Our uneasiness with the fact that quarks were never seen outside of hadrons was only moderately tempered by a physical picture of why quarks are permanently confined. At close distances, quarks exert relatively weak forces on one another. This is the glory domain for theorists, where they can calculate properties of the quark state and the quark's influence on collision experiments. As the quarks separate, however, the force becomes stronger, and the energy required to add distance between them rises rapidly until, long before we have actually separated the quarks, the energy input results in the creation of a new quark-antiquark pair. This curious property is a result of the fact that gluons are not simple, dumb messenger particles. They actually exert forces on each other. This is where QED differs from QCD, since photons ignore each other.
Still, QED and QCD had many close analogies, especially in the high-energy domain. QCD's successes were slow in coming, but steady. Because of the fuzzy long-distance part of the force, calculations were never very precise, and many experiments would conclude with the rather nebulous statement that "our results are consistent with the predictions of QCD."
So what kind of a theory do we have if we can never, ever see a free quark? We can do experiments that sense the presence of electrons and measure them, this way and that, even when they are all bound up in atoms. Can we do the same with quarks and gluons? Bjorken and Feynman had suggested that in very hard collisions of particles, the energized quarks would initially head out and, just before leaving the influence of their quark partners, would mask themselves into a narrow bundle of hadrons—three or four or eight pions, for example, or add some kaons and nucleons. These would be narrowly directed along the path of the parent quark. They were given the name "jets," and the search was on.
With the machines of the 1970s, these jets were not easily distinguished because all we could produce were slower quarks that gave rise to broad jets of a small number of hadrons. We wanted dense, narrow jets. The first success belonged to a young woman experimentalist named Gail Hanson, a Ph.D. from MIT working at SLAC. Her careful statistical analysis revealed that a correlation of hadrons did appear in the debris of a 3 GeV e+ e− collision at SPEAR. She was helped by the fact that what went in were the electrons and what came out were a quark and an antiquark, back to back to conserve momentum. These correlated jets showed up, barely but decisively, in the analysis. When Democritus and I were sitting in the CDF control room, needlelike bundles of ten or so hadrons, two jets 180 degrees apart, were flashed on the large screen every few minutes. There is no reason why there should be such a structure unless the jet is the offspring of a very high energy, very high momentum quark, which dresses itself before going out.
But the major discovery of the 1970s along these lines was made at the PETRA e+ e− machine in Hamburg, Germany. This machine, colliding at the total energy of 30 GeV, also showed, without need for analysis, the two-jet structure. Here one could almost see the quarks in the data. But something else was also seen.
One of the four detectors on-line at PETRA had its own acronym: TASSO, for Two-Armed Solenoidal Spectrometer. The TASSO group was looking for events in which three jets would appear. A consequence of QCD theory is that when e+ and e− annihilate to produce a quark and an antiquark, there is a reasonable probability that one of the outgoing quarks will radiate a messenger particle, a gluon. There is enough energy here to convert "virtual" gluon to real gluon. The gluons share the quarks' shyness and, like quarks, dress themselves before leaving the black box of the encounter domain. Therefore three jets of hadrons. But this takes more energy.
In 1978, runs of total energy of 13 and 17 GeV came out empty, but at 27 GeV, something happened. The analysis was pushed by another woman physicist, Sau Lan Wu, a professor at the University of Wisconsin. Wu's program soon uncovered more than forty events in which there were three jets of hadrons, each jet having three to ten tracks (hadrons). The array looked like the hood ornament of a Mercedes.
The other PETRA groups soon got on the bandwagon. Looking through their data, they also found the three-jet events. A year later, thousands had been collected. The gluon had thus been "seen." The pattern of tracks was calculated by theorist John Ellis at CERN using QCD, and one must credit his intervention in motivating the search. The announcement of the gluon's detection was made at a conference at Fermilab in the summer of 1979, and it was my job to go on the Phil Donahue television show in Chicago to explain the discovery. I put more energy into explaining that the Fermilab buffalo were not roaming the lab as early warning devices for dangerous radiation. But in physics, the real news was the gluons—the bosons, not the bisons.
So now we have all the messenger particles, or gauge bosons as they are more eruditely called. ("Gauge" came from gauge symmetry, and boson is derived from the Indian physicist S. N. Bose, who described the class of particles with integer values of spin.) Whereas the matter particles all have spin of ½ and are called fermions, the messenger particles all have spin 1 and are bosons. We've skipped over some details. The photon, for instance, was predicted by Einstein in 1905 and observed experimentally by Arthur Compton in 1923, using x-rays scattered from atomic electrons. Although neutral currents had been discovered in the mid-1970s, the Ws and Z's were not directly observed until 1983–84, when they were detected in the CERN hadron collider. As mentioned, the gluons were pinned down by 1979.
In this long discussion of the strong force, we should note that we define it as the quark-quark force carried by gluons. But what about the "old" strong force between neutrons and protons? We now understand this as the residual effects of the gluons, sort of leaking out of the neutrons and protons that bind together in the nucleus. The old strong force that is well described by exchange of pions is now seen as a consequence of the complexities of quark-gluon processes.
END OF THE ROAD?
Entering the 1980s, we had figured out all the matter particles (quarks and leptons), and we had the messenger particles, or gauge bosons, of the three forces (excluding gravity) pretty much in hand. Adding the force particles to the matter particles, you have the complete standard model, or SM. Here, then, is the "secret of the universe":
MATTER
First generation Second generation Third generation
QUARKS
u c t?
d s b
LEPTONS
νe νμ ντ
e μ τ
FORCES
GAUGE BOSONS
electromagnetism photon (γ)
weak force W− W+ Z0
strong force eight gluons
Remember that the quarks come in three colors. So if one is nasty, one can count eighteen quarks, six leptons, and twelve gauge boson force carriers. There is also an and table in which all the matter particles appear as antiparticles. That would give you sixty particles total. But who's counting? Stick to the above table; it's all you need to know. At last we believe we have Democritus's a-toms. They are the quarks and leptons. The three forces and their messenger particles account for his "constant violent motion."
It may seem arrogant to sum up our entire universe in a chart, albeit a messy one. Yet humans appear to be driven to construct such syntheses; "standard models" have been a recurrent theme in Western history. The current standard model wasn't given that name until the 1970s, and the term is peculiar to the recent modern history of physics. But certainly there have been other standard models through the centuries. The next page shows just a few of them.
Why is our standard model incomplete? One obvious flaw is that the top quark hasn't yet been seen. Another is that one of the forces is missing: gravity. No one knows how to work this grand old force into the model. Another aesthetic flaw is that it's not simple enough—it should look more like Empedocles' earth, air, fire, and water plus love and strife. There are too many parameters in the standard model, too many knobs to twiddle.
Which is not to say that the standard model is not one of the great accomplishments of science. It represents the work o
f 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, ne
utrinos 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.
The God Particle: If the Universe Is the Answer, What Is the Question? Page 43
