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

emission of a gluon,
just as an electron can shift its phase only by emitting a photon. The gluon,
propagating at the speed of light, is then absorbed by another quark, which will have its color shifted in exactly the way needed to compensate for the original change. Suppose, for example, a red quark changes its color to green and in the process emits a gluon that bears the colors red and antigreen. The gluon is then absorbed by a green quark, and in the ensuing reaction the green of the quark and the antigreen of the gluon annihilate each other, leaving the second quark with a net color of red. Hence in the final state as in the initial state there is one red quark and one green quark.
Because of the continual arbitration of the gluons there can be no net change in the color of a hadron, even though the quark colors vary freely from point to point. All hadrons remain white, and the strong force is nothing more than the system of interactions needed to maintain that condition.
    In spite of the complexity of the gluon fields, quantum electrodynamics and quantum chromodynamics are remarkably similar in form. Most notably the photon and the gluon are identical in their spin and in their lack of mass and electric charge. It is curious, then, that the interactions of quarks are very different from those of electrons.
    Both electrons and quarks form bound states, namely atoms for the electrons and hadrons for the quarks. Electrons,
however, are also observed as independent particles; a small quantity of energy suffices to isolate an electron by ionizing an atom. An isolated quark has never been detected. It seems to be impossible to ionize a hadron, no matter how much energy is supplied. The quarks are evidently bound so tightly that they cannot be pried apart; paradoxically,
however, probes of the internal structure of hadrons show the quarks moving freely, as if they were not bound at all.
    Gluons too have not been seen directly in experiments. Their very presence in the theory provokes objections like those raised against the pure, massless Yang-Mills theory. If massless particles that so closely resemble the photon existed,
they would be easy to detect and they would have been known long ago.
Of course, it might be possible to give the gluons a mass through the Higgs mechanism. With eight gluons to be concealed in this way, however, the project becomes rather cumbersome.
Moreover, the mass would have to be large or the gluons would have been produced by now in experiments with high-energy accelerators; if the mass is large, however, the range of the quarkbinding force becomes too small.
    ----
    POLARIZATION OF THE VACUUM explains to some extent the peculiar force law that seems to allow quarks complete
freedom of movement within a hadron but forbids the isolation of quarks or gluons. In quantum electrodynamics
( below ) pairs of virtual electrons and antielectrons surround any isolated charge,
such as an electron. Because of electrostatic forces the positively charged antielectrons tend to remain nearer
the negtive electron charge and thereby cancel part of it. The observed electron charge is the difference between the "bare"
charge and the screening charge of virtual antielectrons. Similarly, pairs of virtual quarks diminish the strength of
the force between a real quark and a real antiquark. In quantum chromodynamics, however, there is a competing effect
not found in quantum electrodynamics. Because the gluon also has a color charge (whereas the photon has no electric charge),
virtual gluons also have an influence on the magnitude of the color force between quarks. The gluons do not screen the quark
charge but enhance it. As a result the color charge is weak and the quarks move freely as long as they are close. At long range
infinite energy may be needed to separate two quarks.

    Illustration by Allen Beechel
----
    Atentative resolution of this quandary has been discovered not by modifying the color fields but by examining