The God Particle

by Leon Lederman

  THE GAP

  The physics of particle acceleration is simple to explain (watch out!). Connect the terminals of a DieHard battery to two metal plates (also called terminals), positioned, say, a foot apart. This arrangement is called the Gap. Seal the two terminals into a can from which the air is removed. Organize the equipment so that an electrically charged particle—electrons and protons are the prime projectiles—can move freely across the gap. A negatively charged electron will gladly rush toward the positive terminal, gaining an energy of (look at the label on the battery) 12 eV. Thus the Gap produces an acceleration. If the positive metallic terminal is made of wire screen instead of a solid plate, most of the electrons will pass through it, creating a directed beam of 12 eV electrons. Now an electron volt is an extremely small amount of energy. What we need is a billion-volt battery, but Sears doesn't handle such an item. To achieve high voltages requires moving beyond chemical devices. But no matter how big the accelerator, whether we're talking about a 1920s Cockcroft-Walton device or the fifty-four-mile-around Super Collider, the basic mechanism remains the same—the Gap, across which particles gain energy.

  The accelerator takes normal, law-abiding particles and gives them extra energy. Where do we get the particles? Electrons are easy. We heat a wire to incandescence, and electrons pour out. Protons are easy, too. The proton is the nucleus of the hydrogen atom (hydrogen nuclei have no neutrons), so all we need is commercially available hydrogen gas. Other particles can be accelerated, but they must be stable—that is, have long lifetimes—because the acceleration process is time consuming. And they must be electrically charged, since the Gap obviously wouldn't work on a neutral particle. The leading candidates for acceleration are protons, antiprotons, electrons, and positrons (anti-electrons). Heavier nuclei such as deuterons and alpha particles can also be accelerated, and they have their special uses. An unusual machine under construction on Long Island in New York will accelerate uranium nuclei to billions of electron volts.

  THE PONDERATOR

  What does the acceleration process do? The easy but incomplete answer is that it speeds up the lucky particles. In the early days of accelerators, this explanation worked fine. A better description is that it raises the energy of the particles. As accelerators got more powerful, they soon were able to achieve speeds close to the ultimate: the velocity of light. Einstein's 1905 special theory of relativity asserts that nothing can travel faster than light. Because of relativity, "velocity" is not a very useful concept. For example, one machine may accelerate protons, say, to 99 percent of the velocity of light, while a much more expensive one, built ten years later, can achieve 99.9 percent of the velocity of light. Big deal. Go explain this to the congressman who voted all that dough just to achieve another 0.9 percent!

  It's not speed that sharpens the Democritan knife and yields new domains of observation. It's energy. A 99-percent-of-the-velocity-of-light proton has an energy of about 7 GeV (the Berkeley Bevatron, 1955), whereas a 99.95 percent proton has 30 GeV (Brookhaven AGS, 1960), and a 99.999 percent proton has 200 GeV (Fermilab, 1972). So Einstein's relativity, which rules the changes in velocity and energy, makes it silly to talk about speed. What is important is energy. A related attribute is momentum, which, for a high-energy particle, can be considered directed energy. Incidentally, the particle being accelerated also gets heavier because of E = mc2. In relativity a particle at rest still has the energy given by E = m0c2, where m0 is defined as the "rest mass" of the particle. When the particle is accelerated its energy, E, and hence its mass increase. The closer to the velocity of light we get, the heavier the object becomes, and consequently the more difficult it is to increase its speed. But the energy keeps going up. Conveniently, a proton's rest mass is about 1 GeV. So the mass of a 200 GeV proton is more than two hundred times that of the proton resting comfortably in the hydrogen gas bottle. Our accelerator is actually a "ponderator."

  MONET'S CATHEDRAL, OR THIRTEEN WAYS OF LOOKING AT A PROTON

  Now, how do we use these particles? Simply said, we cause them to make collisions. Since this is the core process by which we learn about matter and energy, we must go into detail. It's okay to forget the various particulars about the machinery and how the particles are accelerated, interesting as these may be. But remember this part. The whole point of the accelerator is the collision.

  Our technique of observing and eventually comprehending the abstract world of the subnuclear domain is similar to how we comprehend anything—a tree, for example. What is the process? First, we need light. Let's use sunlight. The photons from the sun stream toward the tree and reflect off leaves and bark, twigs and branches, and some fraction of these photons is collected by our eyeball. The photons, we can say, are scattered by the object toward the detector. The lens of the eye focuses the light on the retina at the back of the eye. The retina detects the photons and sorts out the various qualities: color shade, intensity. This information is organized and sent to the on-line processor, the occipital lobe of the brain, which specializes in visual data. Eventually, the off-line processor comes to a conclusion: "By Jove, a tree! How lovely."

  The information coming to the eye may be filtered through spectacles or sunglasses, adding to the distortion that the eye has already introduced. It's up to the brain to correct these distortions. Let's replace the eye with a camera, and now, a week later, with a greater degree of abstraction, the tree is seen projected in a family slide show. Or a video recorder can convert the data provided by scattered photons into digital electronic information: zeroes and ones. To enjoy this, one plays it through the TV, which converts the digital information back to analog—a tree shows up on the screen. If one wanted to send "tree" to our scientist colleagues on the planet Ugiza, the digital information might not be converted to analog, but it could convey, with maximum precision, the configuration that earthlings call a tree.

  Of course, it's not so simple in an accelerator. Different kinds of particles are used in different ways. Still, we can push the metaphor for nuclear collisions and scattering another step. The tree looks different in the morning, at noon, in the setting sun. Anyone who has seen Monet's numerous paintings of the entrance to the cathedral at Rouen at different times of the day knows what a difference the quality of light makes. What is the truth? To the artist the cathedral has many truths. Each shimmers in its own reality—the hazy morning light, the stark contrasts of the noontime sunshine, or the rich glow of the late afternoon. In each of these lights a different aspect of truth is exhibited. Physicists work with the same bias. We need all the information we can get. The artist employs the sun's changing light. We employ different particles: a stream of electrons, a stream of muons or neutrinos—at ever-changing energies.

  Here's how it works.

  What is known about a collision is what goes in and what comes out—and how it comes out. What happens in that tiny volume of the collision? The maddening truth is that we can't see. It's as if a black box covers the collision region. The inner mechanistic details of the collision are not observable—are hardly even capable of being imagined—in the spooky, shimmering quantum world. What we do have is a model for the forces at play and, where relevant, for the structure of the colliding objects. We see what goes in and what comes out, and we ask if the patterns are predictable by our model of what is in the box.

  In a Fermilab education program for ten-year-olds, we confront them with this problem. We give them an empty square box to look at, shake, weigh. Then we put something in the box, such as a wooden block or three steel balls. Then we ask the students again to weigh, shake, tilt, listen, and to tell us everything they can about the objects: size, shape, weight ... It's an instructive metaphor for our scattering experiments. You'd be surprised how often the kids get it right.

  Let's switch to grownups and particles. Let's say we want to find out the size of protons. So we take a tip from Monet. We look at them in different forms of "light." Could protons be points? To find out, physicists hit protons with o
ther protons at very low energy to explore the electromagnetic force between the two charged objects. Coulomb's law says that this force reaches out to infinity, decreasing in strength as the square of the distance. The target proton and the accelerated proton are, of course, both positively charged, and since like charges repel, the slow proton is readily repelled by the target proton. It never gets very close. In this kind of "light," the proton does in fact look like a point, a point of electric charge. So we increase the energy of the accelerated protons. Now the deviations in the patterns of scattered protons indicate that the penetrations are getting deep enough to touch what's called the strong force, the force that we now know holds the proton's constituents together. The strong force is a hundred times stronger than the Coulomb electrical force, but unlike the electrical force, its range is anything but infinite. The strong force reaches out only to a distance of about 10−13 centimeters, then fades quickly to zero.

  By increasing the energy of the collision, we unearth more and more details about the strong force. As the energy increases, the wavelength of the protons (remember de Broglie and Schrodinger) shrinks. And, as we have seen, the smaller the wavelength, the more detail that can be discerned in the particle being studied.

  Some of the best "pictures" of the proton were taken in the 1950s by Robert Hofstadter of Stanford University. There the "light" used was a beam of electrons rather than protons. Hofstadter's team aimed a well-organized beam of, say, 800 MeV electrons at a small vat of liquid hydrogen. The electrons bombarded the protons in the hydrogen, resulting in a scattering pattern, the electrons emerging in a variety of directions relative to their original motion. Not too different from what Rutherford did. Unlike the proton, the electron does not respond to the strong nuclear force. It responds only to the electric charge in the proton, so the Stanford scientists were able to explore the shape of the charge distribution in the proton. In effect, this revealed the proton's size. It was clearly not a point. The radius was measured to be 2.8 × 10−13 centimeters, with the charge piling up at the center and fading out at the edges of what we call a proton. Similar results were obtained when the experiments were repeated with muon beams, which also ignore the strong force. Hofstadter was awarded a Nobel Prize in 1961 for his "photograph" of the proton.

  About 1968, physicists at the Stanford Linear Accelerator Center (SLAC) bombarded protons with electrons at a much higher energy—8 to 15 GeV—and got a vastly different set of scattering patterns. In this hard light, the proton presented quite a different picture. The relatively low-energy electrons that Hofstadter used were able to see only a "blurry" proton, a smooth distribution of charge that made the proton look like a mushy little ball. The SLAC electrons probed harder and found little guys running around inside the proton. This was the earliest indication of the reality of quarks. The new data and the old data were consistent—like morning and evening paintings by Monet—but the low-energy electrons could reveal only average charge distributions. The visualization provided by the higher-energy electrons showed that our proton contains three rapidly moving, pointlike constituents. Why did the SLAC experiment show this detail, while the Hofstadter study did not? A collision with high enough energy (determined by what goes in and what comes out) freezes the quarks in place and "feels" the pointlike force. It's the virtue of short wavelengths again. This force promptly induces large-angle scattering (remember Rutherford and the nucleus) and large energy changes. The formal name for this phenomenon is "deep inelastic scattering." In Hofstadter's earlier experiments, the quark motion was blurred out and the protons looked "smooth" and uniform inside because of the lower energy of the probing electrons. Think of taking a photograph of three rapidly vibrating, tiny light bulbs using a one-minute time exposure. The film would show one big blurry undifferentiated object. The SLAC experiment, in a crude sense, used a faster shutter, freezing the spots of light so that they could easily be counted.

  Since the quark interpretation of the higher energy electron scattering was very far out and of tremendous importance, these experiments were repeated at Fermilab and at CERN (an acronym for the European Center for Nuclear Research), using muons of ten times the SLAC energy (150 GeV) as well as neutrinos. Muons, like electrons, test the electromagnetic structure of the proton, but neutrinos, impervious to both the electromagnetic and the strong forces, test what's called the weak-force distribution. The weak force is the nuclear force responsible for radioactive decay, among other things. These huge experiments, carried out in heated competition, each came to the same conclusion: the proton is made of three quarks. And we learned some details about how the quarks move about. Their motion defines what we call "proton."

  Detailed analysis of all three types of experiments—electron, muon, and neutrino—also succeeded in detecting a new kind of particle, the gluon. Gluons are carriers of the strong force, and without them the data just could not be explained. The same analysis gave quantitative details on how the quarks whirl about each other in their proton prison. Twenty years of such study (the technical term is structure functions) has given us a sophisticated model that accounts for all the collision experiments in which protons, neutrons, electrons, muons, and neutrinos as well as photons, pions, and antiprotons are aimed at protons. This is Monet with a vengeance. Perhaps Wallace Stevens's poem "Thirteen Ways of Looking at a Blackbird" would be more to the point.

  As you can see, we learn many things in order to account for what-goes-in-and-what-comes-out. We learn about the forces and how these forces result in complex structures such as protons (made of three quarks) and mesons (made of a quark and an antiquark). With so much complementary information, it becomes less and less important that we can't see inside the black box where the collision actually takes place.

  One can't help being impressed by the sequence of "seeds within seeds." The molecule is made of atoms. The core of the atom is the nucleus. The nucleus is made of protons and neutrons. The proton and neutron are made of quarks. The quarks are made of ... whoops, hold it. The quarks can't be broken down, we think, but of course we are not sure. How dare we say we've come to the end of the road? Nevertheless, that is the consensus—at present—and after all, Democritus can't live forever.

  NEW MATTER: SOME RECIPES

  We have yet to discuss an important process that can take place during a collision. We can make new particles. This happens all the time around the house. Look at the lamp that is valiantly trying to illuminate this dark page. What is the source of the light? It is electrons, agitated by the electrical energy squirting into the filament of the bulb or, if you are energy efficient, into the gas of the fluorescent lamp. The electrons emit photons. That's the process. In the more abstract language of the particle physicist, the electron in the process of a collision can radiate a photon. The energy is provided to the electron (via the wall plug) by an accelerating process.

  Now we have to generalize. In the process of creation, we are constrained by the laws of conservation of energy, momentum, charge, and respect for all of the other quantum rules. Also, the object that is somehow responsible for creating a new particle has to be "connected" to the particle being created. Example: a proton collides with another proton, and a new particle, a pion, is made. We write it like this:

  p+ + p+ → p+ + π+ + n

  That is, protons collide and produce another proton, a positive pion (π+), and a neutron. These particles are all connected via the strong force, and this is a typical creation process. Alternatively, one can view this as a proton, "under the influence" of another proton, dissolving into a "pi plus" and a neutron.

  Another kind of creation, a rare and exciting process called annihilation, takes place when matter and antimatter collide. The term annihilation is used in its strictest dictionary sense of putting something out of existence. When an electron collides with its antiparticle, the positron, the particle and antiparticle disappear, and in their place energy, in the form of a photon, appears momentarily. The conservation laws don't like this
process, so the photon is temporary and must soon create two particles in its place—for example, another electron and a positron. Less frequently the photon may dissolve into a muon and an antimuon, or even a positive proton and a negative antiproton. Annihilation is the only phenomenon that is fully efficient in converting mass to energy in accordance with Einstein's law, E = mc2. When a nuclear bomb explodes, for instance, only a fraction of 1 percent of the atomic mass is converted into energy. When matter and antimatter collide, 100 percent of the mass disappears.

  When we're making new particles, the primary requirement is that there be enough energy, and E = mc2 is our accounting tool. For example, we mentioned that a collision between an electron and a positron can result in a proton and an antiproton, or a p and a p-bar, as we call them. Since the rest mass energy of a proton is about 1 GeV, the particles in the original collision must bring in at least 2 GeV to produce a p/p-bar pair. More energy increases the probability of this result and gives the newly produced objects some kinetic energy, making them easier to detect.

  The glamorous nature of antimatter has given rise to the science fiction notion that it may solve the energy crisis. Indeed, a kilogram (2.2 pounds) of antimatter would provide enough energy to keep the United States going for a day. This is because the entire mass of antiproton (plus the proton it takes with it to total annihilation) is converted to energy via E = mc2 In the burning of coal or oil, only one billionth of the mass is converted to energy. In fission reactors this number is 0.1 percent, and in the long-awaited fusion energy supply (don't hold your breath!) it is about 0.5 percent.