To reach the energies required to produce massive particles such as the W or Z bosons, two opposing beams of particles must collide. In this case the total center-of-mass energy is simply twice the energy of each beam. If each colliding beam of particles has an energy of one hundred times the rest mass of a proton, this then yields 200 GeV of energy to be converted into the mass of new particles.
Why, then, produce accelerators with stationary targets and not colliders? The answer is quite simple. If I am shooting a bullet at a barn door, I am more or less guaranteed to hit something. If I shoot a bullet at another incoming bullet, however, I’d have to be a much better shot than probably anyone else alive and have a better gun than any now made to be guaranteed to hit it.
This was the challenge facing experimentalists in 1976, by which time they took the electroweak model seriously enough that they thought it worth the time, effort, and money to try to test it.
But no one knew how to build a device with the appropriate energy. Accelerating individual beams of particles or antiparticles to high energies had been achieved. By 1976 protons were being accelerated to 500 GeV, and electrons up to 50 GeV. At lower energies, collisions of electrons and their antiparticles had successfully been carried out, and this is how the new particle containing the charmed quark and antiquark had been discovered in 1974.
Protons, having greater mass and thus more rest energy initially, are easier to accelerate to high energies. In 1976 a proton accelerator at the European Organization for Nuclear Research (CERN) in Geneva, the Super Proton Synchrotron (SPS), had just been commissioned as a conventional fixed-target accelerator operating with a proton beam at 400 GeV. However, another accelerator at Fermilab, near Chicago, had already achieved proton beams of 500 GeV by the time the SPS turned on. In June of that year, physicists Carlo Rubbia, Peter McIntyre, and David Cline made a bold suggestion at a neutrino conference: converting the SPS at CERN into a machine that collided protons with their antiparticles—antiprotons—would allow CERN to potentially produce W’s and Z’s.
Their bold idea was to use the same circular tunnel to accelerate protons in one direction, and antiprotons in another. Since the two particles have opposite electric charges, the same accelerating mechanism would have opposite effects on each particle. So a single accelerator could in principle produce two high-energy beams circulating in opposite directions.
The logic of such a proposal was clear, but its implementation was not. In the first place, given the strength of the weak interaction, the production of even a few W and Z particles would require the collision of hundreds of billions of protons and antiprotons. But no one had ever produced and collected enough antiprotons to make an accelerator beam.
Next, you might imagine that with two beams traversing the same tunnel in opposite directions, particles would be colliding all around the tunnel and not in the detectors designed to measure the products of the collisions. However, this is far from the case. The cross section of even a small tunnel compared to the size of the region over which a proton and an antiproton might collide is so huge that the problem is quite the opposite. It seemed impossible to produce enough antiprotons and ensure that both they and the protons in the proton beam would be sufficiently compressed so that when the two beams were brought together, steered by powerful magnets, any collisions at all would be observed.
Convincing the CERN directorate to transform one of the world’s most powerful accelerators, built in a circular tunnel almost eight kilometers around at the French-Swiss border, into a new kind of collider would have been difficult for many people, but Carlo Rubbia, a bombastic force of nature, was up to the task. Few people who got in Rubbia’s way were likely to be happy about it afterward. For eighteen years he jetted every week between CERN and Harvard, where he was a professor. His office was two floors down from mine, but I knew when he was in town because I could hear him. Moreover, Rubbia’s idea was good, and in promoting it he was really suggesting to CERN that the SPS move up from an “also-ran” machine to the most exciting accelerator in the world. As Sheldon Glashow said to the CERN directorate when encouraging them to move forward, “Do you want to walk, or do you want to fly?”
Still, to fly one needs wings, and the creation of a new method to produce, store, accelerate, and focus a beam of antiprotons fell to a brilliant accelerator physicist at CERN, Simon van der Meer. His method was so clever that many physicists who first heard about it thought it violated some fundamental principles of thermodynamics. The properties of the particles in the beam would be measured at one place in the circular tunnel, then a signal would be sent for magnets farther down the tunnel to give many small kicks over time to the particles in the beam as they passed by, thus slightly altering the energies and momenta of any wayward particles so that they would eventually all get focused into a narrow beam. The method, called stochastic cooling, helped make sure particles that were wandering away from the center of the beam would be sent back into the middle.
Together van der Meer and Rubbia pushed forward, and by 1981 the collider was working as planned, and Rubbia assembled the largest physics collaboration ever created and built a large detector capable of sorting through billions of collisions of protons and antiprotons to search for a handful of possible W and Z particles. Rubbia’s team was not the only one hunting for a W and a Z, however. Another detector collaboration had been assembled and was also built at CERN. Redundancy for such an important observation seemed appropriate.
Unearthing a signal from the immense background in these experiments was not easy. Remember that protons are made of more than one quark, and in a single proton-antiproton collision a lot of things can happen. Moreover, the W’s and Z’s would not be observed directly, but via their decays—in the case of the W, into electrons and neutrinos. Neutrinos would not be directly observed, either. Rather the experimentalists would tally up the total energy and momentum of each outgoing particle in a candidate event and look for large amounts of “missing energy,” which would signal that a neutrino had been produced.
By December 1982, a W candidate event had been observed by Rubbia and his colleagues. Rubbia was eager to publish a paper based on this single event, but his colleagues were more cautious, for good reason. Rubbia seemed to have a history of making discoveries that weren’t always there. In the meantime he leaked details of the event to a number of colleagues around the world.
Over the next few weeks his “UA1” collaboration obtained evidence for five more W candidate events, and the UA1 physicists designed several far more stringent tests to ascertain with high confidence that the candidates were real. On January 20, 1983, Rubbia presented a memorable and masterful seminar at CERN announcing the result. The standing ovation he received made it clear that the physics community was convinced. A few days later Rubbia submitted a paper to the journal Physics Letters announcing the discovery of six W events. The W had been discovered with precisely the predicted mass.
The search was not over, however. The Z remained to be seen. Its predicted mass was slightly higher than that of the W, and its signal was therefore slightly harder to obtain. Nevertheless, within a month or so of the W announcement, evidence for Z events began to come in from both experiments, and on the basis of a single clear event, on May 27 that year Rubbia announced its discovery.
The gauge bosons of the electroweak model had been found. The significance of these discoveries for solidifying the empirical basis of the Standard Model was underscored when, just slightly over a year after making the announcement, Rubbia and his accelerator colleague van der Meer were awarded the Nobel Prize in Physics. While the teams that had built and operated both the accelerator and the detectors were huge, few could deny that without Rubbia’s drive and persistence and van der Meer’s ingenious invention the discovery would not have been possible.
One big Holy Grail now remained: the purported Higgs particle. Unlike the W and Z bosons, the mass of the Higgs is not fixed by the theory. Its couplings to matter a
nd to the gauge bosons were predicted, as these couplings allow the background Higgs field that presumably exists in nature to break the gauge symmetry and give mass not to just the W and the Z, but also to electrons, muons, and quarks—indeed to all the fundamental particles in the Standard Model save the neutrino and the photon. However, neither the Higgs particle mass nor the strength of its self-interactions was separately determined in advance by then existing measurements. Only their ratio was fixed by the theory in terms of the measured strength of the weak interaction between known particles.
Given conservative estimates of the possible magnitude of the Higgs self-interaction strength, the Higgs particle mass was conservatively estimated to lie within a range of 2 to 2,000 GeV. What set the upper limit was that, if the Higgs self-coupling is too big, then the theory becomes strongly interacting and many of the calculations performed using the simplest picture of the Higgs break down.
Aside from their necessary role in breaking the electroweak symmetry and giving other elementary particles masses, these quantitative details were therefore largely undetermined by experiments up to that time—which is probably why Sheldon Glashow in the 1980s referred to the Higgs as the “toilet” of modern physics. Everyone was aware of its necessary existence, but no one wanted to talk about the details in public.
That the Standard Model didn’t fix in advance many of the details of the Higgs field didn’t dissuade many theorists from proposing models that “predicted” the Higgs mass based on some new theoretical ideas. In the early 1980s, each time accelerators increased their energies, new physics papers would come out predicting a Higgs would be discovered when the machine was turned on. Then a new threshold would be reached, and nothing would be observed. To explore all the available parameter space to see if the Higgs existed, a radically new accelerator would clearly have to be built.
I was convinced during all this time that the Higgs didn’t exist. The spontaneous symmetry breaking of the electroweak gauge symmetry did certainly occur—the W and the Z exist and have mass—but adding a fundamental new scalar field designed by recipe specifically to perform this task seemed contrived to me. First, no other fundamental scalar field had ever been observed to exist in nature’s particle menagerie. Second, I felt that with all of the unknown physics yet to be discovered at small scales, nature would have developed a much more ingenious and unexpected way of breaking the gauge symmetry. Once one posits the Higgs particle, then the next obvious question is “Why that?” or more specifically “Why just the right dynamics to cause it to condense at that scale, and with that mass?” I thought that nature would find a way to break the theory in a less ad hoc fashion, and I expressed this conviction fairly strongly when I was interviewed for my eventual position at the Society of Fellows at Harvard after getting my PhD.
Let’s recall now what the existence of the Higgs implies. It requires not just a new particle in nature but an invisible background field that must exist throughout all of space. It also implies that all particles—not just the W and the Z particles but also electrons and quarks—are massless in the fundamental theory. These particles that interact with the Higgs background field then experience a kind of resistance to their motion that slows their travel to less than the speed of light—just as a swimmer in molasses will move more slowly than a swimmer in water. Once they are moving at sub-light-speed, the particles behave as if they are massive. Those particles that interact more strongly with this background field will experience a greater resistance and will act as if they are more massive, just as a car that goes off the road into mud will be harder to push than if it were on the pavement, and to those pushing it, it will seem heavier.
This is a remarkable claim about the nature of reality. Remembering that in superconductors the condensate that forms is a complicated state of bound pairs of electrons, I was skeptical that things would work out so much more simply and cleanly on fundamental scales in empty space.
So how to explore such a remarkable claim? We use the central property of quantum field theory that was exploited by Higgs himself when he proposed his idea. For every new field in nature, at least one new type of elementary particle must exist with that field. How, then, to produce the particles if such a background field exists throughout space?
Simple. We spank the vacuum.
By this I mean that if we can focus enough energy at a single point in space, we can excite real Higgs particles to emerge and be measured. One can picture this as follows. In the language of elementary particle physics, using Feynman diagrams, we can think of a virtual Higgs particle emerging from the background Higgs field, giving mass to other particles. The left diagram corresponds to particles such as quarks and electrons scattering off a virtual Higgs particle and being deflected, thus experiencing resistance to their forward motion. The right diagram represents the same effect for particles such as the W and the Z.
We can then simply turn this picture around:
In this case energetic particles such as W’s and Z’s or quarks and/or antiquarks or electrons and/or positrons appear to emit virtual Higgs particles and recoil. If the energies of the incoming particles are large enough, then the emitted Higgs could be a real particle. If they aren’t, the Higgs would be a virtual particle.
Now remember that if the Higgs gives mass to particles, then the particles it interacts with most strongly will be the particles that get the largest masses. In turn this means that the particles most likely to spit out a Higgs are the incident particles with the heaviest masses. That means that light particles such as electrons are probably not a good bet to directly create Higgs particles in an accelerator. Instead we can imagine creating an accelerator with enough energy so that we can create heavy virtual particles that will spit out Higgs particles, either virtual or real.
The natural candidates are then protons. Build an accelerator or a collider starting with protons and accelerate them to high enough energies to produce enough virtual heavy constituents so as to produce Higgs particles. The Higgs particles, virtual or real, being heavy, will quickly decay into the lighter particles that the Higgs interacts with most strongly—once again either the top or bottom quarks or W’s and Z’s. These will in turn decay into other particles.
The trick would be to consider events with the smallest number of outgoing particles that could be cleanly detected, then determine their energies and momenta precisely and see if one could reconstruct a series of events traceable to a single massive intermediate particle with the predicted interactions of a Higgs particle. No small task!
These ideas were already clear as early as 1977, even before the discovery of the top quark itself (since the bottom quark had already been discovered, and all the other quarks came in weak pairs—up and down, charm and strange—clearly another quark had to exist, although it took until 1995 to discover it, a whopping 175 times heavier than the proton). But knowing what was required and actually building a machine capable of doing the job were two different things.
Chapter 21
* * *
GOTHIC CATHEDRALS OF THE TWENTY-FIRST CENTURY
The price of wisdom is above rubies.
—JOB 28:18
Accelerating protons to high enough energies to explore the full range of possible Higgs masses was well beyond the capabilities of any machine in 1978—when all the other predictions of the electroweak theory were confirmed—or in 1983 when the W and the Z had been discovered. An accelerator at least an order of magnitude more powerful than the most powerful machine then in existence was required. In short, not a collider, but a supercollider.
The United States, which for the entire period since the end of the Second World War had dominated science and technology, had good reason to want to build such a machine. After all, CERN in Geneva had emerged by 1984 as the dominant particle physics laboratory in the world. American pride was so hurt when both the W and the Z particles were discovered at CERN that six days after the press conference announcing the Z discovery, the Ne
w York Times published an editorial titled “Europe 3, U.S. Not Even Z-Zero”!
Within a week after the Z discovery, American physicists decided to cancel construction of an intermediate-scale accelerator in Long Island and go for broke. They would build a massive accelerator with a center-of-mass energy almost one hundred times greater than the CERN SPS machine. To do so they would need new superconducting magnets, and so their brainchild was named the Superconducting Super Collider (SSC).
After the project was proposed by the US particle physics community in 1983, the traditional scramble proceeded among many different states to get a piece of the enormous fiscal pie for its construction and management. After much political and scientific wrangling a site just south of Dallas, Texas, in Waxahachie, was chosen. Whatever the motivation, Texas seemed particularly appropriate, as everything about the project, which was approved in 1987 by President Reagan, was supersize.
The huge underground tunnel would have been eighty-seven kilometers around, the largest tunnel ever constructed. The project would be twenty times bigger than any other physics project ever attempted. The proposed energy of collisions, with two beams each having an energy twenty thousand times the mass of the proton, would be about one hundred times larger than the collision energy of the machine at CERN that had discovered the W and the Z. Ten thousand superconducting magnets, each of unprecedented strength, would have been required.
Cost overruns, lack of international cooperation, a poor US economy, and political machinations eventually led to SSC’s demise in October 1993. I remember the time well. I had recently moved from Yale to become chair of the Physics Department at Case Western Reserve University, with a mandate to rebuild the department and hire twelve new faculty members over five years. The first year we advertised, in 1993–94, we received more than two hundred applications from senior scientists who had been employed at the SSC and who were now without a job or any prospects. Many of them were very senior, having left full professorships at distinguished universities to spearhead the effort. It was sad, and more than half of those people had to leave the field altogether.
The Greatest Story Ever Told—So Far Page 24
