Collider: The Search for the World's Smallest Particles - Paul Halpern (2009)
Chapter 6. A Tale of Two Rings
the Tevatron and Super
Its true that there were a few flaws in my logic. The rivers of ground water that flowed through their experiments, the walls of piling rusting away, the impossible access, and all without benefit of toilet facilities. But someof the users had their finest moment down in these pitys–the discovery of beauty, the bottom quark, where else?! Alas, as far as I know not one piling has been pulled up, not one pit has yet been refilled with earth.
—ROBERT R. WILSON (LECTURE AT THE THIRD INTERNATIONAL
SYMPOSIUM ON THE HISTORY OF PARTICLE PHYSICS, STANFORD
LINEAR ACCELERATOR CENTER, JUNE 1992)
After finding the misplaced rubber gasket that had stopped up his cyclotron during an important demonstration of its medical uses, Lawrence was absolutely furious. “You get out of this laboratory!” he screamed at the young assistant. “Don’t you ever come back!”1
Robert R. (Bob) Wilson, a graduate student at the Berkeley Radiation Laboratory who would blossom into the designer and leader of the greatest enterprise in the history of American high-energy physics, was absolutely crushed. With the laboratory dressed up in hospital white, and patients ready to be treated for their cancerous tumors, how could he have been so careless? Patients were literally waiting for days while the cyclotron refused to work, all because of his silly mistake. No words could express the depth of his remorse.
Lawrence rehired Wilson, only to fire him again after he ruined a pair of expensive pliers by melting them in a hot flame. The second dismissal wasn’t quite so bad. “I thought I’d probably get back somehow,” Wilson recalled.2
To say that Wilson’s career took many twists and turns before he became the force behind the establishment of Fermilab, the foremost accelerator lab in the United States and for a time in the world, is an understatement. Born in Frontier, Wyoming, in 1914, he came, like Rutherford and Lawrence, from a pioneering family. Wilson’s mother, Edith, was the daughter of a rancher who had come to the region during a gold rush. She married Wilson’s father when he was working in town as a surveyor. A stern, practical man, he could never appreciate the academic proclivities of his son. When Wilson, who was a well-read youth, wanted to head off to Berkeley for college, his father tried to forbid him—insisting that he go into business instead. Unsupported by his dad, Wilson set out at the age of eighteen for a life of adventure in particle physics.
At Berkeley, eyeing Lawrence’s lab for the first time, Wilson marveled like a child at a Christmas display. He was awestruck by the fancy equipment and the researchers’ exuberance. Although on a personal level he found Lawrence egotistical, at least initially, he decided to pursue working at the Rad Lab for his undergraduate research project. Nervously, he traipsed over to Lawrence’s office to ask about a position, and was greatly relieved when the great director replied, “Oh, yes, yes, yes.”
Wilson became an expert at cyclotron design, particularly with regard to producing stable particle orbits. He completed his undergraduate studies at Berkeley and continued as Lawrence’s graduate student. He witnessed the Rad Lab becoming a shining example for high-energy research around the world—its physicists respected for their experience.
Wilson also learned a great deal from Lawrence’s leadership skills. “I’m sure he had a profound influence on me,” Wilson recalled. “His style of running that laboratory was very impressive. He led by example. . . . We were infected by his enthusiasm and optimism, and his sense of priorities and of pushing hard.”3
In 1940, after getting his Ph.D. from Berkeley, Wilson got married to California native Jane Scheyer and moved east with her to Princeton, New Jersey. He served there as instructor for three years before being recruited to Los Alamos to work on the Manhattan Project. After the war, he spent a year at Harvard, then became appointed director of the Laboratory for Nuclear Studies at Cornell, where he served from 1947 to 1967. There he ran four different electron synchrotrons—the final one yielding energies up to 12 GeV—and established a reputation as an outstanding supervisor.
Synchrotrons, invented in the 1940s, offer far more flexibility than conventional cyclotrons by stepping up their magnetic fields in tandem with the requirements of the packs of particles rounding their tracks. Particles are injected into a synchrotron in bunches, like cyclists clustered together during a race. As each grouping reaches higher energies, the magnetic field is ramped up so that instead of spiraling outward the particles maintain the same radial distance. Fiercer creatures require stronger leashes.
Another key characteristic of a synchrotron is that its central magnet is replaced by bending magnets placed at equal intervals around the beam path. These serve a similar purpose but enable the device to encompass a wide area (a football field or farmlands, for example), thus permitting a much larger radius and increasing its power well beyond room-size instruments.
Yet another difference between synchrotrons and the original cyclotrons has to do with variations in the driving electric field. While the electric fields of cyclotrons vary periodically, offering a constant cadence of boosts, those of synchrotrons keep pace with the particle packets—preventing them from falling out of step once they reach relativistic speeds. It’s like a father who is pushing his son on a swing increasing his rhythm for greater effect after it has sped up. Similarly, synchrotrons are flexible enough to raise the energy bar of already energetic particles.
Wilson’s tenure at Cornell coincided with a dramatic rise of the use of synchrotrons in particle physics. The extraordinary power and flexibility of synchrotrons would prove critical for the discovery of massive new types of particles. Large synchrotrons would ultimately supply the dynamos for mighty particle colliders that would be used to search for evidence of unity—such as identifying the exchange particles for electroweak unification. Over decades, researchers at various laboratories would find ways of increasing synchrotrons’ ring sizes and improving the focusing power of their magnets to make them more effective at producing high-energy particles.
The 1950s and 1960s were a golden age for synchrotron design. In Berkeley, two engineers under Lawrence’s supervision, William Brobeck and Edward Lofgren, constructed a concert-hall-size proton synchrotron called the Bevatron. Completed in 1954, it could reach energies of up to 6 GeV. Unfortunately, its costs were driven up by a doughnut-shaped vacuum chamber (and surrounding magnet) with such wide openings, it seemed made for race cars rather than for particles.
Another early synchrotron, the Cosmotron, built on a converted army base in bucolic Brookhaven, New York, was more efficiently designed—possessing apertures that, though narrow, had sufficient room to accommodate the particle beams. Team leaders M. Stanley Livingston, Ernest Courant, John Blewett, G. Kenneth Green, and others managed to perform this feat through the use of 288 C-shaped magnets that carefully guided the proton pulses through the pipe of the seventy-five-foot diameter accelerator. It took but a second for the protons to travel 135,000 miles (through millions of revolutions) and reach energies of 3 GeV before smashing into targets.4 When the Cosmotron first came on line in May 1952, the New York Times applauded its inaugural “Billion Volt Shot.”5
Courant’s experience with adjusting the magnets of the Cosmotron to focus the beam as tightly as possible led him to a critical insight that paved the way for the next generation of machines. He calculated that by switching adjacent magnets to face in opposite directions—alternatively inward and outward—he could greatly augment their focusing power. His finding, called strong focusing, paved the way for the construction at Brookhaven of the Alternating Gradient Synchrotron, an even mightier accelerator that opened in 1960 and is still in use today.
Meanwhile, at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland, Niels Bohr smashed a bottle of champagne and inaugurated the Proton Synchrotron (PS), another strong-focusing accelerator. It was a triumph for the reemergence of European science after the war. CERN had been established a decade earlier, through a resolution put forth by Rabi at the fifth UNESCO conference authorizing that agency, “to assist and encourage the formation and organization of regional centres and laboratories in order to increase and make more fruitful the international collaboration of scientists.”6
By the time the PS opened, the CERN council, consisting of representatives of Belgium, Denmark, France, (West) Germany, Greece, Italy, the Netherlands, Norway, Spain, Sweden, Switzerland, the United Kingdom, and Yugoslavia, had met numerous times, and established its scientific laboratory near the village of Meyrin—part of the Geneva canton, close to the French border. The smashing new accelerator bolstered the center’s reputation as an international hub of high-energy research.
Wilson worked hard at Cornell to compete with the burgeoning laboratories at Berkeley, Brookhaven, and CERN. He established a reputation as a highly capable leader. Yet he wanted to be more than just a scientist and administrator. In a highly unusual move for a scientist of such ambition, Wilson took time off to pursue a second career as a sculptor. In 1961, he traveled to Rome and enrolled at the Accademia di Belle Arti, where he learned how to create modern sculpture. He was also interested in architecture and other aspects of design.
All of Wilson’s passions beautifully merged together when, in 1967, he was offered the supreme challenge of designing the foremost accelerator laboratory in the United States, to be built among the cornfields of rural Batavia, Illinois—about thirty-five miles west of Chicago. Originally called the National Accelerator Lab, it was renamed in 1974 after Fermi. In taking on the responsibility, Wilson set out to make the lab as user-friendly as possible—open to whoever wanted to conduct experiments requiring high energies, without regard for hierarchy. He wanted to avoid the restrictions of having just a handful of leaders, in the mode of Rutherford and Lawrence, setting the course for all of the lab’s projects.
A second goal of Wilson’s, meshing well with the democratic spirit he established, was to keep construction and operating costs as low as possible. The U.S. Atomic Energy Commission requested that construction time be kept to less than seven years, with a maximum expense of $250 million (reduced, due to cost-cutting measures, from an original allotment of $340 million). Miraculously, Wilson finished ahead of schedule and within the tight budget—all while doubling the accelerator’s energy from an anticipated 200 GeV to more than 400 GeV. He truly wanted the greatest bang for the buck.
Despite financial restraints, Wilson fervently aspired to bring aesthetics to lab design. He involved himself in all aspects of planning the lab’s architecture—including a futuristic central tower of concrete and glass—and even crafted innovative sculpture to beautify the landscape. A painter was recruited to bathe the equipment in bright colors. Unprecedented for an experimental facility, its art won accolades from the New Republic’s critic Kenneth Everett, who called it a “rare combination of artistic and scientific aspiration.”7 With tastes reflecting the utilitarian spirit of the 1960s, Wilson’s creations managed to impress while not sapping budgets.
Finally, as a born frontiersman, he believed in living in harmony with the land and was an early environmentalist. He recycled many of the original barns by converting them to buildings used for social functions, housing, and other purposes. Wildlife from mallard ducks to muskrats found refuge in the ponds, the fields, and even the machinery. As a reminder of his roots, and also as a symbol of the pioneering nature of modern physics, he brought in a herd of bison to graze freely in a grove. Their shaggy descendants still roam the grounds today.
Wilson felt very much at home wandering Fermilab on horseback—as if it were a ranch that happened to be raising protons and mesons instead of cattle and sheep. Dressed in jeans, a windbreaker, cowboy boots, and a black fedora, he’d mount his gray mare, Star, and ride her around his enterprise—as if warming up for the Preakness—to inspect its fine details.
An aerial view of Fermi National Accelerator Laboratory (Fermilab) showing the main ring and principal office tower.
No aspect of the accelerator center was too trivial for Wilson to tweak—from the distinctive geometric rooftops (a geodesic dome in one case) to the no-frills dirt floors—even the specifics of how the kitchen was run. On a limited budget choices needed to be made—roofs versus floors, for instance—and Wilson thought that he needed to make these himself, lest he incur the wrath of the Atomic Energy Commission.
As one of the bulwarks against excess spending, Wilson hired a hardheaded administrative assistant named Priscilla Duffield, who was formerly Lawrence’s secretary at the Rad Lab and then J. Robert Oppenheimer’s secretary at Los Alamos during the Manhattan Project. J. David Jackson, acting head of theoretical physics at Fermilab from 1972 to 1973, remembered her as a “tall, imposing, no-nonsense woman.” She would become incensed by the mere hint of unauthorized expenditure. Jackson recalled her sharp reaction when she found out about a wine-and-cheese seminar he helped organize.
She stormed into my office, looking for my scalp. “What do you think you’re doing, serving wine at that seminar? Don’t you know it’s illegal to spend government money on such things?” I said that I wasn’t spending government money on the wine. She said, “Well who is paying for it?” I said, “I am.” And she said, “Oh.” It was the one time I saw Priscilla just a little bit penitent.8
Not every decision Wilson made worked out for the best, due to his passionate effort to cut costs. By steaming single-mindedly ahead with certain structural choices, he almost launched the whole project over a precipice. Not considering that magnets might sometimes need to be replaced, he had them welded to the beam line running through the tunnel of the main ring.9 Neglecting to protect the tunnel from the humid Illinois summers, some of the magnets acquired moisture and started to crack. Imagine the horror of the eager researchers primed for discovery when right before the accelerator was about to be turned on many of its magnets failed and couldn’t easily be removed. Fortunately, a stalwart team of experimentalists gathered quickly and resolved the problem.
In contrast to the permanently secured magnets, Wilson made the opposite choice for the places where physicists would take measurements. To ensure minimal cost and maximal flexibility, he designed the working areas to be as provisional as anthills. He came to realize that his makeshift structures, though thrifty, were not very popular. As Wilson remarked:
These enclosures are indeed rough-and ready places. . . . Indeed some of the users were advised by their older colleagues to “abandon all hope, ye who enter here!” I fear that I bear the responsibility for this fiasco. In the frenzy of saving big bucks, I had the fantasy of not putting up (or down) any laboratory building at all. Instead the idea was that, once an experiment had been accepted, an outline of the necessary space would be drawn in an empty field at the end of one of the proton beams, then steel interlocking piles would be driven . . . down to the necessary depth. . . . The experimental equipment would be lowered to a luxurious graveled floor, and finally a removable steel roof would be covered with the requisite thickness of earth. . . . Simple and inexpensive, is it not? I still find it difficult to understand why all those users stopped speaking to me.10
Wilson nicknamed the experimental area for proton studies, constituting the termini of beams shunted from the four-mile-long main synchrotron ring, the “proton pits”; other research zones were dedicated to mesons and neutrinos. He was especially proud of the fifteen-foot bubble chamber, lauded by Berkeley physicist Paul Hernandez as the “Jewel in the Crown” for its type of detector.11
A bubble chamber consists of a large vat of liquid hydrogen surrounded by an immense guiding magnet. After protons collide, the magnet would steer charged debris along swirling paths through the fluid. The hydrogen along the tracks would bubble away, enabling experimentalists to photograph these trails and calculate the properties of the particles that produced them. Because their different charges would experience opposite magnetic forces, positive and negative particles would spiral in opposite directions.
The Big European Bubble Chamber, a device for tracking particles, on display at CERN’s Microcosm museum.
Other types of detectors commonly used in high-energy physics include scintillation counters, photomultipliers, Cherenkov detectors, calorimeters, spark chambers, and drift chambers. The reason for such a diverse toolbox of measuring instruments is to glean as much information as quickly as possible. Many particles, once born, are extremely short lived, and decay almost immediately into other particles. Sometimes the only signs of an interaction are missing energy, momentum, and other conserved quantities. Like detectives at a crime scene, physicists investigating possible culprits must cordon off the collision site—by surrounding as much of it as possible with data-collecting instruments—and rapidly gather a cache of evidence. Only then can they hope to reconstruct the event and determine what actually transpired.
Scintillation counters, an update on Rutherford’s favorite technique for spotting particles by means of flashes in fluorescent materials, rely on atomic electrons becoming energized by passing particles and then releasing this energy as light. Moldable, fluorescent plastics and liquid additives called fluors have served well for this purpose. Photomultipliers are electronic devices that amplify faint light (from scintillators, for example) so it is much more discernable.
Cherenkov detectors depend on a different physical property, called the Cherenkov effect. Discovered in 1934 by physicist Pavel Cherenkov, of the Lebedev Physical Institute in Moscow, it is a phenomenon that occurs when a particle travels faster than light does in a particular material. Although nothing can exceed the speed of light in a vacuum, light moves slower in certain substances and can thereby be outpaced. Like jets creating a sonic boom (an audible shock wave) when they exceed sound’s velocity in air, particles racing past light’s material speed emit a cone of radiant energy in the direction of motion called Cherenkov radiation. Conveniently, the angle of the cone directly depends on the particle’s velocity, offering a practical way of measuring this important factor.
Another class of apparatus, called calorimeters, enables researchers to record the energies of particles. These are dense materials that trigger a cascade of decays, through processes such as pair production (creation of electron-positron companions) and bremstrahlung (radiation generated when particles slow down), releasing a storehouse of energy in the process. By trapping some or all of this energy, physicists can try to determine how energetic the original must have been. While electromagnetic calorimeters rely on electromagnetic processes to produce the cascades, hadronic calorimeters depend on the strong interaction instead.
Hadrons are particles that experience the strong force, such as protons, neutrons, various types of mesons, and an assortment of heavier particles. They are each composed of quarks. Leptons such as electrons, positrons, muons, and neutrinos, on the other hand, are particles that ignore the strong force. They are not made of quarks but, rather, are fundamental. Hadronic calorimeters capture the energy of hadrons but not leptons.
Along with bubble chambers, various other types of instruments can be used to measure particle trajectories. Spark chambers, useful for charged particles, involve electrical signals passing like lightning through regions of a gas ionized by particles whizzing past. Drift chambers are more sophisticated devices that use electronics to record the time particles take to move from one point to another.
The invention of the computer provided a vital tool for high-energy research. It allowed researchers the luxury of sifting through vast amounts of data and accessing which subset displayed the fingerprints of potentially interesting events. Otherwise finding rare decay products would be a hopeless task—like locating a single four-leaf clover in the vast expanse of the American prairie.
By the time Fermilab became operational in the early 1970s, one of its principal features was already out of date. Along the lines of Rutherford’s experiments, beams produced by the accelerator slammed into fixed targets. According to conservation principles, most of the collision energy served to channel secondary particles along a tight path past the target. Merely a fraction of the energy could be used to produce the new particles themselves. Technically, this is because the useful energy for a fixed-target collision increases at the relatively slow rate of the square root of the beam energy. If, with improvements to a fixed-target device, protons were accelerated to one hundred times more energy, for instance, its effective energy would increase only tenfold. Not only was this situation inefficient, the narrowness of the particle jets produced made it difficult for researchers to examine what was created.
As far back as 1953, Wideröe, with his extraordinary foresight, had patented a design for a far more efficient type of accelerator, now known as a collider.12 He recognized that by smashing particles together head-on, a much larger portion of the collision energy would be transformative rather than kinetic (engendering motion). Because he was working as an industrial engineer at the time, the physics community did not take note of his patent. Three years later, however, a team of synchrotron developers, led by physicist Donald Kerst, independently proposed the idea of colliding particle beams with each other. Published in the leading journal Physical Review and discussed at a 1956 CERN symposium in Geneva, Kerst’s proposal stimulated efforts to boost the usefulness of conventional accelerators by turning them into colliders.
We can envision the difference between fixed-target accelerators and colliders by imagining two different kinds of accidents involving diesel locomotives. In the first case, picture an engine car racing out of control and hitting the back of a boxcar that’s sitting at the junction of two tracks. Conceivably, the impact of the engine car could cause the boxcar to roll down one of the tracks, the engine car down the other, and both would escape unscathed. The collision energy would be mainly kinetic.
However, suppose two engine cars (of comparable size and speed), traveling in opposite directions, plow into each other head on. In that case it would be hard to picture a happy outcome. The bulk of the energy would likely end up producing a burning wreck. What would be horrific for a transportation engineer would work out nicely for high-energy physicists in their quest to add more fuel to the fire and spark the creation of new particles.
At the same CERN conference, Princeton physicist Gerald O’Neill proposed a clever way of implementing the collider idea by use of linked storage rings. Particles accelerated in a synchrotron, he envisioned, could be directed into two different storage rings, where they would orbit in opposite directions before smashing together at a designated intersection point. His idea formed the basis of several important electron-positron collision projects during the 1960s and early 1970s, culminating in the completion of the SPEAR ring at SLAC in 1972—where Burton Richter, SPEAR’s developer, would codiscover the J/psi particle (a heavy meson composed of quarks with properties called “charm” and “anticharm”), and Martin Perl would discover the ultraheavy tau lepton, among other findings.
These discoveries would help provide evidence that quarks and leptons are organized into three distinct generations: the up and down quarks, electron and neutrino in the first; the strange and charm quarks, muon and muon neutrino in the second; and the tau lepton (along with the later-discovered top and bottom quarks and tau neutrino) in the third. Either individually, as in the case of leptons, or grouped into various combinations to form different hadrons, as in the case of quarks, these constitute the basis of matter.
In 1971, CERN inaugurated the world’s first hadron collider: the Intersecting Storage Rings (ISR). CERN’s then existing Proton Synchrotron accelerated bunches of protons to energies of 28 GeV, upon which an injection system whisked them off to one of two storage carousels. There, they were “stacked,” a process involving timing the proton injections so that groups are packed in closely together but still flowing smoothly. It’s a bit like a traffic signal allowing cars to merge onto a highway only at particular intervals to pace them just right and increase the road’s capacity. Through stacking, the proton beams circulating around the rings increase their luminosity, or rate of collisions per area, a function of the beam intensity. Boosting the luminosity is akin to upping the firing rate and focus of a machine gun to maximize its chances of hitting a target. A higher collision rate increases the chances of exceptional events taking place, such as the production of rare particles.
Shortly after the ISR came online, CERN researchers decided to test out a novel method for increasing luminosity, called stochastic cooling. Developed by Dutch physicist Simon van der Meer, who was in charge of the steering magnets at CERN, it offered a way to tighten up the bunches of protons into denser clusters, allowing them to be stacked much closer together. The basic idea is to test how far particles deviate from the average of their group and kick them back in line if they stray too far. These correcting nudges cause each bunch to have less prominent fluctuations and “cool down” to a more tightly packed state—creating more room to stack more clusters and increase the beam intensity. Van der Meer’s enhancement of beam luminosity represented such an important enhancement for colliders—opening the door to pivotal discoveries—that it would earn him the 1984 Nobel Prize in Physics (along with Italian physicist Carlo Rubbia).
Anticipating the competition CERN would provide with its radically improved methods, Wilson argued for upgrades to the Fermilab accelerator that would at least double its effective energy. The reason was clear. A critical advance in theoretical physics, the unification of electromagnetism and the weak interactions into a single quantum theory, had triggered an intense race to discover the massive particles it predicted. The chance to verify a stunning new form of unity inspired a whole generation of experimentalists to join teams at Fermilab, CERN, and elsewhere and dedicate themselves to an epic search through unprecedented quantities of data generated through extraordinary energies.
The Standard Model of electroweak unification, proposed independently in 1967 by Steven Weinberg and Abdus Salam, predicts four new massive bosons, to supplement the familiar massless photon. Two of these, the W+ and W-, serve as the exchange bosons for the weak interaction involving positive and negative charge transfers respectively (for example, interactions involving electrons and neutrinos, or positrons and antineutrinos). A third, the Z0, conveys the neutral version of the weak interaction. This was inserted, based upon Sheldon Glashow’s work, to make it a mathematically balanced theory, even though no one at the time had ever observed a neutral weak current. Together, the W+, W-, and Z0 are known as the intermediate vector bosons, the designation “vector” referring to their particular transformative properties. The fourth predicted particle is the Higgs boson, which through its spontaneous symmetry breaking (as discussed in chapter 2), supplies mass to the W+, W-, and Z0 bosons, along with the quarks and leptons.
The scenario sketched by Weinberg and Salam meant that finding these new bosons wouldn’t be easy. At high-enough temperatures—during the initial instants of the Big Bang, for example—the theory’s symmetry would be unbroken and the W and Z bosons would be massless too. However, below a critical temperature—today’s conditions, for example—the spontaneous breaking of the original symmetry would give ample mass to these bosons. To detect them, therefore, would require the extraordinarily energetic conditions of the world’s mightiest accelerators.
In 1970, three intrepid experimentalists—Carlo Rubbia, then at Harvard, Alfred K. Mann, of the University of Pennsylvania, and David Cline, of the University of Wisconsin—initiated a fledgling effort at Fermilab to find the W boson. Nicknamed the HPWF collaboration, after the initials of the universities involved (and Fermilab), the group set up shop in the neutrino building. The geodesic roof of that metal structure leaked badly during rainstorms and the floors were dirt, so team members often needed to wade through muddy puddles to get to their equipment. Wilson’s cost-cutting measures had led to working conditions suitable for one of Dante’s lower circles.
The following year, a young virtuoso in field theory, Gerard ’t Hooft of the University of Utrecht, Holland, working under the supervision of Martinus Veltman, proved that the Weinberg-Salam theory could be renormalized (infinite terms canceled out), just like quantum electrodynamics. This made the theory extremely attractive. Giddy from these remarkable results, Weinberg was eager to have one of the basic predictions of electroweak theory tested: the existence of neutral weak currents. As Rubbia later recounted, Weinberg “brainwashed” the HPWF team to switch course and look for neutral currents instead.13
Rubbia asked Larry Sulak, a colleague from Harvard working with the group, to install a new trigger for the detector that would be sensitive to neutral current events. These would involve fermions keeping their own identities as they interact with each other through the weak force—for example, electrons remaining electrons and protons remaining protons. The problem was that common electromagnetic interactions similarly preserve particle characteristics; electrons stay electrons during those events too. Therefore, the major challenge was to find the weak neutral needle among the haystack of electromagnetic events that similarly conserve charge and mass. Neutrino events offered the best chance for this, because as light neutral leptons their principal mode of interaction is the weak force. If a neutral hadron, such as a neutron, interacted with a neutrino in an event that kept both particles the same, the weak neutral current would be the natural culprit.
A competing team from CERN, led by Jack Fry and Dieter Haidt, also took up the gauntlet. Using the Gargamelle heavy-liquid bubble chamber, pumped full of freon, that had recently been installed at CERN’s Proton Synchrotron inside a colossal superconducting magnet, they spent the fall and winter of 1972 searching for neutrino-induced neutrons. As skiers etched zigzag tracks in the snowy slopes near Geneva, Fry and Haidt examined the frozen trails of cascading particles—their specific paths in the bubble chamber marking their interactions and properties. The summer brought new joys. By July 1973, the group had collected sufficient evidence of neutral current events to present its work. The HPWF collaboration announced its own promising results around the same time.
Alas, summer’s heat sometimes shapes cruel mirages. After modifying its equipment and retesting its data, the HPWF team’s findings vanished amid the desert sands of statistical insignificance. Skeptics wondered if electroweak unity was simply a beautiful illusion.
Nervous that its own results would become similarly wiped out, the Gargamelle group set out for more testing, and found, to its delight, that its findings were on firm footing. Meanwhile, the HPWF group reexamined its results one more time, resolved the issues that had plagued its analysis, and proclaimed success as well. For the first time in the history of science, a wholly new mode of interaction—the neutral weak current—had been anticipated by theory before being found by experiment.
Haidt later described the impact of his team’s findings: “The discovery of weak neutral currents . . . brought CERN a leading role in the field. The new effect marked the experimental beginning of the Standard Model of electroweak interactions, and triggered huge activity at CERN and all over the world, both on the experimental and theoretical sides.”14
From the neutral current results, theorists developed fresh estimates of the mass of the W boson, stimulating an international race to find that particle. Leading the pack was an energized CERN, anxious to prove that its neutral current victory was no fluke. The European community had already identified the land and allocated the funds to begin constructing the Super Proton Synchrotron (SPS), a four-mile-long accelerator intended to be—at 300 GeV—the most energetic in the world. In the midst of construction, however, Fermilab’s Main Ring surpassed the SPS’s intended energy—a major disappointment for the Europeans.
When you are locked in battle, even the smallest delay can offer the other side an opening for victory. In the case of the race to identify the weak bosons, CERN’s opportunity arose when Rubbia became exasperated by the failure of Fermilab to commit to building a proton-antiproton collider—an idea initially suggested by his young Harvard colleague Peter McIntyre and then developed in a 1976 paper by Cline, McIntyre, and himself. The three of them urged Wilson and the Fermilab program committee to plan out a means for running proton and antiproton beams through the same ring in opposite directions. Through enough high-energy smash-ups, they proposed, perhaps somewhere in the debris would lie the sought-after particles.
At that time, Wilson was committed instead to building the Tevatron—the world’s first synchrotron with superconducting magnets guiding the beams. The name Tevatron derives from its goal of energizing protons up to 1 TeV (one teravolt or one trillion electron volts). The Tevatron would indeed be used as a collider, but until the superconducting technology was tested, the fiscally conservative director didn’t want to offer a firm guarantee.
Like a persistent salesman, Rubbia knocked on CERN’s door next. Because he had worked there in the 1960s, he was very familiar with the organization. Born in Gorizia, Italy, in 1934, he had come to CERN for the first time when he was twenty-six, following a university education in Pisa and Milan and a year and a half at Columbia in the United States. Since the early 1970s, Rubbia had kept up an impossibly frantic schedule, spending time at Harvard, Fermilab, and CERN. These formative experiences, coupled with a natural self-assurance, offered him the clout to suggest CERN’s next move.
In shaping a new direction for CERN, Rubbia found the perfect partner in Simon van der Meer. Rubbia realized that the Dutch physicist’s stochastic cooling technique would offer an ideal means of fashioning dense proton and antiproton beams. This would enable the two beams to circle in contrary ways through the SPS—greatly augmenting its center-of-mass energy by transforming it into a collider. Persuaded by Rubbia’s cogent reasoning, Leon van Hove, CERN’s codirector at the time, helped shepherd the concept through the bureaucracy. With amazing speed, researchers assembled the Antiproton Accumulator, a revolutionary means of building up an intense beam of those particles, and linked it to the SPS. By 1981, only five years after Cline, McIntyre, and Rubbia had proposed the idea, the SPS became operational as a proton-antiproton collider—the major purpose for which it was used throughout the decade. Although the more powerful Tevatron opened in 1983, its collider operations wouldn’t begin until 1985, offering CERN a significant head start.
With the SPS and later the Tevatron came the rise of “supergroups” of researchers—teams representing dozens of institutions and hundreds of experimentalists each. High-energy physicists gravitated to the two centers hoping to share the glory of wrapping up the Standard Model, completing the third generation of fundamental particles (supplementing the tau lepton and the bottom quark—the latter found by Leon Lederman and his colleagues at Fermilab in 1977), and perhaps even discovering wholly unexpected new ones. Gone were the days of Rutherford and Lawrence—where experimental papers included but a few authors—perhaps the supervising professor, a postdoctoral researcher, and a couple of graduate students. With the burgeoning teams, some articles even included the long list of authors as lengthy footnotes. You practically needed a magnifying glass to see which contributors lent their expertise to which projects.
Moreover, because of the increasing specialization associated with the new particle-production “factories,” and the many years often required to obtain results, research supervisors began to adopt a more flexible attitude toward what constituted acceptable Ph.D. theses. For example, completing Monte Carlo simulations (using a random-number generator to predict possible outcomes), writing software, building and testing new detectors, and so forth, could constitute elements of approved dissertations. Otherwise, not only wouldn’t there have been enough theses to accommodate all of the graduate students working in each experimental supergroup, but also the time to get such degrees while waiting for final data would have, in many cases, been inordinately long.
Upon the reinauguration of the SPS as a collider, two detectors were readied for service—each supported by its own extensive team. The first, UA1 (Underground Area 1), was Rubbia’s brainchild—an extraordinarily complex instrument that made use of state-of-the-art electronics to probe collisions from almost every possible angle. The property of covering almost the entire solid angle, known as “hermeticity,” became a mainstay of detectors from that point on. No one had seen such a massive detector before—at approximately two thousand tons it was truly a Goliath. Its complexity and bulk girth inspired a French newspaper to root for the second, smaller detector UA2 (Underground Area 2), as the dexterous “David” that would slay the unwieldy giant. Reportedly, this characterization infuriated Rubbia, who considered himself the true maverick.15
The initial run, in December 1981, was dedicated to testing some of the predictions of quantum chromodynamics (QCD), the leading gauge theory of the strong interaction. Developed in the 1970s, QCD models the interactions among the quarks in hadrons, mediated via exchange particles called gluons. Through volleying gluons among one another, quarks of different colors cement their connections—forming baryons or mesons. The gluon concept replaced the idea of pion exchange, which failed to explain why quarks like to assemble in certain groupings and are never found roaming freely. The UA1 and UA2 detectors looked for the hard knocks of quark kernels against one another, measured the energy produced, and compared the results to theoretical QCD models. Splendidly, particularly in the UA2 results, many of the QCD predictions proved right on the mark.
After savoring the delectable appetizer of the QCD findings, it would be time for the main course. The W and Z bosons were ripe for the plucking and—thanks to the capabilities of the upgrading SPS—it would finally not be a stretch to reach for such exotic fruit. The sensitive detectors of each group were primed to taste the characteristic flavor combinations of the rare morsels.
In the case of the W bosons, the researchers expected that the quarks and antiquarks from the protons and antiprotons (up and antidown, for example) would unite briefly at high energies to produce these exchange particles. These would be extremely short lived, almost immediately decaying into charged leptons and neutrinos. Particles too fleeting to be directly detected are called resonances—manifesting themselves only through peaks in production at particular energies corresponding to their masses. It’s like trying to find signs of a snowman built (from freezer scrapings) during a hot summer day; a sufficiently sized puddle of water would be a giveaway.
Around Christmas 1982, the SPS was colliding protons and antiprotons with astonishing beam luminosities of more than 1029 (1 followed by 29 zeroes) incident particles per square inch each second. In the UA1 detector, these yielded about 1 million events interesting enough to trigger data collection. Of these, six events met the criteria (particular amounts of energy and momentum associated with electrons fleeing at certain angles) to represent W boson candidates. Further data narrowed down the mass of the W boson to be approximately 81 GeV/c2 (divided by the speed of light squared, in accordance with Einstein’s famous mass-energy equation). Meanwhile, UA2 gathered four candidate events, confirming the important discovery.
Snagging the Z boson happened just a few months later, during a run in April/May 1983. This time, the teams looked for a different signal: the production of electron-positron pairs of particular energies. UA1 found the mass of the Z to be approximately 95.5 GeV/c2—with the UA2 group corroborating this result. Papers in Physics Letters B triumphantly announced these findings, much to the delight of the physics community around the world. The discoveries were so telling, no one from that point on could question the reality of electroweak unity.
As CERN researcher Daniel Denegri, a member of the UA1 collaboration, recalled the exhilaration of the day: “This period, around the end of 1982 and throughout 1983, was an amazing time from both a professional and personal point of view. It was an unforgettable time of extreme effort, tension, excitement, satisfaction and joy.”
The boost to European morale because of the weak boson findings cannot be overestimated. After decades of looking to the United States as the main innovator in high-energy physics, the continent of Einstein, Bohr, and the Curies finally got its groove back. As Denegri noted, “The discovery of the W and Z at CERN . . . signaled that the ‘old side’ of the Atlantic regained its eminence in particle physics.”16
Although American researchers were happy for their colleagues across the ocean, and pleased that electroweak unification held up under close scrutiny, they could not conceal their disappointment that CERN had beaten them to the punch. Like baseball, accelerator physics had become an American pastime, so it was like losing the World Series to Switzerland. A New York Times editorial laid out the score: “Europe 3, U.S. Not Even Z-Zero.”17
The first major repercussion of the European triumph was the cancellation of ISABELLE, a proton-proton collider then under construction at Brookhaven. Although hundreds of millions of dollars had already been spent on the project and its tunnel had already been excavated, in July 1983, a subpanel of the High Energy Physics Advisory Panel of the Department of Energy decided that the anticipated energy of the collider, around 400 GeV, would be insufficient to generate new discoveries beyond what had just been found.
With the W and Z identified, the next step would be to find the remaining ingredients of the Standard Model including the top quark, the tau neutrino, and the Higgs. Other goals included finding hypothetical new particles predicted in models seeking to extend the Standard Model into more comprehensive unification schemes.
For example, in the 1970s and 1980s, a number of researchers developed Grand Unified Theories—schemes designed to incorporate QCD along with the electroweak interaction into a single theory. The idea was that at high enough energies, such as in the nascent moments of the Big Bang, all of these interactions would be comparable in strength. With the cooling of the universe, these would bifurcate during two distinct phase transitions into the strong and electroweak interactions and then the strong, weak, and electromagnetic interactions. Thus the original perfect symmetry would break down over time as the vacuum changed its fundamental character.
Even farther reaching unification schemes proposed around that period included supersymmetry, the hypothesized means of uniting fermions and bosons into a comprehensive theory. Each fermion, according to this hypothesis, has a boson counterpart, called its superpartner. Similarly, each boson has a fermion companion. When the universe was a steaming primordial soup, the partners and superpartners were on equal footing, but since then, with the universe cooling, supersymmetry has spontaneously broken—rendering the superpartners too massive to be readily observed. Following a tradition in particle physics of zany nomenclature, the hypothetical boson superpartners of electrons were christened “selectrons” and those of quarks, “squarks.” The fermion counterparts of photons were named “photinos,” those of gluons, “gluinos,” and those of the W and Z, the bizarre-sounding “winos” and “zinos.” Theorists hoped signs of the lightest of these superpartners would turn up in collider debris.
As a result of these and other novel theories, by the mid-to-late 1980s, though many of the major predictions of the Standard Model had been verified, no one could complain that there weren’t enough projects in high-energy physics to go around. The problem would be cranking out enough juice to spawn the sought-after particles. The SPS collider, at 450 GeV, had quickly reached its limits, with no sign to be found of coveted gems such as the top quark or the Higgs, let alone more exotic particles.
Another CERN project, the Large Electron-Positron Collider (LEP), proved ambitious in size, if not in overall energy. A ring seventeen miles in circumference and hundreds of feet deep, it extended CERN’s reach far beyond the Geneva suburbs into the verdant countryside across the French-Swiss border. One of the reasons it was built so large was to reduce the amount of radiation emitted by the electrons and positrons—the greater the radius, the smaller the radiative energy lost.
The LEP’s construction required some adjustments to CERN operations. The SPS was adapted to serve as a source for electrons and positrons, which were injected into the LEP ring in countercircling beams before being brought to crash together in a crescendo of energy. By knowing the ring radius, the frequency of the electrons and positrons, and other factors, researchers could calculate the total energy of each collision, allowing for precise determinations of the masses of particles produced.
During its eleven-year run (1989-2000), the LEP was the most powerful lepton collider in the world—but, because electrons are so much lighter than protons, lepton colliders are generally weaker than comparably sized hadron colliders. Its energy ranged from just under 100 GeV (when it opened) to slightly over 200 GeV (after upgrades)—insufficient, as it turned out, to find the Higgs or to beat competitors to the top quark. Nevertheless, it was an adept factory for manufacturing W and Z bosons, pinning down their masses with a jeweler’s precision.
The top quark would be located among the same Illinois cornfields where the bottom quark was found almost two decades earlier. (The two quarks are sometimes also called “truth” and “beauty.”) Nobody expected the wait would be so long and that the second member of the third quark family would be quite so heavy. Its 1995 discovery would be the pinnacle (so far) of the Tevatron’s impressive run.
Wilson had stepped down from Fermilab’s directorship in 1978, shortly after the upsilon—the first particle known to house a bottom quark—was discovered. After pouring his heart and soul into the Tevatron, he had become incensed when the Department of Energy initially didn’t offer enough funding that year to keep it on schedule for a speedy completion. 18 To buttress his argument, he had handed in his resignation and astonishingly it was accepted. Fresh off his bottom quark discovery, Lederman was appointed the new director. He would preside over the Tevatron’s opening and chart its course throughout the 1980s. (In 1989, John Peoples became director—succeeded by Michael Witherell and then Pier Oddone. Lederman has maintained a role as director emeritus.)
Physicists around the world celebrated the Tevatron’s inauguration, keenly aware that its superior power would offer the best chance to advance scientific understanding of the subatomic realm. On July 3, 1983, twelve hours after its beam was turned on, it reached 512 GeV and broke the world record for energy produced in an accelerator. Among the applauders, Herwig Schopper, CERN’s director-general at the time, sent Lederman a gracious message by telex:
Our warmest congratulations for the extraordinary achievement to accelerate protons for the first time in a superconducting ring to energies never obtained before. Fermilab pioneered the construction of superconducting magnets, opening up a new domain of future accelerators. Please convey our admiration to all the staff concerned.19
Though CERN remained Fermilab’s major competitor, much of the competition at the Tevatron was internal. With the success of the vying UA1 and UA2 groups as models for a Darwinian approach to discovery, Lederman advocated competing teams at the Tevatron, too. Each would use its own designated detector and analyze its own data. The natural advantage of such a strategy was to subject each group’s findings to independent verification by the other.
The first Tevatron team, called the Collider Detector at Fermilab (CDF) Collaboration, consisted of thousands of researchers and technicians from the United States, Canada, Italy, Japan, and China—representing three dozen universities and other institutions. The sheer number of people involved meant that the first few pages of each paper the collaboration published consisted simply of a long list of names.
Still in operation today, the CDF is a multifaceted, hundred-ton device that surrounds one of the beam intersection points and sifts through collision debris for interesting events. As in the case of the SPS, only a minute percentage of all collisions are suitable for analysis. Only if the quark and antiquark constituents of the proton and antiproton beams strike each other directly, interact, and produce debris flying off at large angles, are the collisions worth talking about. Otherwise they are of little note, akin to commuters inadvertently brushing past one another on the way to their trains.
If particle offspring meet the large angle criteria, the CDF, like all complex detectors, subjects them to a barrage of tests. As in traditional devices such as cloud chambers, ionization plays a major role in modern methods for tracking charged particles. Using thin materials technology, a delicate instrument called a Silicon Vertex Tracker sniffs out the subtlest whiff of charge, pinpointing the locations of fleeing subatomic bodies within ten-thousandths of an inch. An immerse superconducting magnet surrounds another unit, called the Central Tracking Chamber, steering charged particles in a way that allows their momenta to be gauged.
Like marines on their first day of boot camp, that’s just the beginning of the rigors the particles must face. The next ordeal includes two different energy-trapping devices, electromagnetic and hadronic calorimeters. These force the particles through hurdles (lead sheets and iron plates, respectively), induce jets or showers, and record the energy they sweat off in the process. Those with the endurance to continue through the calorimetry without losing much energy are likely muons and are picked up by the muon tracker just beyond.
Certain initial readings act as triggers, signaling that something significant may be in the works. Immediately, the full data collection machinery kicks in and records everything that can possibly be known about the event—positions, momenta, and energies—tens of thousands of bits of information in many cases. Otherwise, rare, sought-after processes would be buried in an avalanche of mundane decays.
The final step takes place not in the actual detector at all, but rather in the virtual world of computers. Like crime scene detectives, sophisticated software reconstructs what likely happened. Each event of potential interest is dissected—with any missing energy or momentum duly noted. Because energy and momentum are normally conserved, their absence might point to unseen subatomic thieves such as neutrinos. Thus reconstruction offers the only way of filling in the gaps and representing the full picture of what happens during collisions.
Around the Tevatron ring from the CDF another formidable group of experimentalists, called the D0 (pronounced “dee zero”) Collaboration, collects vital data with its own detector. Like the CDF it has a tracking system, calorimeters, and muondetection and triggering systems—with somewhat more emphasis on calorimetry than tracking. Researchers working on D0 projects have hailed from countries around the globe—from Argentina and Brazil to the United Kingdom and the United States.
One of the consistent contributors to the D0 Collaboration is Stony Brook University in New York. As a graduate student there during the early preparatory stages for the project, I witnessed how much testing and calibration takes place to make sure each detector element performs optimally. Calibration involves comparing a piece of equipment’s readings to known values. For example, a researcher could calibrate a temperature sensor by seeing if its readings match those of an analogue thermometer. Without calibration, a detector’s results could well be flawed—like a scale improperly balanced and indicating the wrong weight. To help calibrate a Cherenkov detector, I recall having to render a room the size of a closet perfectly light-tight, so that only cosmic rays could enter. It took numerous hours in the dark and many layers of duct tape to make sure no stray photons could be seen. That was only one of many thousands of tests performed by many thousands of researchers over many thousands of days before actual runs could even begin. Like raising exotic orchids, high-energy physics certainly requires patience—making the blossoming all the lovelier.
The extraordinary hard work and persistence of the CDF and D0 teams wonderfully paid off when on March 2, 1995, at a specially arranged meeting at Fermilab, both groups reported incontrovertible evidence that they had identified the top quark. Each had released preliminary results earlier—but they wanted to be certain before announcing firm conclusions. Their proof came from evaluating the energy and other properties of leptons and jets produced in numerous candidate events. Based on these values, each team found the mass to be about 175 GeV, the heaviest particle known to date—weighing as much as a gold atom. No wonder it took such a powerful collider to produce it!
A bittersweet aspect of the discovery is that it was announced barely two years after the cancellation of what would have been the next logical step for American physics. What would have been the largest, most powerful collider in the world was axed in a budget-cutting frenzy. High-energy physics in the United States would never be the same.