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.
P-bars don't grow on trees, and you can't buy them at Ace Hardware. In the 1990s Fermilab is the world's largest repository of antiprotons, which are stored in a magnetic ring. A futuristic study by the U.S. Air Force and the Rand Corporation has determined that one milligram (one thousandth of a gram) of antiprotons would be an ideal rocket fuel, for it would contain the energy equivalent of about two tons of oil. Since Fermilab is the world leader in antiproton production (1010 per hour), how long would it take to make a milligram? At the present rate, a few million years of twenty-four-hour operation. Some incredibly optimistic extrapolations of technology might reduce this to a few thousand years. So my advice is not to invest in Fidelity's P-Bar Mutual Fund.
The Fermilab collider scheme works as follows. The old 400 GeV accelerator (the main ring), operating at 120 GeV, throws protons against a target every two seconds. Each collision of about 1012 protons makes some 10 million antiprotons heading in the right direction with the right energy. With each p-bar there are thousands of unwanted pions, kaons, and other debris, but these are all unstable and they go away sooner or later. The p-bars are focused into a magnetic ring called the debuncher ring, where they are processed, organized, and compressed, then transferred to the accumulator ring. Both rings are about 500 feet around and store p-bars at 8 GeV, the same energy as the booster accelerator. It takes five to ten hours to accumulate enough p-bars to inject back into the accelerator complex. Storage is a delicate affair as all of our equipment is made out of matter (what else?), and the p-bars are antimatter. If they come into contact with matter—annihilation. So we must be fastidious in keeping the p-bars orbiting near the center of the vacuum tube. And the quality of the vacuum must be extraordinary—the best "nothing" that technology can buy.
After accumulation and continued compression for about ten hours, we are ready to inject the p-bars back into the accelerator from whence they came. In a procedure reminiscent of a NASA launch, a tense countdown ensues to make sure that every voltage, every current, every magnet, and every switch is correct. The p-bars are zapped into the main ring, where they circulate counterclockwise because of their negative charge. They are accelerated to 150 GeV and transferred, again by magnetic legerdemain, to the Tevatron superconducting ring. Here the protons, recently injected from the booster via the main ring, have been patiently waiting, circulating tirelessly in the customary clockwise direction. Now we have two beams, running in opposite directions around the four-mile ring. Each beam is composed of six bunches of particles, with about 1012 protons and a somewhat smaller number of p-bars per bunch.
Both beams are accelerated from 150 GeV, the energy imparted to them from the main ring, to the full Tevatron energy of 900 GeV. The final step is "squeeze." Because the beams are counter-circulating in the same small tube, they inevitably have been crossing each other during the acceleration phase. However, their density is so low that very few collisions between particles occur. "Squeeze" energizes special superconducting quadrupole magnets that compress beam diameters from soda straws (a few millimeters) to human hairs (microns). This increases the density of particles enormously. Now, when the beams cross, there is at least one collision per crossing. Magnets are tweaked to make sure the collisions take place at the center of the detectors. The rest is up to them.
Once we have established stable operation, the detectors turn on and begin collecting data. Typically this continues for ten to twenty hours while more p-bars are being accumulated with the help of the old main ring. In time the proton and antiproton bunches become depleted and more diffuse, cutting the event rate. When the luminosity (the number of collisions per second) has gone down to about 30 percent, and if there are enough new p-bars stored in the accumulator ring, the beams are dumped and another NASA countdown ensues. It takes about a half hour to refill the Tevatron collider. About 200 billion antiprotons is considered an okay number to inject. More is better. These face some 500 billion protons, far easier to come by, to produce about 100,000 collisions per second. Improvements to all of this, designed for installation in the 1990s, will increase these numbers by about a factor of ten.
In 1990 the CERN p/p-bar collider retired, leaving the field to the Fermilab facility with its two powerful detectors.
WATCHING THE BLACK BOX: THE DETECTORS
We learn about the subnuclear domain by observing, measuring, and analyzing the collisions induced by high-energy particles. Ernest Rutherford locked his team up in a dark room so they could see and count the scintillations generated by alpha particles hitting zinc sulfide screens. Our techniques of particle counting have evolved considerably since then, especially in the post-World War II period.
Prior to World War II the cloud chamber was a major tool. Anderson used it to discover the positron, and it was found in cosmic ray laboratories around the world. My assignment at Columbia was to build a cloud chamber to operate with the Nevis cyclotron. As an absolutely green graduate student, I was unaware of the subtleties of cloud chambers and was competing with experts at Berkeley, Cal Tech, Rochester, and other such places. Cloud chambers are finicky devices, susceptible to "poisoning"—impurities that create unwanted droplets, which compete with those that delineate the particle tracks. No one at Columbia had any experience with these loathsome detectors. I read all the literature and adopted all the superstitions: clean the glass with sodium hydroxide and wash with triple distilled water; boil the rubber diaphragm in 100 percent methyl alcohol; mutter the right incantations ... A little prayer can't hurt.
In desperation, I tried to get a rabbi to bless my cloud chamber. Unfortunately, I picked the wrong rabbi. He was Orthodox, very religious, and when I asked him to say a brucha (Hebrew: "blessing") for my cloud chamber, he demanded to know what a cloud chamber was. I showed him a photograph, and he was furious at my suggested sacrilege. The next guy I tried, a Conservative rabbi, upon seeing the picture, asked how the cloud chamber worked. I explained. He listened, nodded, stroked his beard, and finally said sadly that he just couldn't do it. "The law..
." So I went to the Reform rabbi. He was just getting out of his Jaguar XKE when I came to his house. "Rabbi, can you say a brucha for my cloud chamber?" I pleaded. "Brucha?" he responded. "What's a brucha?" So I was worried.
Finally I was ready for the big test. At this point everything should have worked, but each time the chamber was operated, I got dense, white smoke. At this stage Gilberto Bernardini, a true expert, arrived at Columbia and began looking over my shoulder.
"Whatsa de brass rod, poking into de chamber?" he asked.
"That's my radioactive source," I said, "to give tracks. But all I get it white smoke."
"Tay-ka id oud."
"Take it out?"
"Si, si, oud!"
So take it out I did, and a few minutes later ... tracks! Beautiful wavy threads of tiny droplets tearing through the chamber. The most beautiful sight I'd ever seen. What happened was that my millicurie source was so strong it was filling the chamber with ions, and each grew its own drop. The result: dense, white smoke. I didn't need a radioactive source. Cosmic rays, omnipresent in the space around us, kindly provide enough radiation. Ecco!
The cloud chamber turned out to be a very productive instrument because one could photograph the trail of tiny droplets formed along the track of particles passing through it. Equipping it with a magnetic Held caused the tracks to curve, and measuring the radius of this curvature gave us the momentum of the particles. The closer the tracks were to being straight (less curvature) the more energetic the particles. (Remember the protons in Lawrence's cyclotron, which gained momentum and then described larger circles.) We took thousands of pictures that revealed a variety of data on the properties of pions and muons. The cloud chamber—looked at as an instrument rather than as a source of my Ph.D. and tenure—allowed us to observe some dozen tracks in each photograph. The pions take about a billionth of a second to pass through the chamber. We can provide a dense plate of material in which a collision can take place, which happens perhaps once in every hundred photographs. Because pictures can be taken only about one per minute, the data accumulation rate is further limited.
The God Particle: If the Universe Is the Answer, What Is the Question? Page 31
