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Elementary Particles
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Everything you see and experience—all of the 100 or so types of atoms that make up your body and everything in the universe and all of the forces that keep the atoms intact and allow them to interact with one another to form molecules, life, stars, and galaxies—all of this is made up of the fundamental particles listed in this table.

These particles fall into two main categories—matter particles (fermions) and force particles (bosons). There are three families of fermions. Family 1 accounts for practically all of the everyday matter we encounter in everyday life.


 

The Elegant Universe homepage

Chart of elementary particles
 

The entire universe can be represented by the few particles included in the table shown on this page.* It would be even more amazing, then, if these 19 were made of the same elemental constituent. If superstring theory turns out to be true, this will be the case. According to the theory, every one of the "fundamental" particles are made of identical strings, with the only difference between the strings being their vibrational patterns (see Resonance in Strings for more on this).


The Standard Model, Plus Gravity

On one hand there is quantum mechanics and the standard model, which describes the world of the very small. On the other hand there is general relativity, which describes gravity and the world of the very large. With one exception, all of the particles listed on the table are explained by the standard model. The exception is the graviton, the force particle behind gravity.

Superstring theorists believe they are on their way to developing a framework that incorporates both general relativity and quantum mechanics. If they succeed, they will have discovered a theory that solves the greatest problem in physics—that of unifying the laws of nature.


Particles

Graviton
The graviton is the theorized force carrier of gravity. Gravity is by far the weakest of the four forces—much weaker even than the weak force. The reason gravity can seem like a substantial force is that it can only attract, and the more the mass is concentrated in an area, the stronger the gravitational force. Electromagnetism, on the other hand, is a repulsive force as well as an attractive force. Its positive and negative charges tend to cancel each other out, which neutralizes the overall force.

The graviton has not yet been observed or otherwise proven to exist. However, the rate at which the spin of neutron stars slows down is consistent with calculations that include the radiation of gravitons.


Photon
The photon is the carrier of all electromagnetic radiation, including radio waves, light, X-rays, and gamma rays.

Photons and their associated electromagnetic field also keep atoms together from the attraction among negatively charged electrons and positively charged atomic nuclei. The electromagnetic force is also responsible for the interaction between atoms and molecules—without this force, your finger would pass through your computer mouse (not to mention the atoms and molecules that make up your body would no longer be bound together).


Weak gauge bosons
W+, W-, and Z0 are the weak gauge bosons, carriers of the weak force.

The weak force triggers radioactive decay by causing a down quark within a neutron to change to an up quark. This change transforms the neutron into a proton, releasing an electron in the process. The weak force can also transform a proton into a neutron.

Carlo Rubbia, Simon Van der Meer, and a team of 130 physicists discovered the weak gauge bosons in 1983.


Gluon
The gluon, which is the carrier of the strong force, works at tiny distances—10-13 cm or less. The strong force works like glue, tightly binding quarks together in groups of threes to form protons and neutrons and in other combinations to form other particles.

The strong force also holds together the protons and neutrons that comprise an atomic nucleus. Without the strong force, the protons would move away from each other due to the repulsion caused by their like electrical charges.

Indirect evidence of the gluon was first detected at the Deutsches Elektronen Synchrotron in Hamburg, Germany, in 1979.


Higgs boson
Particle physicists expected the weak gauge bosons to be massless particles. Instead, these bosons have been shown by experiment to have substantial mass. The Higgs boson was postulated to explain this mass.

According to the hypothesis, the Higgs field slows down the weak gauge bosons, which would otherwise travel at the speed of light. This slowing down gives the bosons an effective mass. At high enough temperatures, the effects of the Higgs field disappear, allowing the weak gauge bosons to travel at the speed of light and to become massless.

The Higgs boson, named after the man who postulated its existence, Peter Higgs, explains why the weak gauge bosons are not massless particles.


Family 1 Leptons and Quarks

Electron
The electron is a negatively charged particle that surrounds the nucleus of an atom. Because of its negative electrical charge, the electron is attracted to the positively charged protons within the atom's nucleus, keeping them bound to the nucleus. The way in which electrons assemble in the shells around an atom helps to determine the chemical characteristics of that atom.

The electron's negative charge also keeps atoms from passing through one another—the repulsion caused by the like charges keeps the atoms apart.

The electron, discovered in 1897 by J. J. Thomson, has the distinction of being the first subatomic particle detected.


Electron neutrino
The neutrino is able to travel great distances through matter without interacting with it—about 600 trillion neutrinos from the sun passes through your body every second. Neutrinos shoot through the Earth virtually unimpeded, so the number moving through you is the same even if it is nighttime and the sun is on the other side of the Earth. The reason is that they are massless or nearly massless and that they interact with matter only through gravity and the weak force.

In 1930 Wolfgang Pauli predicted the existence of the neutrino. The particle was shown to exist by Clyde Cowan and Fred Reines in 1956.


Up quark
The least massive of the six quarks, the up quark combines with the down quark to make up the matter that we experience in daily life.

Although individual quarks have never been observed, there is indirect evidence that convinces particle physicists that they do exist.

In 1964 Murray Gell-Mann and George Zweig independently announced the theoretical existence of quarks.


Down quark
Down quarks and up quarks make up the ´protons and neutrons that form atomic nuclei and therefore they are responsible for the vast bulk of all matter that we experience and see.

Two down quarks and an up quark comprise a neutron (-1/3 + -1/3 + 2/3 = 0). One down quark and two up quarks comprise a proton (-1/3 + 2/3 + 2/3 = 1).

Murray Gell-Mann's and George Zweig's quark hypothesis was confirmed in 1968 at the Stanford Linear Accelerator Center.


Family 2 Leptons and Quarks

Muon
Like the electron, the muon is a charged particle. Unlike the electron, it is more massive and it is unstable; two-thirds of all muons decay into an electron, a muon neutrino, and an electron anti-neutrino within about two microseconds after they come into existence.

The muon was the next elementary particle discovered after the discoveries of the electron, the proton, and the neutron.

Jabez C. Street and Edward C. Stevenson found evidence of the muon in 1937.


Muon neutrino
Discovered in 1961, the muon neutrino was shown to be a different particle than the electron neutrino.

Electron neutrinos are associated with a type of radioactive decay that produces an electron along with a neutrino. The type of radioactive decay that produces muon neutrinos also produces muons, which are elementary particles that are like electrons but more massive.

Credit for the discovery of the muon neutrino goes to Jack Steinberger, Melvin Schwartz, and Leon Lederman.


Charm quark
The charm quark, which is similar to the up quark but more massive, was the fourth quark found, following the discoveries of the up, down, and strange quarks.

The charm quark was indirectly discovered simultaneously in 1974 at the Brookhaven National Laboratory and at the Stanford Linear Accelerator Center.


Strange quark
The strange quark was given its name because of its "strange" behavior. When it was first discovered, it lived much longer than had been predicted.

Like other quarks, evidence of the strange quark's existence was indirect—it was found within a larger particle that contained it along with an up quark and a down quark.

In 1947 George Rochester and C.C Butler discovered the "V" particle (from a cosmic ray), which was later shown to be associated with the strange quark.


Family 3 Leptons and Quarks

Tau
The tau lepton is identical to the electron, except that it is 3,500 times heavier and unstable. It exists for less than a trillionth of a second before it decays into other particles.

The tau lepton was discovered in 1975 by Martin Perl and a team of 30 physicists at the Stanford Positron-Electron Asymmetric Ring.


Tau neutrino
Only recently discovered by an international collaboration of physicists, the tau neutrino is the most massive of the three types of neutrinos. Its existence was confirmed when a particle that could only have been a tau neutrino hit the nucleus of an atom and created a tau lepton.

The first direct evidence for the tau neutrino was discovered in 2000 at the Fermi National Accelerator Laboratory in Illinois.


Top quark
The top quark was the sixth and final quark to be detected. The mass of a single top quark is equivalent to that of the nucleus of an atom of gold, which contains 197 protons and neutrons. (These 197 protons and neutrons are made up of 591 up and down quarks!)

The top quark was discovered in 1995 at the Fermi National Accelerator Laboratory.


Bottom quark
The bottom quark was the fifth quark detected. Its discovery led physicists to predict that there would be a sixth quark (the top quark), which was later discovered in 1995.

The bottom quark was discovered in 1976 at the Fermi National Accelerator Laboratory.


More About this Table

Context
Listed here are the main forces, matter, and actions that each particle is generally associated with. Exceptions are the fermions that comprise Family 2 and Family 3 particles. These fermions are, for the most part, detected only in particle accelerators.

Mass
According to superstring theory, each elementary particle is made of a tiny, fundamental string. And because mass and energy are equivalent (as described by Einstein's famous equation, E=Mc2), the mass of each "particle" is determined by the string's energy—the higher the energy of the string, the greater the mass of the particle. The figures displayed here are in units of millions of electron volts (MeV) when the particles are at rest.

Charge
A charge of 1 means that the particle has a positive electrical charge, a charge of -1 indicates that the particle has a negative charge, and a charge of 0 means that the particle does not interact with the electromagnetic force. Quarks, which have fractional charges, can combine in threes to form protons and neutrons, which have charges of 1 and 0, respectively. Quarks can also combine in pairs to create particles with charges of +1, 0, or -1.

Spin
Experiments reveal that every elementary particle has an intrinsic angular momentum—or a property called spin. All fermions have a spin of 1/2, while all bosons have a spin that is a whole integer. According to string theory, each elementary particle is made of a tiny string, and each string's spin is associated with the string's vibration.

Strength
The four forces, which are carried by bosons and operate over different distances, vary in strength, with the strong force being the strongest and gravity weakest. The figures displayed here are all relative to the strong force (which is why its strength is listed as "1") at a distance of 10-13 cm. Strength is not applicable to fermions because they do not carry forces.

Range
Some force-carrying particles—such as the photon and the graviton—have infinite range, while others work over only very short distances. The values shown are in centimeters. Range is not applicable to fermions because they do not carry forces.

Observed?
With the help of instruments such as particle accelerators, physicists are able to see deep within the sub-atomic world and verify the existence of various particles. Sometimes there is only indirect evidence of particles, as is the case with all types of quarks and the gluon.

Sparticles
Unlike the other data displayed in this column, the information shown here does not show attributes of the elementary particles but rather the "super particles" (sparticles) associated with each particle. Sparticles are alternate, perhaps more massive, versions of elementary particles, predicted by supersymmetry. Physicists have not yet verified the existence of these particles.


*The table lists 19 known particles (17 of which have been experimentally verified, though some of these indirectly). The matter and forces we experience, however, can be represented by about half that number. This table does not list any antiparticles.

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Rick Groleau is managing editor of NOVA online.



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