BUBBLE, BUBBLE, TOIL AND TROUBLE
The next advance was the bubble chamber, invented in the mid-1950s by Donald Glaser, then at the University of Michigan. The first bubble chamber was a little thimble of liquid ether. The evolution of liquid hydrogen chambers up to the 15-foot monster, retired from Fermilab in 1987, was led by the famed Luis Alvarez at die University of California.
In a chamber filled with liquid, often liquefied hydrogen, tiny bubbles form along the trail of particles passing through. The bubbles indicate the onset of boiling due to a sudden deliberate lowering of the pressure in the liquid. What this does is put the liquid above the boiling point, which depends on both temperature and pressure. (You may have experienced the difficulty of cooking an egg in your mountain chalet. At the low pressure of mountaintops, water boils well below 100 degrees C.) A clean liquid, no matter how hot, will resist boiling. For example, if you heat some oil in a deep pot above its normal boiling temperature, and if everything is really clean, it won't boil. But toss in a single piece of potato, and explosive boiling takes place. So to produce bubbles, two things are required: temperature above the boiling point and some kind of impurity to encourage the formation of a bubble. In the bubble chamber, the liquid is superheated by the sudden decrease in pressure. The charged particle, in its numerous gentle collisions with atoms of the liquid, leaves a trail of excited atoms that, after the pressure is lowered, are ideal for nudeating the bubbles. If a collision occurs between the incident particle and a proton (nucleus of hydrogen) in the vat, all the emerging charged products are also rendered visible. Since the medium is a liquid, dense plates are not necessary, and the collision point can be seen clearly. Researchers around the world took millions of photographs of collisions in bubble chambers, their analysis aided by automated scanners.
So here is how it works. The accelerator shoots a beam of particles toward the bubble chamber. If this is a charged particle beam, ten or twenty tracks begin to crowd the chamber. Within a millisecond or so after the passage of the particles, a piston is rapidly moved, lowering the pressure and thereby beginning the formation of bubbles. After another millisecond or so of growth time, a light is flashed, film is moved, and we are ready for another cycle.
It is said that Glaser (who won the Nobel Prize for his bubble chamber and promptly became a biologist) got his idea for nucleation of bubbles by studying the trick of increasing the head on a glass of beer by adding salt. The bars of Ann Arbor, Michigan, thus spawned one of the more successful instruments used to track the God Particle.
There are two keys to collision analysis: space and time. We would like to record a particle's trajectory in space and its precise time of passage. For example, a particle comes into the detector, stops, decays, and gives rise to a secondary particle. A good example of a stopping particle is a muon, which can decay into an electron, separated in time by a millionth or so of a second from the stopping event. The more precise your detector, the more information. Bubble chambers are excellent for space analysis of the event. The particles leave tracks, and in bubble chambers we can locate points on those tracks to an accuracy of about 1 millimeter. But they provide no time information.
Scintillation counters can locate particles in both space and time. Made of special plastics, they produce a flash of light when struck by a charged particle. The counters are wrapped in light-tight black plastic, and each tiny light flash is funneled to an electronic photomultiplier that converts the signal, indicating passage of a panicle, into a sharply defined electronic pulse. When this pulse is superimposed on an electronic train of clock pulses, the arrival of a particle can be recorded to a precision of a few billionths of a second. If a number of scintillation strips are used, a particle will strike several in succession, leaving a series of pulses that describe its path in space. The space location depends on the size of the counter, which typically establishes the location to a precision of a few inches.
A major breakthrough was the proportional wire chamber (PWC), the invention of a prolific Frenchman working at CERN, Georges Charpak. A World War II hero of the Resistance and a concentration camp prisoner, Charpak became the preeminent inventor of particle detector devices. In his PWC, an ingenious, "simple" device, a number of fine wires, only a few tenths of an inch apart, are stretched across a frame. Typically the frame is two feet by four feet, with a few hundred two-foot-long wires strung across the four-foot span. Voltages are organized so that when a particle passes near a wire, it generates an electrical pulse in the wire, and the pulse is recorded. The accurately surveyed location of the struck wire locates one point on the trajectory. The time of the pulse is obtained by comparison with an electronic clock. By further refinements, the space and time definition can be pinpointed to approximately 0.1 millimeters and 10−8 seconds. With many such planes stacked in an airtight box filled with an appropriate gas, one can precisely define the trajectories of particles. Because the chamber is active for only a short interval of time, random background events are suppressed and very intense beams can be used. Charpak's PWCs have been a part of every major particle physics experiment since about 1970. In 1992 Charpak was awarded the Nobel Prize (alone!) for his invention.
All of these different particle sensors and more were incorporated into the sophisticated detectors of the 1980s. The CDF detector at Fermilab is typical of one of the most complex systems. Three stories high, weighing 5,000 tons, and built at a cost of $60 million, it is designed to observe the head-on collisions of protons and antiprotons in the Tevatron. Here some 100,000 sensors, which include scintillation counters and wires in exquisitely designed configurations, feed streams of information in the form of electronic pulses to a system that organizes, filters, and finally records data for future analysis.
As in all such detectors, there is too much information to handle in real time—that is, immediately—so the data are encoded in digital form and organized for recording on magnetic tape. The computer must decide which collisions are "interesting" and which are not, since there are over 100,000 collisions per second in the Tevatron, and this is expected to increase in the early 1990s to one million collisions per second. Now, most of these collisions are of no interest. The jewels are those in which a quark in one proton really smacks an antiquark or even a gluon in the p-bar. These hard collisions are rare.
The information-handling system has less than a millionth of a second to examine a particular collision and make a fateful decision: is this event interesting? To a human this is mind-boggling speed, but not to a computer. It is all relative. In one of the big cities, a turtle was attacked and robbed by a gang of snails. When later questioned by the police, the turtle said: "I don't know. Everything happened so fast!"
To alleviate the electronic decision making, a system of sequential levels of event selection has evolved. The experimenters program the computers with various "triggers," indicators that tell the system which events to record. For example, a common trigger would be an event that discharges a large amount of energy into the detector, for new phenomena are most likely to occur at high rather than low energies. The setting of triggers is a sweaty-palm business. Make them too loose, and you overwhelm the capability and logic of the recording technology. Set them too tight, and you may miss some new physics, or you may have done the entire experiment for nothing. Some triggers will flip "on" when an energetic electron is detected emerging from the collision. Another trigger will be convinced by the narrowness of a jet of particles, and so on. Typically there are ten to twenty different configurations of collision events that are allowed to set off a trigger. The total number of events passed by these triggers may be 5,000 to 10,000 in a second, but now the event rate is low enough (one every ten-thousandth of a second) to "think" and examine—er, have the computer examine—the candidates more carefully. Do you really want to record this event? The screening goes on through four or five levels until it gets down to about ten events per second.
Each of these events is recorded on magne
tic tape in full detail. Often, at the stages where we are rejecting events, a sampling of, say, one in a hundred is recorded for future study to determine if important information is being lost. The entire data acquisition system (DAQ) is made possible by an unholy alliance of physicists who think they know what they want, clever electronic engineers who try hard to please, and, oh yes, a revolution in commercial microelectronics based on the semiconductor.
The geniuses in all of this technology are too numerous to list, but in my subjective view, one of the leading innovators was a shy electronics engineer who functioned in a garret at Columbia's Nevis Lab, where I grew up. William Sippach was way ahead of his physicist controllers. We specified; he designed and built the DAQ. Time and again I would telephone him at three in the morning crying that we'd come up against a serious limitation in his (it was always his when we had trouble) electronics. He would listen quietly and ask a question: "Do you see a microswitch inside the cover plate of rack sixteen? Activate it and your problem will be solved. Good night." Sippach's fame spread, and in a typical week, visitors from New Haven, Palo Alto, Geneva, and Novosibirsk would drop in to talk to Bill.
Sippach and the many others who helped develop these complex systems continue a great tradition that began in the 1930s and '40s when the circuits for the early particle detectors were invented. These in turn become the key ingredients in the first generation of digital computers. These, in turn, begat better accelerators and detectors, which begat...
The detectors are the bottom line in this whole business.
WHAT WE FOUND OUT: ACCELERATORS AND PHYSICS PROGRESS
You now know everything you need to know about accelerators—perhaps more. You may in fact know more than most theorists. This is not a criticism, just a fact. More important is what these new machines told us about the world.
As I've mentioned, the synchrocyclotrons of the 1950s enabled us to learn much about pions. Hideki Yukawa's theory suggested that by exchanging a particle with a particular mass, one could create a strong attractive counterforce that would bind protons to protons, protons to neutrons, and neutrons to neutrons. Yukawa predicted the mass and lifetime of this particle being exchanged: the pion.
The pion has a rest mass energy of 140 MeV, and it was produced prolifically in the 400 to 800 MeV machines on university campuses around the world in the 1950s. Pions decay into muons and neutrinos. The muon, which was the great puzzle of the 1950s, seemed to be a heavier version of the electron. Richard Feynman was one of the prominent physicists who agonized over two objects that behave in all respects identically, except that one weighs two hundred times as much as the other. The unraveling of this mystery is one of the keys to our entire thrust, a clue to the God Particle itself.
The next generation of machines produced a generational surprise: hitting the nucleus with billion-volt particles was doing "something different." Let's review what you can do with an accelerator, especially since the final exam is coming soon. Essentially, the vast investment in human ingenuity described in this chapter—the development of the modern accelerator and particle detector—allows us to do two kinds of things: to scatter objects or—and this is the "something different"—to produce new objects.
1.Scattering. In scattering experiments we look at how incident particles after collision fly off in various directions. The technical term for the end product of a scattering experiment is angular distribution. When analyzed according to the rules of quantum physics, these experiments tell us a good deal about the nucleus that is scattering the particles. As the energy of the incoming particle from the accelerator increases, the structure comes into better focus. So we learned about the composition of nuclei—neutrons and protons and how they are arranged and how they jiggle around to maintain their arrangement. As we further increase the energy of our protons, we can "see" into the protons and the neutrons. Boxes inside boxes.
To make things simple, we can use single protons (hydrogen nuclei) as targets. Scattering experiments told us about the proton's size and about how the positive electric charge is distributed. A clever reader will ask whether the probe—the particle hitting the target—itself contributes to the confusion, and the answer is yes. So we use a variety of probes. Alpha particles from radiation gave way to protons and electrons fired from accelerators, and later we used secondary particles: photons derived from electrons, pions derived from proton-nucleus collisions. As we got better at doing this in the 1960s and '70s, we began using tertiary particles as the bombarding particles; muons from decays of pions became probes, as did neutrinos from the same source, and lots more.
The accelerator laboratory became a service center with a variety of products. By the late 1980s, Fermilab's sales force advertised to potential customers that the following hot and cold running beams were available: protons, neutrons, pions, kaons, muons, neutrinos, antiprotons, hyperons, polarized protons (all spinning in the same direction), tagged photons (we know their energy), and if you don't see it, ask!
2.Producing new particles. Here the object is to see if a new energy domain results in the creation of new, never-before-seen particles. If there is a new particle, we want to know everything about it—its mass, spin, charge, family, and so on. We also need to know its lifetime and what other particles it decays into. Of course, we have to know its name and what role it plays in the great architecture of the particle world. The pion was discovered in cosmic rays, but soon we found that it doesn't spring fully grown from the forehead of cloud chambers. What happens is that cosmic-ray protons from outer space enter the earth's atmosphere where they collide with nuclei of nitrogen and oxygen (today we also have more pollutants), and out of these collisions pions are created. A few other weird objects were also identified in cosmic-ray studies, such as particles called K+ and K− and objects called lambda (the Greek letter Λ). When more powerful accelerators took over starting in the mid-1950s and then with a vengeance in the 1960s, various exotic particles were created. The trickle of new objects soon became a flood. The huge energies available in collisions uncovered the existence of not one or five or ten but hundreds of new particles, undreamt of in most of our philosophies, Horatio. These discoveries were group efforts, the fruits of Big Science and a mushrooming of technologies and techniques in experimental particle physics.
Each new object was given a name, usually a Greek letter. The discoverers, typically a collaboration of sixty-three and a half scientists, would announce the new object and give as many of its properties—mass, charge, spin, lifetime, and a long list of additional quantum properties—as were known. They would then pass Go, collect two hundred dollars, write up a thesis or two, and wait to be invited to give seminars, conference papers, be promoted, all of that, Most of all, they were eager to follow up and to make sure others confirmed their results, preferably using some other technique so as to minimize instrumental biases. That is, any particular accelerator and its detectors tend to "see" events in a particular way. One needs to have the event confirmed by a different set of eyes.
The bubble chamber served as a powerful technique for discovering particles since many of the details of a close encounter could be seen and measured. Experiments using electronic detectors were generally aimed at more specific processes. Once a particle had made it to the list of confirmed objects, one could design specific collisions and specific devices to provide data on other properties, such as its lifetime—all the new particles were unstable—and decay modes. Into what does it disintegrate? A lambda decays into a proton and a pion; a sigma decays into a lambda and a pion; and so on. Tabulate, organize, try not to be overwhelmed by the data. These were the guidelines to sanity as the subnuclear world exposed deeper and deeper complexity. Collectively all the Greek-letter particles created in strong-force collisions were called hadrons —Greek for heavy—and there were hadrons by the hundreds, literally. This was not what we wanted. Instead of a single, tiny, uncuttable particle, the search for the Democritan a-tom had turned up hundreds of heavy, ve
ry cuttable particles. Disaster! We learned from our biology colleagues what to do when you don't know what to do: classify! And this we did with abandon. The results—and consequences—of this classification are taken up in the next chapter.
THREE FINALES: TIME MACHINE, CATHEDRALS, AND THE ORBITING ACCELERATOR
We close this chapter with a new view of what actually happens in accelerator collisions. This view comes to us courtesy of our colleagues in astrophysics. (There is a small but very funny group of astrophysicists ensconced at Fermilab.) These people assure us—and we have no reason to doubt them—that the world was created about 15 billion years ago in a cataclysmic explosion, the Big Bang. In the earliest instants after creation, the infant universe was a hot, dense soup of primordial particles colliding with one another with energies (equivalent to temperatures) vastly higher than anything we can imagine reproducing, even with acute megalomania, double time. But the universe is cooling as it expands. At some point, about 10−12 seconds after creation, the average energy of the particles in the hot universe soup was reduced to 1 trillion electron volts, or 1 TeV, about the same energy that Fermilab's Tevatron produces in each beam. Thus we can look at accelerators as time machines. The Tevatron replicates, for a brief instant during head-on collisions of protons, the behavior of the entire universe at age "a millionth of a millionth of a second." We can calculate the evolution of the universe if we know the physics of each epoch and the conditions handed to it by the previous epoch.
The God Particle: If the Universe Is the Answer, What Is the Question? Page 32
