The God Particle: If the Universe Is the Answer, What Is the Question?

by Leon Lederman

  What Galileo saw was a profound simplicity in the laws of motion, provided we could remove complicating factors such as air resistance and friction, things that are very much a part of the real world but that hide the simplicity. Galileo saw mathematics—parabolas, quadratic equations—as the way the world must really be. Neil Armstrong, the first astronaut on the moon, dropped a feather and a hammer on the airless lunar surface, demonstrating the Tower experiment for all the world's viewers. With no resistance, the two objects dropped at the same rate. And a ball rolling on a horizontal surface would, in fact, roll forever if there were no friction. It rolls much farther on a highly polished table, and farther yet on an air track or on slippery ice. It takes some ability to think abstractly, to imagine motion without ait, without rolling friction, but when you do, the reward is a new insight into the laws of motion, of space and time.

  Since that heartwarming story, we have learned about hidden simplicity. It is nature's way to hide the symmetry, simplicity, and beauty that can be described by abstract mathematics. What we now see, in place of Galileo's air resistance and friction (and equivalent political obstructions), is our standard model. To track this idea to the 1990s, we have to pick up the story of the heavy messenger particles that carry the weak force.

  STANDARD MODEL, 1980

  The decade of the eighties opens with a large measure of theoretical smugness. The standard model sits there, with its pristine summary of three hundred years of particle physics, challenging the experimentalists to "fill in the blanks." The W+, W−, and Z0 have not yet been observed, nor has the top quark. The tau neutrino requires a three-neutrino experiment, and such experiments have been proposed, but the arrangements are complicated, with small chance of success. They have not been approved. Experiments on the charged tau lepton strongly indicate that the tau neutrino must exist.

  The top quark is the subject of research at all the machines, the electron-positron colliders as well as the proton machines. A brand-new machine, Tristan, is under construction in Japan (Tristan—what is the deep connection between Japanese culture and Teutonic mythology?). It is an e+ e− machine that can produce top plus antitop, if the mass of the top quark is no heavier than 35 GeV, or seven times heavier than its differently flavored cousin bottom, weighing in at 5 GeV. The experiment and the expectations of Tristan, at least insofar as top is concerned, are doomed. The top is heavy.

  THE CHIMERA OF UNIFICATION

  The search for the W was the all-out effort of the Europeans, determined to show the world that they had come into their own in this business. To find the W required a machine energetic enough to produce it. How much energy is required? This depends on how heavy the W is. Responding to the insistent and forceful arguments of Carlo Rubbia, CERN set out to build a proton-antiproton collider in 1978 based upon their 400 GeV proton machine.

  By the late 1970s, the W and Z were estimated by the theorists to be "a hundred times heavier than the proton." (The rest mass of the proton, remember, is close enough to a convenient 1 GeV.) This estimate of the W and Z masses was made with such confidence that CERN was willing to invest $100 million or more on a "sure thing," an accelerator capable of delivering enough energy in a collision to make W's and Z's and a set of elaborate and expensive detectors to observe the collisions. What gave them this arrogant confidence?

  There was a euphoria arising from the sense that a unified theory, the ultimate goal, was close at hand. Not a world model of six quarks and six leptons and four forces, but a model of perhaps only one class of particles and one grand—oh, so grand—unified force. This would surely be the realization of the ancient Greek view, the objective all along as we proceeded from water to air to earth to fire to all four.

  Unification, the search for a simple and all-encompassing theory, is the Holy Grail. Einstein, as early as 1901 (at age twenty-two) wrote about the connections between molecular (electrical) forces and gravity. From 1925 to his death in 1955, he sought in vain for a unified electromagnetic-gravitadonal force. This huge effort by one of the greatest physicists of his, or any other, time failed. We now know that there are two other forces, the weak and the strong. Without these forces Einstein's efforts toward unification were doomed. The second major reason for Einstein's failure was his divorce from the central achievement of twentieth-century physics (to which he contributed strongly in its formative phases), the quantum theory. He never accepted this radical and revolutionary concept, which in fact provided the framework for unification of all the forces. By the 1960s three of the four forces had been formulated in terms of a quantum field theory and had been refined to the point where "unification" cried out.

  All the deep theorists were after it. I remember a seminar at Columbia in the early fifties when Heisenberg and Pauli presented their new, unified theory of elementary particles. The seminar room (301 Pupin Hall) was densely crowded. In the front row were Niels Bohr, I. I. Rabi, Charles Townes, T. D. Lee, Polykarp Kusch, Willis Lamb, and James Rainwater—the present and future laureate contingent. Postdocs, if they had the clout to be invited, violated all the fire laws. Grad students hung from special hooks fastened to the rafters. It was crowded. The theory was over my head, but my not understanding it didn't mean it was correct. Pauli's final comment was an admission. "Yah, this is a crazy theory." Bohr's comment from the audience, which everyone remembers, went something like this: "The trouble with this theory is that it isn't crazy enough." Since the theory vanished like so many other valiant attempts, Bohr was right again.

  A consistent theory of forces must meet two criteria: it must be a quantum field theory that incorporates the special theory of relativity and gauge symmetry. This latter feature and, as far as we know, only this guarantees that the theory is mathematically consistent, renormalizable. But there is much more; this gauge symmetry business has deep aesthetic appeal. Curiously, the idea comes from the one force that has not yet been formulated as a quantum field theory: gravity. Einstein's gravity (as opposed to Newton's) emerges from the desire to have the laws of physics be the same for all observers, those at rest as well as observers in accelerated systems and in the presence of gravitational fields, such as on the surface of the earth, which rotates at 1,000 miles per hour. In such a whirling laboratory, forces appear that make experiments come out quite differently than they would in smoothly moving—nonaccelerated—labs. Einstein sought laws that would look the same to all observers. This "invariance" requirement that Einstein placed on nature in his general theory of relativity (1915) logically implied the existence of the gravitational force. I say this so quickly, but I worked so hard to understand it! The theory of relativity contains a built-in symmetry that implies the existence of a force of nature—in this case, gravitation.

  In an analogous way, gauge symmetry, implying a more abstract invariance imposed upon the relevant equations, also generates, in each case, the weak, the strong, and the elecromagnetic force.

  THE GAUGE

  We are on the threshold of the private driveway that leads to the God Particle. We must review several ideas. One has to do with the matter particles: quarks and leptons. They all have a spin of one half in the curious quantum units of spin. There are the force fields that can also be represented by particles: the quanta of the field. These particles all have integral spin—a spin of one unit. They are none other than the messenger particles and gauge bosons we have often discussed: the photons, the W's and the Z, and the gluons, all discovered and their masses measured. To make sense out of this array of matter particles and force carriers, let's reconsider the concepts of invariance and symmetry.

  We've tap-danced around this gauge symmetry idea because it's hard, maybe impossible, to explain fully. The problem is that this book is in English, and the language of gauge theory is math. In English we must rely on metaphors. More tap-dancing, but perhaps it will help.

  For example, a sphere has perfect symmetry in that we can rotate it through any angle about any axis without producing any change in the sy
stem. The act of rotation can be described mathematically; after the rotation the sphere can be described with an equation that is identical in every detail to the equation before rotation. The sphere's symmetry leads to the invariance of the equations describing the sphere to the rotation.

  But who cares about spheres? Empty space is also rotationally invariant, like the sphere. Thus the equations of physics must be rotationally invariant. Mathematically, this means that if we rotate an x-y-z-coordinate system through any angle about any axis, that angle will not appear in the equation. We have discussed other such symmetries. For example, an object positioned on a flat infinite plane can be moved any distance in any direction, and again the system is identical (invariant) to the situation before the motion. This movement from point A to point B is called a translation, and we believe that space is also invariant to translation; that is, if we add 12 meters to all distances, the 12 will drop out of the equations. Thus, continuing the litany, the equations of physics must display invariance to translations. To complete this symmetry/conservation story, we have the law of conservation of energy. Curiously, the symmetry with which this is associated has to do with time, that is, with the fact that the laws of physics are invariant to translation in time. This means that in the equations of physics, if we add a constant interval of time, say 15 seconds, everywhere that time appears, the addition will wash out, leaving the equation invariant to this shift.

  Now for the kicker. Symmetry reveals new features of the nature of space. I referred to Emmy Noether earlier in the book. Her 1918 contribution was the following: for every symmetry (showing up as the inability of the basic equations to notice, for example, space rotations and translations and time translation), there is a corresponding conservation law! Now conservation laws can be tested experimentally. Noether's work connected translation invariance to the well-tested law of conservation of momentum, rotation invariance to conservation of angular momentum, and time translation to conservation of energy. So these experimentally unassailable conservation laws (using the logic backward) tell us about the symmetries respected by time and space.

  The parity conservation discussed in Interlude C is an example of a discrete symmetry that applies to the microscopic quantum domain. Mirror symmetry amounts to a literal reflection in a mirror of all coordinates of a physical system. Mathematically, it amounts to changing all z-coordinates to -z where z points toward the mirror. As we saw, although the strong and electromagnetic forces respect this symmetry, the weak force doesn't, which of course gave us infinite joy back in 1957.

  So far, most of this material is review and the class is doing well. (I feel it.) We saw in Chapter 7 that there can be more abstract symmetries not related to geometry, upon which our examples above have so far depended. Our best quantum field theory, QED, turns out to be invariant to what looks like a dramatic change in mathematical description—not a geometric rotation, translation, or reflection, but a much more abstract change in describing the field. The name of the change is gauge transformation, and any more detailed description is not worth the math anxiety it would induce. Suffice it to say that the equations of quantum electrodynamics (QED) are invariant to gauge transformation. This is a very powerful symmetry in that one can derive all the properties of the electromagnetic force from it alone. That's not the way it was done in history, but some graduate textbooks do it that way today. The symmetry ensures that the force carrier, the photon, is massless. Because the masslessness is connected to the gauge symmetry, the photon is called a "gauge boson." (Remember that "boson" describes particles, often messenger particles, that have integer spin.) And because it has been shown that QED, the strong force, and the weak force are described by equations that exhibit gauge symmetry, all the force carriers—photons, the W's and the Z, and gluons—are called gauge bosons.

  Einstein's thirty years of fruitless effort to find a unified theory was bested in the late 1960s by Glashow, Weinberg, and Salam's successful unification of the weak force and the electromagnetic force. The major implication of the theory was the existence of a family of messenger particles: the photon, the W+ and W− and Z0.

  Now comes the God Particle theme. How do we have heavy W's and Z's in a gauge theory? How do such disparate objects as the zero-mass photon and the massive Ws and Z's appear in the same family? Their huge mass differences account for the large differences in behavior between the electromagnetic and the weak force.

  We will come back to this teasing introduction later; too much theory exhausts my spirit. And besides, before the theorists can go off to answer this question we must find the W. As if they wait.

  FIND THE W

  So CERN put down its money (or, more correctly, gave it to Carlo Rubbia), and the quest for the W was on. I should note that if the W is about 100 GeV in mass, one needs a good deal more than 100 GeV of collision energy available. A 400 GeV proton colliding with a proton at rest can't do it, for only 27 GeV is available for making new particles. The rest of the energy is used to conserve momentum. That is why Rubbia proposed the collider route. His idea was to make an antiproton source, using the injector to the CERN 400 GeV Super Proton Synchroton (SPS) to manufacture p-bars. When an adequate number had been accumulated, he'd put them into the SPS magnet ring more or less as we explained it back in Chapter 6.

  Unlike the later Tevatron, the SPS was not a superconducting accelerator. This means that its maximum energy was limited. If both beams, protons and antiprotons, were accelerated to the full energy of the SPS, 400 GeV, you would have 800 GeV available—enormous. But the energy selected was 270 GeV in each beam. Why not 400 GeV? First, the magnets would then have to carry a high current for a long time—hours—during the collision time. CERN's magnets were not designed for this and would overheat. Second, remaining for any length of time at high field is expensive. The SPS magnets were designed to ramp their magnetic fields up to the full energy of 400 GeV, dwell for a few seconds while delivering beams to customers doing fixed-target experiments, and then reduce the field to zero. Rubbia's idea of colliding two beams was ingenious, but his basic problem was that his machine was not designed originally to be a collider.

  The CERN authorities agreed with Rubbia that 270 GeV in each beam—making a total energy of 540 GeV—would probably be enough to make Ws, which "weigh" only 100 GeV or so. The project was approved and an adequate number of Swiss francs were given in 1978. Rubbia assembled two teams. The first was a group of accelerator geniuses—French, Italian, Dutch, English, Norwegian, and an occasional visiting Yankee. Their language was broken English but flawless "acceleratorese." The second team, experimental physicists, had to build a massive detector, named UA-1 in a flight of poetic imagination, to observe the collisions between protons and antiprotons.

  In the p-bar accelerator group, a Dutch engineer, Simon Van der Meer, had invented a method of compressing antiprotons into a small volume in the storage ring that accumulates these scarce objects. Called "stochastic cooling," this invention was the key to getting enough p-bars to make a respectable number of p/p-bar collisions, that is, about 50,000 per second. Rubbia, a superb technician, hurried his group, built his constituency, handled marketing, calls, and propaganda. His technique: have talk, will travel. His presentations are machine-gun style, with five transparencies projected per minute, an intimate mixture of blarney, bravado, bombast, and substance.

  CARLO AND THE GORILLA

  To many in physics, Carlo Rubbia is a scientist of heroic proportions. I once had the job of introducing him before he gave the banquet talk at a well-attended international meeting in Santa Fe. (This was after he won the Nobel Prize for finding the W and the Z.) I introduced him with a story.

  At the Nobel ceremonies in Stockholm, King Olaf pulls Carlo aside and tells him there's a problem. Because of a screwup, the king explains, there's only one medal available this year. To determine which laureate gets the gold, the king has designed three heroic tasks, located in three tents on the field in full view of the assemblage. In
the first tent, Carlo is told, he will find four liters of highly distilled slivovitz, the beverage that helped dissolve Bulgaria. The assigned time for drinking all this is 20 seconds! The second tent contains a gorilla, unfed for three days and suffering from an impacted wisdom tooth. The task: remove the offending tooth. The time: 40 seconds. The third tent hides the most accomplished courtesan of the Iraqi army. The task: satisfy her completely. The time: 60 seconds.

  At the starter's gun, Carlo bounds into tent one. The gurgle is heard by all and, in 18.6 seconds, four drained liter bottles of slivovitz are triumphantly displayed.

  Losing no time, the mythical Carlo staggers into the second tent, from which enormous, deafening roars are heard by all. Then silence. And in 39.1 seconds, Carlo stumbles out, wobbles to the microphone and pleads, "All right, where ish the gorilla with the toothache?"

  The audience, perhaps because the conference wine was so generously served, roared with appreciation. I finally introduced Carlo, and as he passed me on his way to the lectern, he whispered, "I don't get it. Explain it later."

  Rubbia did not suffer fools gladly, and his strong control stirred resentment. Sometime after his success, Gary Taubes wrote a book about him, Nobel Dreams, which was not flattering. Once, at a winter school with Carlo in the audience, I announced that the movie rights to the book had been sold and that Sydney Greenstreet, whose girth was roughly the same as Carlo's, had been signed to play him. Someone pointed out that Sydney Greenstreet was dead but would otherwise be a good choice. At another gathering, a summer conference on Long Island, someone put up a sign on the beach: "No Swimming. Carlo is using the ocean."

  Rubbia drove hard on all fronts in the search for the W. He continually urged on the detector builders assembling the monster magnet that would detect and analyze events with fifty or sixty particles emerging from head-on collisions of 270 GeV protons and 270 GeV antiprotons. He was similarly knowledgeable about and active in the construction of the and proton accumulator, or AA ring, the device that would put Van der Meer's idea to work and produce an intense source of antiprotons for insertion and acceleration in the SPS ring. The ring had to have radio-frequency cavities, enhanced water cooling, and a specially instrumented interaction hall where the UA-1 detector would be assembled. A competing detector, UA-2, natch, was approved by CERN authorities to keep Rubbia honest and buy some insurance. UA-2 was definitely the Avis of the situation, but the group building it was young and enthusiastic. Limited by a smaller budget, they designed a quite different detector.