a decades-long theoretical, experimental and technological odyssey, as well as at the beginning of a new era in physics.
The search for this particle grew out of a single phrase in the 1964 paper by physicist Peter Higgs of the University of Edinburgh in Scotland. At the time, what we now call the Standard Model of particle physics, which describes all known elementary particles, was only just starting to coalesce. The Standard Model makes hundreds of testable predictions and, in the decades since its inception, has been proved right every time. The Higgs boson was the last remaining piece of the puzzle, tying together all the known particles of matter (fermions) and the carriers of the forces acting on them (bosons). It paints a compelling picture of how the subatomic world works, but we do not yet know if this picture is just part of a larger canvas.
The Standard Model is based in part on electroweak symmetry, which unites electromagnetism and the weak force. But the particles that carry those forces have very different masses, showing that the symmetry is broken. Theorists were left to explain the divergence of forces. In 1964 three separate papers—by Higgs, by François Englert and Robert Brout, and by Gerald Guralnik, Carl Hagen and Tom Kibble—in our journal, Physical Review Letters, showed that a ubiquitous quantum ocean called a spin-0 field could accomplish the symmetry breaking. Higgs mentioned that this ocean had waves that correspond to a new particle—the boson that came to bear his name.
This particle, key to the Standard Model, has been arguably the hardest to find—it required generations of ever bigger colliders to produce a sufficient number of sufficiently energetic collisions. Yet completing the Standard Model hardly closes the book on particle physics. The discovery of the Higgs may in fact point the way to what lies beyond the realm of this venerated theory.
Experimenters still need to verify that the new particle is a spin-0 Higgs boson. Next, they must test how the Higgs interacts with other particles to high precision. At this writing, its couplings do not quite match predictions, which could be just a statistical fluctuation or a sign of some deeper effect. Meanwhile experimenters have to keep taking data to see whether more than one Higgs boson exists.
These are important tests because theorists have constructed many hypothetical models that put the Standard Model in a broader framework, and many of these predict multiple bosons or deviations from the usual couplings. The models include extra fermions, extra bosons and even extra dimensions of space. The most studied broader framework is supersymmetry, which hypothesizes that each known fermion has an undiscovered partner boson and that each known boson has an undiscovered partner fermion. If supersymmetry is correct, there is not one Higgs boson but at least five. So we are just beginning to explore a new realm.