Much of contemporary research in elementary particle physics is focused on the search for a particle called the Higgs boson. This particle is a critical missing piece of the present theoretical understanding of the fundamental forces of nature, which describe the interactions of the elementary constituents of matter. The forces include gravity and electromagnetism as well as two additional forces—the strong and weak nuclear forces. The nuclear forces are short-ranged and can be felt only over extremely small subatomic distance scales.
The discovery of nuclear forces and the development of a comprehensive theory to explain them have been one of the profound achievements of twentieth-century physics. The strong nuclear force is responsible for the binding of protons and neutrons inside the nucleus. The weak nuclear force is a little more mysterious. Two consequences of the weak force are beta decay (a form of natural radioactivity) and hydrogen fusion (which ultimately is responsible for the energy received from the Sun). However, initial attempts to arrive at a sound theoretical description of this force ran into trouble. Eventually, it became clear that the existence of the weak force demanded the existence of new elementary particles, which had not yet been observed in atomic experiments.
Several times in the development of the theory of the weak force and associated phenomena, theoretical physicists "invented" new elementary particles that were later discovered in the laboratory. Wolfgang Pauli invented the neutrino in 1930 in order to explain certain anomalies in beta-decay radioactivity. Twenty-six years after his bold prediction, the neutrino was discovered in the laboratory by Frederick Reines and Clyde Cowen. By 1961 a theory of the weak force had been formulated by Sheldon Glashow (and others), which invoked the existence of a new set of fundamental particles, called W and Z bosons. Indeed, the W and Z were detected for the first time in high-energy particle collisions in 1982, and their theoretically predicted properties were verified. The term boson describes a class of particles whose interactions with ordinary matter transmit a force of attraction or repulsion. For example, the electromagnetic force between charged particles is transmitted by the photon (the quantum of light). Likewise, the W and Z transmit the weak force, which is responsible for beta decay. However, unlike the photon (which has no mass), the W and Z must be very massive in order to explain the short-ranged nature of the weak force. This means that producing the W and Z in the laboratory requires very high-energy colliding particle beams. In the collision process, energy is converted to mass (as predicted by Einstein's relativity theory that asserted the equivalence of mass and energy) with sufficient collision energy to create the heavy W and Z bosons.
One aspect of Glashow's theory was troubling. The photon is massless because a deep theoretical principle, called gauge invariance, that underlies the theory of electromagnetism. Glashow's theory of the weak interactions was constructed from the same set of principles, and so it seemed to require that the W and Z should also be massless. This was inconsistent with the short-ranged nature of the weak force, which requires the W and Z to be very massive (as experimentally observed). Thus, Glashow's theory of the weak force was incomplete, since it did not provide an explanation for the masses of the W and Z particles. It was not possible to modify the theory by simply adding masses "by hand" for the W and Z . Such modifications either violate Einstein's principle of relativity or lead to nonsense predictions such as negative probabilities for scattering processes.
The key to overcoming this dilemma was found independently in 1964 by Peter Higgs; by Tom Kibble, Gerald Guralnik, and C. Richard Hagen; and by Robert Brout and Francois Englert. These physicists showed that the physics of electromagnetic fields in superconductors, as clarified by Yochiro Nambu, could be generalized to address the problem of mass generation for the carriers of forces. In a superconductor, pairs of electrons condense and organize themselves macroscopically. The superconducting metal then repels the magnetic field. The mechanism can be described mathematically as resulting from the generation of a mass for the photons that propagate within the superconducting material. Higgs and others showed that this mechanism can lead to a sensible relativistic theory with massive W and Z particles. To construct a realistic model, it was necessary to postulate further new particles to play the role of the electron condensate of the super-conductor. At a minimum, one new particle is required. It is the Higgs boson.
In 1967 Steven Weinberg and Abdus Salam constructed a theory of weak interactions based on the Higgs mechanism. The model incorporated Glashow's theory and added a Higgs boson. In doing so, they combined the theory of the electromagnetic and weak forces into a unified description, called the electroweak theory. They showed that in this theory, masses are generated for the W and Z , but the photon remains massless, exactly as required. That is, the symmetry of the gauge boson masses (which are all zero prior to invoking the Higgs mechanism) has been broken. In this case, it is said that the Higgs boson is responsible for electroweak symmetry breaking.
Further examination of the electroweak theory showed that the Higgs boson has just the right properties such that its condensate can also give mass to the electron (and other charged leptons). A subsequent generalization of the model (to incorporate the weak interactions of quarks) showed that quark masses could also be generated in a similar fashion.
It is tempting to suppose that all mass is ultimately due to the interactions of the Higgs bosons. However, that is incorrect. For example, most of the mass of the proton results from the interaction energy of the strong force among its constituent quarks. Perhaps the Higgs boson may be dispensed with entirely by generating mass for the leptons, quarks, and the W and Z bosons in a similar manner, say, by inventing a new strong subnuclear force (not yet discovered). Many theorists have tried to do this, but the results so far have been unsatisfactory. In particular, any theory of electroweak symmetry breaking is significantly constrained by experimental data that provide precision tests of electroweak phenomena. These data are in very good agreement with the simplest theory of electroweak symmetry breaking in which a single Higgs boson is added to the known fundamental particles—if the mass of the Higgs boson is less than approximately twice the mass of the Z boson. To confirm or refute this theory, one must determine whether or not the Higgs boson exists.
The most comprehensive search for the Higgs boson has been undertaken at the large electron-positron (LEP) collider at the European Laboratory for Particle Physics (CERN) in Geneva, Switzerland, with collisions above 200 billion volts of energy. If the mass of the Higgs boson were less than 1.25 times the mass of the Z , then it would have been possible to create Higgs bosons at LEP. This would have been achieved by colliding electrons and positrons, which annihilate into pure energy and then materialize as a Z boson and Higgs boson. Both the Z and the Higgs bosons are unstable, and both decay almost instantaneously into lighter elementary particles with probabilities that can be predicted from the electroweak theory. The theory of Z decay has been tested and verified to high precision at the LEP collider. After a dedicated search for the Higgs boson, the experimental collaborations at LEP announced that there was no definitive evidence of Higgs boson production in their data.
Two colliders now take aim at the potential discovery of the Higgs boson. The Fermilab Tevatron is a proton-antiproton collider, with collisions of 2 trillion volts of energy. If the Tevatron can achieve a sufficient number of collisions between 2002 and 2007, then calculations show that it may be possible to discover the Higgs boson at the Tevatron if the Higgs boson mass lies in its expected mass range. Otherwise, for a definitive discovery, physicists must wait for the Large Hadron Collider (LHC) now under construction at CERN, which is expected to begin operations in 2007. The LHC is a proton-proton collider, which will operate with collisions of 14 trillion volts of energy. When two protons collide, probability exists that some of the constituents of the two protons will annihilate into a Higgs boson. In this way, the Higgs boson will be prolifically produced— perhaps one million Higgs bosons per year! However, its discovery will not be easy.
Although produced in great numbers, each Higgs boson decays immediately into lighter elementary particles. To prove that the Higgs boson has been produced, one must reconstruct its presence from the debris it has left behind. This is not an impossible task; nevertheless, it requires particle detectors of a very specialized nature as well as extremely sophisticated data analyses. Much work has already been devoted to developing the tools and techniques necessary for this task. For example, if the Higgs boson mass is up to 1.5 times the mass of the Z (but beyond the reach of the LEP collider), then the following technique will be employed. The electroweak theory predicts that roughly one time out of a thousand, the Higgs boson (in this mass range) will decay into two photons. This would be a very distinctive event, in which the two photons have a definite and reproducible total mass equal to that of the Higgs boson. However, one must statistically differentiate such events from other more mundane (so-called "background") events in which photons are produced from the interactions of ordinary matter. Simulations have been performed suggesting that with one year of data collection at the LHC, it should be possible to discover the Higgs boson in this way. Other techniques have also been developed if the Higgs boson turns out to be heavier. Ultimately, it will be possible to discover the Higgs boson at the LHC if its mass is less than approximately ten times the mass of the Z .
Does the Higgs boson exist? Or, does the existence of mass require fundamentally new phenomena that await discovery in future experiments? The answers, although not yet known, will be discovered during the first decade of the twenty-first century.
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Howard E. Haber