The God Particle

by Leon Lederman

  Which is not to say that the standard model is not one of the great accomplishments of science. It represents the work of a lot of guys (of both genders) who stayed up late at night. But in admiring its beauty and scope, one can't help feeling uneasy, and desirous of something simpler, a model that even an ancient Greek could love.

  Listen: do you hear a laugh emanating from the void?

  8. THE GOD PARTICLE AT LAST

  And the Lord looked upon Her world, and She marveled at its beauty—for so much beauty there was that She wept. It was a world of one kind of particle and one force carried by one messenger who was, with divine simplicity, also the one particle.

  And the Lord looked upon the world She had created and She saw that it was also boring. So She computed and She smiled and She caused Her Universe to expand and to cool. And lo, it became cool enough to activate Her tried and true agent, the Higgs field, which before the cooling could not bear the incredible heat of creation. And in the influence of Higgs, the particles suckled energy from the field and absorbed this energy and grew massive. Each grew in its own way, but not all the same. Some grew incredibly massive, some only a little, and some not at all. And whereas before there was only one particle, now there were twelve, and whereas before the messenger and the particle were the same, now they were different, and whereas before there was only one force carrier and one force, now there were twelve carriers and four forces, and whereas before there was an endless, meaningless beauty, now there were Democrats and Republicans.

  And the Lord looked upon the world She had created and She was convulsed with wholly uncontrolled laughter. And She summoned Higgs and, suppressing Her mirth, She dealt with him sternly and said:

  "Wherefore hast thou destroyed the symmetry of the world?"

  And Higgs, shattered by the faintest suggestion of disapproval, defended thusly:

  "Oh, Boss, I have not destroyed the symmetry. I have merely caused it to be hidden by the artifice of energy consumption. And in so doing I have indeed made it a complicated world.

  "Who could have foreseen that out of this dreary set of identical objects, we could have nuclei and atoms and molecules and planets and stars?

  "Who could have predicted the sunsets and the oceans and the organic ooze formed by all those awful molecules agitated by lightning and heat? And who could have expected evolution and those physicists poking and probing and seeking to find out what I have, in Your service, so carefully hidden?"

  And the Lord, hard put to stop Her laughter signed forgiveness and a nice raise for Higgs.

  —The Very New Testament 3:1

  IT WILL BE OUR TASK in this chapter to convert the poetry(?) of the Very New Testament to the hard science of particle cosmology. But we cannot abandon our discussion of the standard model just yet. There are a few loose ends to tie up—and a few we can't tie up. Both sets are important in the story of the standard-model-and-beyond, and I must recount a few additional experimental triumphs that firmly established our current view of the microworld. These details provide a feeling for the model's power as well as its limitations.

  There are two kinds of bothersome flaws in the standard model. The first has to do with its incompleteness. The top quark is still missing as of early 1993. One of the neutrinos (the tau) has not been directly detected, and many of the numbers we need are imprecisely known. For example, we don't know if the neutrinos have any rest mass. We need to know how CP symmetry violation—the process of the origin of matter—enters, and, most important, we need to introduce a new phenomenon, which we call the Higgs field, in order to preserve the mathematical consistency of the standard model. The second kind of flaw is a purely aesthetic one. The standard model is complicated enough to appear to many as only a way station toward a simpler view of the world. The Higgs idea, and its attendant particle, the Higgs boson, is relevant to all the issues we have just listed, so much so that we have named this book in its honor: the God Particle.

  A FRAGMENT OF STANDARD-MODEL AGONY

  Consider the neutrino.

  "Which neutrino?"

  Well, it doesn't matter. Let's take the electron neutrino—the garden-variety, first-generation neutrino—since it has the lowest mass. (Unless, of course, all neutrino masses are zero.)

  "Okay, the electron neutrino."

  It has no electric charge.

  It has no strong or electromagnetic force.

  It has no size, no spatial extent. Its radius is zero.

  It may not have a mass.

  Nothing has so few properties (deans and politicians excepted) as the neutrino. Its presence is less than a whisper.

  As kids we recited:

  Little fly upon the wall

  Have you got no folks at all?

  No mother?

  No father?

  Pooey on you, ya bastard!

  And now I recite:

  Little neutrino in the world

  With the speed of light you're hurled.

  No charge, no mass, no space dimension?

  Shame! You do defy convention.

  Yet the neutrino exists. It has a sort of location—a trajectory, always heading in one direction with a velocity close (or equal) to that of light. The neutrino does have spin, although if you ask what it is that's spinning you expose yourself as one who has not yet been cleansed of impure prequantum thinking. Spin is intrinsic to the concept of "particle," and if the mass of the neutrino is indeed zero, its spin and its constant, undeviating velocity of light combine to give it a unique new attribute called chirality. This forever des the direction of spin (clockwise or counterclockwise) to the direction of motion. It can have "right-handed" chirality, meaning that it advances with clockwise spin, or it can be left-handed, advancing with a counterclockwise spin. Therein lies a lovely symmetry. The gauge theory prefers all particles to have zero mass and universal chiral symmetry. There is that word again: symmetry.

  Chiral symmetry is one of these elegant symmetries that describe the early universe—one pattern that repeats and repeats and repeats like wallpaper, but unrelieved by corridors, doors, or corners—unending. No wonder She found it boring and ordered in the Higgs field to give mass and break chiral symmetry. Why does mass break chiral symmetry? Once a particle has mass, it travels at speeds less than that of light. Now you, the observer; can go faster than the particle. Then, relative to you, the particle has reversed its direction of motion but not its spin, so a left-handed object to some observers becomes right-handed to others. But there are the neutrinos, survivors perhaps of the war on chiral symmetry. The neutrino is always left-handed, the antineutrino always right-handed. This handedness is one of the very few properties the poor little fellow has.

  Oh yes, neutrinos have another property, the weak force. Neutrinos emerge from weak processes that take forever (sometimes microseconds) to happen. As we have seen, they can collide with another particle. This collision requires so close a touch, so deep an intimacy, as to be exceedingly rare. For a neutrino to collide hard in an inch-thick slab of steel would be as likely as finding a small gem buffeted randomly in the vastness of the Atlantic Ocean—that is, as likely as catching it in one cup of the Atlantic's water, randomly sampled. And yet for all its lack of properties, the neutrino has enormous influence on the course of events. For example, it is the outrush of huge numbers of neutrinos from the core that instigates the explosion of stars, scattering heavier elements, recently cooked in the doomed star throughout space. The debris of such explosions eventually coalesces and accounts for the silicon and iron and other good stuff we find in our planets.

  Recently, strenuous efforts have been made to detect the mass of the neutrino, if indeed it has any. The three neutrinos that are a part of our standard model are candidates for what astronomers call "dark matter," material that, they say, pervades the universe and dominates its gravitationally driven evolution. All we know so far is that neutrinos could have a small mass ... or they could have zero mass. Zero is such a very special number that even
the very slightest mass, say a millionth that of the electron, would be of great theoretical significance. As part of the standard model, neutrinos and their masses are an aspect of the open questions that lie therein.

  HIDDEN SIMPLICITY: STANDARD-MODEL ECSTASY

  When a scientist, say of the British persuasion, is really, really angry at someone and is driven to the extremes of expletives, he will say under his breath, "Bloody Aristotelian." Them's fightin' words, and a deadlier insult is hard to imagine. Aristotle is generally credited (probably unreasonably) with holding up the progress of physics for about 2,000 years—until Galileo had the courage and the conviction to call him out. He shamed Aristotle's acolytes in full view of the multitudes on the Piazza del Duomo, where today the Tower leans and the piazza is lined with souvenir sellers and ice cream stands.

  We've reviewed the story of things falling from crooked towers—a feather floats down, a steel ball drops rapidly. That seemed like good stuff to Aristotle, who said, "Heavy falls fast, light falls slow." Perfectly intuitive. Also, if you roll a ball, it eventually comes to rest. Therefore, said Ari, rest is "natural and preferred, whereas motion requires a motive force keeping it moving." Eminently clear, confirmed by our everyday experience, and yet ... wrong. Galileo saved his contempt, not for Aristotle, but for the generations of philosophers who worshiped at Aristotle's temple and accepted his views without question.

  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 system. 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.