The Higgs Boson: Searching for the God Particle by Scientific American Editors

Yang-Mills particle, which is a neutral one,
remains massless: it is the photon of electromagnetism. Of the Higgs particles, the three that lend mass to the Yang-Mills particles become ghosts and are therefore unobservable, but the last Higgs particle is not absorbed, and it should be seen if enough energy is available to produce it.
    The most intriguing prediction of the model was the existence of the Z 0 , a particle identical with the photon in all respects except mass, which had not been included in any of the earlier, provisional accounts of the weak force. Without the Z 0 any weak interaction would necessarily entail an exchange of electric charge. Events of this kind are called charged-weak-current events. The Z 0 introduced a new kind of weak interaction,
a neutral-weak-current event. By exchanging a Z 0 , particles would interact without any transfer of charge and could retain their original identities.
Neutral weak currents were first observed in 1973 at CERN .
    The elaboration of a successful gauge theory of the strong interactions, which are uniq ue to hadrons, could not be undertaken until a fundamental fact about the hadrons was understood: they are not elementary particles. A model of hadrons as composite objects was proposed in 1963 by Murray Gell-Mann of the California Institute of Technology;
a similar idea was introduced independently and at about the same time by Yuval Ne'eman of Tel Aviv University and George Zweig of Cal Tech. In this model hadrons are made up of the smaller particles Gell-Mann named quarks. A hadron can be built out of quarks according to either of two blueprints.
Combining three q uarks gives rise to a baryon, a class of hadrons that includes the proton and the neutron.
Bind ing together one quark and one antiq uark makes a meson, a class typified by the pions. Every known hadron can be accounted for as one of these allowed combinations of quarks.
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    QUARK MODEL describes all hadrons, indluding the proton and the nuetron, as being composite particles
made up of the similar entitites called quarks. In the original form of the model the quarks were assumed
to come in three "flavors," labled u , d and s ,
each of which is now said to have three possible "colors," red,
green and blue. There are also antiquarks with the corresponding anticolors cyan, magenta and yellow. The
interactions of the quarks are now described by means of a guage theory based on invariance with respect
to local transformations of color. Sixteen fields are needed to hold this invariance. They are taken in pairs
to make up eight massless vector bosons, called gluons, each bearing a combination of color and anticolor.

    Illustration by Allen Beechel
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    In the original model there were just three kinds of quark, designated "up,"
"down" and "strange." James D. Bjorken of the Stanford Linear Accelerator Center and Sheldon Lee Glashow of Harvard soon proposed adding a fourth quark bearing a property called charm.
In 1971 a beautiful argument by Glashow,
John Iliopoulos of Paris and Luciano Maiani of the University of Rome showed that a quark with charm is needed to cure a discrepancy in the gauge theory of weak interactions. Charmed quarks, it was concluded, must exist if both the gauge theory and the quark theory are correct. The discovery in 1974 of the J or psi particle, which consists of a charmed quark and a charmed antiquark,
s upported the Weinberg-SalamWard model and persuaded many physicists that the quark model as a whole should be taken seriously. It now appears that at least two more "flavors," or kinds, of quark are needed; they have been labeled "top" and "bottom."
    The primary task of any theory of the strong interactions is to explain the peculiar rules for building hadrons out of quarks. The structure of a meson is not too difficult to account for: since the meson consists of a quark and an antiquark, it is merely necessary to assume that the quarks carry some property analogous to electric charge.