Boson

I

Introduction

Boson, one of the two basic divisions of elementary particles, the basic units of matter and energy. Some bosons, called elementary bosons, are fundamental particles, meaning they cannot be divided into anything smaller. These bosons carry energy between particles of matter, affecting the behavior of matter particles and holding the particles together in larger structures. Mesons are bosons that are made of more than one particle. Bosons are named for Indian physicist Satyendra Bose, who (with German-born American physicist Albert Einstein) developed a set of equations that describe the way bosons behave. See also Elementary Particles.

II

How Bosons Differ from Fermions

Bosons differ from the other basic group of elementary particles, called fermions, in that bosons do not obey a rule of physics called the Pauli exclusion principle. The Pauli exclusion principle, developed by Austrian-born Swiss physicist Wolfgang Pauli, states that fermions that have the same characteristics cannot occupy the same region of space. Bosons do not obey the exclusion principle, so two identical bosons can occupy the same space.

Bosons also differ from fermions in a characteristic called spin. Spin is the measurement of the rotation of a particle. Rotation is measured in multiples of the constant number h/2p, where h is a number called Planck’s constant, equal to 6.626 × 10^-34 joules-sec. (The number 6.626 × 10^-34 is very small—written out, it would be a decimal point followed by 33 zeroes, then the digits 6626.) The number p is a constant approximately equal to 3.14. Different types of particles have different rotations, but the rotations of all particles are equal to a multiple of h/2p. The number that, when multiplied by h/2p represents the particle’s rotation, is called the particle’s spin. Bosons have spins that are whole numbers (0, 1, 2, and so on). Fermions have spins that are odd multiples of y (y, 1y, 2y, and so on).

III

Types of Bosons

Bosons fall into two main groups. One group contains the elementary bosons, or bosons that are not made up of other particles. Elementary bosons play a crucial role in transferring energy between the fermions that compose matter. The other group is called the mesons. Mesons are composite particles—that is, they are made up of other particles. Mesons play an important role in holding together the particles in atoms.

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A

Elementary Bosons

Elementary bosons are also called mediators. Mediators carry the four fundamental forces in nature between particles. The four fundamental forces are the electromagnetic force, the strong force, the weak force, and the gravitational force. The electromagnetic force controls interactions between particles with electric charge. The boson that carries the electromagnetic force is called the photon. The strong force holds together particles called quarks. Quarks form the particles that make up the nuclei of atoms (protons and neutrons) as well as other particles. Gluons are bosons that carry the strong force. The weak force controls how particles change into other particles, or decay. Three bosons, the W⁺, W^-, and Z bosons, carry the weak force. The gravitational force is the attraction between any objects with mass (see Gravitation). On the scale of elementary particles and atoms, the gravitational force is the weakest of the four forces. The gravitational force’s boson is the graviton. Particles interact with each other by exchanging mediating bosons. All of the known elementary bosons are mediating bosons, but physicists suspect that several more elementary bosons may exist.

The photon, the mediator of the electromagnetic force, has a spin of 1, zero mass, and no electric charge. Photons make particles with the same electric charge repel each other and particles with opposite charges attract each other. They drive particles with the same electric charge (such as two electrons, tiny negatively charged fermions) apart, because photons carry momentum. In classical physics, momentum is the product of mass and velocity. Photons have no mass, but they move very fast (at the speed of light—3 × 10⁸ m/sec, or 1 × 10⁹ ft/sec). The speed of a photon gives it enough momentum to have an impact on an electron. When two electrons repel each other, one of the electrons emits a photon, and the other electron absorbs it. The exchange of the photon pushes the electrons apart, just as two people standing on a slick surface would slide apart if they tossed a heavy object back and forth.

A gluon has a spin of 1 and no mass or electrical charge. Gluons do have another property, however, called color charge. Color charge is similar to electric charge, but it is important to the strong force instead of to the electromagnetic force. It comes in three colors and three anticolors. The possible colors are red, blue, and green. The three possible anticolors are antired (also called cyan), antiblue (also called yellow), and antigreen (also called magenta). Color charge has nothing to do with colors in the everyday world. Physicists named this property color charge because the three possible values have parallels to the three primary colors of light. For example, the combination of all three possible color charges creates a particle with no color charge, just as the three primary colors of light combine to make white light. The combination of a color and its anticolor is also colorless.

Eight possible color-charge combinations exist for gluons. Six of the possibilities are one color and a different anticolor (for example, red and antiblue, or green and antired). The two other possibilities are complicated combinations of colors and anticolors. Gluons carry the strong force between particles called quarks. Quarks, connected by gluons, make up particles called hadrons. Two families of hadrons exist: mesons and baryons. Mesons are composed of a quark and an antiquark. Antiquarks are particles whose electric charge and color charge are opposite from those of quarks. Baryons are composed of three quarks or three antiquarks. Baryons include protons and neutrons, the heavy positively charged and electrically neutral (respectively) particles that make up atoms. Gluons hold quarks together in these particles by carrying color charges between quarks, making the quarks constantly change color.

The W and Z bosons that carry the weak force are the only known elementary bosons with mass. They each have spins of 1 and no color charge. The W⁺ and W^- bosons have masses of 80 GeV/c², and the Z boson has a mass of 91 GeV/c². Physicists measure the masses of such small particles in electron volts (eV) divided by the speed of light (c) squared. One eV/c² is equal to 1.78 × 10^-36 kg (3.92 × 10^-36 lb). One GeV/c² is equal to 1 billion eV/c². The W⁺ and W^- bosons have electric charges of +1 and –1, respectively. The Z boson has no electric charge.

In a weak interaction, or an event that involves the weak force, a decaying particle changes form and emits one of the bosons that mediates the weak force. The weak boson then decays into other particles. One of the most common weak interactions is beta decay, when a proton turns into a neutron. The process begins when one of the quarks in the proton changes to another type of quark. In doing so, it emits a W⁺ boson. The change also causes the proton to become a neutron. The W⁺ boson decays almost immediately into a positron (a tiny particle with a positive charge of +1) and a particle called an electron neutrino, a very light particle with no electric charge (see Neutrino).

Physicists have not detected the graviton, the particle that may mediate the gravitational force. Theories of elementary particles support the existence of the graviton. If it exists, the graviton has a spin of 2 and no electric charge or mass. Physicists suspect that the graviton transfers gravitational force between particles much like the photon transfers electromagnetic force between particles.

Physicists suspect that several more types of elementary bosons may exist. One of the most important and most theoretically supported particles is called the Higgs boson. The theory that describes elementary particles and their interactions is called the standard model of particle physics. The standard model contains nothing that explains how or why some particles have mass and others do not. It also does not explain how particles get mass. The existence of the Higgs boson would solve that problem. Some physicists believe that the Higgs boson carries mass between particles, just as the mediating bosons carry forces between particles. The mass of the Higgs boson is expected to be relatively large compared to that of the other elementary bosons.

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