Colliding-beam accelerators — will they reveal the ultimate particles? (Mar, 1980)
This is pretty cool. The last paragraph talks about looking for the Higgs particle. Guess it didn’t work out.
Colliding-beam accelerators — will they reveal the ultimate particles?
Giant, high-energy devices can help reveal the forces that bind matter together
By PETER GWYNNE
GENEVA, SWITZERLAND The security guard studied our passes carefully.
I was sitting in a car with engineer Vince Hatton at the entrance to a tunnel in the spacious grounds of the Centre Europeen pour la Recherche Nucleaire, known universally by its acronym CERN, in Geneva.
Despite its title, CERN has nothing to do with nuclear power. It is a center for the study of high-energy physics, the science that reveals the fundamental basis of matter. The security guard who stopped us was more concerned with checking passports than flushing out terrorists. For after he approved our papers, and Vince drove the few hundred yards through the tunnel, we emerged in France. CERN and its huge accelerator known as the Super Proton Synchrotron (SPS) stretch across the boundary between Switzerland and France, and the special tunnel allows scientists to move themselves and their equipment easily within the installation without having to pass through the passport and customs posts above ground.
Improbable as the idea of a particle accelerator located in two countries may sound, it is no more unlikely than physicists’ plans for the huge ring-shaped machine. Within three years, they expect to inject into it two beams of elementary particles that will whirl around in opposite directions at almost the speed of light, and then crash into each other time after time at six “crossing points” inside the ring. It’s rather like asking William Tell to fire his arrow, not at an apple, but at another arrow already in flight.
Twentieth-century monuments The CERN scientists aren’t working in isolation. Engineers are refining or constructing multimillion-dollar particle-collision machines at locations ranging from Stanford, Calif., to Hamburg, West Germany. Buried up to 40 feet underground, the machines will eventually appear to latter-day archaeologists as monuments to man’s search for the underlying secrets of nature, just as the ancient stone circles dotted across Ireland testify to the early Celts’ quest for spiritual perfection.
Everything about the particle accelerators is strictly twentieth century. Swathed in brightly painted magnets, the circular cavities that actually carry the particles extend up to four miles, inside tunnels 10 feet tall. Connected to the tunnels are smaller rings that generate the particle beams, and cavernous experimental halls filled with bubble chambers, magnetic detectors, and other huge devices that monitor the beams’ catastrophic crashes. In brightly lit control rooms, engineers order up realtime data on the beams’ behavior on banks of television screens, and order changes in response to computer-controlled alarms.
The purpose of the effort is profound. By monitoring and examining the subnuclear particles that emerge from the collisions between the beams, physicists hope to learn more about the fundamental nature of matter and the four forces (gravity, electromagnetism, and the weak and strong nuclear forces) that govern the universe.
The new accelerators will subject elementary particles to greater forces than any previous manmade machines and will almost certainly, say physicists, reveal fresh and totally unexpected insights into what matter really is. “Historically, every time we’ve had a new energy region to investigate, we’ve seen more of the peculiarities that weren’t observable before,” Phil Livdahl, of the Fermilab National Accelerator Laboratory outside Chicago, told me.
Stretching technology’s limits Attaining the ultrahigh energies requires technological inspiration and achievement of a high order. Engineers and physicists building the new machines are pressing old technology to its limits—and designing new technology that hasn’t yet been tested. New methods of focusing pencil-thin beams of particles, fresh ways of creating ultrahigh vacuums, and totally untried superconducting magnets operating at temperatures close to absolute zero are among the ingredients that will mean success or failure for the new wave of particle accelerators.
The very idea of crunching two beams of particles together contrasts with the more traditional designs of atom smashers. In the past, experimenters shot high-speed, high-intensity beams of protons, electrons, or other particles into solid metallic targets, and monitored the new particles produced by the bombardment. But that process is rather inefficient. The beams use up most of their energy in pushing back the targets. Only a small proportion remains to create the fresh forms of matter that provide tantalizing clues to the basic structure of nature.
At Fermilab, for example, the giant accelerator, two miles in diameter, creates beams of protons with energies of 400 GeV. (GeV stands for giga electron volts, or billions of electron volts. If one billion electron volts of energy were all transmuted into matter, it would produce enough mass to make a proton.) But when the Fermilab beam smashes into its solid target, only 28 GeV is actually available for forming new particles.
Colliding-beam machines, by contrast, work with total efficiency. When two beams collide almost head-on, all their energy goes into creating new particles. So a relatively small machine that imparts just 15 GeV to each of two colliding beams of particles causes collisions involving 30 GeV of energy—more than the monstrous Fermilab device produces with its fixed targets. And as the amount of energy in the beams increases, so do the chances that their collisions will produce the rare and unusual new particles that physicists seek. “There’s a whole new domain of research to come out of this work,” Italian physicist Carlo Rubbia told me at CERN.
The machine builders, and the scientists who will design experiments for the machines, know that they are in a race for glory. The major target for the colliding-beam machines is a particle called the intermediate vector boson. Theoretical physicists think that the particle—or a series of three or more similar particles—is responsible for the weak nuclear force that is involved in some types of radioactive decay. By detecting the so-far-unseen particle, and learning its physical characteristics, the accelerator users could confirm once and for all the theory that won the 1979 Nobel prize in physics. That theory, devised by Steven Weinberg and Sheldon Glashow of Harvard, and Abdus Salam of Imperial College, London, among others, links the weak nuclear force and the electromagnetic force. It predicts that the intermediate vector boson should emerge at energy levels within the capacity of most of the new colliding-beam machines.
Of course, high-energy physics involves much more routine work than spectacular discoveries. Nevertheless, the big finds delight both scientists and administrators. “If CERN can pull off something like that every so often,” CERN director John Adams told me, “it improves the faith of the politicians in us.”
The politicians need to have faith because colliding-beam accelerators cost plenty—and the taxpayers pick up the tab. Among the major machines now under construction, Stanford University’s positron-electron project (PEP) comes in cheapest—at $78 million. The ambitious double- ringed Isabelle (for Intersecting Storage Accelerator) at Long Island’s Brookhaven National Laboratory carries a price tag of over a quarter of a billion dollars.
All the new machines share basic principles and ways of working. Experiments will take place in four stages: injection, acceleration, collision, and detection..
First comes injection. The two thin beams, normally generated in smaller atom smashers, are fired into the accelerators’ main ring or rings. The rings, a few inches in internal diameter, are surrounded by magnets and located in spacious tunnels through which technicians can drive as they trouble-shoot problems. In most cases, the beams will consist of bunches of particles no more than a few feet long. Isabelle engineers, however, expect to generate continuous beams that will girdle each ring in their mammoth machine.
Next, acceleration. The magnets around the tubes will accelerate the beams from starting energies of a few tens of GeV up to hundreds of GeV, while keeping the beams sharply focused.
The job takes two different types of magnets. Dipole magnets (which, as their name implies, consist of a north and a south pole) accelerate the beams and bend them around their ring-shaped tracks. Quadrupole magnets, with two north poles and two souths, prevent the beams from splaying out to hit the sides of the tubes.
Most machines use three or four dipole magnets for each quadrupole— and the largest accelerators require truly spectacular numbers of magnets. The SPS at CERN, for example, contains 744 dipolar magnets and 216 quadrupoles; Isabelle will carry more than one thousand magnets when it’s completed in the middle 1980’s. The magnets will force the beams to make billions of revolutions each day.
Subnuclear rumbles Once the beams are traveling sufficiently fast and energetically, they will smash together. Engineers using computer controls will force them to collide at anywhere from one to six regions around the rings. The head-on crashes will occur over regions up to two feet long and minute fractions of an inch thick—about the diameter of pencil lead.
Finally, complex instruments costing up to $14 million apiece and weighing perhaps thousands of tons will monitor all the particles created by the subnuclear rumbles. Directed by yet more magnets, the products of the collisions will stream out in series of beams into experimental halls where the detectors are mounted.
For all the basic similarities, no two colliding-beam machines are alike. They use different particles, different levels of energy, and different types of magnets to achieve their goal of contributing to high-energy physics—and making the significant finds before their rivals.
Machines that bring together beams of electrons and positrons (the latter being, in effect, electrons with positive, instead of negative, electric charges) have proved quickest off the mark in the high-energy physics stakes. Research teams at the Deutsches Elektronen Synchrotron, known as DESY, have already made some notable discoveries with their $52 million PETRA collider, which slams beams with 19 GeV of energy into each other. Scientists started injecting beams into a segment of Stanford’s PEP, which resembles PETRA, last fall—although the ring remains incomplete. At Cornell University, project head Boyce McDaniel reported that the new Cornell Electron Storage Ring (CESR) “has given very encouraging results” in its early tests.
Because it’s difficult to accelerate electrons, none of those machines generates enough energy to produce the long-sought intermediate vector bo- son. That task will fall to a collection of huge accelerators using protons, which come on line within the next half-dozen years.
Scientists at CERN will undoubtedly take the first crack at the task. They are adapting their Super Proton Synchrotron, which presently slams a single beam of protons into fixed targets, to accept a second beam of anti-protons—the particles whose fundamental properties are the exact opposite of those of protons. “The hardware for the proton-antiproton experiment should he ready by 1981,” said John Adams. “Then,” he added confidently, “it’s just a matter of time before we see the intermediate vector boson.”
U.S. labs giving chase But technical difficulties, scientific problems, or just plain bad luck could throw off that schedule. The teams at Brookhaven and Fermilab haven’t yet given up the chase, although they expect that their machines will be competing with CERN’s to see unexpected new finds rather than the elusive intermediate vector boson. “It’s clear that if Isabelle were coming in two years earlier, CERN wouldn’t have such an opportunity,” lamented Brookhaven physicist Nick Samios.
Isabelle has certain advantages. For a start, it will slam protons into other protons, unlike any other large machine on the drawing board. And the two beams will each possess 400 GeV of energy—appreciably above the 270 GeV per beam planned for the CERN collider and the 80-100 GeV believed necessary to detect the intermediate vector boson. Why such a margin of error? “We chose 400 by 400 because we distrust our theoretical friends,” Samios confided to me.
Even Isabelle won’t be the most energetic colliding-beam machine. That honor will go to Fermilab, which hopes to use a new ring now being installed with superconducting magnets to cause crashes between a beam of protons and another of antiprotons, each carrying an astonishing 1000 GeV of energy. “1984 would be a reasonable date for start-up,” Fermilab scientist Alvin Tollestrup told me.
Energy isn’t everything in high-energy physics, though. Experimenters try not only to make things happen at very high energies, but also to make enough things happen that their instruments will detect the events. The secret is to squeeze as many particles together in the thin regions in which the beams collide. Physicists have coined the term “luminosity” to indicate the number of individual collisions between individual particles in two beams meeting head-on, and ma- chine designers try to raise the luminosities as high as possible. “If you’re looking for a needle in a haystack,” project leader Jim Sanford explained as we drove around the 2V4 miles of land excavated for Isabelle, “it helps to have several needles.”
When completed, in about 1986, Isabelle will create more needles than any other colliding-beam device. Experts expect its luminosity to reach 1033—that’s one followed by 33 zeroes—collisions per square centimeter per second. The figure represents an increase of one thousand over the luminosities forecast for the CERN and Fermilab colliders, and outscores the electron-positron machines by a factor of ten.
Scalpels vs. sledgehammers Electron-positron machines have their own unique advantages. Because both particles are truly fundamental and indivisible, collisions between them tend to yield relatively small numbers of easily detected new particles. Protons and antiprotons, by contrast, have their own substructure; theorists think that they each consist of three of the elementary particles called quarks, linked together by evanescent particles known as gluons. Thus a crash between a proton and an antiproton produces a huge shower of new entities—enough to tax the most sophisticated detector. Collisions between two protons are even more productive.
Because of their particular properties, experimenters plan to use the different particles in different ways. “Electron-positron collisions are like scalpels, and proton-antiproton ones like sledgehammers,” explained Nick Samios as he ran through the list of machines on his blackboard at Brookhaven. Thus physicists expect to make fresh discoveries with proton machines, and then characterize and refine the finds with electron devices. That’s already happened once. In late 1974, teams of researchers at Brook-haven’s alternating-gradient synchrotron, a proton machine, and the two-mile Stanford Linear Accelerator, an electron device, simultaneously spotted a particle that became known as the J/psi (or gypsy). Just what the particle was became clear when the Stanford researchers re-examined it with their electron machine. It plainly contained a new type of quark that had been forecast 10 years previously by Harvard’s Sheldon Glashow.
One of the new electron-positron colliding machines has already notched a major achievement. Last summer, scientists working with Hamburg’s PETRA reported evidence for the existence of gluons. The cleanness of the collision between an electron and a positron made the discovery possible. When those two particles collide, theorists believe, they annihilate each other, converting all the matter into a great burst of energy. Almost immediately, new particles emerge from the cloud, in the form of two quarks. The creation of the pairs of quarks manifests itself as small jets of particles emerging from the collision region as the quarks quickly change into other, more identifiable, particles. Instead of two such jets, however, the PETRA researchers spotted three. The most likely reason: The third jet represented the breakup of gluons, which linked the pairs of quarks produced in the head-on collision.
The thrill of new discovery has eluded other centers of large colliding-beam machines so far. Their engineers face the more mundane concerns of getting the machines to work. Stanford University’s PEP has encountered the most irritating problems. For while new accelerators have a long tradition of starting up earlier than scheduled and coming in under their budgets, PEP is costing more and moving more slowly than expect- ed. Engineers expected to run the first beam around the PEP ring last October. Now, they say, that won’t happen until March.
Difficulties and delays John Rees, who heads the project, lays the blame squarely on American industry. “The vendors are performing in a way that, 20 or 30 years ago, I’d have said was scandalous,” he complained when I toured the still incomplete facility. “There are firms that take advantage of the fact that we’re virtually required to take the low bidder. They bid low to get the job, and then try to improve their profit by claiming in court that the conditions of the contract have been changed.”
Rees complains mainly about well-established technology, such as electrical installations. So it’s not surprising that engineers developing entirely new technology for their machines have also encountered difficulties and delays.
The main challenge is to produce superconducting magnets. Both Fermilab and Brookhaven have opted for this brand-new technology, because it reduces the amount of power required to create the colliders’ magnetic fields by up to 80 percent. But the technology is so novel that it’s scarcely out of the laboratory. “You start from scratch, and you don’t understand anything,” commented Fermilab engineer Tim Toohig.
Both laboratories face problems with their superconducting magnets. At one point, the factory at Fermilab had produced 100 magnets, but could use only 12. Brookhaven had to set up its own magnet factory after industry proved incapable of making the magnets. Even its own products aren’t up to standard. They seem unable to generate magnetic fields higher than 40 kilogauss. Unfortunately, plans for Is- abelle require the magnets to generate 50 kilogauss apiece—100,000 times the Earth’s magnetic field. “The magnets don’t respond in a predictable manner day in and day out,” Jim Sanford told me as we watched technicians wind coils for the magnets to tolerances of a millimeter.
Keeping the magnets frigid enough to operate as superconductors puts engineers in cold sweats. Every one of the more than a thousand magnets ringing Isabelle and the new Fermilab ring must be bathed in liquid helium to keep it within a few degrees of absolute zero, 273 degrees Celsius below freezing. “It turns out that refrigerators of the sort we need, working seven days a week under remote control, just don’t exist,” shrugged Tim Toohig. So Fermilab has jury-rigged a system with a huge central refrigerator that pumps an astonishing 1057 gallons (4000 liters) of liquid helium per minute—a machine that they picked up as surplus from Vandenberg Air Force Base—complemented by 24 smaller refrigerators installed in buildings around the giant accelerator ring.
Administrators at CERN avoided such headaches when they decided to use conventional magnets for their Super Proton Synchrotron, which carried its first beam of protons in May 1976. But they now have to rely on the ingenuity of their scientists to make the spectacular machine into a colliding-beam accelerator.
The problem, shared with Fermilab, is how to create and focus beams of antiprotons. Unlike protons and electrons, these particles can’t be easily generated in large numbers.
Both laboratories plan to make their antiprotons by smashing beams of protons into targets, and then feeding the small number of antiprotons that result into a storage device called an accumulator. In addition to holding the particles until sufficient numbers have built up to feed into the main machine to collide with protons, the accumulators will “cool” the antiprotons—that is, squeeze them into a well-focused beam.
The experts are only just learning how to carry out the cooling. A group at CERN spent nine months last year refining a technique called stochastic cooling. This is a kind of statistical trick that uses sensors to detect the positions of all the antiprotons in the accumulator, and devices called “kickers” to knock out-of-line particles back into the main beam. Fermilab has opted for another approach, “electron cooling,” which involves running a beam of electrons alongside the beam of antiprotons. By removing some energy from the antiprotons, the electrons straighten out the antiproton beam.
Particle riches The technological headaches will undoubtedly continue for many years. But once the machines start up, a surge of scientific discoveries will provide1 perfect analgesics. Physicists will start probing an entirely new region of nature, with only a hazy idea of what they will find there. Beyond the intermediate vector boson may come entities called Higgs particles, predicted by theories that unify the forces of nature. Studies of cosmic rays indicate that extraordinarily energetic particles may exist within detection range of the new machines. In fact, the greatest surprise to physicists would occur if the colliding-beam machines fail to turn up any surprises. The experts uniformly assume that a whole world of new wonders awaits the new generation of atom smashers—and that there will be more than enough riches for everyone.