by David Kaiser
Late in November 2009, the laboratory team celebrated a new world record: they had achieved the highest-energy particle interactions ever recorded in an Earth-bound accelerator, edging past the previous record set by the smaller machine at Fermilab. Even that record-setting energy, however, remained roughly ten times lower than the anticipated peak energy for which the LHC had been designed. Once again bitter disappointment quickly eclipsed the momentary cheer. The lab announced in spring 2010 that it would be able to operate the LHC at only half capacity until the end of 2011, before taking the entire machine off-line for a new round of costly repairs. The culprit again appeared to be electrical shielding around the delicate superconducting magnets. A few years later—in an encounter that Aesop surely would have relished—a tiny weasel shut the monstrous machine down after scaling a fence, gnawing through some electrical cables, and receiving a jolt of 18,000 volts.12
Weasels notwithstanding, the LHC has flourished at CERN while the unfinished SSC recedes further into memory. When Congress halted funding for the SSC in October 1993, the federal government had already spent $2 billion on the project and excavated nearly fifteen miles of underground tunnel; Congress had to appropriate another $1 billion just to cover shutdown costs. The Cold War model, which had once seemed so self-evident to scientists and policymakers—that huge research projects would be funded to ensure that generations of scientists would be well trained and at the ready, in case the Cold War ever turned hot—had at last run its course. CERN, on the other hand, had been founded in 1954 on a rather different premise. Its purpose was to bring together scientists from many countries across Europe, despite at-times considerable political differences, to provide a platform for international collaboration. From the start, projects at CERN have been shielded from any whiff of potential military relevance, let alone classified research.13 It was hardly obvious, back in the early 1950s, that European governments would pay such large sums to build enormous particle accelerators—but once that commitment had been made, the arguments undergirding CERN’s efforts proved more adaptable to the political realities of a post-Soviet world.
Today, even multinational projects like a next-generation LHC face enormous hurdles, given the colossal price tags required to smash particles together at still-higher energies. And so particle physicists’ hopes focus, at least for now, on the frozen vacuum of the LHC, buried deep underground, as they wonder whether the massive machine will be the last of its kind.
11
Something for Nothing
Just after New Year’s Day, early in 1964, Murray Gell-Mann submitted a short paper to the journal Physics Letters that forever changed physicists’ lexicon. He had been trying to make sense of curious patterns in the masses and interactions of dozens of newly discovered particles. In his brief, two-page article, he suggested that these exotic nuclear particles—as well as more familiar particles, like protons and neutrons—might themselves consist of smaller particles. Lifting a nonsense word from James Joyce’s Finnegans Wake, Gell-Mann called the new particles “quarks.” (Joyce’s novel appears as Ref. 6 in Gell-Mann’s brief reference list.) At nearly the same time, George Zweig at CERN wrote a lengthy paper introducing the same basic idea; he called the hypothetical entities “aces.” Gell-Mann’s paper was published within three weeks of submission; five years later, the Nobel Prize committee cited the work when awarding Gell-Mann the physics prize. Meanwhile, Zweig’s paper was rejected; neither that paper nor a longer, follow-up study ever made it into print. And so it was that physicists around the world added the jabberwockian term “quark” to their everyday speech.1
Figure 11.1. Murray Gell-Mann introduced the term “quark” into physicists’ lexicon early in 1964 and helped build particle physicists’ Standard Model of elementary particles and interactions. (Source: AIP Emilio Segrè Visual Archives, Physics Today Collection.)
Ten years after Gell-Mann pirated Joyce, physicists had developed a full-fledged theory of how quarks behave. More than that: they had learned how to combine the new description of quarks with other, fresh ideas about the particles and forces that roil the nuclear realm. A product of many authors, the new coagulation of ideas became known simply as the “Standard Model”—a far cry from Gell-Mann’s whimsical and idiosyncratic terminology, reflecting instead its composition by committee. The Standard Model describes the forces and interactions among all the known subatomic particles, from quarks and electrons to their most exotic cousins, which are seen only in carefully controlled experiments. For nearly half a century, the model has served as both compass and polestar, enabling physicists to navigate complicated experimental results and guiding new inquiries. The Standard Model has been subjected to high-precision tests since the 1980s; nearly all the tests have shown exquisite agreement with predictions. (The only tantalizing discrepancy to date concerns neutrinos, whose tiny but nonzero masses had not been incorporated into the original model.) Even the long-elusive Higgs boson—the last hypothetical particle described by the model to be detected—provided a close match to predictions. The Standard Model is almost certainly the most boringly titled exciting development in the history of science.2
For all its successes, however, physicists agree that the Standard Model cannot be the final word. For one thing, it suffers from a series of arbitrary, unexplained features. Why does one particle, the muon—otherwise so similar to the electron—happen to weigh precisely 206.7683 times more than its lightweight sibling? Why do two particular interactions have strengths in the ratio 0.23120 rather than, say, 1 or 0.25 or 17? By sticking those parameters in by hand, physicists can match experiments to extraordinary accuracy. But accounting for why those values must be put in, just so, remains an open question. For several decades, high-energy physicists have striven to account for those parameters in a first-principles sort of way—that is, to develop some larger framework into which the Standard Model might be subsumed, in terms of which the arbitrary features might appear natural, even necessary. Beyond the arbitrary parameters, meanwhile, most physicists consider the Standard Model glaringly incomplete. It incorporates three of the four basic forces of nature: the forces that cause electric charges to attract or repel; that cause nuclear particles to clump densely into atomic nuclei; and that cause some of those nuclei to disintegrate via radioactivity. But the Standard Model has nothing at all to say about gravity, which, on cosmic scales, is by far the most important force of all.
Most of the efforts to redress these shortcomings—to rectify the arbitrariness of the existing Standard Model and to glean some way to smuggle gravity in—focus on symmetry. Symmetry means that a system remains unchanged even as you shake it up or twist it around or, as physicists say, perform a transformation. Imagine playing a Bach fugue on the piano. Unbeknownst to you, some mischievous gremlins have shifted your keyboard up by a major third. Every time you play what looks like middle C, the piano key’s hammer actually strikes the strings for E; you hit a D but the hammer sounds an F-sharp, and so on. If the gremlins’ actions affect each note in the same way, independent of their position on the keyboard—and if they don’t change the rules over time—then they have performed a “global transformation.” The relative intervals between notes have remained intact, but the piece is not truly unchanged. Someone with perfect pitch could detect the difference.
The fugue would remain unchanged—symmetric under this global transformation—if, equally unbeknownst to you, tiny elves living inside the piano hooked up an elaborate contraption of pulleys and gears. Every time a piano key’s hammer begins to fall, the elves’ wheelworks redirect it to the originally intended strings. By adding in more machinery—new types of particles and interactions—the elves compensate for the gremlins’ transformation, leaving the composition completely unchanged from the original.
So much for symmetry under global transformations. More complicated transformations are possible, too. For example, the gremlins inside the piano might dream up a distinct transposition for every note on the k
eyboard: middle C moves up to E, while D moves down to B-flat, and so on. Even worse, the gremlins might change their minds and make up different transpositions over time, so that later the middle-C hammer strikes a G while the D hammer strikes a D-sharp. Physicists call such maneuvers “local transformations.” With the right combination of gears and pulleys, the elves could still render your Bach fugue unchanged from the original, if the elves constantly adjusted their machinery to compensate for the gremlins’ complicated transformations. In the parable, the elves’ machinery enforces the symmetry. More than that: the whole reason to dream up the elves in the first place, with their pulleys and gears—to posit that the world really contains more kinds of stuff than we originally thought—is to protect the hypothetical symmetry. On this telling, thought-stuff—specific, mathematical symmetries—would conjure up physical things in the world, populating the subatomic realm with particular kinds of particles.3
The forces described by the Standard Model—forces that bind nuclei together or make them fall apart—remain symmetrical under local transformations. Decades ago, physicists postulated that special particles might exist that generate compensating nuclear forces which, just like the elves’ impressive bric-a-brac, guarantee the overall symmetry. The types of transformations the particles needed to overcome helped to fix what properties they should have, if indeed they existed at all. And, lo and behold, when experimenters went looking for particles to match those descriptions in the early 1980s, using large particle accelerators at Fermilab, CERN, and elsewhere, there the particles were: pretty much exactly as the theorists had expected they would be.4
Figure 11.2. Carlo Rubbia and colleagues conducted a series of experiments at CERN, code-named UA1 and UA2 (named for “Underground Area”), that succeeded in finding evidence of the hypothetical W and Z force–carrying particles of the Standard Model in 1983. (Source: CERN, courtesy of AIP Emilio Segrè Visual Archives.)
That was a remarkable congruence. Small hints gleaned from various experiments suggested that some underlying symmetry might govern specific forces of nature. Mental gymnastics—often far more elaborate, far-fetched, and just plain bizarre than my gremlin-elf-piano story—led theorists to predict that some new, tiny things might be out there scurrying around, shoring up the underlying symmetry. New experiments then aimed to catch a fleeting glimpse of those tiny elves at work, or at least to find empirical data that might plausibly be attributed to those dreamed-up interactions. The Standard Model was pieced together that way between the 1960s and the 1980s, a frenetic zigzag between theory and experiment.
One of the astonishing successes of the Standard Model has been to account for why objects have mass. The Higgs mechanism (to which I turn in the next chapter) describes a process by which basic constituents such as quarks and electrons acquire mass from their immersion in a vat of Higgs-field goo. But what about conglomerations of quarks, like protons and neutrons? Nearly all the matter we know—you, me, just about everything we can see here or in the heavens—consists of protons and neutrons. (There seems to be quite a lot of matter in outer space that we can’t see, known as “dark matter,” which is not composed of protons and neutrons, but let us tackle one cosmic mystery at a time.) Only about 5 percent of the mass of ordinary particles like protons and neutrons can be accounted for by the mass of their indwelling quarks. Ninety-five percent of a proton’s mass—and, by extension, 95 percent of the mass of you and me—comes from raw energy. Mass does not arise from glomming lots of heavy items together. It comes very nearly from nothing: from the feverish quantum dance of massless particles.5
The main participants in this dance are gluons. Gluons are one species of elves from my Bach parable: they skitter around enforcing a particular symmetry. The symmetry they regulate governs the strong nuclear force, that is, the force that binds quarks together into composites like protons and neutrons. As their name implies, they are nuclear glue. Gluons do not have any mass of their own—they avoid all those jostlings with the Higgs field—but they interact with each other and with quarks all the time. Most important, they stay true to their elven ways. If you try to disturb the symmetry they guard—for example, by placing a lone quark in isolation—gluons leap into action, dredging up other quarks with compensating nuclear charges to cancel out the first quark’s charge and restore the overall symmetry.
The cancellation would be complete if the compensating quarks could be forced to sit directly on top of the original quark; then no quark-charge would spill out to threaten the nuclear-force symmetry. But a competing factor hinders any complete cancellation. The Heisenberg uncertainty principle, that central pillar of quantum theory, stipulates a mandatory trade-off between how precisely a quantum object’s position and momentum may be specified. In other words, nothing—not even gluons—can force quarks to sit perfectly still in a fixed location. The more gluons act to keep the new quarks affixed squarely on top of the original, the more energetically those quarks jump around, like so many toddlers pitching a tantrum. At the natural balancing point between those two tendencies—canceling the original quark’s charge as much as possible while minimizing the new quarks’ thrashing—some residual energy remains. Thanks to Einstein’s E = mc2, we see that energy in the form of a proton’s mass. Our mass, you might say, is nothing but a cosmic accounting error.
The basic mechanism of this mass-inducing process had been hypothesized in the 1970s, just a decade after Gell-Mann had first dreamed up the idea of quarks. Yet it took years to evaluate the idea. For one thing, calculating the quantitative details of the quark-gluon interactions proved formidable. Only in 2008 could the first compelling computer simulations tackle realistic-enough scenarios to allow physicists to compare theoretical predictions for the mass of the proton with experimental data. The match was remarkable, the accounting secure. Those imagined elves really seem to be out there, performing their tasks just as the Standard Model prescribes.6
So here we are: teeming collections of atoms, which are mostly empty space, their subatomic constituents acquiring heft from the symmetry-preserving whirl of a gluonic quantum dance. “Standard Model,” indeed.
12
Higgs Hunting
Particle physics is at once the most elegant and the most brutish of sciences. Elegant because of its sweeping symmetries and exquisite mathematical structures. Brutish because the principal means of acquiring information about the subatomic realm is by revving up tiny bits of matter to extraordinary energies and smashing them together. Richard Feynman once likened particle physicists’ methods to trying to discern the inner workings of a finely crafted pocket watch—carefully gauged springs and gears, all arranged just so—by hurling two watches at each other and watching the detritus that comes flying apart.1 In particle physics, there’s an added twist: some of the detritus was never contained within the original matter. It’s as if, in addition to the springs and gears of the smashed watches, out flew pulleys, ropes, the odd coin, and a yo-yo or two. The new objects that come flying out when subatomic particles smash together are coagulations of raw energy: some of the energy carried by the two colliding particles becomes transmuted, thanks to Einstein’s famous equation, E = mc2, into little chunks of matter. These colossal transmutations occur billions of times per second in hulking machines like the Large Hadron Collider (LHC) at the CERN laboratory near Geneva.
As soon as the LHC came on line, in September 2008, huge teams of experimentalists began looking for one particle in particular: the Higgs boson. (“Boson” is a generic label for particles that carry whole-number units of spin, or intrinsic angular momentum, such as a photon of light. Most of the particles that make up ordinary matter, such as protons and electrons, carry half-integer units of spin and are known as “fermions.”) The Higgs has been dubbed “the God particle,” though I have never understood why this particular bit of matter is presumed to be holier than all the others. I prefer a more descriptive nickname: the “billion-dollar boson,” since for decades the challenge of findi
ng the Higgs particle served as a major argument in favor of building larger and larger particle accelerators.2
The idea of the Higgs boson emerged more than fifty years ago, an idea born of desperation. Experiments had suggested that the weak nuclear force—responsible for phenomena such as radioactive decay—obeyed specific symmetries. Several theorists recognized that they could model such a force if, as for electromagnetism, the weak force arose when subatomic particles exchanged special force-carrying particles. But there was a catch. The symmetries of the weak force demanded that the hypothetical force-carrying particles have no mass, just like the massless photon that gives rise to electromagnetic forces. Unlike electromagnetism, however, the weak force has a very short range: it is effective only when particles are very close to each other (such as packed tightly within an atomic nucleus); the short range, in turn, seemed to imply that the force-carrying particles should be very massive. Hence the conundrum: theorists could model the symmetries of the weak force or its short range, but not both.3
Figure 12.1. Reconstruction of particle paths from a single event captured in the ATLAS detector at CERN in late May 2012. Protons that had been accelerated to near light speed collided, forming a short-lived Higgs boson. Before it could leave a measurable track in the detector, the Higgs particle quickly decayed into two tau-mesons, which in turn decayed into an electron (thin line pointing nearly straight up from the collision region) and a muon (thin line pointing diagonally up and to the left). (Source: ATLAS Collaboration, courtesy of CERN.)
Several physicists proposed a clever workaround. What if the force-carrying particles of the weak force really were massless but were always slogging through some medium—a medium that fills all of space and slows the force-carriers’ motion, like marbles rolling through molasses? Several versions of that idea appeared in the journal Physical Review Letters over the summer and fall of 1964, in short papers by François Englert and Robert Brout (received at the journal on 26 June and published on 31 August), by Peter Higgs (received on 31 August and published on 19 October), and by Gerald Guralnik, Carl Hagen, and Thomas Kibble (received on 12 October and published on 16 November). Higgs noted in his paper that the molasses-like medium implied that there should exist a new particle, associated with the medium, which became known as the “Higgs boson.”4