The Higgs Boson: Searching for the God Particle (3 page)

Camouflage

Within this polarized vacuum, however, the quark itself continuously emits and reabsorbs gluons, thereby changing its color. The color-charged gluons propagate to appreciable distances.
In effect they spread the color charge throughout space, thus camouflaging the quark that is the source of the charge. The smaller an arbitrary region of space centered on the quark is, the smaller will be the proportion of the quark's color charge contained in it. Thus the color charge felt by a quark of another color will diminish as it approaches the first quark. Only at a large distance will the full magnitude of the color charge be apparent.

In QCD the behavior of the strong force represents the net effect of screening and camouflage. The equations of QCD yield a behavior that is consistent with the observed paradox of quarks: they are both permanently confined and asymptotically free. The strong interaction is calculated to become extraordinarily strong at appreciable distances, resulting in quark confinement, but to weaken and free quarks at very close range.

In the regime of short distances that is probed in high-energy collisions, strong interactions are so enfeebled that they can be described using the methods developed in the context of QED for the much weaker electromagnetic interaction. Hence some of the same precision that characterizes QED can be imparted to QCD. The evolution of jets of hadrons from a quark and an antiquark generated in electron-positron anhthilation, for example, is a strong interaction. QCD predicts that if the energy of the collision is high enough, the quark and the antiquark moving off in opposite d irections may generate not two but three jets of hadrons. One of the particles will radiate a gluon, moving in a third direction. It will also evolve into hadrons, giving rise to a third distinct jet–a feature that indeed is common'
ly seen in high-energy collisions.

The three jets continue along paths set by quarks and gluons moving within an extremely confined space, less than 10
^-13
centimeter. The quark-antiquark pair cannot proceed as isolated particles beyond that distance, the limit of asymptotic freedom. Yet the confinement of quarks and of their interactions is not absolute. Although a hadron as a whole is color-neutral, its quarks do respond to the individual color charges of quarks in neighboring hadrons. The interaction, feeble compared with the color forces within hadrons, generates the binding force that holds the protons and neutrons together in nuclei.

Moreover, it seems likely that when hadronic matter is compressed and heated to extreme temperatures, the hadrons lose their individual identities.
The hadronic bubbles of the image used above overlap and merge, possibly freeing their constituent quarks and gluons to migrate over great distances.
The resulting state of matter, called q uark-gluon plasma, may exist in the cores of collapsing supernovas and in neutron stars. Workers are now studying the possibility of creating quark-gluon plasma in the laboratory through collisions of heavy nuclei at very high energy.

Electroweak Symmetry

Understanding of the third interaction that elementary-particle physics must reckon with, the weak interaction, also has advanced by analogy with QED. In 1933 Enrico Fermi constructed the first mathematical description of the weak interaction, as manifested in beta radioactivity, by direct analogy with QED. Subsequent work revealed several important differences between the weak and the electromagnetic interactions. The weak force acts only over distances of less than 10
^-16
centimeter (in contrast to the long range of electromagnetism), and it is intimately associated with the spin of the interacting particles.
Only particles with a left-handed spin are affected by weak interactions in which electric charge is changed, as in the beta decay of a neutron, whereas right-handed ones are unaffected.

In spite of these distinctions theorists extended the analogy and proposed that the weak interaction, like electromagnetism, is carried by a force particle, which came to be known as the intermediate boson, also called the
W
(for weak) particle. In order to mediate decays in which charge is changed, the
W
boson would need to carry electric charge. The range of a force is inversely proportional to the mass of the particle that transmits it;
because the photon is massless, the electromagnetic interaction can act over infinite distances. The very short range of the weak force suggests an extremely massive boson.

A number of apparent connections between electromagnetism and the weak interaction, including the fact that the mediating particle of weak interactions is electrically charged, encouraged some workers to propose a synthesis. One immediate result of the proposal that the two interactions are only different manifestations of a single underlying phenomenon was an estimate for the mass of the
W
boson.
The proposed unification implied that at very short distances and therefore at very high energies the weak force is equal to the electromagnetic force.
Its apparent weakness in experiments done at lower energies merely reflects its short range. Therefore the whole of the difference in the apparent strengths of the two interactions must be due to the mass of the
W
boson. Under that assumption the
W
boson's mass can be estimated at about 100 times the mass of the proton.

To advance from the notion of a synthesis to a viable theory unifying the weak and the electromagnetic interactions has required half a century of experiments and theoretical insight, culminating in the work for which Sheldon Lee Glashow and Steven Weinberg, then at Harvard University, and Abdus Salam of the Imperial College of Science and Technology in London and the International Center for Theoretical Physics in Trieste won the 1979 Nobel prize in physics. Like QED itself, the unified, or electroweak, theory is a gauge theory derived from a symmetry principle, one that is manifested in the family groupings of quarks and leptons.

Not one but three intermediate bosons, along with the photon, serve as force particles in electro weak theory.
They are the positively charged
W+
and negatively charged
W-
bosons, which respectively mediate the exchange of positive and negative charge in weak interactions, and the Z
⁰
particle, which mediates a class of weak interactions known as neutral current processes. Neutral current processes such as the elastic scattering of a neutrino from a proton, a weak interaction in which no charge is exchanged, were predicted by the electroweak theory and first observed at CERN in 1973 . They represent a further point of convergence between electromagnetism and the weak interaction in that electromagnetic interactions do not change the charge of participating particles either.

To account for the fact that the electromagnetic and weak interactions, although they are intimately related, take different guises, the electroweak theory holds that the symmetry uniting them is apparent only at high energies.
At lower energies it is concealed. An analogy can be drawn to the magnetic behavior of iron. When iron is warm, its molecules, which can be regarded as a set of infinitesimal magnets, are in hectic thermal motion and therefore randomly oriented. Viewed in the large the magnetic behavior of the iron is the same from all directions, reflecting the rotational symmetry of the laws of electromagnetism. When the iron cools below a critical temperature, however, its molecules line up in an arbitrary direction, leaving the metal magnetized along one axis. The symmetry of the underlying laws is now concealed.

The principal actor in the breaking of the symmetry that unites electromagnetism and the weak interaction at high energies is a postulated particle called the Higgs boson. It is through interactions with the Higgs boson that the symmetry-hiding masses of the intermediate bosons are generated. The Higgs boson is also held to be responsible for the fact that quarks and leptons within the same family have different masses. At very high energies all quarks and leptons are thought to be massless; at lower energies interactions with the Higgs particle confer on the quarks and leptons their varying masses. Because the Higgs boson is elusive and may be far more massive than the intermediate bosons themselves, experimental energies much higher than those of current accelerators probably will be needed to produce it.

The three intermediate bosons required by the electro weak theory, however, have been observed. Energies high enough to produce such massive particles are best obtained in head-on collisions of protons and antiprotons.
In one out of about five million collisions a quark from the proton and an antiquark from the antiproton fuse, yielding an intermediate boson.
The boson disintegrates less than 10
^-24
second after its formation. Its brief existence, however, can be detected from its decay products.

In a triumph of accelerator art, experimental technique and theoretical reasoning, international teams at CERN led by Carlo Rubbia of Harvard and Pierre Darriulat devised experiments that in 1983 detected the
W
bosons and the Z
⁰
particle. An elaborate detector identified and recorded in the debris of violent proton-antiproton collisions single electrons whose trajectory matched the one expected in a W- particle's decay; the detector also recorded electrons and positrons traveling in precisely opposite directions, unmistakable evidence of the Z
⁰
particle.
For their part in the experiments and in the design and construction of the proton-antiproton collider and the detector Rubbia and Simon van der Meer of CERN were awarded the 1984 Nobel prize in physics.

Unification

With QCD and the electroweak theory in hand, what remains to be understood?
If both theories are correct, can they also be complete? Many observations are explained only in part, if at all, by the separate theories of the strong and the electroweak interactions.
Some of them seem to invite a further unification of the strong, weak and electromagnetic interactions.

Among the hints of deeper patterns is the striking resemblance of quarks and leptons. Particles in both groups are structureless at current experimental resolution. Quarks possess color charges whereas leptons do not, but both carry a half unit of spin and take part in electromagnetic and weak interactions.
Moreover, the electroweak theory itself suggests a
relation between quarks and leptons. Unless each of the three lepton families
(the electron and its neutrino, for example) can be linked with the corresponding family of quarks
(the
u
and
d
quarks, in their three colors) the electroweak theory will be beset with mathematical inconsistencies.

What is known about the fundamental forces also points to a
unification.
All three can be described by gauge theories, which are similar in their mathematical structure.
Moreover, the strengths of the three forces appear likely to converge at very short distances, a
phenomenon that would be apparent only at extremely large energies.
We have seen that the electromagnetic charge grows strong at short distances, whereas the strong, or color, charge becomes increasingly feeble.
Might all the interactions become comparable at some gigantic energy?

If the interactions are fundamentally the same, the distinction between quarks, which respond to the strong force, and leptons, which do not, begins to dissolve.
In the simplest example of a unified theory, put forward by Glashow and Howard Georgi of Harvard in 1974, each matched set of quarks and leptons gives rise to an extended family containing all the various states of charge and spin of each of the particles.

The mathematical consistency of the proposed organization of matter is impressive.
Moreover, regularities in the scheme require that electric charge be apportioned among elementary particles in multiples of exactly 1/3, thereby accounting for the electrical neutrality of stable matter.
The atom is neutral only because when quarks are grouped in threes, as they are in the nucleus, their individ ual charges combine to give a
charge that is a
precise integer, eq ual and opposite to the charge of an integral number of electrons.
If quarks were unrelated to leptons, the precise relation of their electric charges could only be a
remarkable coincidence.