Particle physics: What exactly is the Higgs boson? Why are physicists so sure that it really exists?

The central challenge in particle physics today is to understand what differentiates electromagnetism from the weak interactions that govern radioactivity and the energy output of the Sun. The fundamental interactions derive from symmetries we have observed in Nature. One of the great recent achievements of modern physics is a quantum field theory in which weak and electromagnetic interactions arise from a common symmetry. This "electroweak theory" has been validated in detail, especially by experiments in the Large Electron Positron Collider (LEP) at CERN, the European Laboratory for Particle Physics. Although the weak and electromagnetic interactions are linked through symmetry, their manifestations in the everyday world are very different. The influence of electromagnetism extends to infinite distances, while the influence of the weak interaction is confined to subnuclear dimensions, less than about 10­15 centimeters. That is to say, the photon, the force carrier of electromagnetism, is massless, whereas the W and Z particles that carry the weak forces are about a hundred times the mass of the proton.

What hides the symmetry between the weak and electromagnetic interactions? That is the question we hope to answer through experiments at the Large Hadron Collider (LHC) at CERN. When the LHC is commissioned around the year 2005, it will enable us to study collisions among quarks at energies approaching 1 TeV, or 1012 electron volts. A thorough exploration of the 1-TeV energy scale will determine the mechanism by which the electroweak symmetry is hidden and teach us what makes the W and Z particles massive.

The simplest guess goes back to theoretical work by Peter Higgs and others in the 1960s. According to this picture, the giver of mass is a neutral particle with zero spin that we call the Higgs boson. In today's version of the electroweak theory, the W and Z particles and all the fundamental constituents­quarks and leptons­get their masses by interacting with the Higgs boson. But the Higgs boson remains hypothetical; it has not been observed. That is why particle physicists often use the search for the Higgs boson as a shorthand for the campaign to learn the agent that hides electroweak symmetry and endows other particles with mass.

If the answer is the Higgs boson, we can say enough about its properties to guide the search. Unfortunately, the electroweak theory does not predict the mass of the Higgs boson­though consistency arguments require that it weigh less than 1 TeV. Experimental searches already carried out tell us that the Higgs must weigh more than about 60 GeV = 0.06 TeV.

If the Higgs is relatively light, it may be seen soon in electron-positron annihilations at LEP, produced in association with the Z. The Higgs boson would decay into a b quark and a b antiquark. In a few years, experiments at Fermilab's Tevatron should be able to extend the search to higher masses, looking for Higgs plus W or Higgs plus Z in proton-antiproton collisions. If the Higgs mass exceeds about 130 GeV, our best hope is the LHC. Heavy Higgs bosons would be observed by their decay into WW or ZZ. Higher energy electron-positron colliders, or even muon colliders, could also play an important role.

Our inability to predict the mass of the Higgs boson is one of the reasons many of us believe that this picture cannot be the whole story. We are searching for extensions to the electroweak theory that make it more coherent and more predictive. Two approaches seem promising. Both of them imply a rich harvest of new particles and new phenomena at the energies we are just beginning to explore at Fermilab and CERN. One is a supersymmetric generalization of the electroweak theory that associates new particles with all the known quarks and leptons and force particles. Supersymmetry entails several Higgs bosons, and it is natural for one of them to lie in the region LEP is starting to survey. In the other approach, called dynamical symmetry breaking, the Higgs boson is not an elementary particle, but a composite whose properties we may hope to compute once we understand its constituents and their interactions.

Over the next fifteen years, we should begin to find a real understanding of the origin of mass. The interest doesn't merely lie in the arcana of accelerator experiments, but suffuses everything in the world around us, for mass is what determines the range of forces and sets the scale of all the structures we see in Nature.

See also:
Chris Quigg, "Elementary Particles and Forces," Scientific American (April 1985), pages 84-95.
Martinus J. G. Veltman, "The Higgs Boson," Scientific American (November 1986), pages 76-84.

In 1993, British Science Minister William Waldegrave challenged particle physicists to explain on a single page what the Higgs boson is and why they are so eager to find it. He awarded bottles of champagne to the authors of five winning entries at the annual meeting of the British Association for the Advancement of Science. The prize-winning papers range from serious to whimsical. They appeared in the September 1993 issue of Physics World, the monthly magazine of the British Institute of Physics, and are available online.

Related link: The Particle Adventure,  An interactive tour from the Contemporary Physics Education Project.

Chris Quigg, Theoretical Physics, Fermi National Accelerator Laboratory