Probing the Proton

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Particles from the Sky

In 1932, a new elementary particle appeared. Unlike the electron, proton, and neutron, which had been found in the laboratory, this particle was created with radiation from outer space.

Soon after discovering radioactivity in rocks, people found another source of naturally occurring radioactivity--the sky. In 1910, the Jesuit priest Theodor Wulf took an electroscope to the top of the Eiffel Tower, 300 meters above the ground. He measured more radioactivity there than at ground level.

The Austrian physicist Victor Francis Hess took his electroscope up in a balloon. Five kilometers above the ground, he measured four times as much radiocativity. That meant charged particles were coming from outer space. The American physicist Robert Andrews Millikan named these extraterrestrial particles "cosmic rays."

What sort of particles were cosmic rays? To find out, physicists took pictures of their tracks in a cloud chamber.

The cloud chamber, invented by the Scottish physicist Charles Thomson Rees Wilson, is a jar with a piston. The jar is full of moist air. Pulling out the piston expands and cools the air, allowing droplets to form. Water droplets need a particle of dust or an ion to form. If a charged particle passes through the chamber, droplets will form around the ions it leaves in its path.

To measure the momentum of the particles, physicists put a magnet near the cloud chamber. The magnetic field bends the particles' tracks, and a camera takes their picture. From the curvature and length of the tracks, physicists can calculate the mass and energy of the particles that left them.

In 1932, Carl David Anderson of the California Institute of Technology noticed a peculiar track. It looked just like the track of an electron, but it curved in the opposite direction. Anderson concluded it was a particle with the mass of an electron and the charge of a proton.

Patrick Maynard Stuart Blackett and Giuseppe Occhialini of the Cavendish Laboratory saw the same kind of track in their cloud chamber. They agreed that it was a positive electron, or positron.

In 1936, Anderson and his colleague, Seth Neddermeyer, took a cloud chamber to the top of Pikes Peak, 4300 meters (14,000 feet) above sea level. In their photographs they found tracks of a particle heavier than an electron but lighter than a proton. They called it the meson, meaning intermediate, because its mass was between that of the electron and the proton.

After World War II, some physicists began to use a new kind of photographic emulsion to take pictures of cosmic rays without using a cloud chamber. The charged particles ionized the emulsion, leaving a trail of blackened grains. Looking through a microscope, physicists could count the grains and measure the length of a track. From their measurements they could calculate a particle's energy and mass.

Occhialini and three other physicists exposed plates in the Bolivian Andes, 5500 meters above sea level. When they examined the plates, they found that there were two kinds of meson. They called the heavier one the pi-meson, and the lighter one the mu-meson.

In 1947, the English physicists George D. Rochester and Clifford C. Butler noticed two cloud chamber photographs with forked tracks. They guessed that the tracks came from unstable particles. An unstable particle is one that disintegrates, or decays, into two or more lighter particles.

In the next few years, the English and American physicists photographed dozens of forked tracks. Blackett referred to the particles that left them as "V-particles" because the forked tracks looked like the letter V. By 1953, physicists had identified four types of V-particles: the kaons, the lambda, the cascades, and the sigmas.

The list of elementary particles now included the positron, the muon, the pi-mesons, and the V-particles, in addition to the electron proton and neutron. And this was only the beginnning. Around the world, physicists were building huge machines that would enable them to create particles at will.

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The Big Machines

The physicists who studied cosmic rays had to wait for a chance collision to produce a new particle. Nuclear physicists wanted a source of radiation that they could control. Among these were John Douglas Cockcroft and Ernest T. S. Walton of the Cavendish Laboratory.

In 1931, Cockcroft and Walton built a tower thirty centimeters in diameter and four meters high. The tower produced an electric potential of 800,000 volts. The tower was connected to an experimental tube. Inside the experimental tube was a beam of protons. The huge voltage from the tower created an electric force that increased the speed of the protons. The protons gained enough energy to disintegrate an atomic nucleus.

Cockcroft and Walton had built a proton accelerator. An accelerator uses electrical forces to increase the speed of charged particles. Because physicists can use them to break atoms apart, accelerators are sometimes called "atom smashers."

The accelerator was a great improvement over radioactive alpha sources. It produced an intense, controllable beam. The main problem with Cockcroft and Walton's machine was the high voltages. Often the materials between the electrodes would break down, and a spark would punch a hole in their experimental tube.

Ernest Orlando Lawrence, from the University of California at Berkeley, figured out how to accelerate particles without using high voltages. In 1931, he and his student M. Stanley Livingston built a cyclotron.

A cyclotron consists of two semi-circular hollow plates, called dees, with a gap between them. The dees sit between the poles of a powerful magnet. The magnet forces charged particles to travel in a circular path inside the dees.

An oscillating electric field charges the dees. When positively charged particles cross the gap from one dee to another, the first dee is positive and the second is negative. The negative charge attracts the particles and they speed up. By the time they circle through the second dee to re-cross the gap, the electric field has reversed. Now the second dee is positive and the first is negative, so the particles speed up a little more. As they travel faster, the radius of their path increases. They spiral from the center to the edge of the cyclotron.

The first cyclotron was 27 centimeters in diameter. It produced protons with an energy of one million electron volts.

By 1948, physicists at Berkeley had built a 4.67 meter (184-inch) cyclotron, which accelerated alpha particles to an energy of 380 million electron volts. When they bombarded a carbon target with the accelerated alpha particles, they produced a beam of pi-mesons. Physicists no longer had to wait for a chance event to create a meson. With cyclotrons, they could create them at will.

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The Particle Zoo

In 1953, at the Brookhaven National Laboratory, Luke Yuan and Sam Lindenbaum shot a high-energy pion beam into a hydrogen target. They noticed that when the pions had energies from 180 to 200 million electron volts, the target absorbed many more pions than when the pion energy was outside of this range. Their measurements were a typical example of what physicists call resonance.

You may be more familiar with the phenomenon of resonance in music. If you take a pipe, like an organ pipe, and play notes of various pitch into it, you will find there are certain pitches for which the sound becomes very loud. The lowest of these is the resonant frequency of the pipe. Many electrical and mechanical systems have resonant frequencies. It seemed that particle interactions had them as well. Lindenbaum and Yuan called their resonance the delta.

Before long, other resonances appeared. In 1960 Stan Wojcicki and Bill Graziano found a resonance they named the Y*. In 1961, the rho meson appeared at Brookhaven, and the omega meson at Berkeley. In 1962, physicists at Brookhaven discovered the f meson, and the phi meson, and physicists at Berkeley discovered the Xi-star. There seemed to be no limit on the number of resonances waiting to be discovered.

Resonances turned out to be elementary particles. A resonanace exists for such a short period of time that it can't leave a track in a detector. The only way to detect it is by finding its decay products.

In another kind of big machine, a very different sort of particle turned up. The big machine was a nuclear reactor, and the particle was the neutrino.

The first hint of the neutrino's existence came in 1914. James Chadwick noticed that the beta rays from any given radioactive isotope did not always come out with a uniform energy. The electrons could emerge with an energy anywhere between zero and a maximum value peculiar to the isotope. In contrast, alpha and gamma rays always emerged with fixed energies.

Physicists believed that nuclei, like atoms, made quantum jumps from one state to another. Each quantum jump corresponded to a definite amount of energy. In beta decay, some of the energy was missing.

The Swiss professor Wolfgang Pauli thought of a possible explanation for the missing energy. Perhaps a decaying nucleus emitted a neutral particle along with an electron. The electron would carry away part of the energy, and the neutral particle would carry the rest. The sum of their energies would be constant for any isotope. The Italian physicist Enrico Fermi named the hypothetical particle the "neutrino," meaning "little neutron."

The neutrino was nearly impossible to detect. Because it was neutral, it did not leave a trail of ions in a detecter. It was not massive enough to eject a particle from an atom. A neutrino went right through matter as if it wasn't there.

In 1953, the American physicists Frederick Reines and Clyde Cowan realized that if a neutron could turn into a proton, then a proton might sometimes turn into a neutron. In a reaction called inverse beta decay, a proton captures a neutrino, spits out a positron, and becomes a neutron.

Inverse beta decay is rare. In order to see it, Reines and Cowan needed to put a lot of protons in the path of their neutrinos. They put 300 liters of cadmium chloride dissolved in water next to a nuclear reactor. Neutrinos from the reactor passed through the liquid. A few of the neutrinos combined with protons to turn them into neutrons.

Reines and Cowan were able to detect the neutron and the positron from inverse beta decay. At last, they had proven that the ghostly neutrino exists.

In 1962, a team of physicists led by Melvin Schwartz of Columbia University found that there were actually two kinds of neutrinos. The kind that Reines and Cowan had found is associated with electrons and positrons. It is called the electron neutrino. The other kind of neutrino is associated with muons, and is called the muon neutrino.

The world of subatomic particles was getting crowded. By 1957 there were thirty. By 1964 there were more than eighty. Bewildered physicists referred to them as "the particle zoo."

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The Eightfold Way

To understand the growing number of elementary particles, physicists tried to classify them. At first they classified particles by their mass. Light particles like the electron and positron were leptons, from the Greek word for "light." Heavy particles like the proton and the lambda were baryons. The intermediate particles were mesons.

As they studied particles further, physicists began to classify them by the way they interacted with other particles. By 1933 physicists had identified four fundamental interactions: gravitation, electromagnetism, the strong interaction, and the weak interaction.

Both gravity and electromagnetism are familiar from the everyday world. Gravity is the force that attracts any two objects with mass. Electromagnetism acts on all charged particles, and can be attractive or repulsive.

The other two interactions affect only subatomic particles. The weak interaction governs beta decay of nuclei, and other interactions involving neutrinos. The strong interaction holds the atomic nucleus together. Particles that participate in the strong interaction are called hadrons. Protons, neutrons, pi-mesons, K-mesons, and most of the other elementary particles are hadrons.

Particles that do not participate in the strong interaction are now called leptons. Electrons, positrons, muons and neutrinos are leptons.

Particle physicists divide hadrons into two classes: mesons and baryons. Any number of mesons can be created and destroyed in a given interaction, as long as there is enough energy. In contrast, the total number of baryons in the universe seems to be conserved. Pions and kaons are mesons. Protons, neutrons, lambdas, and deltas are baryons.

Conservation of baryon number was a clue that helped the physicists of the 1950's and 1960's unravel the mystery of the hadrons. Another clue was the peculiar behavior of the V-particles.

Since the strong interaction of pions and protons produced V-particles, physicists knew they were hadrons. The V-particles should have decayed through the strong interaction as well. Yet they decayed instead through the much slower weak interaction. This was so strange that somebody called them "strange" particles. Strange particles were always produced in pairs.

In 1953, the American physicist Murray Gell-Mann and the Japanese physicist Kazuhiko Nishijima suggested that strangeness might be a property like electric charge or baryon number. Gell-Mann and Nishijima thought that the strong interaction could not create or destroy strangeness. The only way for a strange particle to decay into two non-strange particles was through the weak interaction.

Another clue was the hadron masses. Hadrons can grouped into families of particles with nearly the same mass but different electric charge. One such family is the proton and neutron. Another is the positive, negative, and neutral pions.

Murray Gell-Mann, and the Israeli physicist Yuval Ne'eman, grouped some of these families together into superfamilies, or "multiplets." The proton's multiplet includes the neutron, the positive, negative and neutral sigmas, the lambda, and the negative and neutral cascade particles. Gell-Mann and Ne'eman called the new classification scheme "the eightfold way."

The eightfold way refers to eight ways of transforming one member of a multiplet into another. It also refers to the Buddhist religion's eightfold way to enlightenment, or nirvana.

The eightfold way was an abstract and mathematical way of thinking about hadrons. Gell-Mann and the CalTech physicist George Zweig offered a more down-to-earth explanation. They said that hadrons were made of pointlike entities called quarks. There were three types, or "flavors," of quark: up, down, and strange.

The most peculiar property of quarks was their electric charge. The up quark carried two-thirds of a unit of positive charge, where the unit is the charge of a proton. The down and strange quarks each had a charge of minus one-third.

Gell-Mann considered quarks to be a purely mathematical concept. He did not expect anyone to find one in a laboratory. In 1964 he wrote, "A search for stable quarks . . . at the highest energy accelerators would help to reassure us of the non-existence of real quarks."

Early searches for quarks in the debris of nuclear collisions seemed to prove Gell-Mann was right. But a few years later, a group of physicists at the Stanford Linear Accelerator Center found something that looked a lot like real quarks.

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Probing the Proton

The Stanford Linear Accelerator Center, or SLAC, was the culmination of years of invention and hard work. The story of SLAC begins in 1947, when a group of physicists led by William Hanson built a travelling wave linear accelerator called the MARK I.

In a travelling wave accelerator, a wave of electromagnetic force travels from one end to the other. Electrons ride on this wave the way a surfer rides an ocean wave. As they travel along with the wave, they gain speed and energy.

The MARK I was a tube about one meter (three feet) long and 3 1/2 inches in diameter. Its successor, the MARK II, completed in 1949, was 4.2 meters (14 feet) long. The MARK III, completed in 1950, started out as a 9.1 meter (30 foot) machine. By 1953 it had grown to 64 meters (210 feet), and accelerated electrons to an energy of 400 million electron-volts.

A group of physicists led by Robert Hofstadter used the MARK III to bombard targets with high-energy electrons. They then counted the electrons scattered at different angles.

If atomic nucleii were infinitely small, Hofstadter should have seen the same angular distribution of scattered particles as Rutherford. When he compared his measurements to the theoretical prediction, he found that fewer electrons were scattered at large angles. Some of the electrons were going right through the atomic nucleii. That meant the nucleus was not an infinitely small point, but rather a "fuzzy ball."

When the Stanford physicists shot their electrons into hydrogen, whose nucleus is a single proton, they found that the proton too had a measurable size. It seemed to be a sphere of charge with a radius of about 10^-13 centimeters, a tenth of a trillionth of a centimeter.

Rutherford's alpha particles had not been able to approach the nucleus closely enough to measure its size. Hofstadter's electrons were more than fifty times as energetic as the alpha particles. They had enough energy to penetrate the nucleus.

The results of his experiments inspired Hofstadter to dream of an even larger machine. He and twenty-two colleagues began to plan a linac three kilometers (two miles) long.

In July, 1962, workers broke ground for the colossal machine. Buried 7.7 meters (25 feet) underground, was a copper tube 10 centimeters in diameters. Above ground were generators called klystrons which created the electromagnetic wave that accelerated the electrons. In 1966, the Stanford Linear Accelerator was ready to deliver beam to experiments.

Among the physicists using the accelerator were three men who had worked with Hofstadter. Their names were Henry Kendall, Jerome Friedman, and Richard Taylor. Kendall, Friedman, Taylor, and nineteen other physicists formed the SLAC-MIT collaboration.

At the end of its two-mile journey, the electron beam from the Stanford Linear Accelerator emerged into a large pit called the switchyard. Inside the switchyard was a windowless concrete block. Inside that block electrons smashed into a hydrogen target. The scattered electrons then entered a magnetic spectrometer, an enormous machine on a curved railroad track. Physicists could move the spectrometer to find out how many electrons were being scattered at different angles.

The magnetic spectrometer enabled the physicists of the SLAC-MIT collabaration to measure the momentum of the scattered electrons. A beam of particles travelling through a magnetic spectrometer spreads out the way light spreads out in a prism. The more its path is bent, the lower its momentum.

To "see" the electrons, the physicists placed plastic scintillator around their spectrometer. Like the zinc sulfide of Rutherford's apparatus, the plastic emitted flashes of light when high energy particles passed through it. Instead of their eyes, the experimenters used phototubes to record the flashes and send the information to a computer.

The SLAC-MIT collaboration studied the phenomenon of inelastic scattering. Hofstadter had measured elastic scattering. When two particles scatter elastically, their momentum changes, but their nature does not. An electron remains an electron and a proton remains a proton throughout the interaction. The electron may emerge in a different direction, but it loses little energy.

In inelastic scattering, on the other hand, the electron may lose a great deal of energy. Sometimes this energy transforms the proton into a resonance. Sometimes the energy creates additional particles, such as pions.

The physicists of the SLAC-MIT collaboration expected that inelastic scattering would show the same results as elastic scattering. They expected the nucleus to look like a fuzzy ball of positive electric charge.

When the SLAC-MIT physicists analyzed their data, they noticed a number of electrons were being scattered at large angles. It was the same phenomenon Rutherford had observed inside the atom. Like the atom, the proton was made of particles very much smaller than itself.

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The Plum Pudding Proton

The SLAC experiment helped other physicists understand some puzzling observations. Physicists at the Italian laboratory Frascati had found that when they smashed a beam of electrons into a beam of positrons, they produced more pions than they had expected. Similarly, a group of physicists working with a neutrino beam at the European laboratory CERN had observed a large number of inelastic reactions. Knowing that hadrons were made of pointlike constituents, the physicists now understood their experimental results.

CalTech professor Richard Feynman named the constituent particles "partons." Were these partons quarks? Many physicists thought so. But the explanation was not so simple.

Further experiment showed that the hadrons could not be composed solely of quarks. To explain the measurements, theoretical physicists postulated a "sea" of virtual quarks and antiquarks. Virtual particles are particles that don't last long enough to be measured. The quarks whose flavor determines the nature of the particle, for example the up, up, and down quarks of the proton, are called "valence quarks." The valence quarks float in the sea of virtual quarks. A new theory called quantum chromodynamics, or QCD, added neutral particles called gluons to the hadrons constituents. According to QCD, gluons are the particles that bind quarks together.

The picture of a proton as a sea of gluons and virtual quarks with three valence quarks floating in it reminded theoretical physicist James D. Bjorken of Thomson's model of the atom. He referred to it as "the plum-pudding proton."

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Return to The Electron Centennial Page
Go on to Particle Physics Today


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Return to The Electron Centennial Page
Go on to Particle Physics Today