The God Particle

by Leon Lederman

  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. Th
e 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.

  Rubbia's third front was to keep the CERN authorities enthusiastic, roil the world community, and set the stage for the great W experiment. All of Europe was rooting for this, for it meant the coming of age of European science. One journalist claimed that a failure would crush "popes and prime ministers."

  The experiment got under way in 1981. Everything was in place—UA-1, UA-2, the AA ring—tested and ready. The first runs, designed as checkout trials of everything in the complex system of collider plus detector, were reasonably fruitful. There were leaks, mistakes, accidents, but eventually, data! And all at a new level of complexity. The 1982 Rochester Conference was to be in Paris, and the CERN lab went all out to get results.

  Ironically, UA-2, the afterthought detector, made the first splash by observing jets, the narrow bundles of hadrons that are the signatures of quarks. UA-1, still learning, missed this discovery. Whenever David beats Goliath, everyone except Goliath feels warm. In this case Rubbia, who hates to lose, recognized that the observation of jets was a real triumph for CERN—that all of the effort in machines, detectors, and software had paid off in a strong indicator. It all worked! If jets were seen, W's were soon.

  A RIDE ON NO. 29

  Perhaps a fantastic voyage can best illustrate the way detectors work. Here I will switch over to the CDF detector at Fermilab because it is more modern than UA-1, although the general idea of all the "four pi" detectors is the same. (Four pi—4π—means that the detector completely surrounds the point of collision.) Remember that when a proton and an antiproton collide, a spray of particles comes off in all directions. On the average, one third are neutral, the rest charged. The task is to find out exactly where each particle goes and what it does. As with any physical observation, one is only partially successful.

  Let's ride on one particle. Say it's track No. 29. It zips out at some angle to the line of the collision, encounters the thin metal wall of the vacuum vessel (the beam tube), zips through this, no sweat, and for the next twenty or so inches passes through a gas containing an immense number of very thin gold wires. Although there is no sign, this is Charpak territory. The particle may pass close to forty or fifty of these wires before reaching the end of the tracking chamber. If the particle is charged, each nearby wire records its passage, together with an estimate of how close it came. The accumulated information from the wires defines the particle's path. Since the wire chamber is in a strong magnetic field, the charged particle's path is curved, and a measurement of this curve, calculated by the on-board computer gives the physicist the momentum of particle No. 29.

  Next the particle passes through the cylindrical wall defining the magnetic wire chamber and passes into a "calorimeter sector" which measures particle energy. Now the particle's subsequent behavior depends on what it is. If it is an electron, it fragments on a series of closely spaced thin lead plates, giving up its entire energy to sensitive detectors that provide the meat for the lead sandwiches. The computer notes that the progress of No. 29 ceases after three or four inches of lead-scintilla tor calorimeter and concludes: electron! If, however, No. 29 is a hadron, it penetrates ten to twenty inches of calorimeter material before exhausting all of its energy. In both cases the energy is measured and cross-checked against the momentum measurement, determined by the particle's curvature in the magnet. But the computer graciously leaves it up to the physicist to draw a conclusion.

  If No. 29 is a neutral particle, the tracking chamber doesn't record it at all. When it turns up in the calorimeter, its behavior is essentially the same as that of a charged particle. In both cases the particle produces nuclear collisions with calorimeter materials, and the debris produces further collisions undl all the original energy is exhausted. So we can record and measure neutrals, but we can't chart the momentum, and we lose precision in the direction of motion since no track is left in the wire chamber. One neutral particle, the photon, can be easily identified by its relatively quick absorption by the lead, like the electron. Another neutral, the neutrino, leaves the detector entirely, carrying away its energy and its momentum, leaving behind not even a hint of its fragrance. Finally, the muon moves through the calorimeter leaving a small amount of energy (it has no strong nuclear collision). When it emerges, it finds some thirty to sixty inches of iron, through which it passes only to find a muon detector—wire chambers or scintillation counters. This is how muons are tagged.

  One does all this for forty-seven particles, or whatever the number is, in this one particular event. The system stores the data, close to one million bits of information—equivalent to the' amount of information in a hundred-page book—for each event. The data collection system must quickly decide whether this event is interesting or not; it must discard or record the event or pass the data into a "buffer" memory and clear all registers in order to be ready for the next event. This arrives on the average of a millionth of a second later if the machine is working very well. In the most recent full run at the Tevatron (1990–91), the total amount of information stored on magnetic tape of the CDF detector was equivalent to the text of one million novels or five thousand sets of Encyclopaedia Britannica.

  Among the outgoing particles are some with very short lifetimes. These may move only a few tenths of an inch away from the collision point in the beam tube before spontaneously disintegrating. W's and Z's are so short lived that their flight distance is unmeasurable, and one must identify their existence from measurements on the particles to which they give rise. These are often hidden among the debris that typically flies out of each collision. Since the W is massive, the decay products have higher than average energy, which helps locate them. Such exotics as a top quark or a Higgs particle will have a set of expected decay modes that must be extra
cted from the mess of emerging particles.

  The process of converting enormous numbers of electronic data bits to conclusions about the nature of the collisions takes impressive efforts. Tens of thousands of signals have to be checked and calibrated; tens of thousands of lines of code must be inspected and verified by looking at events that have to "make sense." Small wonder that it takes a battalion of highly skilled and motivated professionals (even though they may officially be classified as graduate students or postdocs) armed with powerful work stations and well-honed analysis codes two or three years to do justice to the data collected in a Tevatron collider run.

  TRIUMPH!

  At CERN, where collider physics was pioneered, it all worked, validating the design. In January 1983, Rubbia announced W's. The signal was five clear events that could be interpreted only as the production and subsequent disintegration of a W object.

  A day or so later UA-2 announced that it had four additional events. In both cases, the experimenters had to sort through about one million collisions that produced all manner of nuclear debris. How does one convince oneself as well as the multitude of skeptics? The particular W decay most conducive to discovery is W+ → e+ + neutrino, or W− → e− + antineutrino. In a detailed analysis of this kind of event one has to verify (1) that the single observed track is indeed an electron and not anything else, and (2) that the electron energy adds up to about half the mass of the W. The "missing momentum," which the invisible neutrino carries off, can be deduced by adding up all the momentum seen in the event and comparing it to "zero," which is the momentum of the initial state of colliding particles. The discovery was greatly facilitated by the lucky accident that W's are made almost at rest under the CERN collider parameters. To discover a particle, lots of constraints must be satisfied. An important condition is that all the candidate events yield the same value (within allowable measurement errors) for the W mass.