The God Particle

by Leon Lederman

  The proton synchrotron solves the problem in an even more elegant way. It is a little complicated but depends on the fact that the speed of the particle (99 point whatever percent of the speed of light) is essentially constant. Suppose the particle crosses the gap at that part of the radio-frequency cycle when the accelerating voltage is zero. No acceleration. We now increase the magnetic field a bit. The particle bends in a tighter circle and arrives a bit early at the gap, and now the radio frequency is in a phase to accelerate. Thus the mass grows, the orbit radius increases, and we are back to where we started but with higher energy. The system is self-correcting. If the particle gains too much energy (mass), its radius will increase and it will arrive later at the gap and see a decelerating voltage, which will correct the error. Raising the magnetic field has the effect of increasing the mass energy of our hero-particle. This method depends on "phase stability," which is discussed later in this chapter.

  IKE AND THE PIONS

  One early accelerator was near and dear to me—Columbia University's 400 MeV synchrocyclotron, built on an estate in Irvington-on-Hudson, New York, within commuting distance of Manhattan. The estate, named after the ancestral Scottish mountain Ben Nevis, was established in colonial times by Alexander Hamilton. Later it was owned by a branch of the Du Pont family and then by Columbia University. The Nevis cyclotron, built between 1947 and 1949, was one of the most productive particle accelerators in the world during its twenty-some years of operation (1950–1972). It also produced more than a hundred and fifty Ph.D.'s, about half of whom stayed in the field of high-energy physics and became professors at Berkeley, Stanford, Cal Tech, Princeton, and many other such fly-by-night institutions. The other half went everywhere: small teaching institutions, government labs, science administration, industrial research, investment banking ...

  I was a graduate student when President (of Columbia) Dwight Eisenhower dedicated the new facility in June of 1950, in a small ceremony on the lawn of the lovely estate—magnificent trees, shrubbery, a few red brick outbuildings—sloping down to the stately Hudson River. After appropriate speechifying, Ike threw a switch and out of the loudspeakers came the amplified "cheeps" of a Geiger counter, indicating radiation. The cheeps were produced by a radioactive source I held near a particle counter because the machine had chosen that moment to crash. Ike never found out.

  Why 400 MeV? The hot particle of 1950 was the pion, or pi meson, as it's also called. The pion had been predicted in 1936 by a Japanese theoretical physicist, Hideki Yukawa. It was thought to be the key to the strong force, which in those days was the big mystery. Today we think of the strong force in terms of gluons. But back then pions, which fly back and forth between the protons and neutrons to hold them together tightly in the nucleus, were the key, and we needed to make and study them. To produce pions in nuclear collisions, the particle coming in from the accelerator must have an energy greater than m(pion)c2, that is, greater than the pion's rest mass energy. Multiplying the pion's rest mass by the speed of light squared, we get 140 MeV, its rest mass energy. Since only a fraction of the collision energy goes into the production of new particles, we needed extra energy, and we settled on 400 MeV. The Nevis machine became a pion factory.

  BEPPO'S LADIES

  But wait. First a word on how we found out about pions in the first place. In the late 1940s, scientists at the University of Bristol in England noticed that when an alpha particle passes through a photographic emulsion coated on a glass plate, it "activates" the molecules in its path. After developing the film, you can see a track defined by grains of silver bromide. The track is easily discerned through a low-power microscope. The Bristol group sent batches of very thick emulsion up in balloons almost to the top of the atmosphere, where the intensity of cosmic rays is much higher than at sea level. This source of "naturally" occurring radiation far exceeded in energy Rutherford's puny 5 MeV alphas. It was in these emulsions exposed to cosmic rays in 1947 that the pion was first discovered by Cesare Lattes, a Brazilian, Giuseppe Occiallini, an Italian, and C. F. Powell, the resident professor in Bristol.

  The most colorful of the above trio was Occiallini, known as Beppo to his friends. An amateur speleologist and compulsive practical joker Beppo was the driving force of the group. He trained a bevy of young women to do the painstaking work of studying the emulsions under a microscope. My thesis supervisor Gilberto Bernardini, a close friend of Beppo's, visited him one day in Bristol. Following directions given to him in unbroken English, a language he found very difficult, Bernardini quickly got lost. Finally he stumbled into a lab where several very proper English ladies were staring into microscopes and cursing in Italian argot that would be outlawed on the docks of Genoa. "Ecco!" cried Bernardini. "Dissa is Beppo's lab!"

  What the tracks in those emulsions showed was a particle, the pion, entering at high speed, gradually slowing down (the density of the grains of silver bromide increases as the particle slows), and coming to rest. At the end of the track a new, energetic particle appears and races off. A pion is unstable, decaying within one hundredth of a microsecond into a muon (the new particle at the end of the track) and something else. The something else turned out to be a neutrino, which doesn't leave a track in the emulsion. The reaction is written π → μ + ν. That is, a pion (eventually) gives rise to a muon and a neutrino. Since the emulsion provides no time sequence information, it took careful analysis of the tracks on half a dozen of these- rare occurrences to understand what the particle was and how it decayed. The new particle had to be studied, but using cosmic rays yields only a handful of such events per year. As with nuclear disintegrations, accelerators with high enough energy were required.

  At Berkeley, Lawrence's 184-inch cyclotron began to produce pions, as did the Nevis machine. Soon synchrocyclotrons in Rochester Liverpool, Pittsburgh, Chicago, Tokyo, Paris, and Dubna (near Moscow) were studying the pion in its strong interactions with neutrons and protons as well as the weak force in the pion's radioactive decay. Other machines at Cornell, Cal Tech, Berkeley, and the University of Illinois used electrons to produce pions, but the most successful machines were the proton synchrocyclotrons.

  THE FIRST EXTERNAL BEAM: PLACE YOUR BETS!

  So there I was in the summer of 1950 with a machine going through birth pains and me needing data so I could get a Ph.D. and earn a living. Pions were the name of the game. Hit a piece of something—carbon, copper, anything containing nuclei—with the 400 MeV protons from the Nevis machine and you'd generate pions. Berkeley had hired Lattes, who showed the physicists how to expose and develop the very sensitive emulsions used so successfully in Bristol. They inserted a stack of emulsions into the beam vacuum tank and allowed the protons to hit a target near the stack. Remove the emulsions through an air lock, develop them (a week of effort), and then subject them to microscopic study (months!). All this effort had given the Berkeley team but a few dozen pion events. There had to be an easier way. The trouble was that the particle detectors had to be installed inside the machine, in the region of the strong accelerator magnet, to record the pions, and the only device that was practical was the stack of emulsions. In fact, Bernardini was planning an emulsion experiment on the Nevis machine similar to what the Berkeley folks had done. The large, elegant cloud chamber I had built for my Ph.D. project was a much better detector, but it would never fit between the poles of a magnet inside an accelerator. Nor would it survive as a particle detector in the intense radiation inside the accelerator. Between the cyclotron magnet and the experimental area was a ten-foot-thick concrete wall to confine the stray radiation.

  A new postdoc, John Tinlot, had arrived at Columbia from Bruno Rossi's famed cosmic ray group at MIT. Tinlot was the quintessential physicist. In his late teens he had been a violinist of concert quality, but he had put his violin away after an agonizing decision to study physics. He was the first young Ph.D. I had ever worked with, and I learned enormously from him. Not only physics. John was a genetically infected horse player and gambler: long
shots, blackjack, craps, roulette, poker—lots of poker. We played during experiments while the data were being collected. We played on vacation, on trains and airplanes. It was a moderately expensive way to learn physics, my losses being moderated by the other players—students, technicians, and security guards whom John would recruit. He had no pity.

  John and I sat on the floor of the not-yet-really-working accelerator, drinking beer and discussing the world. "What really happens to the pions that come flying off the target?" he asked suddenly. I had learned to be cautious. John was a gambler in physics as well as in horses. "Well, if the target is inside the machine [and it had to be, we didn't know how to get the accelerated protons out of the cyclotron], the powerful magnet will spray them in all directions," I answered cautiously.

  JOHN: Some will come out of the machine and hit the shielding wall?

  ME : Sure, but all over the place.

  JOHN: Why don't we find out?

  ME: How?

  JOHN: We do magnetic tracing.

  ME: That's work. [It was 8 P.M. on a Friday.]

  JOHN: Do we have the table of measured magnetic fields?

  ME: I'm supposed to go home.

  JOHN: We'll use those huge rolls of brown wrapping paper and draw the paths of the pions on a scale of one to one ...

  ME: Monday?

  JOHN: You do the slide-rule work [this was 1950] and I'll draw the paths.

  Well, by 4 A.M. Saturday we had made a fundamental discovery that would change the way cyclotrons were used. We had traced eighty or so fictional particles emerging from a target in the accelerator with plausible directions and energies—we used 40, 60, 80, and 100 MeV. To our amazement, the particles didn't "just go everywhere." Instead, because of the properties of the magnetic field near and beyond the rim of the cyclotron magnet, they curved around the machine in a tight beam. We had discovered what became known as "fringe field focusing." By rotating the large sheets of paper—that is, by picking a specific target location—we could get the beam of pions in a generous energy band around 60 MeV to head right for my brand-new cloud chamber. The only catch was the wall of concrete between the machine and the experimental area where my princess chamber sat.

  No one had anticipated our discovery. Monday morning we were camped outside the director's office waiting to pounce on him with it. We had three simple requests: (1) a new target location in the machine; (2) a much thinner window between the beam vacuum chamber of the cyclotron and the outside world to minimize the influence of a one-inch-thick stainless steel plate on the emerging pions; and (3) a new hole about four inches high by ten inches wide, we guessed, through the ten-foot-thick concrete wall. All this from a lowly graduate student and a postdoc!

  Our director, Professor Eugene Booth, was a Georgia gentleman and a Rhodes scholar who rarely said "gosh darn." He made an exception for us. We argued, we explained, cajoled. We painted visions of glory. He would be famous! Imagine an external pion beam, the first ever!

  Booth threw us out, but after lunch he called us in again. (We had been weighing the advantages of strychnine versus arsenic.) Bernardini had dropped in, and Booth had tried out our idea on this eminent visiting professor. My guess is that the details, expressed in Booth's Georgian lilt, were too much for Gilberto, who once confided in me, "Booos, Boosth, who can pronounce dese American names?" However, Bernardini supported us with typical Latin exaggeration, and we were in.

  A month later it all worked—just like the wrapping paper sketches. In a few days my cloud chamber had registered more pions than all the other labs in the world put together. Each photograph (we took one each minute) had six to ten beautiful tracks of pions. Every three or four photographs would show a kink in a pion track as it disintegrated into a muon and "something else." I used the pion decays as my thesis. Within six months we had constructed four beams, and Nevis was in full production as a data factory for the properties of pions. At the earliest opportunity, John and I went to the racetrack in Saratoga where, continuing his roll, he hit a 28-to-l shot in the eighth race, on which he had wagered our dinner and return-home gas money. I really loved that guy.

  John Tinlot must have had extraordinary insight to suspect the fringe field focusing that everyone else in the cyclotron business had missed. He went on to a distinguished career as a professor at the University of Rochester but died of cancer at the age of forty-three.

  A SOCIAL SCIENCE DIVERSION: THE ORIGIN OF BIG SCIENCE

  World War II marked a crucial watershed between pre-WWII and post-WWII scientific research. (How's that for a controversial statement?) But it also marked a new phase in the search for the a-tom. Let's count some of the ways. The war generated a leap forward in technology, much of this centered in the United States, which was unencumbered by the loud noises of nearby explosions that Europe was experiencing. The wartime development of radar, electronics, the nuclear bomb (to use its proper name) all provided examples of what a collaboration between science and engineering could do—as long as it was unconstrained by budget considerations.

  Vannevar Bush, the scientist who led U.S. science policy during the war, spelled out a new relationship between science and government in an eloquent report to President Franklin D. Roosevelt. From that time on, the U.S. government was committed to supporting basic research in the sciences. Support for research, basic and applied, climbed so rapidly that we can laugh at the $1,000 grant E. O. Lawrence worked so hard to get in the early 1930s. Even adjusting for inflation, that amount pales in contrast to federal support of basic research in 1990—some $12 billion! World War II also saw a flood of scientist refugees from Europe become a crucial part of the research boom in the United States.

  In the early 1950s some twenty universities had accelerators capable of carrying out research in nuclear physics at the cutting edge. As we came to understand the nucleus better, the frontier shifted to the subnuclear domain, where larger—more expensive—machines were required. The era became one of consolidation—scientific mergers and acquisitions. Nine universities banded together to build and manage the accelerator laboratory at Brookhaven, Long Island. They commissioned a 3 GeV machine in 1952 and a 30 GeV machine in 1960. Princeton University and the University of Pennsylvania banded together to build a proton machine near Princeton. MIT and Harvard built the Cambridge Electron Accelerator, a 6 GeV electron machine.

  Over the years, as the consortia grew in size, the number of front-line machines diminished. We needed ever higher energy to address the question of "What's inside?" and to search for the true a-toms—or the zero and one of our library metaphor. As new machines were proposed, older ones were phased out to free up funds, and Big Science (a term often used as an expletive by ignorant commentators) grew bigger. In the 1950s, one could do maybe two or three experiments a year with groups of two to four scientists. In the following decades, the collaborations got larger and larger, and the experiments took longer and longer, driven in part by the necessity to build ever more complex detectors. By the 1990s the Collider Detector Facility alone at Fermilab comprised 360 scientists and students from twelve universities, two national labs, and institutions in Japan and Italy. Scheduled runs stretched to a continuous year or more of data taking—with time off for Christmas, the Fourth of July, or whenever something broke down.

  Supervising the evolution from a tabletop science to one based on accelerators measured in miles around was the U.S. government. The World War II bomb program gave rise to the Atomic Energy Commission (AEC), a civilian agency that oversaw nuclear weapons research, production, and stockpiling. It was also given the mission, as a national trust, of funding and overseeing basic research in nuclear and what was later to become particle physics.

  The case for Democritus's a-tom even reached the halls of Congress, which created the Joint (House and Senate) Committee on Atomic Energy to provide oversight. The committee's hearings, published in dense, government-green booklets, are a Fort Knox of information for science historians. Here one reads the testim
ony of E. O. Lawrence, Robert Wilson, I. I. Rabi, J. Robert Oppenheimer, Hans Bethe, Enrico Fermi, Murray Gell-Mann, and many others patiently responding to questions about how the search for the ultimate particle was going—and why did it require yet another machine? The interchange at the beginning of this chapter between Fermilab's flamboyant founding director, Robert Wilson, and Senator John Pastore was taken from one of those green books.

  To complete the alphabet soup, the AEC dissolved into the ERDA (Energy Research and Development Agency), which soon gave way to the DOE (U.S. Department of Energy), which at this writing oversees the national laboratories where atom smashers operate. Presently there are five such high-energy labs in the U.S.: SLAC, Brookhaven, Cornell, Fermilab, and the Superconducting Super Collider lab, now under construction.

  Accelerator labs are generally owned by the government, but operated by a contractor; which can be a university, such as Stanford in SLAC's case, or a consortium of universities and institutions, as is the case with Fermilab. The contractors appoint a director, and then they pray. The director runs the lab, makes all the important decisions, and often stays on the job too long. As Fermilab director from 1979 to 1989, my major task was to implement Robert R. Wilson's vision: the construction of the Tevatron, the first superconducting accelerator. We also had to create a proton-antiproton collider and humongous detectors that would observe head-on collisions near 2 TeV.