Bosons

Type of physical science: Elementary particle (high-energy) physics

Field of study: Systematics (particle physics)

A boson is a particle with integer spin that is not subject to the Pauli exclusion principle. The four fundamental interactions in nature are each mediated by a boson: electromagnetism is mediated by the photon, the weak interaction by the intermediate vector bosons (W⁺, W^-, and Z⁰), the strong interaction by the gluon, and the gravitational interaction by the graviton.

Overview

All elementary particles—electrons, photons, neutrons, and protons—possess an intrinsic angular momentum called spin. Spin is an important distinguishing feature of elementary particles, and there is a separate quantum number assigned to a particle in order to indicate the exact value of its spin. While the spin is physically related to the internal angular momentum, it can also be understood as an indicator of the symmetry of the particle. A spin 0 particle—that is, one with an internal angular momentum equal to zero—has complete symmetry, meaning that it looks the same from any angle. This is the type of symmetry possessed by a point or a sphere. A particle with one unit of angular momentum, called a spin 1 particle, has symmetry through a rotation of 360 degrees. A spin 2 particle has symmetry through a rotation of 180 degrees.

As quantum mechanics developed, so many different particles were discovered that physicists became discouraged and began to refer to the rather disorganized catalog as the "particle zoo." Great efforts were made to find groups or families within the vast population of particles to bring some order to the zoo. One very important step in this direction was the realization that the vast array of particles could be naturally divided into two fundamental classes, based on the numerical value of the spin. Particles with a half-integer spin (s =1/2) are called fermions and obey the Pauli exclusion principle, which states that no two identical fermions can be in the same quantum state. (A quantum state is a particular physical configuration with unique values of energy, angular momentum, and spin.) Particles with integral spin (s = 0, 1, 2) are called bosons and are not subject to the exclusion principle. Fermions, consisting of leptons and quarks, are the building blocks of matter. Bosons, on the other hand, are the force carriers, traveling back and forth between the fermions and carrying the messages that tell the particles how to behave in response to the various forces at work.

Fermionic matter, following the instructions of bosonic force messengers, causes all the diverse physical phenomena in the universe, from telling neutrons when to decay to creating black holes. It is difficult to visualize the action of the bosonic force carriers. A repulsive force can be compared to two free-floating astronauts tossing a ball back and forth in space. Each time they catch or throw the ball, they move further apart. Without the ball, they cannot move. In this analogy, the ball is the force-carrying boson and the astronauts are the fermions. An attractive force can be compared to the two astronauts pulling the ball from each other.

It was an extremely significant development in fundamental quantum physics when theory and experiment converged with the notion that the fundamental interactions in nature are mediated by special force-carrying particles, all of which are bosons. Physicists began an aggressive search for comprehensive theories that would explain exactly how this process works. The first theory to be so articulated was quantum electrodynamics, developed in the 1950s by Richard P. Feynman, Shin'ichiro Tomonaga, and Julian Schwinger, who won the 1965 Nobel Prize in Physics for their work.

Quantum electrodynamics, or QED as it is more often called, states that the familiar electromagnetic interaction, first described classically by James Clerk Maxwell in 1864, is communicated between charged particles through the exchange of photons, massless bosons with one unit of spin. Thus, when two electrons interact and are influenced by their mutual electrical repulsion, this interaction is communicated through photons that travel back and forth between the two electrons at the speed of light, pushing them apart. These photons are not directly detectable because they are "virtual" photons. Virtual particles have a very short lifetime, their brief existence permitted by a temporary violation of the conservation of energy through the Heisenberg uncertainty principle. This principle allows a particle such as a virtual photon to come into existence out of the vacuum as long as the product of its energy and its lifetime is less than Planck's constant. Occasionally, an electromagnetic disturbance, such as the de-excitation of an atom, will be large enough to release some energy, and a "real" (nonvirtual) photon will be produced with no upper limit on its lifetime. This photon can be detected using a simple experimental apparatus such as a photomultiplier tube or even photographic film. The photon was the first of the force-carrying bosons to be discovered, although it was not initially assigned this function.

Because the photon has no mass, it can travel at the speed of light and is unrestrained by the speed limits imposed on mass-possessing particles by special relativity. It also has an unlimited range, which is the reason that the electromagnetic force extends to infinity. The single unit of spin possessed by the photon is the result of the specific nature of the electromagnetic interaction, which is a dipole (two-pole) interaction. A typical example of a dipole interaction is when an electron moves from a large to a small orbit in an atom. Because this movement involves a rearrangement of opposite charges (the positively charged nucleus and the negatively charged electron), the laws of quantum mechanics show that the particle emitted during the rearrangement will have a single unit of spin.

The explanation of the electromagnetic force provided by the theory of QED was so compelling and in such extraordinary agreement with experiments that QED became the model that inspired theories of the other fundamental forces, the first of which was the strong force. The strong force holds quarks together through the exchange of bosonic force carriers known as gluons. The source of the force, in analogy with the electrical charge of QED, is the color "charge" of the quarks. This color charge, which is unrelated to anything visible, gives this theory its name: quantum chromodynamics, or QCD ("chromo" meaning "color"). QCD is a more complicated theory than QED and requires eight different gluons to mediate the various interactions associated with the color of the quarks.

Unlike photons, which can travel an infinite distance, gluons are confined to a very small region of space about the size of the nucleus, or 3 x 10¹⁵ meters. Thus, the force does not extend very far in space and, in fact, is confined within the nucleus, making the strong nuclear force completely undetectable under normal conditions. According to QCD, gluons and the quarks that they bind are confined inside the nucleus and can never be observed directly. There is considerable indirect evidence, however, for the existence of both quarks and gluons. Gluons are massless and possess no electrical charge, although they do possess color charge.

The weak nuclear interaction is responsible for radioactive decay and is communicated through particles known as weak bosons (weakons) or, more typically, intermediate vector bosons. There are three intermediate vector bosons, identified as W⁺, W^-, and Z⁰, with electrical charges of +1, -1, and 0, respectively. These bosons were predicted on the basis of theory and were discovered by Carlo Rubbia and Simon van der Meer at the European Organization for Nuclear Research (CERN) in 1983. Rubbia and van der Meer won the 1984 Nobel Prize in Physics, an unusually prompt recognition of an unusually significant discovery.

Intermediate vector bosons are very heavy and require more than one billion electronvolts of energy to produce. Because of their great mass, these bosons have a very short lifetime, about ten to twenty seconds, after which they decay into other particles, and they cause the range of the weak interaction to be very short, less than 10^-15 meter. The short lifetime of these particles prevents them from traveling very far, thus accounting for the short range of the weak interaction.

The most elusive of the bosonic force carriers is the graviton, the carrier of the gravitational force. This particle has never been observed, and there is only a speculative theoretical argument for its existence, based largely on the assumption that there must exist a theory of quantum gravity with the graviton as its messenger. It is known that Albert Einstein's theory of general relativity breaks down under the extreme conditions of small size or tremendous force, such as can be found in black holes. Many theoretical physicists believe that it should be possible to replace the classical theory of gravitation with a quantum theory that would not break down at these extremes. Such a theory might arise out of the long-sought union of the gravitational interaction with the other quantum interactions.

The graviton, if it exists, must have a spin of two, because it is produced by quadrupole (four-pole) interactions, compared to the other force-carrying bosons, which are all spin 1 because they are produced by dipole interactions. It is widely expected to be massless, like the photon, since it is known that the range of gravity is infinite, although some physicists believe that it may have a very small but nonzero mass instead. It is evident that the graviton is not confined like the gluon and is not short of range like intermediate vector bosons. Therefore, there should be numerous gravitons around, since the gravitational interaction is rather commonplace. Yet finding a graviton has not proved to be easy. The problem lies in the great strength of the earth's gravity, which overwhelms any experimental attempts to isolate an individual gravitational interaction and study it. Nevertheless, many experimental physicists believe that the graviton exists and eagerly search for it.

Applications

Scientists are very interested in the elementary constituents of the universe and the laws governing their interactions. The study of gravity began when Sir Isaac Newton first wrote down his universal law of gravitation in the seventeenth century. Yet Newton was aware that while he had a good description of gravity, he did not have an explanation. He still wondered how the gravitational interaction was conveyed from one mass to another and across such tremendous distances in the universe. In the same way, while the other interactions in nature—electromagnetic, strong nuclear, and weak nuclear—have been known for some time, there has always been a certain amount of mystery surrounding their behavior. How do the quarks inform one another of their locations so that the strong interaction can occur? How does the electron in an atomic orbit know where the nucleus is? What tells the neutron to decay into other particles?

A good theory of the four fundamental interactions must provide satisfactory answers to these questions. The enunciation of the boson quartet has seemingly provided such an explanation by suggesting, either theoretically or experimentally, that a force-carrying particle is responsible for these fundamental interactions. In addition, each of the theories associated with each of these particles seems to explain the nature of the interaction that it conveys.

Gravity is by far the weakest of the four interactions, as it is about forty-one orders of magnitude weaker than the strong interaction. How, then, can gravity be responsible for all the large-scale structures in the universe, while the strong force, by comparison, barely makes a contribution at all? The theories describing bosons answer these and other important questions.

Gravity is the most significant interaction on the scale of the universe because its range is infinite and its effects are cumulative. Unlike the nuclear interactions, which are of a very short range, or the electromagnetic interaction, which tends to have an average of zero because there can be both positive and negative interactions, gravity goes on forever and all the individual interactions add together. Thus, one can talk about the total mass in the universe and how it affects the expansion of the universe.

The third strongest interaction is the weak nuclear force, which is 10²⁸ times as strong as gravity. Its effects, however, are restrained to a very small region inside the nucleus. Because the particles that convey the weak force are massive, they decay rapidly into other particles that do not convey it. Next in line is the electromagnetic interaction, which is 10³⁹ times as strong as gravity. It is infinite in range but not very powerful on the scale of the universe, or any large scale. Because the positive charge in a region generally tends to be balanced by the negative charge, a net cancellation of the electromagnetic interaction results. For example, the earth and the sun do not have an appreciable electromagnetic interaction because neither possesses a net charge. The strongest of the four interactions is the strong nuclear force, which is 10⁴¹ times as strong as gravity. This force is confined to the nucleus, however, so its effects do not reach the macroscopic world. The theory of QCD explains how the strong force becomes stronger with distance rather than weaker, thus containing its power within the nucleus.

Context

Perhaps the most significant dimension to the study of elementary particles and fundamental interactions has been the prospect of constructing a single unified theory that would contain all the partial fundamental theories within it. Such a theory is known as a "theory of everything," and the task of developing such a theory is, according to Stephen Hawking, the end of physics. Progress on this daunting task started inadvertently in 1864 when Maxwell showed that electricity and magnetism, universally believed to be fundamentally distinct interactions, were really two manifestations of the more general theory that came to be known as electromagnetism. The simplification provided by this unification was so attractive that Einstein spent the latter portion of his career trying (unsuccessfully) to duplicate the feat by uniting his general theory of relativity with Maxwell's electromagnetism.

The modern understanding of bosonic force carriers is an essential component of the search for a theory of everything. Already theorists have been able to show that the electromagnetic and weak nuclear interactions are actually manifestations of a more comprehensive electroweak interaction. In electroweak theory, the photon joins with the intermediate vector bosons as one of the four conveyors of the electroweak interaction.

The large mass of the intermediate vector bosons was initially problematic, as theoretically, due to the symmetry of the electroweak interaction, all of its bosons should be massless, not just photons. In the 1960s, several physicists proposed the existence of the Higgs mechanism, in which a field called the Higgs field exists throughout the universe and breaks the symmetry of the electroweak force. The observed differences between the W⁺, W^-, and Z⁰ and the photon, including the mass of the former, are the result of this broken symmetry; when the symmetry is restored, at temperatures approximately 10¹⁵ kelvins and above, the photon becomes indistinguishable from the other particles.

The Higgs mechanism remained purely theoretical until 2012, when scientists at CERN confirmed the existence of the Higgs boson, a previously hypothetical particle that would have to exist in order for the corresponding Higgs field to exist. While the discovery of the Higgs boson does not prove that the Higgs mechanism is correct, it does add validity to the model.

There have been a number of attempts to merge the electroweak theory with the strong interaction, which would entail the adoption of the electroweak-interaction carriers by the gluon family. This larger unified theory is called a grand unified theory (GUT), and several versions of GUTs await experimental confirmation, some through the undetected decay of the proton. Interactions in nature are conveyed by means of bosons, and these bosons seem to be related to one another in ways that suggest that they are a part of some large family. Even though a GUT has still not been confirmed, some physicists attempt to go beyond it to a true theory of everything as they search for some way to join the undiscovered graviton to the GUT family. The search for simplicity in nature could lead physicists to the construction of a true "family tree": a way to link all the bosons to some simple ancestor.

Principal terms

BOSON: a particle with an integer spin that obeys the laws of Bose-Einstein statistics; all the particles in nature that mediate interactions are bosons

GLUON: the massive, colored carrier of the strong nuclear interaction; the eight different gluons all have one unit of spin

GRAVITON: the hypothetical massless, chargeless carrier of the gravitational interaction that is believed to have two units of spin; this particle has not yet been observed experimentally

INTERMEDIATE VECTOR BOSON: the massive, charged carrier of the weak nuclear interaction; the three different intermediate vector bosons (the W⁺, W^-, and Z⁰, with charges of +1, -1, and 0, respectively) all have one unit of spin and have been directly observed in high-energy accelerator experiments

PHOTON: the massless, chargeless carrier of the electromagnetic interaction that has one unit of spin

SPIN: the quantum-mechanical property of elementary particles that indicates the internal angular momentum of a particle; spin is also closely related to a particle's symmetry

Bibliography

ATLAS Collaboration. "Observation of a New Particle in the Search for the Standard Model Higgs Boson with the ATLAS Detector at the LHC." Physics Letters B 716.1 (2012): 1–29. Print.

Carrigan, Richard A., and W. Peter Trower, eds.Particles and Forces: At the Heart of the Matter. New York: Freeman, 1990. Print. This book is a collection of articles reprinted from Scientific American, and many of the articles are excellent introductions to bosons and other elementary particles. In particular, "Elementary Particles and Forces" by Chris Quigg and "Quarks with Color and Flavor" by Sheldon Glashow are superb. Contains several other relevant articles as well.

CMS Collaboration. "Observation of a New Boson at a Mass of 125 GeV with the CMS Experiment at the LHC." Physics Letters B 716.1 (2012): 30–61. Print.

Crease, Robert P., and Charles C. Mann. The Second Creation: Makers of the Revolution in Twentieth-Century Physics. Rev. ed. New Brunswick: Rutgers UP, 1996. Print. An extraordinary presentation of particle physics in the twentieth century. The authors interviewed many of the scientists involved and have produced a colorful introduction to the theory that is accessible to the nonscientist. Highly recommended.

Grissom, Thomas. The Physicist's World: The Story of Motion and the Limits to Knowledge. Baltimore: Johns Hopkins UP, 2011. Print.

Ne'eman, Yuval, and Yoram Kirsh. The Particle Hunters. 2nd ed. New York: Cambridge UP, 1996. Print. At a slightly more technical level, this book presents an insider's view of particle physics. Very well written, with many figures and diagrams.

Ostdiek, Vernon J., and Donald J. Bord. Inquiry into Physics. 7th ed. Boston: Brooks, 2013. Print. This is a well-written standard physical-science text that presents a cross section of physical ideas in nonmathematical language. The sections on particle physics are particularly helpful and contain a variety of excellent diagrams.

Riordan, Michael. The Hunting of the Quark: A True Story of Modern Physics. New York: Simon, 1987. Print. Written by a physicist who was involved in many of the experiments that helped to establish the modern theory of particles and their interactions. Riordan knows all the major scientists personally and has provided an interesting insider's view.

Trefil, James S. From Atoms to Quarks: An Introduction to the Strange World of Particle Physics. Rev. ed. New York: Anchor, 1994. Print. An excellent introduction to the physics of particles and their interactions by one of the most gifted popularizers of science. The numerous diagrams make the interactions between the particles very clear.

Trippe, Sascha. "A Simplified Treatment of Gravitational Interaction on Galactic Scales." Journal of the Korean Astronomical Society 46.1 (2013): 41–47. Print.

Grand Unification Theories and Supersymmetry

Group Theory and Elementary Particles

Leptons and the Weak Interaction

Quarks and the Strong Interaction

The Unification of the Weak and Electromagnetic Interactions