The God Particle

by Leon Lederman

  PARTICLES FROM THE VOID

  Another way of thinking about these things is to imagine that all space, even empty space, is awash with particles, all that nature in her infinite wisdom can provide. This is not a metaphor. One of the implications of quantum theory is that these particles do in fact pop in and out of existence in the void. The particles, in all sizes and shapes, are all temporary. They are created and then quickly disappear—a bazaar of seething activity. As long as they occur in empty space, vacuum, nothing really happens. This is quantum spookiness, but perhaps it can help to explain what happens in a collision. Here a pair of charmed quarks (a certain kind of quark and its antiquark) appears and disappears; there a bottom quark and its anti-bottom mate. And wait, over there, what's that? Well, whatever: an × and an anti-X appear, something we have no knowledge of in 1993.

  There are rules in this chaotic madness. The quantum numbers must add to zero, the zero of the void. Another rule: the heavier the objects, the less frequent their evanescent appearance. They "borrow" energy from the void to appear for the minutest fraction of a second, then disappear because they must pay it back in a time specified by Heisenberg's uncertainty relations. Now here is the key: if energy can be provided from the outside, then the transient virtual appearance of these vacuum-originated particles can be converted to real existence, existence that can be detected by bubble chambers or counters. How provided? Well, if an energetic particle, fresh out of the accelerator and shopping for new particles, can afford to pay the price—that is, at least the rest mass of the pair of quarks or X's—then the vacuum is reimbursed, and we say that our accelerated particle has created a quark-antiquark pair. Obviously, the heavier the particles we want to create, the more energy we need from the machine. In Chapters 7 and 8 you'll meet many new particles that came into being in just such a fashion. Incidentally, this quantum fantasy of an all-pervading vacuum filled with "virtual particles" has other experimental implications, modifying the mass and magnetism of electrons and muons, for example. We'll explain further when we get to the "g minus 2" experiment.

  THE RACE

  Beginning in the Rutherford era, the race was on to make devices that could reach very high energies. The effort was helped along in the 1920s by the electric utility companies, because electrical power is transmitted most efficiently when the voltage is high. Another motivation was the creation of energetic x-rays for cancer therapy. Radium was already being used to destroy tumors, but it was enormously expensive and higher energy radiation was thought to be a great advantage. Thus the electric utilities and medical research institutes supported the development of high voltage generators. Rutherford characteristically took the lead when he issued a challenge to England's Metropolitan-Vickers Electrical Company to "give us a potential on the order of ten million volts which can be accommodated in a reasonably sized room ... and an evacuated tube capable of withstanding this voltage."

  German physicists tried to harness the huge voltage of Alpine lightning storms. They hung an insulated cable between two mountain peaks, siphoning off charges as high as 15 million volts and inducing huge sparks that jumped 18 feet between two metal spheres—spectacular, but not too useful. This approach was abandoned when a scientist was killed while adjusting the apparatus.

  The failure of the German team illustrated that one needed more than power. The terminals of the gap had to he housed in a beam tube or vacuum chamber that was a very good insulator. (High voltages love to arc across insulators unless the design is very precise.) The tube also had to be strong enough to withstand having its air pumped out. A high-quality vacuum was essential; if there were too many residual molecules floating around inside the tube they would interfere with the beam. And the high voltage had to be steady enough to accelerate lots of particles. These and other technical problems were worked on from 1926 to 1933 before they were solved.

  Competition was intense throughout Europe, and American institutions and scientists joined the fray. An impulse generator built by Allgemeine Elektrizität Gesellschaft in Berlin reached 2.4 million volts but produced no particles. The idea was transported to General Electric in Schenectady, which improved the energy to 6 million volts. At the Carnegie Institution in Washington, D.C., physicist Merle Tuve drove an induction coil to several million volts in 1928 but didn't have an appropriate beam tube. Charles Lauritsen at Cal Tech succeeded in building a vacuum tube that would hold 750,000 volts. Tuve adopted Lauritsen's tube and produced a beam of 1013 (10 trillion) protons per second at 500,000 volts, theoretically enough particles and energy to probe the nucleus. Tuve did in fact achieve nuclear collisions, but not until 1933, by which time two other efforts had beaten him to the punch.

  Another runner-up was Robert Van de Graaff, of Yale and then MIT, who built a machine that carried electric charge along an endless silk belt up to a large metal sphere, gradually increasing the voltage of the sphere until, at a few million volts, he drew a tremendous arc to the wall of the building. This was the now famous Van de Graaff generator familiar to high school physics students across the land. Enlarging the radius of the sphere postponed the discharge. Encasing the entire sphere in dry nitrogen gas helped increase the voltage. Ultimately, Van de Graaff generators would be the machines of choice in the under-10-million-volt category, but it took years to perfect the idea.

  The race continued through the late 1920s and early '30s. It was a couple of Rutherford's Cavendish gang, John Cockcroft and Ernest Walton, who won, though by a whisker. And (here I have to groan) they were given invaluable help by a theorist. Cockcroft and Walton, after numerous failures, were attempting to reach the one million volts that was perceived to be necessary to probe the nucleus. A Russian theorist, George Gamow, had been visiting Niels Bohr in Copenhagen and decided to hop over to Cambridge before heading home. There he got into an argument with Cockcroft and Walton, telling the experimenters that they didn't need all the voltage they were playing with. He argued that the new quantum theory permitted successful nuclear penetrations even if the energy was not high enough to overcome the electrical repulsion of the nucleus. He explained that the quantum theory gave the protons wave properties, which can tunnel through the nuclear charge "barrier," as we discussed in Chapter 5. Cockcroft and Walton finally took note and redesigned their device for 500,000 volts. Using a transformer and a voltage multiplier circuit, they accelerated protons obtained from a discharge tube of the type that J. J. Thomson used to generate cathode rays.

  In Cockcroft and Walton's machine, bursts of protons, about a trillion per second, accelerated down the evacuated tube and smashed into targets of lead, lithium, and beryllium. The year was 1930, and nuclear reactions had finally been induced by accelerated particles. Lithium was disintegrated by protons of only 400,000 eV, far below the millions of electron volts that had been thought necessary. It was a historic event. A new style of "knife" was now available, although still in its most primitive form.

  A MOVER AND SHAKER IN CALIFORNIA

  The action now switches to Berkeley, California, where Ernest Orlando Lawrence, a native of South Dakota, had arrived in 1928 after a brilliant beginning in physics research at Yale. E. O. Lawrence invented a radically different technique of accelerating particles in a machine called a cyclotron, for which he was awarded the Nobel Prize in 1939. Lawrence was familiar with the clumsy electrostatic machines, with their huge voltages and frustrating electrical breakdowns, and he figured there had to be a better way. Searching through the literature for ways to achieve high energy without high voltages, he came across the papers of a Norwegian engineer, Rolf Wideröe. Wideröe noted that one could double the energy of a particle without doubling the voltage by passing it through two gaps in a row. Wideröe's idea is the basis for what is now called the linear accelerator. One gap is positioned after another down a line, the particles picking up energy at each gap.

  Wideröe's paper; however; gave Lawrence an even better idea. Why not use a single gap with modest voltage but use it over and over
again? Lawrence reasoned that when a charged particle moves in a magnetic field, its path is curved into a circle. The circle's radius is determined by the strength of the magnet (strong magnet, small radius) and the momentum of the charged particle (high momentum, large radius). Momentum is simply the particle's mass times its speed. What this means is that a strong magnet will guide a particle to move in a tiny circle, but if the particle gains energy and therefore also momentum, the radius of the circle will increase.

  Picture a hatbox sandwiched between the north and south poles of a large magnet. Make the box out of brass or stainless steel, something strong but nonmagnetic. Pump the air out of the box. Inside it are two hollow D-shaped copper structures that almost fill the box: the straight sides of the D open and facing each other across a small gap, the round sides closed. Suppose one D is positively charged, the other negatively charged, with the difference of potential being, say, 1,000 volts. A stream of protons generated (never mind how) near the center of the circle is aimed across the gap from the positive D to the negative D. The protons gain 1,000 volts and their radius now increases since the momentum is higher. The protons sweep around inside the D, and when they return to the gap, thanks to clever switching, they again see a negative voltage across the gap. Again they are accelerated, and now they have 2,000 eV. The process continues. Every time the protons cross the gap, they gain 1000 eV. As they gain momentum they fight against the constricting power of the magnet, and the radius of their path continues to increase. The result is that the protons spiral out from the center of the box toward the perimeter. There they strike a target, a collision takes place, and the research begins.

  The key to acceleration in the cyclotron is to make sure that the protons always see a negative D on the other side of the gap. The polarity has to flip-flop rapidly from D to D in exact synchronization with the rotation of the protons. But, you may be asking yourself, isn't it difficult to synchronize the alternating voltage with the protons, whose path continues to describe larger and larger circles as the acceleration continues? The answer is no. Lawrence discovered that by God's cleverness, the spiraling protons compensate for their longer path by speeding up. They complete each half circle in the same time, a process known as resonant acceleration. To match the proton orbits, one needs a fixed-frequency alternating voltage, a technology that was well known in radio broadcasting. Hence the name of the switching acceleration mechanism: radio-frequency generator. In this system the protons arrive at the edge of the gap just as the opposite D has maximum negative voltage.

  Lawrence worked out the theory of the cyclotron in 1929 and 1930. Later he designed, on paper, a machine in which the protons made a hundred turns with a generation of 10,000 volts across the D-gap. That would give him a beam of 1 MeV protons (10,000 volts × 100 turns = 1 MeV). Such a beam would be "useful for the studies of atomic nuclei." The first model, actually constructed by Stanley Livingston, one of Lawrence's students, came up considerably short, reaching 80 KeV (80,000 volts). Lawrence then went big-time. He obtained a huge grant ($1,000!) to build a machine that could produce nuclear disintegrations. The pole pieces (the north and south pole pieces of the magnet) were ten inches in diameter and in 1932 the machine accelerated protons to an energy of 1.2 MeV. These were used to produce nuclear collisions in lithium and other elements only a few months after Cockcroft and Walton's group at Cambridge. Second place, but Lawrence still lit a cigar.

  BIG SCIENCE AND THE CALIFORNIA MYSTIQUE

  Lawrence was a mover and shaker of enormous energy and ability. He was the father of Big Science. The term refers to huge, centralized facilities of great complexity and expense that are shared by a large number of scientists. In its evolution, Big Science created new ways of carrying out research with teams of scientists. It also created exquisite sociological problems, about which more later. The likes of Lawrence had not been seen since Tycho Brahe, the Lord of Uraniborg, the laboratory on Hven. In the experimental arena, Lawrence established the United States as a serious player in world physics. He contributed to the California mystique, the love of technological extravaganzas, complex and expensive undertakings. These were alluring challenges for young California and, indeed, for the young United States.

  By 1934 Lawrence was producing beams of 5 MeV deuterons with a thirty-seven-inch cyclotron. The deuteron, a nucleus consisting of one proton and one neutron, had been discovered in 1931, and had proved to be a more efficient projectile than the proton for producing nuclear reactions. In 1936 he had an 8 MeV deuteron beam. In 1939 a sixty-inch machine operated at 20 MeV. A monster started in 1940 and completed after the war had a magnet that weighed 10,000 tons! Cyclotrons were built all over the world because of their ability to unravel the mysteries of the nucleus. In medicine they were used to treat tumors. The beam of particles, directed at a tumor, deposits enough energy in the malignancy to destroy it. In the 1990s over a thousand cyclotrons are in use in hospitals across the United States. Basic research in particle physics, however, has abandoned the cyclotron in favor of a new type of machine.

  THE SYNCHROTRON: AS MANY TURNS AS YOU WANT

  The drive to create ever higher energies intensified and spread worldwide. At each new energy domain new discoveries were made. New puzzles were also created, increasing the desire to attain even higher energies. Nature's richness seemed to be hidden in the nuclear and subnuclear microworld.

  The cyclotron is limited by its design. Because the particles spiral outward, the number of orbits is obviously limited by the circumference of the device. To get more orbits and more energy, you need a bigger cyclotron. The magnetic field must be applied to the entire spiral area, so the magnets must be large ... and expensive. Enter the synchrotron. If the particles' orbit, instead of spiraling out, could be kept to a constant radius, then the magnet would be needed only along the narrow path of the orbit. As the particles gained energy, the magnetic field could be increased synchronously to keep them imprisoned in an orbit of constant radius. Clever! Tons and tons of iron could be saved, because the magnetic pole pieces, transverse to the path of the beam, could be reduced to inches instead of feet.

  Two important details must be mentioned before we proceed to the 1990s. In a cyclotron the charged particles (protons or deuterons) travel through what became thousands of turns in a vacuum chamber clamped between the poles of a magnet. To keep the particles from spreading out and striking the chamber walls, it was absolutely essential to have some kind of focusing process. Just as a lens focuses the light from a flashlight into a (nearly) parallel beam, magnetic force is used to squeeze the particles into a tight beam.

  In the cyclotron this focusing action is provided by the way the magnetic field changes as the protons move toward the outer edge of the magnet. Robert R. Wilson, a young student of Lawrence's and later the builder of the Fermilab accelerator, was the first to understand the subtle but crucial effect the magnetic forces had in keeping the protons from spraying out. In the early synchrotrons, the pole pieces were shaped to provide these forces. Later, specially designed quadrupole magnets (with two north poles and two south poles) were used to focus the particles, while separate dipole magnets steered them in a fixed orbit.

  Fermilab's Tevatron, a trillion-electron-volt machine completed in 1983, is a good example. The particles are steered into a circular orbit by powerful superconducting magnets, much as tracks guide a train around a turn. The highly evacuated beam pipe is a stainless steel (nonmagnetic) oval-shaped tube about 3 inches wide and 2 inches high, centered between the north and south poles of the magnets. Each dipole (steering) magnet is 21 feet long. The "quads" are 5 feet long. More than a thousand magnets are needed to cover the length of the tube. The beam pipe and magnet combination complete a circle that has a radius of 1 kilometer, or 0.6 miles—quite a change from Lawrence's first 4-inch model. You can see the advantage of the synchrotron design here. One needs a lot of magnets, but they're relatively skinny, just wide enough to cover the vacuum pipe. If the Tevatron were a cyclot
ron, we would need a magnet with pole pieces 1.2 miles in diameter to cover the 4-mile-around machine!

  Particles make 50,000 orbits in one second around this 4-mile track. In 10 seconds the particles have traveled 2 million miles. Each time they pass a gap—actually a series of specially constructed cavities—a radio-frequency voltage kicks up the energy by about 1 MeV. The magnets that keep the particles focused allow them to deviate from their appointed rounds by less than one eighth of an inch over the entire trip. It's not perfect, but it's good enough. Like aiming a riñe at a mosquito sitting on the moon but hitting it in the wrong eye. To keep the protons in the same orbit while they are being accelerated, the strength of the magnets must increase in precise synchronism with the proton's gain in energy.

  The second important detail has to do with the theory of relativity: protons become detectably heavier as their energy rises above 20 or so MeV. This increase in mass destroys the "cyclotron resonance" that Lawrence had discovered, in which the spiraling protons exactly compensate for their longer path by speeding up. This allows the rotation to be synchronized with a fixed frequency of the accelerating voltage across the gap. At higher energy the rotation time increases, and one can no longer apply a constant radio-frequency voltage. To counter the slowdown, the applied frequency has to decrease, so frequency-modulated (FM) accelerating voltages are used to track the increasing mass of the protons. The synchrocyclotron, a frequency-modulated cyclotron, was the earliest example of the influence of relativity on accelerators.