The Higgs Boson: Searching for the God Particle Read online

  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 Z0 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 Z0 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 Z0 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

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  KINSHIP OF ALL MATTER is implied by unified theories of the fundamental forces; one branch of a unified family of elementary particles is shown here. Particles that are equivalent within a theory can metamorphose into one another. Because leptons, such as the electron and the neutrino, respond to the electroweak force alone whereas quarks also respond to the strong force, the two kinds of particle are not equivalent in current theory, and transformations of one into the other have not been observed (left). If the simplest unified theories are correct and the fundamental forces are ultimately identical, then at some very high energy quarks and leptons are interconvertible (right). Known transformations are mediated by force particles such as the W bosons and the glouns; transitions between the quark and lepton groups would be mediated by new force particles, here given as X and Y.

  Illustration by Andrew Christie

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  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.

  In such a unification only one gauge to describe all the interactions of matter. In a gauge theory each particle in a set can be transformed into any other particle. Transformations of quarks into other quarks and of leptons into other leptons, mediated by gluons and intermediate bosons, are familiar. A unified theory suggests that quarks can change into leptons and vice versa. As in any gauge theory, such an interaction would be mediated by a force particle: a postulated X or Y boson. Like other gauge theories, the unified theory describes the variation over distance of interaction strengths. According to the simplest of the unified theories, the separate strong and electroweak interactions converge and become a single interaction at a distance of 10-29 centimeter, corresponding to an energy of 1024 electron volts.

  Such an energy is far higher than may ever be attained in an accelerator, but certain consequences of unification might be apparent even in the lowenergy world we inhabit. The supposition that transformations can cross the boundary between quarks and leptons implies that matter, much of whose mass consists of quarks, can decay. If, for example, the two u quarks in a proton were to approach each other closer than 10-29 centimeter, they might combine to form an X boson, which would disintegrate into a positron and a d antiquark. The antiquark would then combine with the one remaining quark of the proton, ad quark, to form a neutral pion, which itself would quickly decay into two photons. In the course of the process much of the proton's mass would be converted into energy.

  The observation of proton decay would lend considerable support to a unified theory. It would also have interesting cosmological consequences. The universe contains far more matter than it does antimatter. Since matter and antimatter are equivalent in almost every respect, it is appealing to speculate that the universe was formed with equal amounts of both. If the number of baryons – three-quark particles such as the proton and the neutron, which constitute the bulk of ordinary matter – can change, as the decay of the proton would imply, then the current excess of matter need not represent the initial state of the universe. Originally matter and antimatter may indeed have been present in equal quantities, but during the first instants after the big bang, while the universe remained in a state of extremely high energy, processes that alter baryon number may have upset the balance.

  A number of experiments have been mounted to search for proton decay. The large unification energy implies that the mean lifetime of the proton must be extraordinarily long—1030 years or more. To have a reasonable chance of observing a single decay it is necessary to monitor an extremely large number of protons; a key feature of proton-decay experiments has therefore been large scale. The most ambitious experiment mounted to date is an instrumented tank of purified water 21 meters on a side in the Morton salt mine near Cleveland. During almost three years of monitoring none of the water's more than 1033 protons has been observed to decay, suggesting that the proton's lifetime is even longer than the simplest unified theory predicts. In some rival theories, however, the lifetime of the proton is considerably longer, and there are other theories in which protons decay in ways that would be difficult to detect in existing experiments. Furthermore, results from other experiments hint that protons can indeed decay.

  Open Questions

  Besides pointing the way to a possible unification the standard model, consisting of QCD and the electroweak theory, has suggested numerous sharp questions for present and future accelerators. Among the many goals for current facilities is an effort to test the predictions of QCD in greater detail. Over the next decade accelerators with the higher energies needed to produce the massive W and Z0 bosons in adequate numbers will also add detail to electroweak theory. It would be presumptuous to say these investigations will turn up no surprises. The consistency and experimental successes of the standard model at familiar energies strongly suggest, however, that to resolve fundamental issues we need to take a large step up in interact ion energy from the several hundred GeV (billion electron volts) attainable in the most powerful accelerators now being built.

  Although the standard model is remarkably free of inconsistencies, it is incomplete; one is left hungry for further explanation. The model does not account for the pattern of quark and lepton masses or for the fact that although weak transitions usually observe family lines, they occasionally cross them. The family pattern itself remains to be explained. Why should there be three matched sets of quarks and leptons? Might there be more?

  Twenty or more parameters, constants not accounted for by theory, are required to specify the standard model completely. These include the coupling strengths of the strong, weak and electromagnetic interactions, the masses of the quarks and leptons, and parameters specifying the interactions of the Higgs boson. Furthermore, the apparently fundamental constituents and force particles number at least 34: 15 quarks (five flavors, each in three colors), six leptons, the photon, eight gluons, three intermediate bosons and the postulated Higgs boson. By the criterion of simplicity the standard model does not seem to represent progress over the ancient view of matter as made up of earth, air, fire and water, interacting through love and strife. Encouraged by historical precedent, many physicists account for the diversity by proposing that these seemingly fundamental particles are made up of still smaller particles in varying combinations.

  There are two other crucial points at which the standard model seems to falter. Neither the separate theories of the strong and the electroweak interactions nor the conjectured unification of the two takes any account of gravity. Whether gravity can be described in a quantum theory and unified with the other fundamental forces remains an open question. Another basic deficiency of the standard model concerns the Higgs boson. The electroweak theory requires that the Higgs boson exist but does not specify precisely how the particle must interact with other particles or even what its mass must be, except in the broadest terms.

  The Superconducting Supercollider

  What energy must we reach, and what new instruments do we need, to shed light on such fundamental problems? The questions surrounding the Higgs boson, although they are by no means the only challenges we face, are particularly well defined, and their answers will bear on the entire strategy of unification. They set a useful target for the next generation of machines.

  It has been proposed that the Higgs boson is not an elementary particle at all but rather a composite object made up of elementary constituents analogous to quarks and leptons but subject to a new kind of strong interaction, often called technicolor, which would confine them within about 10-17 centimeter. The phenomena that would reveal such an interaction would become apparent at energies of about 1 TeV (trillion electron volts). A second approach to the question of the Higgs boson's mass and behavior employs a postulated principle known as supersymmetry, which relates particles that differ in spin. Supersymmetry entails the existence of an entirely new set of elusive, extremely massive particles. The new particles would correspond to known quarks, leptons and bosons but would differ in their spins. Because of their mass, such particles would reveal themselves fully only in interactions taking place at very high energy, probably about 1 TeV.

  Our best hope for producing interactions of fundamental particles at energies of 1 TeV is an accelerator known as the Superconducting Supercollider (SSC). Formally recommended to the Department of Energy in 1983 by the High Energy Physics Advisory Panel, it would incorporate proved technology on an unprecedented scale. A number of designs have been put forward, but all envision a proton-proton or proton-antiproton collider. High-energy beams of protons are prod uced more readily with current technology than beams of electrons and positrons, although electron-positron collisions are generally simpler to analyze; because protons are composite particles, their collisions yield a larger variety of interactions than collisions of electrons and positrons. Another common feature of the designs is the use of supercond
ucting magnets, first employed on a large scale in the Tevatron Collider at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill. The technology increases the field strength and lowers the power consumption of the magnets that bend and confine the beam.

  One of the more compact designs incorporates niobium-titanium alloy magnets cooled to 4.4 degrees Celsius above absolute zero. If the magnets generated fields of five tesla (l00,000 times the strength of the earth's magnetic field), two counterrotating beams of protons accelerated to energies of 20 TeV (needed to produce 1-TeV interactions of the quarks and gluons within the protons) could be confined within a loop about 30 kilometers in diameter. In other designs magnetic fields are lower and the proposed facility is correspondingly larger.

  It is believed such a device could be operational in 1994, at a cost of $3 billion. The Department of Energy has encouraged the establishment of a Central Design Group to formulate a specific construction proposal within three years and is currently funding the development of magnets for the SSC at several laboratories.

  The SSC represents basic research at unprecedented cost on an unmatched scale. Yet the rewards will be proportionate. The advances of the past decade have brought us tantalizingly close to a profound new understanding of the fundamental constituents of nature and their interactions. Current theory suggests that the frontier of our ignorance falls at energies of about 1 TeV. Whatever clues about the unification of the forces of nature and the constituents of matter wait beyond that frontier, the SSC is likely to reveal them.