Subatomic Particles
Subatomic particles are particles that are smaller than an atom. Historically subatomic particles were considered to be electrons, protons, and neutrons. However, the definition of subatomic particles has now been expanded to include elementary particles and all the particles they can be combined to make.
Elementary particles are particles that cannot be divided up into smaller particles. There are two types of elementary particles. One type of elementary particles make up matter. Examples of these particles include electrons and quarks (which make up protons and neutrons). 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. To illustrate this idea, let's say two boys want to play catch. The boys represent the matter particles, and playing catch represents the fundamental force. In this case, the ball would represent the mediator particle.
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, it was Joseph John Thomson who in 1897 measured their velocity and charge-to-mass ratio. The charge-to-mass ratio was found to be relatively large, and independent of the gas 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 the 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 x 10-19 Coulombs.
The first mediator particle to be discovered was the photon. In 1900, Max Planck reported that light came in little packages of energy, which he called "quanta." In 1905 Albert Einstein studied the photoelectric effect and proposed that radiation is quantized by its nature. A photon (the name was coined by chemist Gilbert Lewis in 1926) is one of these quanta, the smallest possible piece of energy in a light wave.
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, or protons, but atomic structure was not understood at the time. Conversely, Thomson thought that protons and electrons were randomly scattered throughout the atom, the so-called "plum-pudding model." In 1909-1911 Ernest Rutherford and his colleagues Hans Wilhelm Geiger and Ernest Marsden, did their famous scattering experiments of alpha particles (two protons and two neutrons) onto gold foil. They found that atoms had relatively hard and small centers, thus discovering the atomic nucleus and disproving the plum-pudding model.
In 1913, the Bohr model of the atom was introduced. 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) of the electron be limited to certain values (that is, that it be "quantized"), in order that the electron doesn't fall into the nucleus. Rutherford gave the proton its name in 1920.
When the principles of quantum mechanics were developed, the Heisenberg uncertainty principle meant the Bohr atom had to be modified. The Heisenberg uncertainty principle tells us that it is impossible to accurately determine both the position and the momentum (mass times velocity) of a subatomic particle at the same time. This means that the electrons in an atom still orbit the nucleus, but their position is scattered throughout a "cloud" instead of in well-defined orbits.
In 1930, scientists started to suspect the existence of another subatomic particle, which came to be known as the neutrino. Neutrinos are considered matter particles, but they do not make up Earth matter by themselves. Neutrinos are very common, but we don't observe them because they don't interact much at all with other particles.
In 1930 a problem with a process called nuclear beta decay had developed. Nuclear beta decay occurs 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 main 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, Wolfgang Pauli proposed that another particle carried off the missing energy that was given off in the decay. In 1933 Enrico Fermi named this hard-to-detect particle the neutrino, and successfully explained the theory of beta decay.
The neutrino was found in a 1956 experiment. 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. In 1998 physicist discovered that at least some of these three types of neutrinos must have a 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 definitely not zero.
In 1931-1932 Carl Anderson experimentally observed the anti-electron, which he called the positron, after its positive charge. The positron is an antiparticle which had been predicted by Paul Adrain Maurice Dirac in 1927-1930. It turns out every particle has an antiparticle partner that has the same properties except for an opposite electric charge (and other less-obvious quantities used in quantum mechanics). Antiparticles make up what is called antimatter. As far as we can tell, matter is much more common in our universe than antimatter, though it is unknown why this is so.
In 1932 James Chadwick discovered another matter particle, the neutron. The neutron is very similar to the proton except that it is electrically neutral. Chadwick found it 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 of protons and neutrons, which was later found to be true.
The second mediator particle discovered was the pion. In 1935 Hideki Yukawa formulated the idea that protons and neutrons were held together by a nuclear force, which was mediated by a particle called a pion. Yukawa described it in detail. In 1937 the first evidence for it was observed 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, 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 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 we would expect the time to make a particle and the time for it to decay to be about the same, but this wasn't true for the kaon. The kaon ended up being the first of many new particles to be detected.
In 1980 Maurice Jacob 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.
Quarks had been postulated much earlier, in 1964, by Murray Gell-Mann and independently by George Zweig. 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 Oscar W. Greenberg suggested each quark has a quality called color. The label "color" for this quark property does not refer to the usual definition of color. It 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 ++ (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, and mass, and spin. The ++ 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.
There are two kind 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 (). For unknown reasons, this lepton pairing is repeated two more times, each time with increasing mass. These leptons are called the muon (- ), and muon neutrino (), and the tau (- ), and tau neutrino (). We say there are three families, or generations, of leptons.
As in the lepton sector, the quark sector has 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 "botton" and "top" quark. Though all matter we see around us contains 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 called 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; it is that the quantity called spin obeys rules that mathematically are similar to those used to describe certain rotation bodies.) The spin of elementary particles is so small it is measured in special units called h-bar (h-bar is Planck's constant divided by 2*pi), and equals 1.1 x 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 this table. 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.
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 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 (g), 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 here. Masses are given in MeV/c2 .
One of the main rules of the standard quark model is that combinations of three quarks are called baryons. Protons (p) and neutrons (n) 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/c2 and spin in terms of h-bar.
The second main idea of the standard quark model is that combinations of one quark and one antiquark are called mesons. Pions () 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, + , 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.
Subatomic particles are very important in many technologies. Television sets use beams of electrons to create their pictures. A television set picks up the television signal which is then sent to electron guns. Electron guns shoot off beams of electrons which hit the face of the picture tube. When electrons hit the tube, it lights up, creating the picture. A common type of smoke detector that uses subatomic particles is an ionization smoke detector. In an ionization smoke detector alpha particles take away or add electrons ("ionize") to groups of atoms called 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, which makes the alarm go off.
In the 1990s, particle physics was 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.) This could be an important breakthrough and may mean that neutrinos play an important part in the ultimate fate of the universe.
The matter that astronomers can observe is called visible matter. There is not enough mass from visible matter in the universe for it to collapse in a big crunch (the opposite of the big bang, the explosion that created our universe) due to its own gravity. If it does not collapse, the universe will continue to expand forever, and everything, including stars, will die. Since neutrinos are not visible matter (to astronomers), they may be considered dark or "missing" matter. If the mass of neutrinos is enough, this neutrino missing matter may be great enough to "close" the universe, allowing it to collapse and be reborn someday. Surprisingly, astronomers found evidence in 1998 that the expansion of the universe is actually accelerating--that there is not enough mass to halt and reverse its current expansion. If true it implies that the universe would probably continue to expand forever.
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