The God Particle

by Leon Lederman

  Sippach and the many others who helped develop these complex systems continue a great tradition that began in the 1930s and '40s when the circuits for the early particle detectors were invented. These in turn become the key ingredients in the first generation of digital computers. These, in turn, begat better accelerators and detectors, which begat...

  The detectors are the bottom line in this whole business.

  WHAT WE FOUND OUT: ACCELERATORS AND PHYSICS PROGRESS

  You now know everything you need to know about accelerators—perhaps more. You may in fact know more than most theorists. This is not a criticism, just a fact. More important is what these new machines told us about the world.

  As I've mentioned, the synchrocyclotrons of the 1950s enabled us to learn much about pions. Hideki Yukawa's theory suggested that by exchanging a particle with a particular mass, one could create a strong attractive counterforce that would bind protons to protons, protons to neutrons, and neutrons to neutrons. Yukawa predicted the mass and lifetime of this particle being exchanged: the pion.

  The pion has a rest mass energy of 140 MeV, and it was produced prolifically in the 400 to 800 MeV machines on university campuses around the world in the 1950s. Pions decay into muons and neutrinos. The muon, which was the great puzzle of the 1950s, seemed to be a heavier version of the electron. Richard Feynman was one of the prominent physicists who agonized over two objects that behave in all respects identically, except that one weighs two hundred times as much as the other. The unraveling of this mystery is one of the keys to our entire thrust, a clue to the God Particle itself.

  The next generation of machines produced a generational surprise: hitting the nucleus with billion-volt particles was doing "something different." Let's review what you can do with an accelerator, especially since the final exam is coming soon. Essentially, the vast investment in human ingenuity described in this chapter—the development of the modern accelerator and particle detector—allows us to do two kinds of things: to scatter objects or—and this is the "something different"—to produce new objects.

  1.Scattering. In scattering experiments we look at how incident particles after collision fly off in various directions. The technical term for the end product of a scattering experiment is angular distribution. When analyzed according to the rules of quantum physics, these experiments tell us a good deal about the nucleus that is scattering the particles. As the energy of the incoming particle from the accelerator increases, the structure comes into better focus. So we learned about the composition of nuclei—neutrons and protons and how they are arranged and how they jiggle around to maintain their arrangement. As we further increase the energy of our protons, we can "see" into the protons and the neutrons. Boxes inside boxes.

  To make things simple, we can use single protons (hydrogen nuclei) as targets. Scattering experiments told us about the proton's size and about how the positive electric charge is distributed. A clever reader will ask whether the probe—the particle hitting the target—itself contributes to the confusion, and the answer is yes. So we use a variety of probes. Alpha particles from radiation gave way to protons and electrons fired from accelerators, and later we used secondary particles: photons derived from electrons, pions derived from proton-nucleus collisions. As we got better at doing this in the 1960s and '70s, we began using tertiary particles as the bombarding particles; muons from decays of pions became probes, as did neutrinos from the same source, and lots more.

  The accelerator laboratory became a service center with a variety of products. By the late 1980s, Fermilab's sales force advertised to potential customers that the following hot and cold running beams were available: protons, neutrons, pions, kaons, muons, neutrinos, antiprotons, hyperons, polarized protons (all spinning in the same direction), tagged photons (we know their energy), and if you don't see it, ask!

  2.Producing new particles. Here the object is to see if a new energy domain results in the creation of new, never-before-seen particles. If there is a new particle, we want to know everything about it—its mass, spin, charge, family, and so on. We also need to know its lifetime and what other particles it decays into. Of course, we have to know its name and what role it plays in the great architecture of the particle world. The pion was discovered in cosmic rays, but soon we found that it doesn't spring fully grown from the forehead of cloud chambers. What happens is that cosmic-ray protons from outer space enter the earth's atmosphere where they collide with nuclei of nitrogen and oxygen (today we also have more pollutants), and out of these collisions pions are created. A few other weird objects were also identified in cosmic-ray studies, such as particles called K+ and K− and objects called lambda (the Greek letter Λ). When more powerful accelerators took over starting in the mid-1950s and then with a vengeance in the 1960s, various exotic particles were created. The trickle of new objects soon became a flood. The huge energies available in collisions uncovered the existence of not one or five or ten but hundreds of new particles, undreamt of in most of our philosophies, Horatio. These discoveries were group efforts, the fruits of Big Science and a mushrooming of technologies and techniques in experimental particle physics.

  Each new object was given a name, usually a Greek letter. The discoverers, typically a collaboration of sixty-three and a half scientists, would announce the new object and give as many of its properties—mass, charge, spin, lifetime, and a long list of additional quantum properties—as were known. They would then pass Go, collect two hundred dollars, write up a thesis or two, and wait to be invited to give seminars, conference papers, be promoted, all of that, Most of all, they were eager to follow up and to make sure others confirmed their results, preferably using some other technique so as to minimize instrumental biases. That is, any particular accelerator and its detectors tend to "see" events in a particular way. One needs to have the event confirmed by a different set of eyes.

  The bubble chamber served as a powerful technique for discovering particles since many of the details of a close encounter could be seen and measured. Experiments using electronic detectors were generally aimed at more specific processes. Once a particle had made it to the list of confirmed objects, one could design specific collisions and specific devices to provide data on other properties, such as its lifetime—all the new particles were unstable—and decay modes. Into what does it disintegrate? A lambda decays into a proton and a pion; a sigma decays into a lambda and a pion; and so on. Tabulate, organize, try not to be overwhelmed by the data. These were the guidelines to sanity as the subnuclear world exposed deeper and deeper complexity. Collectively all the Greek-letter particles created in strong-force collisions were called hadrons —Greek for heavy—and there were hadrons by the hundreds, literally. This was not what we wanted. Instead of a single, tiny, uncuttable particle, the search for the Democritan a-tom had turned up hundreds of heavy, very cuttable particles. Disaster! We learned from our biology colleagues what to do when you don't know what to do: classify! And this we did with abandon. The results—and consequences—of this classification are taken up in the next chapter.

  THREE FINALES: TIME MACHINE, CATHEDRALS, AND THE ORBITING ACCELERATOR

  We close this chapter with a new view of what actually happens in accelerator collisions. This view comes to us courtesy of our colleagues in astrophysics. (There is a small but very funny group of astrophysicists ensconced at Fermilab.) These people assure us—and we have no reason to doubt them—that the world was created about 15 billion years ago in a cataclysmic explosion, the Big Bang. In the earliest instants after creation, the infant universe was a hot, dense soup of primordial particles colliding with one another with energies (equivalent to temperatures) vastly higher than anything we can imagine reproducing, even with acute megalomania, double time. But the universe is cooling as it expands. At some point, about 10−12 seconds after creation, the average energy of the particles in the hot universe soup was reduced to 1 trillion electron volts, or 1 TeV, about the same energy that Fermilab's Tevatron produce
s in each beam. Thus we can look at accelerators as time machines. The Tevatron replicates, for a brief instant during head-on collisions of protons, the behavior of the entire universe at age "a millionth of a millionth of a second." We can calculate the evolution of the universe if we know the physics of each epoch and the conditions handed to it by the previous epoch.

  This time-machine application is really a problem for the astros. Under normal circumstances, we particle physicists would be amused and flattered but unconcerned about how accelerators mimic the early universe. In recent years, however we've begun to see the link. Farther back in time, where the energies are considerably higher than 1 TeV—the limit of our present accelerator inventory—lies a secret that we need. This earlier, hotter universe contains a vital clue to the lair of the God Particle.

  Accelerator as time machine—the astrophysics connection—is one view to consider. Another connection comes from Robert Wilson, the cowboy accelerator-builder, who wrote:

  Familiarly enough, both aesthetic and technical considerations were inextricably combined [in the design of Fermilab]. I even found, emphatically, a strange similarity between the cathedral and the accelerator: The one structure was intended to reach a soaring height in space; the other is intended to reach a comparable height in energy. Certainly the aesthetic appeal of both structures is primarily technical. In the cathedral we see it in the functionality of the ogival arch construction, the thrust and then the counterthrust so vividly and beautifully expressed, so dramatically used. There is a technological aesthetic in the accelerator, too. There is a spirality of the orbits. There is an electrical thrust and a magnetic counterthrust. Both work in an ever upward surge of focus and function until the ultimate expression is achieved, but this time in the energy of a shining beam of particles.

  Thus carried away, I looked into cathedral building a bit further. I found a striking similarity between the tight community of cathedral builders and the community of accelerator builders: Both of them were daring innovators, both were fiercely competitive on national lines, but yet both were basically internationalists. I like to compare the great Maître d'Oeuvre, Suger of St. Denis, with Cockcroft of Cambridge; or Sully of Notre-Dame with Lawrence of Berkeley; and Villard de Honnecourt with Budker of Novosibirsk.

  To which I can only add that there is this deeper connection: both cathedrals and accelerators are built at great expense as a matter of faith. Both provide spiritual uplift, transcendence, and, prayerfully, revelation. Of course, not all cathedrals worked.

  One of the glorious moments in our business is the scene in a crowded control room, where the bosses, on this special day, are at the console, staring at the screens. Everything is in place. The labor of so many scientists and engineers for so many years is now about to hatch as the beam is traced from the hydrogen bottle through the intricate viscera ... It works! Beam! In less time than you can say hooray, the champagne is poured into Styrofoam cups, jubilation and ecstasy written on all faces. In our holy metaphor I see the workmen lowering the last gargoyle into place as priests, bishops, cardinals, and the requisite hunchback stand tensely around the altar to see if it works.

  One must consider the aesthetic qualities of an accelerator as well as its GeVs and other technical attributes. Thousands of years hence, archaeologists and anthropologists may judge our culture by our accelerators. After all, they are the largest machines our civilization has ever built. Today we visit Stonehenge or the Great Pyramids, and we marvel first at their beauty and at the technological achievement of building them. But they had a scientific purpose as well; they were crude "observatories" for tracking astronomical bodies. So we must also stand in awe of how ancient cultures were driven to erect grand structures in order to measure the movements of the heavens in an attempt to understand and to live in harmony with the universe. Form and function combined in the pyramids and Stonehenge to allow their creators to seek scientific truths. Accelerators are our pyramids, our Stonehenge.

  The third finale has to do with the man Fermilab is named for, Enrico Fermi, one of the most famous physicists of the 1930s, '40s, and '50s. He was Italian by birth, and his work in Rome was marked by brilliant advances in both experiment and theory and by a crowd of exceptional students gathered around him. He was a dedicated and gifted teacher. Awarded the Nobel Prize in 1938, he used the occasion to escape from fascist Italy and settle in the U.S.

  His popular fame stems from heading up the team that built the first chain-reacting nuclear pile in Chicago during World War II. At the University of Chicago after the war he again gathered a brilliant group of both theoretical and experimental students. Fermi's students from both his Rome period and his Chicago period dispersed around the world, winning top positions and prizes everywhere. "You can tell a good teacher by how many of his students win Nobel Prizes," goes an ancient Aztec saying.

  In 1954 Fermi gave his retiring address as president of the American Physical Society. With a mixture of respect and satire, he predicted that in the near future we would build an accelerator in orbit around the earth, making use of the natural vacuum of space. He also cheerfully noted that it could be built with the combined military budgets of the United States and the USSR. Using supermagnets and my pocket cost estimator, I get 50,000 TeV for a cost of $10 trillion, not including quantity discounts. What better way to return the world to sanity than by beating swords into accelerators?

  Interlude C

  HOW WE VIOLATED PARITY IN A WEEKEND ... AND DISCOVERED GOD

  I cannot believe God is a weak left-hander.

  —Wolfgang Pauli

  LOOK AT YOURSELF in a mirror. Not too bad, hey? Suppose you raise your right hand, and your image in the mirror also raises its right hand! What? Can't be. You mean left! You'd clearly be in a state of shock if the wrong hand went up. This has never happened with people, as far as we know. But an equivalent act did occur with a fundamental particle called a muon.

  Mirror symmetry had been tested in the laboratory over and over again. The scientific name for mirror symmetry is parity conservation. This is the story of an important discovery, and also of how progress oftentimes involves the killing of an exquisite theory by an ugly fact. It all started at lunch on Friday and was over by about 4 A.M. the following Tuesday morning. A very profound conception of how nature behaved turned out to be a (weak) misconception. In a few intense hours of data taking, our understanding of the way the universe is constructed was changed forever. When elegant theories are disproven, disappointment sets in. It appears that nature is clumsier, more ponderous, than we had expected. But our depression is tempered by the faith that when all is known, a deeper beauty will be revealed. And so it was with the downfall of parity in a few days of January 1957 in Irvington-on-Hudson, twenty miles north of New York City.

  Physicists love symmetry because it has a mathematical and intuitive beauty. Symmetry in art is exemplified by the Taj Mahal or a Greek temple. In nature, shells, simple animals, and crystals of various kinds exhibit symmetrical patterns of great beauty, as does the almost perfect bilateral symmetry of the human body. The laws of nature contain a rich set of symmetries that for years, at least before January 1957, were thought to be absolute and perfect. They have been immensely useful in our understanding of crystals, large molecules, atoms, and particles.

  THE EXPERIMENT IN THE MIRROR

  One of these symmetries was called mirror symmetry, or parity conservation, and it asserted that nature—the laws of physics—could not distinguish between events in the real world and those in the mirror.

  The mathematically appropriate statement, which I'll give for the record, is that the equations describing the laws of nature do not change when we replace the z-coordinates of all objects with −z. If the z-axis is perpendicular to a mirror, defining a plane, this replacement is exactly what happens to any system when it is reflected in the mirror. For example, if you, or an atom, are 16 units in front of a mirror, the mirror shows the image as 16 units behind the mirror. Repla
cing the coordinate z with −z creates a mirror image. If, however, the equations are invariant to this replacement (for example, if the coordinate z always appears in the equation as z2), then mirror symmetry is valid and parity is conserved.

  If one wall of a lab is a mirror and scientists in the lab are carrying out experiments, then their mirror images will be carrying out mirror images of these experiments. Is there any way of deciding which is the true lab and which is the mirror lab? Could Alice know where she is (in front of or behind the looking glass) by some objective test? Could a committee of distinguished scientists examining a videotape of an experiment tell if it was carried out in the real or the mirror lab? In December of 1956 the unequivocal answer was no. There was no way a panel of experts could prove they were watching the mirror image of the experiments being conducted in the real laboratory. At this point a perceptive innocent might say, "But look, the scientists in this movie all have their buttons on the left side of their coats. It must be the mirror view." "No," the scientists answer, "that is just a custom; nothing in the laws of nature insists that buttons be on the right side. We have to put aside all human affectations and see if anything in our movie is against the laws of physics."

  So before January 1957 no such violations had been seen in the mirror-image world. The world and its mirror image were equally valid descriptions of nature. Anything that was happening in the mirror space could in principle and practice be replicated in the laboratory space. Parity was useful. It helped us classify molecular, atomic, and nuclear states. It also saves work. If a perfect human stands, disrobed and half concealed by a vertical screen, by studying the half that you do see, you can pretty much know what is behind the screen. Such is the poetry of parity.