The God Particle

by Leon Lederman

  THE COWBOY LAB DIRECTOR

  The man who deserves much of the credit for Fermilab itself is our first director artist/cowboy/machine designer Robert Rathbun Wilson. Talk about charisma! Wilson grew up in Wyoming, where he rode horseback and studied hard at school, winning a scholarship to Berkeley. There he was a student of E. O. Lawrence's.

  I have already described the architectural feats of this Renaissance man in building Fermilab, but he was technologically sophisticated as well. Wilson became the founding director of Fermilab in 1967 and received an allocation of $250 million to build (so said the specifications) a 200 GeV machine with seven beam lines. Construction, started in 1968, was to take five years, but Wilson completed the machine ahead of schedule in 1972. By 1974 it was working steadily at 400 GeV with fourteen beam lines and with $10 million left over from the original allocation—all this with the most splendid architecture ever seen in a U.S. government installation. I recently calculated that had Wilson been in charge of our defense budget over the past fifteen years with the same skills, the United States would now be enjoying a tidy annual budget surplus and our tanks would be the talk of the art world.

  One story has it that Fermilab first sprang into Wilson's mind in the early 1960s in Paris, where he was an exchange professor. One day he found himself sketching a beautiful, curvaceous nude model with a group of other artists in a public drawing session at the Grande Chaumière. The "200" was being discussed in the United States, and Wilson didn't like what he read in his mail. While others drew breasts, Wilson drew circles for beam tubes and adorned them with calculations. This is dedication.

  Wilson wasn't perfect. He took short cuts when building Fermilab, and not all were successful. He complained bitterly that one blooper cost him a year (he would have finished in 1971), and an extra $10 million. He also gets mad, and in 1978, disgusted with the slow pace of federal funding for his superconducting work, he quit. When I was asked to become his successor I went to see him. He threatened to haunt me if I didn't take the job, and that did it. The prospect of being haunted by Wilson on his horse was too much. So I took the job and prepared three envelopes.

  A DAY IN THE LIFE OF A PROTON

  We can illustrate everything that has been explained in this chapter by describing Fermilab's cascade accelerator which has five sequential machines (seven if you want to count the two rings in which we make antimatter). Fermilab is a complex choreography of five different accelerators, each a step up in energy and sophistication, like ontogeny recapitulating phylogeny (or whatever it recapitulates).

  First we need something to accelerate. We run over to Ace Hardware and buy a pressurized bottle of hydrogen gas. The hydrogen atom consists of an electron and a simple nucleus of one proton. There are enough protons in this bottle to run Fermilab for a year. Cost: about twenty dollars if you return the bottle. The first machine in the cascade is nothing less than a Cockcroft-Walton electrostatic accelerator, 1930s design. Although it is the most ancient of the Fermilab series of accelerators, it is the most futuristic looking, adorned with very large and shiny balls and doughnutlike rings that photographers like to shoot. In the Cockcroft-Walton a spark strips the electron away from the atom, leaving a positively charged proton essentially at rest. The machine then accelerates the protons, creating a 750 KeV beam aimed at the entrance to the next machine, which is a linear accelerator, or linac. The linac sends the protons down a 500-foot-long series of radio-frequency cavities (gaps) to bring them to 200 MeV.

  At this respectable energy they are transferred via magnetic steering and focusing to the "booster," a synchrotron, which whirls the protons around and raises their energy to 8 GeV. Just think: at this point we've produced higher energies than the Berkeley Bevatron, the first GeV accelerator, and we have two rings yet to go. This load of protons is then injected into the main ring, the almost-four-mile-around "200" machine, which in the years 1974–1982 worked at 400 GeV, twice the official energy it was designed for. The main ring was the workhorse of the Fermilab complex.

  After the Tevatron came on-line in 1983, the main ring began taking life a little easier. Now it takes the protons up to only 150 GeV and then transfers them to the superconducting Tevatron ring, which is exactly the same size as the main ring and is just a few feet beneath it. In the conventional application of the Tevatron, the superconducting magnets carry the 150 GeV particle around and around, 50,000 circuits per second, gaining about 700 KeV per turn until, after about 25 seconds, they reach 900 GeV. By this time the magnets, powered by currents of 5,000 amperes, have increased their field strength to 4.1 tesla, more than twice the field that the old iron magnets could provide. And the energy required to maintain the 5,000 amperes is approximately zero! The technology of superconducting alloys is continually improving. By 1990 the 1980 Tevatron technology had been improved so that the Super Collider will use fields of 6.5 tesla, and CERN is working hard to push the technology to what may be a limit for niobium alloys—to 10 tesla. In 1987 a new kind of superconductor was discovered based on ceramic materials that require only liquid nitrogen cooling. Hopes were raised that a cost breakthrough was imminent, but the requisite strong magnetic fields are not there yet, and no one can estimate when and if these new materials will ever replace niobium titanium.

  At the Tevatron, 4.1 tesla is the limit, and now the protons are kicked by electromagnetic forces into an orbit that brings them out of the machine into a tunnel, where they are divided up among some fourteen beam lines. Here experimental teams provide targets and detectors to do their experiments. Some thousand physicists work in the fixed-target program. The machine operates in cycles. It takes about 30 seconds to do all the acceleration. The beam is spilled out over another 20 seconds so as not to crowd the experimenters with too high a rate of particles for their experiments. This cycle is repeated every minute.

  The external beam line is very tightly focused. My colleagues and I set up an experiment in "Proton Center," where a beam of protons is extracted, focused, and steered for about 8,000 feet onto a target 0.01 inches wide, the width of a razor blade. The protons collide with the thin edge. Every minute, day after day for weeks, a burst of protons strikes this target, never shifting by more than a small fraction of its width.

  The other mode of using the Tevatron, the collider mode, is quite different, and we will discuss it in detail. In this mode, the injected protons coast around in the Tevatron at 150 GeV waiting for antiprotons, which in due course are delivered from the p-bar source and sent around the ring in the opposite direction. When both beams are in the Tevatron, we begin ramping up the magnets and accelerating both beams. (More about how this works in a moment.)

  At every phase of the sequence, computers control the magnets and radio-frequency systems, keeping the protons tightly bunched and under control. Sensors give information on currents, voltages, pressures, temperatures, the location of the protons, and the latest Dow Jones averages. A malfunction could send the beam careening out of its vacuum pipe and through the enveloping magnet structure, boring a very neat and very expensive hole. This has never happened—at least not yet.

  DECISIONS, DECISIONS: PROTONS VS. ELECTRONS

  We've been talking a lot about proton machines here, but protons aren't the only way to go. The nice thing about protons is that they are relatively inexpensive to accelerate. We can accelerate them to thousands of billions of electron volts. The Super Collider will accelerate protons to 20 trillion electron volts. In fact, there may be no theoretical limit to what we can do. On the other hand, protons are full of other particles—quarks and gluons. This makes the collisions messy and complicated. That's why some physicists prefer to accelerate electrons, which are pointlike, a-tomlike. Because they are points, their collisions are cleaner than with protons. The downside is that they are low in mass, so they are difficult and expensive to accelerate. Their low mass results in a large amount of electromagnetic radiation when steered around a circle. Much more power must be put in to make up for the radia
tion loss. While this radiation is a waste from the point of view of acceleration, it's a spinoff boon to some researchers because it is very intense and of very short wavelength. Many circular electron accelerators are actually devoted to producing this synchrotron radiation. Customers include biologists who use the intense photon beams to study the structure of huge molecules, electronic chip makers who do x-ray lithography, condensed-matter scientists, who study the structure of materials, and many other practical types.

  One way around this energy loss is to use a linear accelerator such as the 2-mile-long linac at Stanford, built back in the early 1960s. The Stanford machine was originally called "M," for monster, and it was an outrageous machine for its time. It begins on the Stanford campus, about a quarter mile from the San Andreas Fault, and works its way toward San Francisco Bay. The Stanford Linear Accelerator Center owes its existence to the drive and verve of its founder and first director, Wolfgang Panofsky. J. Robert Oppenheimer told the story that the brilliant Panofsky and his equally brilliant twin brother Hans, attended Princeton together both achieving stellar academic records, but one doing just a hair better than the other. From that time on, claimed Oppenheimer, they became "Smart" Panofsky and "Dumb" Panofsky. Which is which? "That's a secret!" says Wolfgang. If truth be told, most of us call him Pief.

  The differences between Fermilab and SLAC are obvious. One does protons; one does electrons. One is circular the other straight. And when we say a linear accelerator is straight, we mean straight. For example, let's say we build a two-mile stretch of road. The surveyors guarantee us that it's straight, but it isn't. It follows the very slight curve of the earth. To a surveyor standing on the surface of the planet, it looks straight, but if viewed from space it's an arc. The beam tube in SLAC, on the other hand, is straight. If the earth were a perfect sphere, the linac would be a two-mile tangent to the earth's surface. Electron machines proliferated around the world, but SLAC remained the most spectacular, accelerating electrons to 20 GeV in 1966 and to 50 GeV in 1987. Then the Europeans took over.

  COLLIDERS VERSUS TARGETS

  Okay, here are our choices so far. You can accelerate protons or electrons, and you can accelerate them in circles or in a straight line. But there's one more decision to make.

  Conventionally, one extracts beams from the confines of the magnetic prison and transports the beams, always in vacuum pipes, up to a target where collisions take place. We've explained how analyzing the collisions provides information about the subnuclear world. The accelerated particle brings in a certain amount of energy, but only a fraction of it is available to explore nature at small distances or to manufacture new particles via E = mc2. The law of conservation of momentum says that some of the input energy will be preserved and given to the final products of the collisions. For instance, if a moving bus hits a stationary truck, much of the energy from the accelerating bus will go into knocking the various bits of sheet metal, glass, and rubber forward. This subtracts from the energy that could demolish the truck more thoroughly.

  If a 1,000 GeV proton strikes a proton at rest, nature insists that whatever particles come off must have enough forward motion to equal the forward momentum of the incident proton. It turns out that this leaves a maximum of only 42 GeV for making new particles.

  We came to realize in the mid-1960s that if one could get two particles, each having the full energy of the accelerator beam, to collide head on, we'd have an extraordinarily more violent collision. Twice the energy of the accelerator would be brought into the collision, and all of it would be available, since the total initial momentum is zero (equal and opposite momenta for the colliding objects). Ergo, in a 1,000 GeV accelerator, a head-on collision of two particles, each having 1,000 GeV, releases 2,000 GeV for the creation of new particles, compared to the 42 GeV when the accelerator is in stationary target mode. There's a penalty, however. A machine gun can easily hit the side of a barn; it is more difficult to have two machine guns shoot at each other and have the bullets collide in midair. This gives you some idea of the challenge of operating a colliding-beam accelerator.

  MAKING ANTIMATTER

  Stanford followed up its original collider with a very productive accelerator called SPEAR, for Stanford Positron-Electron Accelerator Ring, in 1973. Here beams of electrons are accelerated in the two-mile-long linear accelerator to an energy between 1 and 2 GeV and then injected into a small magnetic storage ring. Positrons, Carl Anderson's particles, are produced by a sequence of reactions. First, the intense electron beam impinges on a target to produce, among other things, an intense beam of photons. The debris of charged particles is swept away with magnets, which do not affect the neutral photons. Thus a clean beam of photons is allowed to strike a thin target, for example platinum. The most common result is that the pure energy of the photon converts to an electron and a positron, each sharing the original energy of the photon, minus the rest mass of the electron and positron.

  A magnet system collects some fraction of the positrons, and these are injected into a storage ring in which the electrons have been patiently going around and around. The streams of positrons and electrons, having opposite electrical charges, curve in opposite directions in a magnet. If one stream goes clockwise, the other goes counterclockwise. The result is obvious: head-on collisions. SPEAR made several important discoveries, colliders became very popular, and a plethora of poetic(?) acronyms was unleashed upon the world. Before SPEAR there was ADONE (Italy, 2 GeV); after SPEAR (3 GeV), there was DORIS (Germany, 6 GeV), then PEP (Stanford again, 30 GeV), PETRA (Germany, 30 GeV), CESR (Cornell, 8 GeV), VEPP (USSR), TRISTAN (Japan, 60 to 70 GeV), LEP (CERN, 100 GeV), and SLC (Stanford, 100 GeV). Note that colliders are rated by the sum of the two beam energies. LEP, for example, has 50 GeV in each beam; ergo it's a 100 GeV machine.

  In 1972, proton-proton head-on collisions were made available at the pioneering CERN Intersecting Storage Ring (ISR) facility in Geneva. Here two independent rings entwine around one another, the protons going in opposite directions in each ring and colliding head on at eight different intersection points. Matter and antimatter such as electrons and positrons can be circulated in the same ring because the magnets make them circulate in opposite directions, but two separate rings are needed to slam protons into each other.

  In the ISR each ring is filled with 30 GeV protons from the more conventional CERN accelerator, the PS. The ISR was ultimately very successful. But when it was fired up in 1972, it attained only a few thousand collisions per second in the "high luminosity" collision points. "Luminosity" is the term used to describe the number of collisions per second, and ISR's early troubles demonstrated the difficulty of getting two machine gun bullets (the two beams) to collide. Eventually the machine improved to over 5 million collisions per second. As for physics, some important measurements were made, but, in general, the ISR mostly provided a valuable learning experience about colliders and detection techniques. The ISR was an elegant machine both technically and in appearance—a typical Swiss production. I worked there during my 1972 sabbatical and returned frequently over the next decade. Early on I took 1.1. Rabi, who was visiting Geneva for an "Atoms for Peace" conference, on a tour. As we entered the elegant tunnel of the accelerator, Rabi's jaw dropped, and he exclaimed, "Ah, Patek Philippe!"

  The most difficult collider of all, one that pits protons against antiprotons, was made possible by an invention of a fabulous Russian, Gershon Budker, working in the Novosibirsk Soviet Science City. Budker had been building electron machines in Russia, competing with his American friend Wolfgang Panofsky. Then his operation was transferred to Novosibirsk, a new university research complex in Siberia. As he put it, since Panofsky was not similarly transferred to Alaska, the competition was unfair and he was forced to innovate.

  In Novosibirsk in the 1950s and '60s, Budker ran a thriving capitalistic system of selling small accelerators to Soviet industry in exchange for materials and money to keep his research going. He had been fascinated by the prospects for
using antiprotons, or p-bars, as one of the colliding elements in accelerators, but realized that they are a scarce commodity. The only place to find them is in high-energy collisions, where they are produced, yes, via E = mc2. A machine with many tens of GeV will have a few p-bars among the debris of collisions. To garner enough for useful collision rates, they would have to be accumulated over many hours. But as the p-bars emerge from a struck target, they are moving every which way. Accelerator scientists like to state these motions in terms of their principal direction and energy (just right!) and the superfluous sideways motions that tend to fill up the available space in the vacuum chamber. What Budker saw was the possibility of "cooling" the sideways components of their motions and compressing the p-bars into a much denser beam as they are stored. This is a complicated business. New levels of beam control, magnet stability, and ultra-high vacuum must be achieved. The antiprotons must be stored, cooled, and accumulated for upward of ten hours before there are enough to inject into the collider for acceleration. It was a lyrical idea, but the program was far too complex for Budker's limited resources in Siberia.

  Enter Simon Van der Meer, a Dutch engineer at CERN who advanced this cooling technique in the late 1970s and helped to build the first p-bar source for use with the first proton-antiproton collider. He used CERN's 400 GeV ring as both the storage and collision device, and the first p/p-bar collisions went on-line in 1981. Van der Meer shared the 1985 Nobel Prize with Carlo Rubbia for his contribution of "stochastic cooling" to the program that Rubbia had designed and that resulted in the discoveries of the W+, W−, and Z0 particles, which we'll discuss later.

  Carlo Rubbia is so colorful that he deserves a whole book, and he has at least one. (Nobel Dreams, by Gary Taubes, is about him.) One of the more brilliant graduates of the awesome Scuola Normale in Pisa, where Enrico Fermi was a student, Carlo is a dynamo that can never slow down. He worked at Nevis, at CERN, at Harvard, at Fermilab, at CERN again, and then Fermilab again. Traveling so much, he invented a complex cost-minimizing scheme of interchanging his "to" and "fro" ticket halves. I once briefly convinced him he'd retire with eight tickets left over; all west to east. In 1989 he became director of CERN, by which time the European consortium's lab had held the lead in proton-antiproton collisions for about six years. However, the lead was recaptured by the Tevatron in 1987–88, when Fermilab made significant improvements in the CERN scheme and put into operation its own antiproton source.