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

by Leon Lederman

  At the time the NOVA program was put to bed, neither group had found any evidence for top. In fact, by the time the program aired, the "race" was over in that CERN was just about out of the picture. Each group had analyzed the absence of a signal in terms of top's unknown mass. As we have seen, not finding a particle tells you something about its mass. The theorists knew everything about the production of top and about certain decay channels—everything but the mass. The production probability depends critically on the unknown mass. Fermilab and CERN both set the same limits: the mass of the top quark was greater than 60 GeV.

  Fermilab's CDF continued to run, and slowly the machine energy began to pay off. By the time the collider run was over CDF had run for eleven months and had seen more than 100 billion (1011) collisions—but no top. The analysis gave a limit of 91 GeV for the mass, making the top at least eighteen times heavier than the bottom quark. This surprising result disturbed many theorists working on unified theories, especially in the electroweak pattern. In these models the top quark should be much lower in mass, and this led some theorists to view top with special interest. The mass concept is somehow tied in with Higgs. Is the heaviness of the top quark a special clue? Until we find top, measure its mass, and in general subject it to the experimental third degree, we won't know.

  The theorists went back to their calculations. The standard model was actually still intact. It could accommodate a top quark as heavy as 250 GeV, the theorists figured, but anything heavier would indicate a fundamental problem with the standard model. Experimenters were reinvigorated in their determination to pursue the top quark. But with top's mass greater than 91 GeV, CERN dropped out. The e+ e− machines are too low in energy and therefore useless; of the world's inventory, only Fermilab's Tevatron can make top. What is needed is at least five to fifty times the present number of collisions. This is the challenge for the 1990s.

  THE STANDARD MODEL IS A SHAKY PLATFORM

  I have a favorite slide that pictures a white-gowned deity, with halo, staring at a "Universe Machine." It has twenty levers, each one designed to be set at some number, and a plunger labeled "Push to create universe." (I got this idea from a sign a student put up on the bathroom hand drier: "Push to get a message from the dean.") The idea is that twenty or so numbers must be specified in order to begin the universe. What are these numbers (or parameters, as they are called in the physics world)? Well, we need twelve numbers to specify the masses of the quarks and leptons. We need three numbers to specify the strengths of the forces. (The fourth, gravity, really isn't a part of the standard model, at least not yet.) We need some numbers to show how one force relates to another. Then we need a number for how the CP-symmetry violation enters, and a mass for the Higgs particle, and a few other handy items.

  If we have these basic numbers, all other parameters are derived therefrom—for example, the 2 in the inverse-square law, the mass of the proton, the size of the hydrogen atom, the structure of H20 and the double helix (DNA), the freezing temperature of water, and the GNP of Albania in 1995. I wouldn't have any idea how to obtain most of the derived numbers, but we do have these enormous computers...

  The drive for simplicity leads us to be very sarcastic about having to specify twenty parameters. It's not the way any self-respecting God would organize a machine to create universes. One parameter—or two, maybe. An alternative way of saying this is that our experience with the natural world leads us to expect a more elegant organization. So this, as we have already complained, is the real problem with the standard model. Of course we still have an enormous amount of work to do to pinpoint these parameters accurately. The problem is the aesthetics—six quarks, six leptons, and twelve force-carrying gauge particles, and the quarks come in three colors, and then there are the antiparticles. And gravity waiting in the wings. Where is Thales now that we need him?

  Why is gravity left out? Because no one has yet succeeded in forcing gravity—the general theory of relativity—to conform to the quantum theory. The subject, quantum gravity, is one of the theoretical frontiers of the 1990s. In describing the universe in its present grand scale, we don't need quantum theory. But once upon a time the entire universe was no bigger than an atom; in fact, it was a good deal smaller. The extraordinarily weak force of gravity was enhanced by the enormous energy of the particles that made all the planets, stars, galaxies of billions of stars, all that mass compressed to a pinhead on a pinhead, a size tiny compared to an atom. The rules of quantum physics must apply here in this primal gravitational maelstrom, and we don't know how to do it! Among theorists the marriage of general relativity and quantum theory is the central problem of contemporary physics. Theoretical efforts along these lines are called "super gravity" or "supersymmetry" or "superstrings" or the "Theory of Everything" (TOE).

  Here we have exotic mathematics that curls the eyebrows of some of the best mathematicians in the world. They talk about ten dimensions: nine space and one time dimension. We live in four dimensions: three space dimensions (east-west, north-south, and up-down) and one time dimension. We can't possibly intuit more than three space dimensions. "No problem." The superfluous six dimensions have been "compactified," curled up to an unimaginably small size so as not to be evident in the world we know.

  Today's theorists have a bold objective: they're searching for a theory that describes a pristine simplicity in the intense heat of the very early universe, a theory with no parameters. Everything must emerge from the basic equation; all the parameters must come out of the theory. The trouble is, the only candidate theory has no connection with the world of observation—not yet anyway. It has a brief instant of applicability at the imaginary domain that the experts call the "Planck mass," a domain where all the particles in the universe have energies of 1,000 trillion times the energy of the Super Collider. The time interval of this greater glory lasted for a trillionth of a trillionth of a trillionth of a second. Shortly thereafter, the theory gets confused—too many possibilities, no clear road indicating that we the people and planets and galaxies are indeed a prediction.

  In the middle 1980s, TOE had a tremendous appeal for young physicists of the theoretical persuasion. In spite of the risk of long years of investment for small returns, they followed the leaders (like lemmings, some would say) to the Planck mass. We who stayed home at Fermilab and CERN received no postcards, no faxes. But disillusion began to set in. Some of the more stellar recruits to TOE quit, and pretty soon, buses began arriving back from the Planck mass with frustrated theorists looking for something real to calculate. The entire adventure is still not over, but it has slowed to a quieter pace, while the more traditional roads to unification are tried.

  These more popular roads toward a complete, overarching principle have groovy names: grand unification, constituent models, supersymmetry, Technicolor, to name a few. They all share one problem: there are no data! These theories made a rich stew of predictions. For example, supersymmetry (affectionately shortened to "Susy"), probably the most popular theory, if theorists voted (and they don't), predicts nothing less than a doubling of the number of particles. As I've explained, the quarks and leptons, collectively called fermions, all have one half unit of spin, whereas the messenger particles, collectively called bosons, all have one full unit of spin. In Susy this asymmetry is repaired by postulating a boson partner for every fermion and a fermion partner for every boson. The naming is terrific. The Susy partner of the electron is called "selectron," and the partners of all the leptons are collectively called "sleptons." The quark partners are "squarks." The spin-one-half partners of the spin-one bosons are given a suffix "ino" so that gluons are joined by "gluinos," photons couple with "photinos," and we have "winos" (partner of the W) and "zinos." Cute doesn't make a theory, but this one is popular.

  The search for squarks and winos will go on as the Tevatron increases its power through the 1990s and the machines of the year 2000 come on-line. The Super Collider being built in Texas will enable exploration of the "mass domain" up to
about 2 TeV. The definition of mass domain is very loose and depends on the details of the reaction that makes a new particle. However, a sign of the power of the Super Collider is that if no Susy particles are found in this machine, most Susy protagonists have agreed to abandon the theory in a public ceremony in which they break all their wooden pencils.

  But the SSC has a more immediate goal, a quarry more pressing than the squarks and sleptons. As a compact summary of everything we know, the standard model has two major defects, one aesthetic, one concrete. Our aesthetic sense tells us that there are too many particles, too many forces. Worse, the many particles are distinguished by the seemingly random masses assigned to quarks and leptons. Even the forces differ largely because of the masses of the messenger particles. The concrete problem is one of inconsistency. When the force-field theories, in impressive agreement with all of the data, are asked to predict the results of experiments carried out at very high energies, they churn out physical absurdities. Both problems can be illuminated and possibly solved by an object (and a force) that must be added gingerly to the standard model. The object and the force go by the same name: Higgs.

  AT LAST...

  All visible objects, man, are but as pasteboard masks. But in each event ... some unknown but still reasoning thing puts forth the mouldings of its features from behind the unreasoning mask. If man will strike, strike through the mask!

  —Captain Ahab

  One of the finest novels in American literature is Herman Melville's Moby Dick. It is also one of the most disappointing—at least for the captain. For hundreds of pages we hear about Ahab's quest to find and harpoon a large white oceangoing mammal named Moby Dick. Ahab is pissed. This whale has bitten off his leg, and he wants revenge. Some critics suggest that the whale bit oft a lot more than leg, which would explain more adequately the good captain's pique. Ahab explains to his first mate, Starbuck, that Moby Dick is more than a whale. He is a pasteboard mask; he represents a deeper force in nature that Ahab must confront. So for hundreds of pages Ahab and his men scurry furiously around the ocean, having adventures and misadventures, killing lots of smaller whales of various masses. Finally, thar she blows: the great white whale. And then, in quick succession, the whale drowns Ahab, kills all the other harpooners, then sinks the ship for good measure. End of story. Bummer. Perhaps Ahab needed a bigger harpoon, one denied by nineteenth-century budgetary restraints. Let's not let that happen to us. Moby Particle is within striking distance.

  We have to ask this question about our standard model: is it simply a pasteboard mask? How can a theory be in accordance with all the data at low energy and predict nonsensical effects at high energy? The answer is to suggest that the theory is leaving something out, some new phenomenon which, when installed in the theory, will contribute negligibly to the data at, say, Fermilab energies and therefore will not spoil agreement with experimental data. Examples of what's left out might be a new particle or a change in the behavior of a force. These postulated new phenomena must contribute negligibly at low energy but massively at Super Collider or higher energy. When a theory does not include these terms (because we don't know about them) we get mathematically inconsistent results at these high energies.

  This is somewhat like Newtonian physics, which works very successfully for ordinary phenomena but predicts that we can accelerate an object to infinite velocity; this implausible consequence is totally contradicted when Einstein's special theory of relativity is installed. Relativity theory has infinitesimally tiny effects at the velocities of bullets and rockets. However, as the velocities approach that of light, a new effect appears: the masses of the speeding objects begin to increase, and infinite velocities become impossible. What happens is that special relativity merges into Newtonian results at velocities that are small compared to the velocity of light. The weakness of this example is that whereas the concept of infinite velocity may have been disturbing to Newtonians, it was not nearly as traumatic as what happens to the standard model at high energies. We'll return to this soon.

  THE MASS CRISIS

  I have hinted at the function of the Higgs particle in giving mass to massless particles and thereby disguising the true symmetry of the world. This is a new and bizarre idea. Heretofore, as we have seen in our myth-history, simplicity was gained by finding substructures—the Democritan idea of atomos. And so we went from molecules to chemical atoms to nuclei to protons and neutrons (and their numerous Greek relatives) to quarks. History would lead one to expect that now we reveal the little people inside the quark, and indeed this may still happen. But we really don't think that is the way the long-awaited complete theory of the world will come out. Perhaps it's more like the kaleidoscope I referred to earlier, in which some split mirrors convert a few bits of colored glass into a myriad of seemingly complex designs. Higgs's ultimate purpose (this isn't science, it's philosophy) may be to create a more amusing, more complex world as suggested in the parable that started this chapter.

  The new idea is that all of space contains a field, the Higgs field, which permeates the vacuum and is the same everywhere. This means that when you look up at the stars on a clear night you are looking through the Higgs field. Particles, influenced by this field, acquire mass. This by itself is not remarkable since particles can acquire energy from the (gauge) fields we have discussed, the gravitational field or the electromagnetic field. For example, if you carry a lead block to the top of the Eiffel Tower, the block acquires potential energy because of its altered position in the earth's gravitational field. Since E = mc2, this increase in potential energy is equivalent to an increment in mass, in this case the mass of the earth-lead-block system. Here we have to gently add a small complexity to Einstein's hoary equation. The mass, m, actually has two parts. One is the rest mass, m0, which is what is measured in the laboratory when the particle is at rest. The other part of the mass is "acquired" by the particle by virtue of its motion (like the protons in the Tevatron) or by virtue of its potential energy in a field. We see a similar dynamic in atomic nuclei. For example, if you separate the proton and neutron that make up the deuterium nucleus, the sum of the masses increases.

  But the potential energy acquired from the Higgs field differs in several ways from the action of the more familiar fields. The Higgs-acquired mass is actually rest mass. In fact, in what may be the most intriguing version of the Higgs theory, all rest mass is generated by the Higgs field. Another difference is that the amount of mass soaked up from the field differs for various particles. Theorists say that the masses of the particles in our standard model are a measure of how strongly they are coupled to the Higgs field.

  The Higgs influence on the masses of quarks and leptons reminds one of Pieter Zeeman's discovery, in 1896, of the splitting of the energy levels of an electron in an atom when a magnetic field is applied to the atom. The field (playing the metaphoric role of Higgs) breaks the symmetry of space that the electron had enjoyed. For example, one energy level, influenced by the magnet, splits into three; level A gains energy from the field, level B loses energy, and level C doesn't change at all. Of course, we now understand completely how all of this happens. It is simple quantum electromagnetism.

  So far we have no idea what the rules are that control the Higgsgenerated mass increments. But the question nags: why only these masses—the masses of the W+, W−, and Z0, and the up, down, charm, strange, top, and bottom, as well as the leptons—which form no obvious pattern? The masses vary from that of the electron, at .0005 GeV, to the top quark's, which must be greater than 91 GeV. We should recall that this bizarre idea—Higgs—was used with great success in formulating the electroweak theory. There the Higgs field was proposed as a way of hiding the unity of the electromagnetic and the weak force. In unity there are four massless messenger particles—the W+, W−, Z0, and the photon—that carry the electroweak force. Along comes the Higgs field, and presto, the W's and Z soak up the essence of Higgs and grow heavy; the photon is untouched. The electroweak shatters into the
weak (weak because the messengers are so fat) and the electromagnetic force, whose properties are determined by the massless photon. The symmetry is spontaneously broken, the theorists say. I prefer the description that Higgs hides the symmetry by its mass-giving power. The masses of the W's and the Z were successfully predicted from the parameters of the electroweak theory. And the relaxed smiles of the theorists remind us that't Hooft and Veltman established that this whole theory has no infinities.

  I dwell on this issue of mass in part because it has been with me all during my professional life. In the 1940s the issue seemed well focused. We had two particles that exemplified the puzzle of mass: the electron and the muon. They seemed to be in all respects identical except that the muon weighed two hundred times more than its puny cousin. The fact that these were leptons, ignoring the strong force, made it more intriguing. I became obsessed with the problem and made the muon my favorite object of study. The aim was to try to find some difference, other than mass, in the behavior of the muon and the electron as a clue to the mechanism of mass differences.