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Subatomic Particles

Neutrons, Quark Model, Elementary Mediator Particles, Baryons, Mesons, Current And Future ResearchDiscovery of particles, Subatomic particle classifications

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 An electronic display of the decay of an upsilon, a particle made of a bottom quark and an antiquark, in the CLEO detector at the CESR collider at Cornell University. An electron and a positron have met and annihilated (center); their energy produces an excited upsilon, which lives too briefly to show. It decays into a lower energy state by emitting a photon which converts into a positron (curves away tightly to the left) and an electron (curves to the right). The lower-energy upsilon then decays to its ground state by emitting another photon (detected in the bottom right sector of the grid). The ground-state upsilon finally decays into a high energy electron-positron pair (seen as the two long paths that cut diagonally across the grid in opposite directions from the center). Newman Laboratory of Nuclear Science, Cornell University, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. 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.


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.


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.)


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.


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.


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.

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.


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.

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

1st family 2nd family 3rd family
particle charge mass particle charge mass particle charge mass
νe 0 e 0 νμ 0 e 0 ντ 0 e 0
e- -1 e .511 μ- -1 e 106 τ- -1 e 1777
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

particle charge mass force
γ 0 e 0 Electromagnetic
g 0 e 0 Strong
±1 e 80200 Weak
Z0 0 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.

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