Particle physics is devoted to discovering and investigating the fundamental constituents of all the matter in the universe (at all times), and the forces of nature by which these constituents interact with each other. The theoretical framework that currently aids our understanding is called the Standard Model. Although there is no experimental result that significantly disagrees with the Standard Model, it is not a complete theory; indeed, there are several reasons why the Standard Model is not a theory at all in the normal sense of the word. Perhaps the most important of these is that it contains a large number of parameters (such as the fundamental particle masses) that are not predicted but determined experimentally and then incorporated into the theoretical framework.
A principle mission of particle physics is the development of a complete, fundamental theory that unifies all of the known forces into one. Historically, this process seems to be taking place one pair of forces at a time (for example, James Clerk Maxwell famously unified electricity and magnetism in the 19th century), and an important feature of the Standard Model is that it contains the unification of the short-range weak nuclear force and the long-range electromagnetic force. The strong nuclear force is not yet unified and gravity is completely excluded from the standard model.
Although the successful implementation of electro-weak unification encourages particle physicists to consider the Standard Model as a low energy approximation of a more fundamental theory, electroweak unification comes at a price. The quantum of the electromagnetic field is the photon, which has zero rest mass. The weak field has a chargeless quantum, known as the Z particle, and a charged quantum, known as the W particle. The rest masses of the Z and the W are both almost 100 times the proton rest mass. This huge difference in the masses of the particles associated with the electromagnetic and weak force fields is known as electroweak symmetry breaking, and in order to interpret it, physicists have introduced into the Standard Model an arbitrary mathematical trick.
In a series of papers some 40 years ago, Englert and Brout (and independently, Higgs) pointed out that a symmetry-breaking idea used in the theory of superconductivity could also be used to break the electroweak symmetry. This mathematical trick is known in particle physics as the Higgs mechanism. In the theory of superconductivity, the idea explains how photons appear to acquire mass inside a superconductor. Introducing it into the Standard Model provides an explanation of where the Z and W masses come from.
The Higgs mechanism is most readily viewed in terms of a constant, ubiquitous energy field that has no preferred direction in space. This field is known as the Higgs field. It is this Higgs field that breaks the electroweak symmetry, giving mass to both Z and W while retaining a massless photon. Just as there are well-defined quanta, known as bosons, associated with each of the known force fields, so it is with the hypothetical Higgs field. In the simplest version of the Higgs mechanism, there is just one quantum and it is known as the Higgs particle or Higgs boson.
So, to recap, the Higgs particle is intimately associated with the Higgs field, and the Higgs field is assumed to be responsible for the mass difference between the quanta of the electromagnetic and weak fields. It is not too much of a stretch to expect that the masses of all matter particles (quarks and leptons) somehow get their masses from the Higgs mechanism. Indeed, the coupling of every particle to the Higgs field has strength proportional to its mass, and so this interpretation seems natural. Unfortunately, the Higgs mechanism does not reduce the number of unknown parameters of the Standard Model—there is still one per particle, plus a few more.
Curiously, the assumed Higgs particle has quite well-determined properties. Its mass, like all other particle masses, is not predicted but must be measured. As a function of an assumed mass, however, one can use the Standard Model to calculate the rate at which it will be produced in a high-energy collision of two elementary particles. The Higgs particle is expected to be highly unstable, and it is also straightforward to calculate, using the Standard Model, how it will decay. In other words, what it will decay into and with what relative rates. These Standard Model predictions provide a basis for experimental searches for the Higgs particle.
The particle experimentalists perform experiments in which they collide, at the highest available energy, protons with protons, or protons with antiprotons, or electrons with positrons. They then examine the debris of the collision for evidence of a Higgs particle whose properties match those suggested by the Standard Model.
In this manner, the experimental techniques of particle physics have been able, over the past decade or so, to eliminate the possibility of a Higgs particle with mass below about 120 proton masses or above about 200 proton masses. When the Higgs mechanism was first introduced to particle physics, the mass of the Higgs particle could have been anywhere between zero and about 1000 times the proton mass. One of the remarkable achievements of particle physics over the past few decades has been the elimination of most of this mass region. Of course, this elimination is of a statistical nature and the best thing that physicists can do is to assign a measure of the statistical confidence to these limits. In late 2002, most particle physicists were confident that if the Standard Model Higgs particle exists, its mass is probably in the mass range of approximately 120–200 proton masses (somewhere between the atomic masses of tin and gold). The Tevatron (proton-antiproton collider accelerator at Fermilab with energy close to 2 TeV) has a chance to investigate the lower 10% of this mass range; the Large Hadron Collider (LHC; proton-proton collider accelerator under construction at CERN with energy 14 TeV, scheduled to start operation in or after 2007) will finish the job. Thus, by 2020 at the latest, we will know whether there is a Higgs particle, and therefore whether Nature uses the Higgs mechanism to generate the masses of fundamental particles.
