Higgs Boson
In the current standard model of particle physics (which does not account for the gravitational force), only one ingredient is yet to be directly discovered (the discovery of the tau neutrino the other missing particle was announced in July 2000). This elusive ingredient is known as the Higgs boson. The existence of the Higgs boson is indirectly predicted because most of the elementary particles have mass, which they could not have otherwise. The inclusion of the Higgs field in the standard model is an elegant solution of this problem.
The standard model of particle physics is a gauge field theory, meaning it is invariant under a certain set of symmetry transformations called gauge transformations. In effect, the gauge transformations allow the definitions of the fundamental objects of the theory (the matter fields) to be redefined differently at different points in space, without changing the observable physical effects. The upshot of this purely mathematical concept is that this invariance suggests there is an interaction between the basic objects (particles), carried by particles known as gauge bosons.
One caveat of the gauge principle is that it works only if the particles are initially assumed to be massless. If the particles have mass, then the mathematical terms associated with the particles' mass-energy are not invariant under gauge transformations, and would spoil features of the theory. It initially seemed to scientists, therefore, that gauge theories could not possibly be correct, since most matter particles have mass. In addition, the electroweak W and Z bosons were presumed not to be massless, or else they probably would have been discovered much earlier along with the photon.
Enter the Higgs boson to make order out of the confusion. A new object, called the Higgs field, was introduced. The Higgs field has a special property known as a nonzero vacuum expectation value. Most fields have the value zero when no particles of that field type are around; but the Higgs field has a nonzero constant component that exists even when no Higgs particles are in the vicinity. This constant component yields terms in the matter particles' energy formulas that looks just like mass energy. Therefore, the Higgs field gives mass to the particles. These mass terms are invariant under the gauge symmetry, because there are other Higgs field terms that cancel changes made under gauge transformations.
Because the Higgs boson has not yet been discovered, the exact form of the Higgs theory is not known. Many candidates exist, all with slightly different consequences that have not yet been measured. In the simplest version, the standard model Higgs, there are two Higgs particles, as well as their antiparticles. One of the particles is the neutral Higgs boson, but the other three are consumed by the W and Z bosons when these bosons acquire mass. In more complicated models, there are four Higgs particles and their antiparticles. These theories are called two Higgs doublet models. A special case of the two Higgs doublet models is supersymmetry, which has many other features. Each of these theories match the measured results of the standard model but have other consequences that differ. The exact form of the theory will not be known until Higgs bosons have been detected directly and these consequences have been studied experimentally.
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