Over the past few decades, scientists have developed a successful and well-tested description of our physical world, called the Standard Model of particle physics. It incorporates the strong, weak and electromagnetic forces into a coherent picture and describes all relevant experiments. But one test has not yet been completed: for consistency, and to account for the masses of particles, the theory requires the existence of a new field called the Higgs fieldwhich is analogous to the familiar electromagnetic field but with new kinds of properties. Just as we learn of electromagnetic fields by detecting their quanta, particles called photons, we hope to learn of the Higgs field by detecting its quanta, called Higgs bosons.
If the Higgs boson (named after British physicist Peter Higgs) were discovered, it would actually be one of the most important experimental discoveries of all time, in large part because of the unique role Higgs physics plays. On one hand, it completes the Standard Model, tying together the successful description of the microscopic physical world, which scientists have sought to understand for many centuries. This description tells us how the physical world works. At the same time, the form the Higgs physics takes points to how the Standard Model can be strengthened and extended to describe not only how the world works but why it works that way. The Higgs boson is important because it is the transition to why. The Higgs boson is also a new kind of matter, the first in a century.
Image: FERMI NATIONAL ACCELERATOR LABORATORY
There are currently two pieces of evidence that a Higgs boson does exist. The first is indirect. According to quantum field theory, all particles spend a little time as combinations of all other particles, including the Higgs boson. This changes their properties a little in ways that we know how to calculate and that have been well verified. Studies of the effect the Higgs boson has on other particles reveal that experiment and theory are consistent only if the Higgs boson exists and is lighter than around 170 giga electron volts (GeV), or about 180 proton masses. Because this is an indirect result, it is not rigorous proof. Several unknown contributions could, in principle, combine to mimic the appearance of the Higgs. That is, however, very unlikely. More concrete evidence of the Higgs came from an experiment conducted at the European laboratory for particle physics (CERN) using the Large Electron Positron (LEP) collider in its final days of operation. That research revealed a possible direct signal of a Higgs boson with mass of about 115 GeV and all the expected properties. Together these make a very convincingalthough not yet definitivecase that the Higgs boson does indeed exist.
Now, assuming the Higgs boson has indeed been discovered, let's turn to the implications for creating a more fundamental theory of particle physics. Ill refer to this fundamental theory as string theory without distinguishing various forms. String theory is formulated in 10 dimensions and has other properties that do not hold in our world. It makes no explicit or testable predictions or explanations, and it has no direct connections to Higgs bosons. Before such connections can happen, we will have to learn how our actual world can emerge from the higher dimensional theoryand how the strong, weak and electromagnetic forces, and the quarks and leptons, emerge from string theory.
Still, enough is known about string theory that there are some suggestive connections to our world. Most important, string theory seems to require our world to have a property called supersymmetry. And a supersymmetric Standard Model with string theory boundary conditions has Higgs bosons and explains their properties. Whereas the mass of the Higgs boson cannot be calculated in the Standard Model, in the supersymmetric Standard Model the mass can be calculated approximately to be 9040 GeV, a range that contains the likely discovered value.
Finding a Higgs boson thus strongly supports the supersymmetric Standard Model, which in turn supports the notion that string theory is indeed the right approach to nature. If so, it is very likely that more confirmation of the existence of the Higgs boson will be discovered in the next few years at the Fermi National Accelerator Laboratory, where data collection will begin in March 2001.