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Subatomic Particles

Subatomic particles

Subatomic particles are particles that are smaller than an atom. In 1940, the number of subatomic particles known to science could be counted on the fingers of one hand: protons, neutrons, electrons, neutrinos, and positrons. The first three particles were known to be the building blocks from which atoms are made: protons and neutrons in atomic nuclei and electrons in orbit around those nuclei. Neutrinos and positrons were somewhat peculiar particles discovered outside Earth's atmosphere and of uncertain origin or significance.

That view of matter changed dramatically over the next two decades. With the invention of particle accelerators (atom-smashers) and the discovery of nuclear fission and fusion, the number of known subatomic particles increased. Scientists discovered a number of particles that exist at energies higher than those normally observed in our everyday lives: sigma particles, lambda particles, delta particles, epsilon particles, and other particles in positive, negative, and neutral forms. By the end of the 1950s, so many subatomic particles had been discovered that some physicists referred to their list as a "particle zoo."

The quark model

In 1964, American physicist Murray Gell-Mann (1929 ) and Swiss physicist George Zweig (1937 ) independently suggested a way out of the particle zoo. They suggested that the nearly 100 subatomic particles that had been discovered so far were not really elementary (fundamental) particles. Instead, they suggested that only a relatively few elementary particles existed, and the other subatomic particles that had been discovered were composed of various combinations of these truly elementary particles.

Words to Know

Antiparticles: Subatomic particles similar to the proton, neutron, electron, and other subatomic particles, but having one property (such as electric charge) opposite them.

Atomic mass unit (amu): A unit of mass measurement for small particles.

Atomic number: The number of protons in the nucleus of an atom.

Elementary particle: A subatomic particle that cannot be broken down into any simpler particle.

Energy levels: The regions in an atom in which electrons are most likely to be found.

Gluon: The elementary particle thought to be responsible for carrying the strong force (which binds together neutrons and protons in the atomic nucleus).

Graviton: The elementary particle thought to be responsible for carrying the gravitational force.

Isotopes: Forms of an element in which atoms have the same number of protons but different numbers of neutrons.

Lepton: A type of elementary particle.

Photon: An elementary particle that carries electromagnetic force.

Quark: A type of elementary particle.

Spin: A fundamental property of all subatomic particles corresponding to their rotation on their axes.

The truly elementary particles were given the names quarks and leptons. Each group of particles, in turn, consists of six different types of particles. The six quarks, for example, were given the rather fanciful names of up, down, charm, strange, top (or truth), and bottom (or beauty). These six quarks could be combined, according to Gell-Mann and Zweig, to produce particles such as the proton (two up quarks and one down quark) and the neutron (one up quark and two down quarks).

In addition to quarks and leptons, scientists hypothesized the existence of certain particles that "carry" various kinds of forces. One of those particles was already well known, the photon. The photon is a strange type of particle with no mass that apparently is responsible for the transmission of electromagnetic energy from one place to another.

In the 1980s, three other force-carrying particles were also discovered: the W+, W, and Z0 bosons. These particles carry certain forces that can be observed during the radioactive decay of matter. (Radioactive elements spontaneously emit energy in the form of particles or waves by disintegration of their atomic nuclei.) Scientists have hypothesized the existence of two other force-carrying particles, one that carries the strong force, the gluon (which binds together protons and neutrons in the nucleus), and one that carries gravitational force, the graviton.

Five important subatomic particles

For most beginning science students, the five most important sub-atomic particles are the proton, neutron, electron, neutrino, and positron. Each of these particles can be described completely by its mass, electric charge, and spin. Because the mass of subatomic particles is so small, it is usually not measured in ounces or grams but in atomic mass units (label: amu) or electron volts (label: eV). An atomic mass unit is approximately equal to the mass of a proton or neutron. An electron volt is actually a unit of energy but can be used to measure mass because of the relationship between mass and energy (E = mc2).

All subatomic particles (indeed, all particles) can have one of three electric charges: positive, negative, or none (neutral). All subatomic particles also have a property known as spin, meaning that they rotate on their axes in much the same way that planets such as Earth do. In general, the spin of a subatomic particle can be clockwise or counterclockwise, although the details of particle spin can become quite complex.

Proton. The proton is a positively charged subatomic particle with an atomic mass of about 1 amu. Protons are one of the fundamental constituents of all atoms. Along with neutrons, they are found in a very concentrated region of space within atoms referred to as the nucleus.

The number of protons determines the chemical identity of an atom. This property is so important that it is given a special name: the atomic number. Each element in the periodic table has a unique number of protons in its nucleus and, hence, a unique atomic number.

Neutron. A neutron has a mass of about 1 amu and no electric charge. It is found in the nuclei of atoms along with protons. The neutron is normally a stable particle in that it can remain unchanged within the nucleus for an infinite period of time. Under some circumstances, however, a neutron can undergo spontaneous decay, breaking apart into a proton and an electron. When not contained with an atomic nucleus, the half-life for this changethe time required for half of any sample of neutrons to undergo decayis about 11 minutes.

The nuclei of all atoms with the exception of the hydrogen-1 isotope contain neutrons. The nuclei of atoms of any one element may contain different numbers of neutrons. For example, the element carbon is made of at least three different kinds of atoms. The nuclei of all three kinds of atoms contain six protons. But some nuclei contain six neutrons, others contain seven neutrons, and still others contain eight neutrons. These forms of an element that contain the same number of protons but different numbers of neutrons are known as isotopes of the element.

Electron. Electrons are particles carrying a single unit of negative electricity with a mass of about 1/1800 amu, or 0.0055 amu. All atoms contain one or more electrons located in the space outside the atomic nucleus. Electrons are arranged in specific regions of the atom known as energy levels. Each energy level in an atom may contain some maximum number of electrons, ranging from a minimum of two to a maximum of eight.

Electrons are leptons. Unlike protons and neutrons, they are not thought to consist of any smaller particles but are regarded themselves as elementary particles that cannot be broken down into anything simpler.

All electrical phenomena are caused by the existence or absence of electrons or by their movement through a material.

Neutrino. Neutrinos are elusive subatomic particles that are created by some of the most basic physical processes of the universe, like decay of radioactive elements and fusion reactions that power the Sun. They were originally hypothesized in 1930 by Swiss physicist Wolfgang Pauli (19001958). Pauli was trying to find a way to explain the apparent loss of energy that occurs during certain nuclear reactions.

Neutrinos ("little neutrons") proved very difficult to actually find in nature, however. They have no electrical charge and possibly no mass. They rarely interact with other matter. They can penetrate nearly any form of matter by sliding through the spaces between atoms. Because of these properties, neutrinos escaped detection for 25 years after Pauli's prediction.

Then, in 1956, American physicists Frederick Reines and Clyde Cowan succeeded in detecting neutrinos produced by the nuclear reactors at the Savannah River Reactor. By 1962, the particle accelerator at Brookhaven National Laboratory was generating enough neutrinos to conduct an experiment on their properties. Later, physicists discovered a second type of neutrino, the muon neutrino.

Traditionally, scientists have thought that neutrinos have zero mass because no experiment has ever detected mass. If neutrinos do have a mass, it must be less than about one hundred-millionth the mass of the proton, the sensitivity limit of the experiments. Experiments conducted during late 1994 at Los Alamos National Laboratory hinted at the possibility that neutrinos do have a very small, but nonzero, mass. Then in 1998, Japanese researchers found evidence that neutrinos have at least a small mass, but their experiments did not allow them to determine the exact value for the mass.

In 2000, at the Fermi National Accelerator Laboratory near Chicago, a team of 54 physicists from the United States, Japan, South Korea, and

Greece detected a third type of neutrino, the tau neutrino, considered to be the most elusive member of the neutrino family.

Positron. A positron is a subatomic particle identical in every way to an electron except for its electric charge. It carries a single unit of positive electricity rather than a single unit of negative electricity.

The positron was hypothesized in the late 1900s by English physicist Paul Dirac (19021984) and was first observed by American physicist Carl Anderson (19051991) in a cosmic ray shower. The positron was the first antiparticle discoveredthe first particle that has properties similar to protons, neutrons, and electrons, but with one property exactly the opposite of them.

[See also Antiparticle; Atom; Electron ]

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subatomic particles

subatomic particles Particles smaller than atoms or are of the types that make up atoms. They can be divided into two groups: the hadrons, such as protons and neutrons, which can be further subdivided; and elementary particles, such as quarks and electrons, which cannot be further divided.

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Subatomic Particles

Subatomic Particles

Discovery of particles

Quark model

Subatomic particle classifications

Elementary matter particles

Elementary mediator particles

Baryons

Mesons

Current and future research

Resources

Subatomic particles are particles that are smaller than an atom. Early in the twentieth century, electrons, protons, and neutrons were thought to be the only subatomic particles; these were also thought to be elementary (i.e., incapable of being broken down into yet smaller particles). However, the list of subatomic particles has now been expanded to include a large number of elementary particles and the particles they can be combined to make.

There are two types of elementary particles. One type makes up matter. Examples of these particles include quarks (which make up protons and neutrons) and electrons. Baryons and mesons are combinations of quarks and are considered subatomic particles. The most famous baryons are protons and neutrons.

The other elementary particles are mediators of the fundamental forces. These mediator particles enable the matter particles to interact with each other. That is, when two electrons collide, they do not simply bounce off of each other like two billiard balls: they exchange a photon (one of the mediator particles). All forces, including gravity, are thought to be mediated by particle exchanges.

Discovery of particles

Electrons

The first subatomic particle to be discovered was the electron. Other scientists had deduced the existence of a negatively charged particle in what were called cathode rays (and which are now known to be beams of electrons). However, English physicist J. J. Thomson (18561940), in 1897, measured the velocity and charge-to-mass ratio of these particles. The charge-to-mass ratio was found to be relatively large, and independent of the gas used in his experiments, which indicated to him that he had found a true particle. Thomson gave it the name corpuscle, which was later changed to electron.

The charges of all particles are traditionally measured in terms of the size of the charge of the electron. The electron has a charge, e, of 1.6×10-19 Coulombs (a unit of electrical charge named for French physicist, Charles-Augustin de Coulomb [17361806]).

Photons

The first mediator particle to be discovered was the photon. In 1900, German physicist Max Planck (1858 1947) reported that light came in little packages of energy, which he called quanta. In 1905, German-American physicist Albert Einstein (18791955) studied the photoelectric effect and proposed that radiation is quantized by its naturethat is, transfers energy in minimal packets termed quanta. A photon (the name was coined by American chemist Gilbert Lewis [18751946] in 1926) is one of these quanta, the smallest possible piece of energy in a light wave. (The word wave is applied by physicists to describe some observable aspects of the behavior of light, while the particle terminology of the photon is applied to describe others. Both words convey mental pictures that are useful in some physical applications, but neither picture is sufficient: a photon is not a particle in the sense of a perfectly round, hard, self-contained sphere, nor is light a wave in the sense of being a smooth undulation in some medium.)

Protons

The proton was one of the earliest particles known. (The word proton is Greek for the first one.) In 1906, the first clues to the nature of the proton were seen. Thomson reported detecting positively charged hydrogen atoms. These were in fact, hydrogen nuclei (protons), but atomic structure was not understood at the time. Thomson thought that protons and electrons were randomly scattered throughout the atom, the so-called plum-pudding model. In 19091911, English physicist Ernest Rutherford (18711937) and his colleagues, German physicist Hans Wilhelm Geiger (18821947) and New Zealand physicist Ernest Marsden (18881970) did their famous scattering experiments involving alpha particles (two protons and two neutrons; a helium-atom nucleus) shot through gold foil. From their observations of the angles at which alpha particles were deflected, they deduced that atoms had relatively hard and small centers, thus proving the existence of the atomic nucleus and disproving the plum-pudding model.

In 1913, the Bohr model of the atom was introduced (named after Danish physicist Neils Bohr [18851962]). In this model, the hydrogen atom consists of an electron orbiting the nucleus (a single proton), much as Earth orbits the sun. The Bohr model also requires that the angular momentum (mass times velocity times distance from the orbital center) of the electron be limited to certain values (that is, be quantized) in order that the electron not fall into the nucleus. Though known to have serious defects, the Bohr model still supplies the standard graphic representation of the atom: a solid nucleus around which electrons orbit like tiny planets.

When the principles of quantum mechanics were developed, the Heisenberg uncertainty principle,

discovered by German physicist Werner Heisenberg (19011976) meant the Bohr atom had to be modified. The Heisenberg uncertainty principle states: it is impossible to accurately determine the position and the momentum (mass×velocity) of a subatomic particle at the same time. Indeed, a subatomic particle cannot be thought of as possessing precise values of these quantities simultaneously, measured or not. This means that the electrons in an atom can still be thought of as orbiting, the nucleus, but their position is smeared throughout a wide region, or cloud, rather than confined to well-defined orbits.

Neutrinos

In 1930, scientists started to suspect the existence of another subatomic particle that came to be known as the neutrino. Neutrinos are considered matter particles, but they do not make up normal matter by themselves. In fact, neutrinos are very common about 60 billion neutrinos from the sun pass through every square centimeter of Earths surface every secondbut scientists do not observe them because they interact only rarely with other particles.

In 1930, a problem with a process called nuclear beta decay had developed. Nuclear beta decay is when an unstable, or radioactive, nucleus decays into a lighter nucleus and an electron. Scientists observed that the energy before the beta decay was greater than the energy after the beta decay. This was puzzling because one of the most basic laws of physics, the law of conservation of energy, states that the amount of energy in any process must remain the same. To keep the idea of energy conservation intact, Austrian physicist Wolfgang Pauli (19001958) proposed that a hitherto-unidentified particle carried off the missing energy. In 1933, Italian physicist Enrico Fermi (19011954) named this hard-to-detect particle the neutrino, and used it to successfully explain the theory of beta decay.

One type of neutrino, the electron neutrino, was finally detected in 1956. Later, a second type of neutrino, the muon neutrino, was found, and a third type, called the tau neutrino, was discovered in the late 1990s. For decades, physicists debated the question of whether the neutrino is a mass-less particle, like the photon, or has a finite mass. In 1998, physicists discovered that at least one of these types of neutrinos must have mass. Though it would have to be very tiny, it must at least be greater than 20-billionths of the mass of the electronextremely small, but not zero.

Positrons

In 19311932, American experimental physicist Carl David Anderson (19051991) experimentally observed the anti-electron, which he called the positron, after its positive charge. The positron is an antiparticle which had been predicted by English physicist Paul Dirac (19021984) between 1927 and 1930. Every particle has a corresponding antiparticle that has the same properties except for an opposite electrical properties (charge and magnetic moment). Antiparticles make up what is called antimatter. Matter is much more common in our universe than antimatter, though it is unknown why this is so.

Neutrons

In 1932, English physicist James Chadwick (1891 1974) discovered another matter particle, the neutron. The neutron is very similar to the proton except that it is electrically neutral (i.e., has no charge). Chadwick found the neutron by hitting a chemical called beryllium with alpha particles. When this occurred, highly penetrating radiation was emitted. This radiation turned out to be a stream of neutrons. After Chadwicks experiment, Heisenberg proposed that the nucleus is made up of protons and neutrons, which was later found to be true.

Pion, muons, and kaons

The second mediator particle discovered (after the photon) was the pion. In 1935, Japanese physicist Hideki Yukawa (19071981) formulated the idea that protons and neutrons were held together by a nuclear force that was mediated by a particle called the pion. Yukawa described it in detail. In 1937, the first evidence for the pion was obtained by studying cosmic rays (high-energy particles from space). By 1947, it became clear that cosmic rays did contain Yukawas pions, but also contained another particle, a heavy electron-like particle, which was given the name muon. In 1947, yet another particle was detected from cosmic rays, the kaon. The kaon is like a heavy pion, and decays into two lighter pions. The kaons are considered strange particles because they can be made quickly, but it takes a long time for them to decay. Usually the time to make a particle and the time for it to decay to be about the same, but this is not true for the kaon.

Quarks

In 1980, Maurice Jacob (1933) and Peter Landshoff detected small, hard, objects inside the proton by firing high-energy electrons and protons at it. Most of the high-energy particles seemed to pass right through the proton. However, a few of these high-energy particles were reflected back, as if they had hit something. These and other experiments indicated that the proton contains three small, hard, solid objects. Thus, protons are not elementary, but the objects inside them may be. These objects are now called quarks.

Quark model

Quarks had been postulated much earlier, in 1964, by American physicist Murray Gell-Mann (1929) and, independently, by American physicist George Zweig (1937). The theory describing quarks was called the quark model. In 1964, it was thought that there should be three different quarks. These different quarks each have a unique property called flavor. These first three quarks had flavors that were whimsically named up, down, and strange. Up-flavored quarks have an electric charge of (2/3)e, where e is the fundamental quantum of charge such as that of the negatively-charged electron. Down- and strange-flavored quarks have an electric charge of (-1/3)e. The quark model also says that quarks must remain bound inside their particles in nature, quarks cannot exist by themselves. This idea is called quark confinement, and is based on the experimental observation that a free quark has never been seen. Since scientists cannot isolate quarks, it is very difficult to determine their masses.

In 1964, physicist Oscar Wallace Greenberg (1932) suggested each quark has a quality he termed color. The label color for this quark property does not refer to the usual definition of color, but is just a way to keep track of quarks. Using this idea of color, the improved quark model says only overall-colorless particles can exist in nature. There are only three different kinds of color in the quark model, usually designated red, blue, and green. Color had to be introduced when a particle called the Δ++ (pronounced delta-plus-plus) baryon was discovered to avoid violating the Pauli exclusion principle. The Pauli exclusion principle says that each particle in a system of matter particles must have unique properties like electric charge, mass, and spin.

Table 1. Elementary Matter Particles. (Thomson Gale.)
Elementary matter particles
1st family 2nd family 3rd family
particle charge mass particle charge mass particle charge mass
leptons
ν e0e0νμ0e0ντ0e0
e--1e.511μ--1e106τ--1e1777
quarks
u2/3 e28c2/3 e10001600t2/3 e176000
d-1/3 e515s-1/3 e100300b-1/3 e41004500

The Δ++ baryon is made of three up quarks. Without color, each of its three up quarks cannot have its own properties. Color has been proven experimentally, and a theory called the standard model of elementary particles has updated the quark model.

Subatomic particle classifications

Elementary matter particles

There are two kinds of elementary (indivisible) matter particles, the quarks and the leptons. The two lowest-mass leptons are the electron (e-) and its partner the neutrino, usually called the electron-neutrino (νe). For unknown reasons, this lepton pairing is repeated two more times, each time with increasing mass. These leptons are called the muon (µ-) and muon neutrino (νµ) and the tau (τ-) and tau neutrino (ντ). There are said to be three families, or generations, of leptons.

Like the leptons, the quarks have three families. The first family of quarks is the up and down quarks, the second contains the strange and charmed quarks, and the third the bottom and top quarks. Though all matter scientists see contain only up, down, and strange quarks, physicists have proven the existence of all six flavors of quarks, culminating with the discovery of the top quark in 1995.

Another property of elementary particles is termed spin. Spin is akin to the rotation of a particle on its axis, as the Earth spins on its axis to give Earth day and night. (In actuality, elementary particles do not rotate like spheres; it is only that the particle property termed spin obeys rules that mathematically are similar to those used to describe the rotation of macroscopic bodies.) The spin of elementary particles is measured in special units called h-bar (h-bar is Plancks constant divided by 2π), and equals 1.1×10-34 Joule-seconds. Using the property called spin, all matter particles are fermions, which have spin one-half h-bar or three-halves h-bar. All quarks and leptons have spins of one-half h-bar. The matter particles and some of their properties are summarized in Table 1. Masses are given in units of MeV/c2, where c is the speed of light (the distance light travels in vacuum in one year, or about three-hundred-million meters per second). The quark masses are approximate.

Elementary mediator particles

Bosons are particles defined to have spin of zero h-bar, one h-bar, or two h-bar. The elementary mediator particles are bosons with spins of one h-bar. The force scientists are most familiar with is the electromagnetic force. The electromagnetic force is responsible for keeping electrons and nuclei together to form atoms. The electromagnetic force is mediated by photons, which are mass-less. The mediators of the strong force are called gluons, because they glue quarks together to form mesons and baryons. Like the quarks, the gluons carry the color property, and as a result there are eight different types of gluons.

The weak force is more uncommon. It is responsible for radioactive decays like nuclear beta decay. The mediators of the weak force are the electrically charged W-bosons (W±), and the electrically neutral Z-bosons (Z0), both discovered in 1983. Some properties of the mediator particles are given in Table 2. Masses are given in MeV/c2.

Baryons

one of the main rules of the standard quark model is that combinations of three quarks are called baryons. Protons and neutrons are the most important baryons. Protons are made of two up quarks and one down quark. Neutrons are made of two down quarks and

Table 2. Elementary Mediator Particles. (Thomson Gale. )
Elementary mediator particles
Particle Charge Mass Force
γ0e 0 Electromagnetic
g0e0Strong
W±±1e80200Weak
Z00 e91200Weak
Table 3. Baryons. (Thomson Gale.)
Baryons
baryon quark content spin charge mass
pu u d1/2+1 e938
nu d d1/20 e939
Λu d s1/20 e1116
+u u s1/21 e1189
0u d s1/20 e1192
-d d s1/2-1 e1197
Ξ0u s s1/20 e1315
Ξ-d s s1/2-1 e1321
Λe+u d c1/21 e2281
Δ++u u u3/22 e1232
Δ+u u d3/21 e1232
Δ0u d d3/20 e1232
Δ-d d d3/2-1e1232
*+u u s3/21 e1383
*0u d s3/20 e1384
*-d d s3/2-1 e1387
Ξ*0u s s3/20 e1532
Ξ*-d s s3/2-1 e1535
Ω-s s s3/2-1 e1672

one up quark. Since the quark model requires that naturally occurring particles be colorless, a baryon must be made of a red, a blue, and a green quark. These combine to make a white, or colorless particle. Spin is also important in classifying baryons. Baryons are fermions and so have spins of one-half h-bar or three-halves h-bar. Table 3 summarizes several kinds of baryons, with masses in MeV/c2 (millions of electron-volts divided by the speed of light squared) and spin in terms of h-bar.

Mesons

the second main idea of the standard quark model is that combinations of one quark and one antiquark form mesons. Pions (π) and kaons (K) are examples of mesons. Thus, Yukawas nuclear force mediator particle, the pion, is really a matter particle made of a quark and an antiquark. There are several kinds of pions. For example, the positively charged pion, π+, is made of an up quark and a down antiquark. Similarly, there are several kinds of kaons. One kind of kaon, K+, is made of an up quark and a strange antiquark. The colorless rule requires that mesons must be made of quarks with opposite color, red and anti-red for example. All mesons are bosons and so have spins of zero h-bar or one h-bar.

Current and future research

Subatomic particles are important in all electronic, optical, and nuclear technologies. Cathode-ray tubes, for example, use beams of electrons to create the pictures. A television antenna first picks up the television signala series of radio-frequency photons which is then processed electronically and used to control an electron gun. An electron gun shoots a beam of electrons, which is steered by magnets and hits the coated inner surface of the picture tube. When electrons hit this surface, it lights up, creating the picture as the electron beam is steered rapidly across it. A common type of smoke detector that uses subatomic particles is an ionization smoke detector; in an ionization smoke detector, alpha particles ionize (strip electrons from) air molecules. These ionized air molecules cause electric current to flow in the detector. If there is a fire, other particles enter the detector and interfere with the flow of the electric current, and this makes the alarm go off. Proton beams are used to treat cancer; all technologies involving optics or radio manipulate photons; all electronic devices manipulate electrons; nuclear weapons and nuclear power depend on controlling neutrons so as to produce either an explosive or a controlled nuclear chain reaction, respectively; positron-emitting isotopes are used to image metabolic activity in the human brain in real time; and so on.

In recent years, particle physics has been particularly exciting, with several important experimental developments. Besides the discovery of the W and Z bosons and the top quark, scientists working in Japan in 1998 found evidence that at least some of the three types of neutrinos have a small but nonzero mass. Their experiment did not allow them to determine the exact value for the mass, but subsequent work has shown that the mass of the neutrino is too small to account for the dark matter that astronomers have shown must account for a significant fraction of the mass of the universe.

KEY TERMS

Coulomb The standard unit of electric charge, defined as the amount of charge flowing past a point in a wire in one second, when the current in the wire is one ampere.

Fundamental force A basic force, which has its own elementary mediator particle(s). There are four fundamental forces: the strong force, the electromagnetic force, the weak force, and gravity.

Mega electron volt (MeV) A unit of energy. One MeV is one million Electron Volts. An Electron Volt is the amount of energy an electron gains as it passes through one Volt of potential difference.

Quarks Believed to be the most fundamental units of protons and neutrons.

As of 2002, it was shown that neutrinos masses are so small that neutrinos could account for at most a fifth of the dark matter in the universe. Other research showed that only sterile neutrinos (those formed immediately after the big bang) could possibly make up dark matter. Previously, it seemed that the unknown mass of the neutrino might explain the dark matter mystery. In 2006, however, suspicion centered on dark energy rather than on dark matter as an explanation of the universes non-visible mass.

See also Spin of subatomic particles.

Resources

BOOKS

Anisovich, V.V., et al. Quark Model and High Energy Collisions. River Edge, NJ: World Scientific, 2004.

Blaschke, D., M.A. Evanov, and T. Mannel, eds. Heavy Quark Physics. Berlin, Germany: Springer, 2004.

Folan, Lorcan M. Modern Physics and Technology for Undergraduates. River Edge, NJ: World Scientific, 2003.

Hosaka, Atsushi. Quarks, Baryons, and Chiral Symmetry. Singapore, and River Edge, NJ: World Scientific, 2001.

Rae, Alastair I.M. Quantum Physics: A Beginners Guide. Oxford, UK: Oneworld, 2005.

Smith, Timothy Paul. Hidden Worlds: Hunting for Quarks in Ordinary Matter. Princeton, NJ: Princeton University Press, 2003.

Staley, Kent W. The Evidence for the Top Quark: Objectivity and Bias in Collaborative Experimentation. Cambridge, UK, and New York: Cambridge University Press, 2004.

PERIODICALS

Barnett, R. The Charm of Strange Quarks: Mysteries and Revolutions of Particle Physics. Physics Today 54 (2001)5051.

Pokrovsky, V. Particle Physics: Russian Turmoil Rattles CERN. Science 292 (2001):2414B22415.

Lesley L. Smith

Larry Gilman

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Subatomic Particles

Subatomic particles

Subatomic particles are particles that are smaller than an atom. Early in the twentieth century, electrons, protons, and neutrons were thought to be the only subatomic particles; these were also thought to be elementary (i.e., incapable

of being broken down into yet smaller particles). However, the list of subatomic particles has now been expanded to include a large number of elementary particles and the particles they can be combined to make.

There are two types of elementary particles. One type of makes up matter . Examples of these particles include quarks (which make up protons and neutrons) and electrons. Baryons and mesons are combinations of quarks and are considered subatomic particles. The most famous baryons are protons and neutrons.

The other elementary particles are mediators of the fundamental forces. These mediator particles enable the matter particles to interact with each other. That is, when two electrons collide, they do not simply bounce off of each other like two billiard balls: they exchange a photon (one of the mediator particles). All forces, including gravity, are thought to be mediated by particle exchanges.

Discovery of particles

Electrons

The first subatomic particle to be discovered was the electron . While others had deduced the existence of a negatively charged particle in what were called cathode rays (and which are now known to be beams of electrons), it was English physicist J. J. Thomson (1856–1940), who in 1897 measured the velocity and charge-to-mass ratio of these particles. The charge-to-mass ratio was found to be relatively large, and independent of the gas used in his experiments, which indicated to him that he had found a true particle. Thomson gave it the name "corpuscle," which was later changed to "electron."

The charges of all particles are traditionally measured in terms of the size of the charge of the electron. The electron has a charge, e, of 1.6 × 10-19 Coulombs.

Photons

The first mediator particle to be discovered was the photon. In 1900, German physicist Max Planck (1858–1947) reported that light came in little packages of energy , which he called "quanta." In 1905, German physicist Albert Einstein (1879–1955) studied the photoelectric effect and proposed that radiation is quantized by its nature—that is, transfers energy in minimal packets termed quanta. A photon (the name was coined by U.S. chemist Gilbert Lewis [1875–1946] in 1926) is one of these quanta, the smallest possible piece of energy in a light wave. (The word "wave" is applied by physicists to describe some observable aspects of the behavior of light, while the particle terminology of the "photon" is applied to describe others. Both words convey mental pictures that are useful in some physical applications, but neither picture is sufficient: a photon is not a "particle" in the sense of a perfectly round, hard, self-contained sphere , nor is light a "wave" in the sense of being a smooth undulation in some medium.)


Protons

The proton was one of the earliest particles known. (The word proton is Greek for "the first one.") In 1906 the first clues to the nature of the proton were seen. J. J. Thomson reported detecting positively charged hydrogen "atoms." These were in fact, hydrogen nuclei (protons), but atomic structure was not understood at the time. Thomson thought that protons and electrons were randomly scattered throughout the atom, the so-called "plum-pudding model." In 1909–1911, English physicist Ernest Rutherford (1871–1937) and his colleagues, German physicist Hans Wilhelm Geiger (1882–1947) and New Zealand physicist Ernest Marsden (1888–1970) did their famous scattering experiments involving alpha particles (two protons and two neutrons; a helium-atom nucleus) shot through gold foil. From their observations of the angles at which alpha particles were deflected, they deduced that atoms had relatively hard and small centers, thus proving the existence of the atomic nucleus and disproving the plum-pudding model.

In 1913, the Bohr model of the atom was introduced (named after Danish physicist Neils Bohr, 1885–1962). In this model, the hydrogen atom consists of an electron orbiting the nucleus (a single proton), much as the Earth orbits the Sun . The Bohr model also requires that the angular momentum (mass times velocity times distance from the orbital center) of the electron be limited to certain values (that is, be "quantized") in order that the electron not fall into the nucleus. Though known to have serious defects, the Bohr model still supplies the standard graphic representation of the atom: a solid nucleus around which electrons orbit like tiny planets.

When the principles of quantum mechanics were developed, the Heisenberg uncertainty principle , discovered by German physicist Werner Heisenberg (1901–1976) meant the Bohr atom had to be modified. The Heisenberg uncertainty principle states that it is impossible to accurately determine both the position and the momentum (mass times velocity) of a subatomic particle at the same time. Indeed, a subatomic particle cannot be thought of as having precise values of these quantities simultaneously, measured or not. This means that the electrons in an atom can still be thought of as orbiting, the nucleus, but their position is smeared throughout a wide region or "cloud" rather than confined to well-defined orbits.

Neutrinos

In 1930, scientists started to suspect the existence of another subatomic particle that came to be known as the neutrino . Neutrinos are considered matter particles, but they do not make up normal matter by themselves. In fact, neutrinos are very common–about 60 billion neutrinos from the Sun pass through every square centimeter of the Earth's surface every second–but we do not observe them because they interact only rarely with other particles.

In 1930 a problem with a process called nuclear beta decay had developed. Nuclear beta decay is when an unstable, or radioactive, nucleus decays into a lighter nucleus and an electron. Scientists observed that the energy before the beta decay was greater than the energy after the beta decay. This was puzzling because one of the most basic laws of physics , the law of conservation of energy, states that the amount of energy in any process must remain the same. To keep the idea of energy conservation intact, Austrian physicist Wolfgang Pauli (1900–1958) proposed that a hitherto-unidentified particle carried off the missing energy. In 1933 Italian physicist Enrico Fermi (1901–1954) named this hard-to-detect particle the neutrino, and used it to successfully explain the theory of beta decay.

One type of neutrino, the electron neutrino, was finally detected in 1956. Later, a second type of neutrino, the muon neutrino, was found, and a third type, called the tau neutrino, was discovered in the late 1990s. For decades physicists debated the question of whether the neutrino is a massless particle, like the photon, or has a finite mass. In 1998 physicists discovered that at least one of these types of neutrinos must have mass. Though it would have to be very tiny, it must at least be greater than 20-billionths of the mass of the electron—extremely small, but not zero.

Positrons

In 1931–1932, U.S. physicist Carl Anderson (1905–) experimentally observed the anti-electron, which he called the positron, after its positive charge. The positron is an antiparticle which had been predicted by English physicist Paul Dirac (1902–1984) in 1927–1930. Every particle has a corresponding antiparticle that has the same properties except for an opposite electrical properties (charge and magnetic moment). Antiparticles make up what is called antimatter . Matter is much more common in our universe than antimatter, though it is unknown why this is so.

Neutrons

In 1932 English physicist James Chadwick (1891–1974) discovered another matter particle, the neutron . The neutron is very similar to the proton except that it is electrically neutral (i.e., has no charge). Chadwick found the neutron by hitting a chemical called beryllium with alpha particles. When this occurred, highly penetrating radiation was emitted. This "radiation" turned out to be a stream of neutrons. After Chadwick's experiment, Werner Heisenberg proposed that the nucleus is made up of protons and neutrons, which was later found to be true.

Pion, muons, and kaons

The second mediator particle discovered (after the photon) was the pion. In 1935, Japanese physicist Hideki Yukawa (1907–1981) formulated the idea that protons and neutrons were held together by a nuclear force that was mediated by a particle called the pion. Yukawa described it in detail. In 1937 the first evidence for the pion was obtained by studying cosmic rays (high-energy particles from space ). By 1947 it became clear that cosmic rays did contain Yukawa's pions, but also contained another particle, a heavy electron-like particle, which was given the name muon. In 1947 yet another particle was detected from cosmic rays, the kaon. The kaon is like a heavy pion, and decays into two lighter pions. The kaons are considered strange particles because they can be made fairly quickly, but it takes a long time for them to decay. Usually the time to make a particle and the time for it to decay to be about the same, but this is not true for the kaon.

Quarks

In 1980, Maurice Jacob (1933–) and Peter Lanshoff detected small, hard, objects inside the proton by firing high-energy electrons and protons at it. Most of the high-energy particles seemed to pass right through the proton. However, a few of these high-energy particles were reflected back, as if they had hit something. These and other experiments indicated that the proton contains three small, hard, solid objects. Thus protons are not elementary, but the objects inside them may be. These objects are now called quarks.

Quark model

Quarks had been postulated much earlier, in 1964, by American physicist Murray Gell-Mann (1929–) and, independently, by American physicist George Zweig (1937–). The theory describing quarks was called the quark model. In 1964 it was thought that there should be three different quarks. These different quarks each have a unique property called flavor. These first three quarks had flavors that were whimsically named up, down, and strange. Up-flavored quarks have an electric charge of (2/3)e, where e is the fundamental quantum of charge such as that of the negatively-charged electron. Down- and strange-flavored quarks have an electric charge of (-1/3)e. The quark model also says that quarks must remain bound inside their particles—in nature, quarks cannot exist by themselves. This idea is called quark confinement, and is based on the experimental observation that a free quark has never been seen. Since we cannot isolate quarks, it is very difficult to determine their masses.

In 1964 physicist Oscar W. Greenberg (1932–) suggested each quark has a quality he termed color . The label "color" for this quark property does not refer to the usual definition of color, but is just a way to keep track of quarks. Using this idea of color, the improved quark model says only overall-colorless particles can exist in nature. There are only three different kinds of color in the quark model, usually designated red, blue, and green. Color had to be introduced when a particle called the Δ++ (pronounced delta-plus-plus) baryon was discovered to avoid violating the Pauli exclusion principle . The Pauli exclusion principle says that each particle in a system of matter particles must have unique properties like electric charge, mass, and spin. The Δ++ baryon is made of three up quarks. Without color, each of its three up quarks cannot have its own properties. Color has been proven experimentally, and a theory called the standard model of elementary particles has updated the quark model.

Subatomic particle classifications

Elementary matter particles

There are two kinds of elementary (indivisible) matter particles, the quarks and the leptons. The two lowest-mass leptons are the electron (e-) and its partner the neutrino, usually called the electron-neutrino ( νe). For unknown reasons, this lepton pairing is repeated two more times, each time with increasing mass. These leptons are called the muon ( μ–) and muon neutrino ( νμ) and the tau ( τ–) and tau neutrino (ντ). There are said to be three families, or generations, of leptons.

Like the leptons, the quarks have three families. The first family of quarks are the up and down quarks, the second contains the strange and "charmed" quarks, and the third the "bottom" and "top" quarks. Though all matter we see around us contains only up, down, and strange quarks, physicists have proven the existence of all six

TABLE 1. ELEMENTARY MATTER PARTICLES
1st family 2nd family 3rd family
particle charge mass particle charge mass particle charge mass
leptons
νe 0 e 0 νμ0 e 0 ντ0 e 0
e- -1 e .511 μ- -1 e 106 τ- -1 e 1777
quarks
u 2/3 e 2-8 c 2/3 e 1000-1600 t 2/3 e 176000
d -1/3 e 5-15 s -1/3 e 100-300 b -1/3 e 4100-4500
TABLE 2. ELEMENTARY MEDIATOR PARTICLES
particle charge mass force
γ 0 e 0 Electromagnetic
g 0 e 0 Strong
±1 e 80200 Weak
Z00 e 91200 Weak

flavors of quarks, culminating with the discovery of the top quark in 1995.

Another property of elementary particles is termed "spin." Spin is akin to the rotation of a particle on its axis, as the earth spins on its axis to give us day and night. (In actuality elementary particles do not rotate like spheres; it is only that the particle property termed spin obeys rules that mathematically are similar to those used to describe the rotation of macroscopic bodies.) The spin of elementary particles is measured in special units called "h-bar" (h-bar is Planck's constant divided by 2π), and = 1.1 × 10-34 Joule-seconds. Using the property called spin, all matter particles are fermions which have spin one-half h-bar or three-halves h-bar. All quarks and leptons have spins of one-half h-bar. The matter particles and some of their properties are summarized in Table 1.

Masses are given in units of MeV/c2 , where c is the speed of light (three-hundred-million meters per second). The quark masses are approximate.

Elementary mediator particles

Bosons are particles defined to have spin of zero hbar, one h-bar, or two h-bar. The elementary mediator particles are bosons with spins of one h-bar. The force we are most familiar with is the electromagnetic force. The electromagnetic force is responsible for keeping electrons and nuclei together to form atoms. The electromagnetic force is mediated by photons, which are massless. The mediators of the strong force are called gluons, because they glue quarks together to form mesons and baryons. Like the quarks, the gluons carry the color property, and as a result there are eight different types of gluons.

The weak force is more uncommon. It is responsible for radioactive decays like nuclear beta decay. The mediators of the weak force are the electrically charged W-bosons (W±), and the electrically neutral Z-bosons (Z 0), both discovered in 1983. Some properties of the mediator particles are given in Table 2.

TABLE 3. BARYONS
baryon quark content spin charge mass
p u u d 1/2 +1 e 938
n u d d 1/2 0 e 939
Λ u d s 1/2 0 e 1116
Σ+u u s 1/2 1 e 1189
Σ0u d s 1/2 0 e 1192
Σ-d d s 1/2 -1 e 1197
Ξ0u s s 1/2 0 e 1315
Ξ-d s s 1/2 -1 e 1321
Λe+u d c 1/2 1 e 2281
Δ++u u u 3/2 2 e 1232
Δ+u u d 3/2 1 e 1232
Δ0u d d 3/2 0 e 1232
Δ-d d d 3/2 -1 e 1232
Σ*+u u s 3/2 1 e 1383
Σ*0u d s 3/2 0 e 1384
Σ*-d d s 3/2 -1 e 1387
Ξ*0u s s 3/2 0 e 1532
Ξ*-d s s 3/2 -1 e 1535
Ω-s s s 3/2 -1 e 1672



Baryons

One of the main rules of the standard quark model is that combinations of three quarks are called baryons. Protons and neutrons are the most important baryons. Protons are made of two up quarks and one down quark. Neutrons are made of two down quarks and one up quark. Since the quark model requires that naturally-occurring particles be colorless, a baryon must be made of a red, a blue, and a green quark. These combine to make a white, or colorless particle. Spin is also important in classifying baryons. Baryons are fermions and so have spins of one-half h-bar or three-halves h-bar. Table 3 summarizes several kinds of baryons, with masses in MeV/c 2 (millions of electron-volts divided by the speed of light squared) and spin in terms of h-bar.

Mesons

The second main idea of the standard quark model is that combinations of one quark and one antiquark form mesons. Pions (π) and kaons (K) are examples of mesons. Thus now we see Yukawa's nuclear force mediator particle, the pion, is really a matter particle made of a quark and an antiquark. There are several kinds of pions. For example, the positively charged pion, π+, is made of an up quark and a down antiquark. Similarly there are several kinds of kaons. One kind of kaon, K+, is made of an up quark and a strange antiquark. The colorless rule requires that mesons must be made of quarks with opposite color, red and anti-red for example. All mesons are bosons and so have spins of zero h-bar or one h-bar.


Current and future research

Subatomic particles are important in all electronic, optical, and nuclear technologies. Cathode-ray tubes, for example, use beams of electrons to create the pictures. A television antenna first picks up the television signal—a series of radio-frequency photons—which is then processed electronically and used to control an electron gun. An electron gun shoots a beam of electrons which is steered by magnets and hits the coated inner surface of the picture tube. When electrons hit this surface, it lights up, creating the picture as the electron beam is steered rapidly across it. A common type of smoke detector that uses subatomic particles is an ionization smoke detector; in an ionization smoke detector, alpha particles ionize (strip electrons from) air molecules. These ionized air molecules cause electric current to flow in the detector. If there is a fire, other particles enter the detector and interfere with the flow of the electric current, and this makes the alarm go off. Proton beams are used to treat cancer ; all technologies involving optics or radio manipulate photons; all electronic devices manipulate electrons; nuclear weapons and nuclear power depend on controlling neutrons so as to produce either an explosive or a controlled nuclear chain reaction, respectively; positron-emitting isotopes are used to image metabolic activity in the human brain in real time; and so on.

In recent years, particle physics has been particularly exciting, with several important experimental developments. Besides the discovery of the W and Z bosons and the top quark, scientists working in Japan in 1998 found evidence that at least some of the three types of neutrinos have a small but nonzero mass. Their experiment did not allow them to determine the exact value for the mass, but subsequent work has shown that the mass of the neutrino is too small to account for the "dark matter" which astronomers have shown must account for a significant fraction of the mass of the Universe. Previously, it seemed that the unknown mass of the neutrino might explain the "dark matter" mystery; today, suspicion centers on "dark energy" rather than on "dark matter" as an explanation of the Universe's nonvisible mass.

See also Spin of subatomic particles.

Resources

books

Barnett, Michael R., Henry Möhry, and Helen R. Quinn. TheCharm of Strange Quarks: Mysteries and Revolutions of Particle Physics. New York: Springer-Verlag, 2000.

Gribbin, John. Q is for Quantum: An Encyclopedia of ParticlePhysics. New York: The Free Press, 1998.

Kane, Gordon. The Particle Garden: Our Universe as Understood by Particle Physicists. Reading, MA: Helix Books, 1995.

Weinberg, S. The First Three Minutes. New York: Basic Books, 1977.

Weinberg, Steven. Dreams of a Final Theory. New York: Patheon Books, 1992.


periodicals

Barnett, R. "The Charm of Strange Quarks: Mysteries and Revolutions of Particle Physics." Physics Today 54 (2001)50–51.

Gibbs, W. Wayt. "A Massive Discovery." Scientific American (August 1998).

Kalmus, P. I. P. "Particle physics at the Turn of the Century." Contemporary Physics. 41 (2000):129–142.

Pokrovsky, V. "Particle Physics: Russian Turmoil Rattles CERN." Science 292 (2001):2414B–22415.


Lesley L. Smith
Larry Gilman
K. Lee Lerner

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Coulomb

—The standard unit of electric charge, defined as the amount of charge flowing past a point in a wire in one second, when the current in the wire is one ampere.

Fundamental force

—A basic force, which has its own elementary mediator particle(s). There are four fundamental forces: the strong force, the electromagnetic force, the weak force, and gravity.

Mega electron volt (MeV)

—A unit of energy. One MeV is one million Electron Volts. An Electron Volt is the amount of energy an electron gains as it passes through one Volt of potential difference.

Quarks

—Believed to be the most fundamental units of protons and neutrons.

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