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Tom’s Infinite Science Archive: The Standard Model

Physicists have developed a theory called the Standard Model that attempts to describe all matter and forces in the universe (except for gravity). Its elegance lies in the ability to explain hundreds of particles and complex interactions in terms of a few fundamental particles and interactions.
The Standard Model unifies the nuclear, electromagnetic, and weak forces and enumerates the fundamental building blocks of the universe.

Inside an atom there are two main things, the nucleus and the electrons.

The nucleus lies in the center of the atom, it is composed of protons(p) and neutrons(n).

The electrons orbit the atom's nucleus and along with protons and neutrons, make up atoms and molecules.

Here is a diagram of inside a carbon atom:

The electron shells depicted in the diagrams in this feature represent electrons' energy levels and not how electrons move around an atom's nucleus. The diagrams also do not depict the distance between an electron and the particles in the nucleus, which are relatively great. Physicists have found that electrons do not move along a definite path, and the location of electrons as they move cannot really be measured. Electrons are found within an "electron cloud" which surrounds the nucleus.

All the particles (protons, neutrons, quarks, and electrons) are constantly in motion.

Protons have a charge of +1. Electrons have a charge of -1. Neutrons are neutral, as its name implies.

But protons and neutrons are not fundamental -- they are comprised of more fundamental particles called quarks.

While we know for sure that quarks and electrons are smaller than 10 to the power of (-18) meters, it is possible that they have no size at all. It is also possible that quarks and electrons are not fundamental but are composites of more fundamental particles.

In summary, we know that atoms are made of protons, neutrons, and electrons. Protons and neutrons are made of quarks, which are possibly made of more fundamental particles...

Physicists look for undiscovered particles in order to understand how the universe works. They always wonder if the new particles, as well as known particles, are truly fundamental.

Physicists have discovered about 200 particles (most of which are not fundamental.) To keep track of these particles, they name them with letters from the Roman and Greek alphabets.

The Key Ideas Are:

Force Carrier Particles:
Each type of fundamental force is "carried" by a force-carrier particle (an example of which is a photon.)

Matter Particles:
The Standard Model says that most matter particles we know of are actually composites of more fundamental particles called quarks. There is also another class of fundamental matter particles called leptons (an example of a lepton is the electron).

So, there are two kinds of particles: particles that are matter (like electrons, protons, neutrons, and quarks) and particles that carry forces (like photons).

What makes the Standard Model so comprehensive is that all observed particles can be explained by:

  • 6 types of leptons,
  • 6 types of quarks,
  • Force carrier particles
    And for each of the matter particles, there is a corresponding antimatter particle, (see Antimatter).


    Matter particle

    Lepton, any member of a class of elementary particles that are neutral or have unit charge, are fermions,, and are relatively small. Unlike hadrons, which are composed of quarks, leptons appear not to have any internal structure. Each member of the lepton class has an antiparticle with equal mass and opposite charge. (see Antimatter). Although all leptons are relatively light, they are not alike. The electron, for example, carries a negative charge, but is stable, meaning it does not decay into other elementary particles; the muon, which also has a negative charge, has a mass about 200 times greater than that of an electron and decays into smaller particles. Leptons interact with other particles through the weak force (the force that governs radioactive decay), the electromagnetic force, and the gravitational force. (See Atom and Atomic Theory, Neutrino, Quantum Theory - Coming Soon.)

    There are six leptons, three of which have electrical charge and three of which don't. The best known charged lepton is the electron. The other two charged leptons are the muon and the tau, which are essentially electrons with a lot more mass. The charged leptons are all negative.

    The other three leptons are the very elusive neutrinos. They have no electrical charge and little, if any, mass. There is one type of neutrino for every type of electrically charged lepton.

    Leptons are independent lonely particles, they can exist without the companionship of other particles. Quarks, on the other hand, are only found in groups.


    A type of elementary particle that, along with protons and neutrons, make up atoms and molecules. Electrons play a role in a wide variety of phenomena. The flow of an electric current in a conductor is caused by the drifting of free electrons in the conductor. Heat conduction is also primarily a phenomenon of electron activity. In vacuum tubes a heated cathode emits a stream of electrons that can be used to amplify or rectify an electric current. If such a stream is focused into a well-defined beam, it is called a cathode-ray beam (see Cathode Ray). Cathode rays directed against suitable targets produce X rays; directed against the fluorescent screen of a television tube, they produce visible images. Also, the negatively charged beta particles emitted by some radioactive substances are electrons. (See Atom and Atomic Theory - Particle Accelerators.) Electrons have a rest mass of 9.109 x 10-28 grams, and an electrical charge of negative 1.602 x 10-19 coulombs (see Electrical Units). The charge of the electron is the basic unit of electricity. Electrons are classified as fermions because they have half-integral spin. The antimatter version of the electron is the positron.


    An elementary particle in the lepton family (not a meson), having a mass 209 times that of the electron, a negative electric charge, and a mean lifetime of 2.2 × 10-6 second. Also called mu meson.


    An elementary particle of the lepton family, having a mass about 3,490 times that of the electron, a negative electric charge, and a mean lifetime of 3×10-13 seconds.


    Fundamental nuclear particle that is electrically neutral and of much smaller mass than an electron (see Atom and Atomic Theory). In spite of the neutrino's very small mass, its spin is significant, because in beta-decay processes the emission of electrons occurs in such a way that the total energy, momentum, and spin involved in the process are apparently not conserved. In order to account for this inconsistency, the Austrian physicist Wolfgang Pauli in 1930 inferred the properties of the neutrino.

    Because it has no charge and has negligible mass, the neutrino is extremely elusive; through the measurement of its recoil effect, research confirmed its peculiar properties. Conclusive proof of its existence was obtained in 1956 by the American physicists Frederick Reines and Clyde Lorrain Cowan, Jr…

    The antiparticle of the neutrino is emitted in electron beta decay, whereas the neutrino is emitted in positron beta decay. Some physicists have theorized that in a rare form of radioactivity called double beta decay, two neutrinos may sometimes merge to form a particle called a majoron. Another type of high-energy neutrino, called the muon neutrino, is produced, along with a muon, in the decay of a pion (see Elementary Particles). When a pion decays, a neutral particle must be emitted in the direction opposite that of the muon in order to conserve momentum. The original assumption was that this particle was the neutrino that conserves momentum in beta decay. In 1962, however, researchers proved that the neutrino accompanying pion decay is different. A third type of neutrino, the tau neutrino (and its antiparticle), also exists.

    Of great current interest is the possible existence of oscillating neutrinos, that is, the possibility that neutrinos can oscillate from one form to another. In 1996 physicists at Los Alamos National Laboratory detected the oscillation of a muon antineutrino into an electron antineutrino, suggesting that the neutrino had a rest mass of as yet undetermined value. This has profound implications for the physical sciences and cosmology. If the mass is greater than 25 electron volts (about 20,000 times lighter than an electron), the universe will not continue to expand but will eventually halt due to gravitation attractions and will begin to contract.


    Matter particle

    Any of six hypothetical particles that are believed to form the basic constituents of the elementary particles called hadrons, such as the proton, neutron, and pion.

    Quarks were first classified as three kinds: up, down, and strange. The proton, for example, is believed to be constituted of two up quarks and one down quark. Later theorists postulated the existence of a fourth quark; in 1974 the existence of this quark, named charm, was experimentally confirmed. Thereafter a fifth and sixth quark-called bottom and top, respectively-were hypothesized for theoretical reasons of symmetry. Experimental evidence for the existence of the bottom quark was obtained in 1977. The top quark eluded researchers until March 1995, when two teams of physicists at Fermi National Accelerator Laboratory (Fermilab) announced they had detected and measured the top quark.

    Each kind of quark has its antiparticle, and each kind of quark or antiquark comes in three types of "colors," (see Color Charge & Confinement.) Quarks can be either red, blue, or green, while antiquarks can be either antired, antiblue, or antigreen. These quark and antiquark colors have nothing to do with the colors seen by the human eye. Rather, these colors represent a quantum property. When combining to form hadrons, quarks and antiquarks can only exist in certain color groupings. The hypothetical carrier of the force between quarks is called the gluon.

    Quarks have the unusual characteristic of having fractional electric charge of either 2/3 or -1/3, unlike the -1 charge of an electron and the +1 charge of the proton. Quarks also carry another type of charge called color charge, which we will discuss later.

    Quarks only exist in groups with other quarks. Individual quarks have fractional electric charges. However, these fractional charges are never directly observed because quarks never hang out alone; instead, quarks will form composite particles called hadrons. The sum of the quarks' electric charges in a hadron is always an integer number. While individual quarks carry color charge, hadrons are color-neutral.


    A quark with a charge of +2/3 and a mass about 607 times that of the electron. It is a component of protons and neutrons.


    A quark with a charge of -1/3, a mass about 607 times that of the electron, and a downward spin. It is a component of protons and neutrons.


    A quark with a charge of +2/3, a mass about 2,900 times that of the electron, and a charm of +1.


    A quark with a charge of -13, a mass about 988 times that of the electron, and a strangeness of -1.


    A hypothetical quark with a charge of +2/3 and a mass more than 100,000 times that of the electron. Also called truth quark.

    On March 2, 1995, Fermilab announced the discovery of the top quark, the last of the six predicted quarks. The search began in 1977 when physicists found the fifth quark, bottom, at Fermilab. It took this long because the top quark was much more massive than was originally predicted, so it required a more powerful accelerator to create it.

    Although the top quark decays too fast to be observed, it does leave behind particles that give record to its existence - a top quark "signature". The top quark can decay in more than one way. Since a top quark appears only once in several billion collisions, it was necessary to perform trillions of collisions.

    Physicists still do not understand why the top is so massive. It is 40 times heavier than the next heaviest quark and about 35, 000 times heavier than the up and down quarks that make up most of the matter we see around us. In fact the question still remains why anything has mass at all. Physicists hope that the discovery of the top quark will give them insight to these questions.


    A quark with a charge of -1/3 and a mass about 10,000 times that of the electron. Also called beauty quark.

    There are two classes of hadrons:

    Baryons are any hadron made of three quarks (qqq). For example, protons are 2 up quarks and 1 down quark (uud) and neutrons are 1 up and 2 down quarks (udd).

    Mesons contain one quark:
    and one antiquark:
    For example, a negative pion:

    The Generations of Matter

    Notice that both quarks and leptons exist in 3 distinct sets. We call each of these sets a generation of matter particles. A generation is a set of one of each charge type of quark and lepton. Each generation tends to be heavier than the previous set.

    All visible matter in the universe is made from the first generation of matter particles: up and down quarks, and electrons. Second and third generation particles are unstable, and decay into first generation particles. It is for this reason that all the stable matter in the universe is made from first generation particles.

    The question then arises, if we almost never observe the higher generations of matter particles in our universe, why do they exist at all? Indeed, when the muon was discovered in 1936, the physicist I.I. Rabi asked,

    Without understanding why the second and third generation particles exist, we cannot rule out the possibility that there are more quarks and leptons, with yet larger masses. Possibly, the answer could be that quarks and leptons aren't fundamental, but are made up of yet more elementary particles whose interactions manifest as the different generations of matter.

    In summary...

    Definitions & Descriptions:


    Any of a class of elementary particles characterized by their angular momentum, or spin. According to quantum theory, the angular momentum of particles can take on only certain values, which are either whole-integer or half-integer multiples of a certain constant; h. Fermions, which include electrons, protons, and neutrons, have half-integer multiples of h, as contrasted to bosons, such as mesons, which have 0 spin. Fermions obey the exclusion principle; bosons do not.


    In physics, intrinsic angular momentum of a sub-atomic particle. In particle and atomic physics, there are two types of angular momentum: spin and orbital angular momentum. Spin is a fundamental property of all elementary particles, and is present even if the particle is not moving; orbital angular momentum results from the motion of a particle. For example, an electron in an atom has orbital angular momentum, which results from the electron's motion about the nucleus, and spin angular momentum. The total angular momentum of a particle is a combination of spin and orbital angular momentum.

    The existence of spin was suggested by the Dutch-born American physicists Samuel Abraham Goudsmit and George Eugene Uhlenbeck in 1925. The two physicists noted that certain features of the atomic spectra could not be explained by the quantum theory in use at the time; by adding an additional quantum number-the spin of the electron-Goudsmit and Uhlenbeck were able to provide a more complete explanation of atomic spectra (see Spectrum). Soon the idea of spin was extended to all sub-atomic particles, including protons, neutrons, and antiparticles (see Antimatter). Groups of particles, such as an atomic nucleus, also have spin as a result of the spin of the protons and neutrons that make up the nucleus.

    Quantum theory prescribes that spin angular momentum can only occur in certain discrete values. These discrete values are described in terms of integer or half-integer multiples of the fundamental angular momentum unit h/2p, where h is Plank's constant. In general usage, stating that a particle has spin 1/2 means that its spin angular momentum is 1/2 (h/2p). Fermions, which include protons, neutrons, and electrons, have odd half-integer spin (1/2, 3/2,…); bosons, such as photons, alpha particles, and mesons, have integer spin (0,1,…). Fermions obey the Pauli exclusion principle, while bosons do not.


    Any of a class of particles, such as the photon, pion, or alpha particle, that have zero or integral spin and obey statistical rules permitting any number of identical particles to occupy the same quantum state.

    Bosons are those particles that have an integer spin measured in the units of h-bar (spin = 0, 1, 2...).

    The following are bosons:

  • The carrier particles related to all the fundamental interactions
  • Composite particles with even numbers of fermion constituents (such as mesons)

    The nucleus of an atom is a fermion or boson depending on whether the sum of the number of protons and neutrons is odd or even. This property explains the strange behavior of very cold helium, which is a superfluid (meaning it has no viscosity, among other things) because its nuclei are bosons and may pass through each other.


    Rainbowlike series of colors, in the order violet, blue, green, yellow, orange, and red, produced by splitting a composite light, such as white light, into its component colors. Indigo was formerly recognized as a distinct spectral color. The rainbow is a natural spectrum, produced by meteorological phenomena. A similar effect can be produced by passing sunlight through a glass prism. The first correct explanation of the phenomenon was advanced in 1666 by the English mathematician and physicist Sir Isaac Newton.

    When a ray of light passes from one transparent medium, such as air, into another, such as glass or water, it is bent; upon reemerging into the air, it is bent again. This bending is called refraction; the amount of refraction depends on the wavelength of the light. Violet light, for example, is bent more than red light in passing from air to glass or from glass to air. A mixture of red and violet light is thus dispersed into the two colors when it passes through a wedge-shaped glass prism.

    A device for producing and observing a spectrum visually is called a spectroscope; a device for observing and recording a spectrum photographically is called a spectrograph; a device for measuring the brightness of the various portions of spectra is called a spectrophotometer; and the science of using spectroscopes, spectrographs, and spectrophotometers to study spectra is called spectroscopy. For extremely accurate spectroscopic measurements, an interferometer is used. During the 19th century, scientists discovered that beyond the violet end of the spectrum, radiations could be detected that were invisible to the human eye but that had marked photochemical action; these radiations were termed ultraviolet. Similarly, beyond the red end of the spectrum, infrared radiations were detected that, although invisible, transmitted energy, as shown by their ability to raise the temperature of a thermometer. The definition of spectrum was then revised to include these invisible radiations, and has since been extended to include radio waves beyond the infrared, and X rays and gamma rays beyond the ultraviolet.

    The term spectrum is often loosely applied today to any orderly array produced by analysis of a complex phenomenon. A complex sound such as noise, for example, may be analyzed into an audio spectrum of pure tones of various pitches. Similarly, a complex mixture of elements or isotopes of different atomic weights can be separated into an orderly sequence called a mass spectrum in order of their atomic weights.

    Spectroscopy has not only provided an important and sensitive method of chemical analysis, but has also been the chief tool for discoveries in the apparently unrelated fields of astrophysics and atomic theory. In general, changes in motions of the outer electrons of atoms produce spectra in the visible, infrared, and ultraviolet regions. Changes in motions of the inner electrons of heavy atoms produce X-ray spectra. Changes in the configurations of the nucleus of an atom produce gamma-ray spectra. Changes in the configurations of molecules produce visible and infrared spectra. See Atom and Atomic Theory.

    Different colors of light are similar in consisting of electromagnetic radiations that travel at a speed of approximately 300,000 km per sec (about 186,000 mi per sec). They differ in having varying frequencies and wavelengths, the frequency being equal to the speed of light divided by wavelength. Two rays of light having the same wavelength also have the same frequency and the same color. The wavelength of light is so small that it is conveniently expressed in nanometers, which are equal to one-billionth of a meter, or one-thousandth of a micrometer. The wavelength of violet light varies from about 400 to 450 mµ, and of red light from about 620 to 760 mµ, or from about 0.000016 to 0.000018 in. for violet, and from 0.000025 to 0.000030 in. for red.

    Alpha Particles

    Positively charged nuclear particle, symbol a, consisting of two protons bound to two neutrons. Alpha particles are emitted spontaneously in some types of radioactive decay. They are also produced when helium-4 atoms are completely ionized (Ion).


    Particle formed when a neutral atom or group of atoms gains or loses one or more electrons. An atom that loses an electron forms a positively charged ion, called a cation; an atom that gains an electron forms a negatively charged ion, called an anion. Atoms can be converted to ions by radiation such as X rays or light of sufficient energy. This kind of radiation is thus called ionizing radiation.


    Hadron, any member of a large class of elementary particles that interact by means of the so-called strong force-the force that not only binds protons and neutrons together in atomic nuclei but also governs hadron behavior when high-energy particles are caused to collide with them (see Particle Accelerators). Gravitation, electromagnetism, and the weak force (the force that governs radioactive decay), the other fundamental natural forces, also act on hadrons. Hadrons are composed of two classes of particle: mesons and baryons. All hadrons except protons and nuclear neutrons are unstable and decay into other hadrons. Mesons include the lighter pion and kaon particles; baryons are the heavier particles that include protons, neutrons, atomic nuclei in general, and hyperons, very heavy particles that decay into protons or neutrons. (See Atom and Atomic Theory.)


    Neutrino, fundamental nuclear particle that is electrically neutral and of much smaller mass than an electron (see Atom and Atomic Theory). In spite of the neutrino's very small mass, its spin is significant, because in beta-decay processes the emission of electrons occurs in such a way that the total energy, momentum, and spin involved in the process are apparently not conserved. In order to account for this inconsistency, the Austrian physicist Wolfgang Pauli in 1930 inferred the properties of the neutrino.

    Cathode Ray

    A high-speed electron emitted by the negative electrode of a vacuum tube when an electric current is passed through it. Cathode rays were first generated by means of the Crookes tube, an invention of the British physicist Sir William Crookes. While conducting research, the German physicist Wilhelm Conrad Roentgen in 1895 accidentally discovered that cathode rays striking a metal target produce X rays. Cathode rays can be deflected and focused by magnetic fields or by electrostatic fields. These properties are utilized in the electron microscope, in the cathode-ray oscilloscope, and in the image tube of a television receiver.

    Electrical Units

    Units used to express quantitative measurements of all types of electrostatic and electromagnetic phenomena and of the electrical characteristics of components of electrical circuits. The basic electrical units are part of the centimeter-gram-second system, but because, in most cases, these units are either too large or too small for convenient measurement, a number of practical units have been adopted for use in engineering.

    Electrostatic Units

    The elemental unit of electricity is the absolute charge on a single electron or proton. The symbol for this unit is e. The CGS unit of electrical charge is the electrostatic unit (esu), which is defined as the quantity of electricity that when concentrated at a point in a vacuum will repel a like charge 1 centimeter away with a force of 1 dyne. The esu equals the aggregate charge carried by 2,082,000,000 electrons or protons.

    The basic unit of electrical current, or flow, is the statampere, which is defined as a current of 1 electrostatic unit per second. The statvolt, the basic unit of electromotive force, or potential difference, is the difference in potential that exists between two points when 1 erg of work is required to force 1 electrostatic unit of electricity between those two points.

    Electromagnetic Units

    Besides the electrostatic units of charge, current, and potential difference, a parallel group of basic electromagnetic units exists. The basic magnetic unit, comparable to the elemental unit of electricity, is the unit magnetic pole, defined as a point magnetic pole that in a vacuum will act on a similar pole 1 centimeter away with a force of 1 dyne. The unit used to measure the strength of magnetic fields is the oersted. A field that acts on a unit magnetic pole with a force of 1 dyne has a strength of 1 oersted. The electromagnetic unit of electric current is called the abampere. If a current of 1 abampere flows in a wire 1 centimeter long, the wire is pushed sidewise with a force of 1 dyne by a magnetic field of 1 oersted acting at right angles to the wire. The abcoulomb is the quantity of electricity passing any point in a circuit in 1 second when a current of 1 abampere is flowing in the circuit. The abvolt, the electromagnetic unit of potential difference, is the potential difference between two points when 1 erg of work is necessary to move 1 abcoulomb of electricity from one point to the other.

    The mathematical relationships between the electrostatic and electromagnetic units are as follows: 1 esu equals 3.3356 × 10-11 abcoulombs; 1 statampere equals 3.3356 × 10-11 abamperes; and one statvolt equals 29,979,245,800 abvolts. This last figure is exactly equal to the velocity of light through a vacuum, which is expressed in centimeters per second, as predicted by the electromagnetic-wave theory developed by British physicist James Clerk Maxwell.

    International System Units

    The International System of Units is a system of units that are practical to use in the laboratory. They are commonly referred to as SI units, from the initials of the French words Système International. The SI unit of electrical current is the ampere (amp), which is defined as 0.1 abamperes. The SI unit of electrical quantity is the coulomb, the amount of electricity passing a given point in a circuit in 1 second when a current of 1 ampere is flowing. The volt (V) is the SI unit of potential difference. It is equal to 100 million abvolts and can be defined as the potential difference existing between two points when 1 joule (10 million ergs) of work is required to move 1 coulomb of electricity from one of the points to the other. The SI unit of electrical work is the watt. It represents the generation or use of electrical energy at the rate of 1 joule per second. The kilowatt is equal to 1000 watts.

    Resistance, Capacitance, Inductance

    All components in electrical circuits exhibit one or more of the characteristics of resistance, capacitance, and inductance. The commonly used unit of resistance is the ohm, which is the resistance of a conductor in which a potential difference of 1 volt causes a current flow of 1 ampere. The capacitance of a condenser is measured in farads. A condenser of 1 farad capacitance will exhibit a change in potential difference of 1 volt between its plates when 1 coulomb of electricity is transferred from one plate to the other. The henry (H) is the unit of inductance. A coil has a self-inductance of 1 henry when a change in current of 1 ampere per second produces a countervoltage of 1/V. In a transformer, or in any two magnetically coupled circuits, a mutual induction of 1 henry is that inductance which will induce a voltage of 1 volt in the secondary when there is a change of 1 ampere per second in the primary.

    Unit Standards

    The standards for electrical units are maintained by national standards laboratories. Originally, the volt was defined in terms of a standard voltaic cell, called the Weston cell, which has poles of cadmium amalgam and mercurous sulfate and an electrolyte of cadmium sulfate. A volt was defined as 0.98203 of the potential of this standard cell at 20° C (68° F). This definition is still used by laboratories in daily measurements. For more accurate standards, the Josephson effect, a phenomenon involving discrete voltage steps, is used to define the volt. The ohm was originally defined by using a collection of standard resistors. Today, the quantum Hall effect, which involves a constant resistance that is independent of experimental conditions, is used to define the ohm. The other electrical units are defined based on these more accurate values for the volt and the ohm.

    In all the SI electrical units, the conventional prefixes of the metric system are used to indicate fractions and multiples of the basic units. Thus, a micromicrofarad is a trillionth of a farad, a microampere is a millionth of an ampere, a millivolt is a thousandth of a volt, a millihenry is a thousandth of a henry, a kilowatt is 1000 watts, and a megohm is 1 million ohms.


    An elementary antimatter particle having a mass equal to that of an electron and a positive electrical charge equal in magnitude to the charge of the electron. The positron is sometimes called a positive electron or anti-electron. Electron-positron pairs are formed if cosmic rays or gamma rays of energies of more than 1 million electron volts are made to strike particles of matter. The reverse of the pairing process, called annihilation, is initiated when an electron and a positron interact, destroying each other and producing gamma rays.

    The existence of the positron was first suggested in 1928 by the British physicist Paul Adrien Maurice Dirac as a necessary consequence of his quantum-mechanical theory of electron motion. In 1932 the American physicist Carl David Anderson experimentally confirmed the existence of the positron. See Atom and Atomic Theory; Elementary Particles.


    In particle physics, the mutual destruction of elementary particles by their antiparticles (see Antimatter), with the release of energy in the form of gamma rays or very short-lived particles. An example is the annihilation of an electron when it collides with its positively charged antiparticle, a positron. Annihilation of heavier particles, as in the collision of a neutron and an antineutron, is a more complex type of phenomenon.


    In physics, a hypothetical subatomic particle that mediates the attractive force among quarks. Most particle physicists agree that all the elementary particles in the large class called hadron (which includes the proton) are made of various combinations of (probably) six types of quark. These quarks are in turn thought to be held together by the exchange of possibly eight types of gluon, or field quanta. (Some theorists, however, propose a "diquark" model that does not require gluons.) This branch of particle physics is called quantum chromodynamics.

    Quantum Chromodynamics

    Quantum Chromodynamics or Qcd, in physics, theory that attempts to account for the behavior of the theoretical particles called quarks and gluons in forming the elementary particles known as hadrons. Mathematically, QCD is quite similar to the quantum electrodynamics theory of electromagnetic interactions; it seeks to provide an equivalent basis for the strong nuclear force that binds particles into atomic nuclei. The prefix "chromo-" refers to "color," a mathematical property assigned to quarks.


    Class of heaviest elementary particles that includes nucleons (neutrons and protons) and the heavier, unstable hyperons: the lambda, sigma, xi, and omega particles. The antiparticles of this class are termed antibaryons. The strong force, the force responsible for the cohesion of nucleons in atomic nuclei, expresses itself through an interaction among baryons. The law of conservation of baryons, a fundamental physical law, states that for any interaction of elementary particles, the sum of baryon numbers remains unchanged. Baryons are assigned the baryon number of +1, antibaryons -1, and nonbaryons 0.

    Baryons always contain three quarks, and at any time may also contain some gluons and quark-antiquark pairs. A proton = uud and a neutron = udd.

    Each quark in a baryon rapidly exchanges color charges with other quarks in that baryon. However, baryon (and all hadrons) have no net color charge because the different color charges cancel each other out.

    They have spins of 1/2, 3/2, ... so they are fermions.

    For each baryon there is an antimatter baryon (antibaryon) made of the 3 corresponding antiquarks.


    A proton or a neutron, especially as part of an atomic nucleus.


    Nuclear particle having a positive charge identical in magnitude to the negative charge of an electron and, together with the neutron, a constituent of all atomic nuclei (see Atom and Atomic Theory). The proton is also called a nucleon, as is the neutron. The proton forms, by itself, the nucleus of the hydrogen atom. The mass of a proton is approximately 1836 times that of an electron, or 1.6726 × 10-24 g. Consequently, the mass of an atom is contained almost entirely in the nucleus. The proton has an intrinsic angular momentum, or spin, and thus a magnetic moment. In addition, the proton obeys the exclusion principle. The number of protons in the nucleus of an atom determines what element it is; the atomic number of an element denotes the number of protons in the nucleus. In nuclear physics, the proton is used as a projectile in large accelerators to bombard nuclei to produce fundamental particles (see Particle Accelerators). As the hydrogen ion, the proton plays an important role in chemistry.

    The antiproton, the antiparticle of the proton, is also called a negative proton. It differs from the proton in having a negative charge and not being a constituent of atomic nuclei. The antiproton is stable in a vacuum and does not decay spontaneously. When an antiproton collides with a proton or a neutron, however, the two particles are transformed into mesons, which have an extremely short half-life. Although physicists had postulated the existence of this elementary particle since the 1930s, the antiproton was positively identified for the first time in 1955 at the University of California Lawrence Berkeley National Laboratory.

    Because protons are essential parts of ordinary matter, they are obviously stable. Particle physicists are nevertheless interested in learning whether protons eventually decay after all, on a time scale of many billions of billions of years. This interest derives from current attempts at grand unification theories that would combine all four fundamental interactions of matter in a single scheme. Many of these attempts call for the ultimate instability of the proton, so research groups at a number of accelerator facilities are conducting tests to detect such decays. By the end of the 1980s no clear evidence had yet been found; possible results thus far can also be interpreted in other ways.


    Uncharged particle, one of the fundamental particles of which matter is composed. The mass of a neutron is 1.0086654 atomic mass units (amu). The existence of the neutron was predicted in 1920 by the British physicist Ernest Rutherford and by Australian and American scientists, but experimental verification of its existence was exceedingly difficult because the net charge on the neutron is zero.


    The neutron was first identified in 1932 by the British physicist Sir James Chadwick, who correctly interpreted the results of experiments conducted at that time by the French physicists Irène and Frédéric Joliot-Curie and other scientists. The Joliot-Curies had produced what Chadwick recognized as neutrons by the interaction of alpha particles with beryllium nuclei. When this newly discovered radiation was passed through paraffin wax, collisions between the neutrons and the hydrogen atoms in the wax produced readily detectable protons.


    The neutron is a constituent particle of all nuclei of mass number greater than 1; that is, of all nuclei except ordinary hydrogen (see Atom and Atomic Theory). Free neutrons-those outside of atomic nuclei-are produced in nuclear reactions. They can be ejected from atomic nuclei at various speeds or energies and are readily slowed down to very low energy by a series of collisions with light nuclei, such as those of hydrogen, deuterium, or carbon. (For the role of neutrons in the production of atomic energy. When expelled from the nucleus, the neutron is unstable and decays to form a proton, an electron, and a neutrino. Like the proton and the electron, the neutron possesses angular momentum, or spin. Neutrons act as small, individual magnets; this property enables beams of polarized neutrons to be created. The neutron has a negative magnetic moment of -1.913141 nuclear magnetons. Its half-life was fixed approximately at 10.61 minutes.

    The antiparticle of a neutron, known as an antineutron, has the same mass, spin, and beta-decay constant. These particles are sometimes the result of the collisions of antiprotons with protons, and they possess a magnetic moment equal and opposite to that of the neutron. According to current particle theory, the neutron and the antineutron-and other nuclear particles-are themselves composed of quarks (see Quark).

    Neutron Radiography

    An increasingly important application of reactor-generated neutrons is in neutron radiography, by which information is obtained by determining the absorption of a beam of neutrons emanating from a nuclear reactor or a powerful radioisotope source. The technique resembles X-ray radiography. Many substances, however, such as metals that are opaque to X rays, will transmit neutrons; other substances (particularly hydrogen compounds) that transmit X rays are opaque to neutrons. A neutron radiograph is made by exposing a thin foil to a beam of neutrons that has penetrated the test object. The neutrons leave an invisible radioactive "picture" of the object on the foil. A visible picture is made by placing a photographic film in contact with the foil. A direct, televisionlike technique for viewing images has also been developed.

    First used in Europe in the 1930s, neutron radiography has been employed widely in the U.S. since the 1950s for examining the fuel and other components of nuclear reactors. More recently it has been used in examining explosive devices and the components of space vehicles. Beams of neutrons are widely used now in the physical and biological sciences and in technology, and neutron activation analysis is an important tool in such diverse fields as paleontology, archaeology, and art history.


    A semistable or unstable baryon with mass greater than the neutron.


    A lambda hyperon.


    A sigma hyperon.

    Xi hyperon

    Either of two subatomic particles in the baryon family, one neutral and one negatively charged, with masses of 2,573 and 2,585 times that of the electron and average lifetimes of 2.9 × 10-10 and 1.6 × 10-10 second. Also called xi.

    Omega hyperon

    A subatomic particle in the baryon family having a rest mass of 3,272 times that of the electron, a unit negative electron charge, and an average lifetime of 8 × 10-11 second. Also called omega.


    Cohesion, phenomenon of intermolecular forces holding particles of a substance together. Cohesion differs from adhesion in being the force of attraction between adjacent particles within the same body; adhesion is the interaction between the surfaces of different bodies. The force of cohesion in gases can be observed in the liquefaction of a gas, which is the result of a number of molecules being pressed together to produce forces of attraction high enough to give a liquid structure.

    Cohesion in liquids is reflected in the surface tension caused by the unbalanced inward pull on the surface molecules, and also in the transformation of a liquid into a solid state when the molecules are brought sufficiently close together. Cohesion in solids depends on the pattern of distribution of atoms, molecules, and ions, which in turn depends on the state of equilibrium (or lack of it) of the atomic particles. In many organic compounds, which form molecular crystals, for example, the atoms are bound strongly into molecules, but the molecules are bound weakly to each other.


    Any member of a class of elementary particles that particulate in strong interaction. A meson is composed of a quark, and an antiquark, and binding gluons and have masses generally intermediate between leptons and baryons.

    A meson is a color-neutral object, since its quark and antiquark have opposite color charges. Thus, a meson can be found in isolation. All mesons are unstable.

    Since a meson has an integer spin, it is a boson. The spin is made of the spins of the quarks, plus a contribution from their motion around each other. For example notice that a pion and a rho have the same quarks, but different spins and masses.


    A semistable meson produced either in a neutral form with a mass 264 times that of an electron and a mean lifetime of 8.4 × 10-17 second or in a positively charged form with a mass 273 times that of an electron and a mean lifetime of 2.6 × 10-8 second. Also called pi meson.


    An unstable meson produced either in an electrically charged form with a mass 966 times that of an electron or in a neutral form with a mass 974 times that of an electron as a result of a high-energy particle collision. Also called K-meson.

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