The Higgs Boson: Searching for the God Particle (8 page)

The Weinberg-Salam-Ward model actually embraces both the weak force and electromagnetism. The conjecture on which the model is ultimately founded is a postulate of local invariance with respect to isotopic spin; in order to preserve that invariance four photon-like fields are introduced, rather than the three of the original Yang-Mills theory.
The fourth photon could be identified with some primordial form of electromagnetism.
It corresponds to a separate force, which had to be added to the theory without explanation. For this reason the model should not be called a unified field theory. The forces remain distinct;
it is their intertwining that makes the model so peculiar.

At the outset all four of the fields in the Weinberg-Salam-Ward model are of infinite range and therefore must be conveyed by massless quanta; one field carries a negative electric charge, one carries a positive charge and the other two fields are neutral. The spontaneous symmetry breaking introduces four Higgs fields, each field represented by a scalar particle. Three of the Higgs fields are swallowed by Yang-Mills particles, so that both of the charged Yang-Mills particles and one of the ne utral ones take on a large mass. These particles are collectively named massive intermediate vector bosons, and they are designated
W+, W-
and
Z
⁰
. The fourth 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.

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. The binding of a quark and an antiquark would then be explained on the principle that opposite charges attract, just as they do in the hydrogen atom. The structure of the baryons, however, is a deeper enigma.
To explain how three quarks can form a bound state one must assume that three like charges attract.

The theory that has evolved to explain the strong force prescribes exactly these interactions. The analogue of electric charge is a property called color (although it can have nothing to do with the colors of the visible spectrum). The term color was chosen because the rules for forming hadrons can be expressed succinctly by requiring all allowed combinations of quarks to be "white," or colorless. The quarks are assigned the primary colors red, green and blue;
the antiq uarks have the complementary
"anticolors" cyan, magenta and yellow.
Each of the quark flavors comes in all three colors, so that the introduction of the color charge triples the number of distinct quarks.

From the available quark pigments there are two ways to create white : by mixing all three primary colors or by mixing one primary color with its complementary anticolor. The baryons are made according to the first scheme: the three quarks in a baryon are required to have different colors, so that the three primary hues are necessarily represented.
In a meson a color is always accompanied by its complementary anticolor.

The theory devised to account for these baffling interactions is modeled directly on quantum electrodynamics and is called quant um chromodynamics.
It is a non-Abelian gauge theory.
The gauge symmetry is an invariance with respect to local transformations of quark color.

It is easy to imagine a global color symmetry. The quark colors, like the isotopic-spin states of hadrons, might be indicated by the orientation of an arrow in some imaginary internal space.
Successive rotations of a third of a turn would change a quark from red to green to blue and back to red again. In a baryon,
then, there would be three arrows,
with one arrow set to each of the three colors. A global symmetry transformation,
by definition, must affect all three arrows in the same way and at the same time. For example, all three arrows might rotate clockwise a third of a turn.
As a result of such a transformation all three quarks would change color, but all observable properties of the hadron would remain as before. In particular there would still be one quark of each color, and so the baryon would remain colorless.

Quantum chromodynamics requires that this invariance be retained even when the symmetry transformation is a local one. In the absence of forces or interactions the invariance is obviously lost. Then a local transformation can change the color of one quark but leave the other quarks unaltered, which would give the hadron a net color. As in other gauge theories, the way to restore the invariance with respect to local symmetry operations is to introduce new fields.
In quantum chromodynamics the fields needed are analogous to the electromagnetic field but are much more complicated;
they have eight times as many components as the electromagnetic field has. It is these fields that give rise to the strong force.

The quanta of the color fields are called gluons (because they glue the quarks together). There are eight of them, and they are all massless and have a spin angular momentum of one unit.
In other words, they are massless vector bosons like the photon. Also like the photon the gluons are electrically neutral,
but they are not color-neutral. Each gluon carries one color and one anticolor.
There are nine possible combinations of a color and an anticolor, but one of them is equivalent to white and is excluded, leaving eight distinct gluon fields.

The gluons preserve local color symmetry in the following way. A quark is free to change its color, and it can do so independently of all other quarks, but every color transformation must be accompanied by the 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.