When physicists are forced to give a single-word answer to the question of why we are building the Large Hadron Collider (LHC), we usually reply “Higgs.” The Higgs particle—the last remaining undiscovered piece of our current theory of matter—is the marquee attraction. But the full story is much more interesting. The new collider provides the greatest leap in capability of any instrument in the history of particle physics. We do not know what it will find, but the discoveries we make and the new puzzles we encounter are certain to change the face of particle physics and to echo through neighboring sciences.
In this new world, we expect to learn what distinguishes two of the forces of nature—electromagnetism and the weak interactions—with broad implications for our conception of the everyday world. We will gain a new understanding of simple and profound questions: Why are there atoms? Why chemistry? What makes stable structures possible?
The search for the Higgs particle is a pivotal step, but only the first step. Beyond it lie phenomena that may clarify why gravity is so much weaker than the other forces of nature and that could reveal what the unknown dark matter that fills the universe is. Even deeper lies the prospect of insights into the different forms of matter, the unity of outwardly distinct particle categories and the nature of spacetime. The questions in play all seem linked to one another and to the knot of problems that motivated the prediction of the Higgs particle to begin with. The LHC will help us refine these questions and will set us on the road to answering them.
The Matter at Hand
What physicists call the “Standard Model” of particle physics, to indicate that it is still a work in progress, can explain much about the known world. The main elements of the Standard Model fell into place during the heady days of the 1970s and 1980s, when waves of landmark experimental discoveries engaged emerging theoretical ideas in productive conversation. Many particle physicists look on the past 15 years as an era of consolidation in contrast to the ferment of earlier decades. Yet even as the Standard Model has gained ever more experimental support, a growing list of phenomena lies outside its purview, and new theoretical ideas have expanded our conception of what a richer and more comprehensive worldview might look like. Taken together, the continuing progress in experiment and theory point to a very lively decade ahead. Perhaps we will look back and see that revolution had been brewing all along.
Our current conception of matter comprises two main particle categories, quarks and leptons, together with three of the four known fundamental forces, electromagnetism and the strong and weak interactions. Gravity is, for the moment, left to the side. Quarks, which make up protons and neutrons, generate and feel all three forces. Leptons, the best known of which is the electron, are immune to the strong force. What distinguishes these two categories is a property akin to electric charge, called color. (This name is metaphorical; it has nothing to do with ordinary colors.) Quarks have color, and leptons do not.
The guiding principle of the Standard Model is that its equations are symmetrical. Just as a sphere looks the same whatever your viewing angle is, the equations remain unchanged even when you change the perspective from which they are defined. Moreover, they remain unchanged even when the perspective shifts by different amounts at different points in space and time.
Ensuring the symmetry of a geometric object places very tight constraints on its shape. A sphere with a bump no longer looks the same from every angle. Likewise, the symmetry of the equations places very tight constraints on them. These symmetries beget forces that are carried by special particles called bosons [see “Gauge Theories of the Forces between Elementary Particles,” by Gerard ’t Hooft; Scientific American, June 1980, and “Elementary Particles and Forces,” by Chris Quigg; Scientific American, April 1985].
In this way, the Standard Model inverts Louis Sullivan’s architectural dictum: instead of “form follows function,” function follows form. That is, the form of the theory, expressed in the symmetry of the equations that define it, dictates the function—the interactions among particles—that the theory describes. For instance, the strong nuclear force follows from the requirement that the equations describing quarks must be the same no matter how one chooses to define quark colors (and even if this convention is set independently at each point in space and time). The strong force is carried by eight particles known as gluons. The other two forces, electromagnetism and the weak nuclear force, fall under the rubric of the “electroweak” forces and are based on a different symmetry. The electroweak forces are carried by a quartet of particles: the photon, Z boson, W+ boson and W– boson.
Breaking the Mirror
The theory of the electroweak forces was formulated by Sheldon Glashow, Steven Weinberg and Abdus Salam, who won the 1979 Nobel Prize in Physics for their efforts. The weak force, which is involved in radioactive beta decay, does not act on all the quarks and leptons. Each of these particles comes in mirror-image varieties, termed left-handed and right-handed, and the beta-decay force acts only on the left-handed ones—a striking fact still unexplained 50 years after its discovery. The family symmetry among the left-handed particles helps to define the electroweak theory.
In the initial stages of its construction, the theory had two essential shortcomings. First, it foresaw four long-range force particles—referred to as gauge bosons—whereas nature has but one: the photon. The other three have a short range, less than about 10–17 meter, less than 1 percent of the proton’s radius. According to Heisenberg’s uncertainty principle, this limited range implies that the force particles must have a mass approaching 100 billion electron volts (GeV). The second shortcoming is that the family symmetry does not permit masses for the quarks and leptons, yet these particles do have mass.
The way out of this unsatisfactory situation is to recognize that a symmetry of the laws of nature need not be reflected in the outcome of those laws. Physicists say that the symmetry is “broken.” The needed theoretical apparatus was worked out in the mid-1960s by physicists Peter Higgs, Robert Brout, François Englert and others. The inspiration came from a seemingly unrelated phenomenon: superconductivity, in which certain materials carry electric current with zero resistance at low temperatures. Although the laws of electromagnetism themselves are symmetrical, the behavior of electromagnetism within the superconducting material is not. A photon gains mass within a superconductor, thereby limiting the intrusion of magnetic fields into the material.
As it turns out, this phenomenon is a perfect prototype for the electroweak theory. If space is filled with a type of “superconductor” that affects the weak interaction rather than electromagnetism, it gives mass to the W and Z bosons and limits the range of the weak interactions. This superconductor consists of particles called Higgs bosons. The quarks and leptons also acquire their mass through their interactions with the Higgs boson [see “The Higgs Boson,” by Martinus Veltman; Scientific American, November 1986]. By obtaining mass in this way, instead of possessing it intrinsically, these particles remain consistent with the symmetry requirements of the weak force.
The modern electroweak theory (with the Higgs) accounts very precisely for a broad range of experimental results. Indeed, the paradigm of quark and lepton constituents interacting by means of gauge bosons completely revised our conception of matter and pointed to the possibility that the strong, weak and electromagnetic interactions meld into one when the particles are given very high energies. The electroweak theory is a stunning conceptual achievement, but it is still incomplete. It shows how the quarks and leptons might acquire masses but does not predict what those masses should be. The electroweak theory is similarly indefinite in regard to the mass of the Higgs boson itself: the existence of the particle is essential, but the theory does not predict its mass. Many of the outstanding problems of particle physics and cosmology are linked to the question of exactly how the electroweak symmetry is broken.
Where the Standard Model Tells Its Tale
Encouraged by a string of promising observations in the 1970s, theorists began to take the Standard Model seriously enough to begin to probe its limits. Toward the end of 1976 Benjamin W. Lee of Fermi National Accelerator Laboratory in Batavia, Ill., Harry B. Thacker, now at the University of Virginia, and I devised a thought experiment to investigate how the electroweak forces would behave at very high energies. We imagined collisions among pairs of W, Z and Higgs bosons. The exercise might seem slightly fanciful because, at the time of our work, not one of these particles had been observed. But physicists have an obligation to test any theory by considering its implications as if all its elements were real.
What we noticed was a subtle interplay among the forces generated by these particles. Extended to very high energies, our calculations made sense only if the mass of the Higgs boson were not too large—the equivalent of less than one trillion electron volts, or 1 TeV. If the Higgs is lighter than 1 TeV, weak interactions remain feeble and the theory works reliably at all energies. If the Higgs is heavier than 1 TeV, the weak interactions strengthen near that energy scale and all manner of exotic particle processes ensue. Finding a condition of this kind is interesting because the electroweak theory does not directly predict the Higgs mass. This mass threshold means, among other things, that something new—either a Higgs boson or other novel phenomena—is to be found when the LHC turns the thought experiment into a real one.
Experiments may already have observed the behind-the-scenes influence of the Higgs. This effect is another consequence of the uncertainty principle, which implies that particles such as the Higgs can exist for moments too fleeting to be observed directly but long enough to leave a subtle mark on particle processes. The Large Electron Positron collider at CERN, the previous inhabitant of the tunnel now used by the LHC, detected the work of such an unseen hand. Comparison of precise measurements with theory strongly hints that the Higgs exists and has a mass less than about 192 GeV.
For the Higgs to weigh less than 1 TeV, as required, poses an interesting riddle. In quantum theory, quantities such as mass are not set once and for all but are modified by quantum effects. Just as the Higgs can exert a behind-the-scenes influence on other particles, other particles can do the same to the Higgs. Those particles come in a range of energies, and their net effect depends on where precisely the Standard Model gives way to a deeper theory. If the model holds all the way to 1015 GeV, where the strong and electroweak interactions appear to unify, particles with truly titanic energies act on the Higgs and give it a comparably high mass. Why, then, does the Higgs appear to have a mass of no more than 1 TeV?
This tension is known as the hierarchy problem. One resolution would be a precarious balance of additions and subtractions of large numbers, standing for the contending contributions of different particles. Physicists have learned to be suspicious of immensely precise cancellations that are not mandated by deeper principles. Accordingly, in common with many of my colleagues, I think it highly likely that both the Higgs boson and other new phenomena will be found with the LHC.
Theorists have explored many ways in which new phenomena could resolve the hierarchy problem. A leading contender known as supersymmetry supposes that every particle has an as yet unseen superpartner that differs in spin [see “Is Nature Supersymmetric?” by H. E. Haber and G. L. Kane; Scientific American, June 1986]. If nature were exactly supersymmetric, the masses of particles and superpartners would be identical, and their influences on the Higgs would cancel each other out exactly. In that case, though, physicists would have seen the superpartners by now. We have not, so if supersymmetry exists, it must be a broken symmetry. The net influence on the Higgs could still be acceptably small if superpartner masses were less than about 1 TeV, which would put them within the LHC’s reach.
Another option, called technicolor, supposes that the Higgs boson is not truly a fundamental particle but is built out of as yet unobserved constituents. (The term “technicolor” alludes to a generalization of the color charge that defines the strong force.) If so, the Higgs is not fundamental. Collisions at energies around 1 TeV (the energy associated with the force that binds together the Higgs) would allow us to look within it and thus reveal its composite nature. Like supersymmetry, technicolor implies that the LHC will set free a veritable menagerie of exotic particles.
A third, highly provocative idea is that the hierarchy problem will go away on closer examination, because space has additional dimensions beyond the three that we move around in. Extra dimensions might modify how the forces vary in strength with energy and eventually meld together. Then the melding—and the onset of new physics—might not happen at 1012 TeV but at a much lower energy related to the size of the extra dimensions, perhaps only a few TeV. If so, the LHC could offer a peek into those extra dimensions [see “The Universe’s Unseen Dimensions,” by Nima Arkani-Hamed, Savas Dimopoulos and Georgi Dvali; Scientific American, August 2000].
One more piece of evidence points to new phenomena on the TeV scale. The dark matter that makes up the bulk of the material content of the universe appears to be a novel type of particle [see “The Search for Dark Matter,” by David B. Cline; Scientific American, March 2003]. If this particle interacts with the strength of the weak force, then the big bang would have produced it in the requisite numbers as long as its mass lies between approximately 100 GeV and 1 TeV. Whatever resolves the hierarchy problem will probably suggest a candidate for the dark matter particle.
Revolutions on the Horizon
Opening the TeV scale to exploration means entering a new world of experimental physics. Making a thorough exploration of this world—where we will come to terms with electroweak symmetry breaking, the hierarchy problem and dark matter—is the top priority for accelerator experiments. The goals are well motivated and matched by our experimental tools, with the LHC succeeding the current workhorse, Fermilab’s Tevatron collider. The answers will not only be satisfying for particle physics, they will deepen our understanding of the everyday world.
But these expectations, high as they are, are still not the end of the story. The LHC could well find clues to the full unification of forces or indications that the particle masses follow a rational pattern. Any proposed interpretation of new particles will have consequences for rare decays of the particles we already know. It is very likely that lifting the electroweak veil will bring these problems into clearer relief, change the way we think about them and inspire future experimental thrusts.
Cecil Powell won the 1950 Nobel Prize in Physics for discovering particles called pions—proposed in 1935 by physicist Hideki Yukawa to account for nuclear forces—by exposing highly sensitive photographic emulsions to cosmic rays on a high mountain. He later reminisced: “When [the emulsions] were recovered and developed in Bristol, it was immediately apparent that a whole new world had been revealed.... It was as if, suddenly, we had broken into a walled orchard, where protected trees had flourished and all kinds of exotic fruits had ripened in great profusion.” That is just how I imagine our first look at the TeV scale.