At dawn on a summer morning in 2012, we were on our third round of espresso when the video link connected our office at the California Institute of Technology to the CERN laboratory near Geneva. On the monitor we saw our colleagues on the Razor team, one of many groups of physicists analyzing data from the CMS experiment at CERN's Large Hadron Collider (LHC). Razor was created to search for exotic collisions that would provide the first evidence of supersymmetry, a 45-year-old theory of matter that would supplant the standard understanding of particle physics, solving deep problems in physics and explaining the nature of the universe's mysterious dark matter. After decades of searching, no experimental evidence for supersymmetry has been found.
At CERN, Maurizio Pierini, the Razor team's leader, flashed a plot of new data, and from nine time zones away we could see the raised eyebrows around the room: there was an anomaly. “Somebody should look at this event,” Pierini said matter-of-factly. By “event” he meant a particular proton-proton collision, one of trillions produced at the LHC. Within minutes the two of us had pulled up the full record for this collision on a laptop.
Supersymmetry is an amazingly beautiful solution to the deep troubles that have been nagging at physicists for more than four decades. It provides answers to a series of important “why” questions: Why do particles have the masses they do? Why do forces have the strengths they do? In short: Why does the universe look the way it does? In addition, supersymmetry predicts that the universe is filled with heretofore hidden “superpartner” particles that would solve the mystery of dark matter. It is not an exaggeration to say that most of the world's particle physicists believe that supersymmetry must be true—the theory is that compelling. These physicists' long-term hope has been that the LHC would finally discover these superpartners, providing hard evidence that supersymmetry is a real description of the universe.
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As we pulled up the interesting collision, we immediately saw that it appeared to be a smoking-gun signal of supersymmetry. Two clusters of very energetic particles were observed moving one way, recoiling against something unseen—perhaps a superpartner? Yet soon enough we noticed a big red spike on the readout. Could this be a fake signal from a detector malfunction? And so it turned out—another disappointment in the seemingly unending quest to find supersymmetry.
Indeed, results from the first run of the LHC have ruled out almost all the best-studied versions of supersymmetry. The negative results are beginning to produce if not a full-blown crisis in particle physics, then at least a widespread panic. The LHC will be starting its next run in early 2015, at the highest energies it was designed for, allowing researchers at the ATLAS and CMS experiments to uncover (or rule out) even more massive superpartners. If at the end of that run nothing new shows up, fundamental physics will face a crossroads: either abandon the work of a generation for want of evidence that nature plays by our rules, or press on and hope that an even larger collider will someday, somewhere, find evidence that we were right all along.
Of course, the story of science has many examples of long quests succeeding triumphantly—witness the discovery of the long-sought Higgs boson at the LHC. But for now most particle theorists are biting their nails, as LHC data are about to test the foundations of the mighty cathedral of theoretical physics that they have built up over the past half-century.
The Need for Supersymmetry
Supersymmetry is part of a broader attempt to understand the big mysteries of quantum weirdness. We have a fantastically successful and predictive theory of subatomic physics, prosaically known as the Standard Model, which combines quantum mechanics with Einstein's special theory of relativity to describe particles and forces. Matter is made of one variety of particles called fermions (after Enrico Fermi) and held together by forces related to another type of particle called bosons (after Satyendra Bose).
The Standard Model provides an excellent description of what goes on in the subatomic world. But we begin to get into trouble when we ask the questions of why the Standard Model has the features that it does. For example, it holds that there are three different types of leptons (a type of fermion): the electron, muon and tau. Why three? Why not two, or four, or 15? The Standard Model does not say; we need to explore a deeper level of nature to discover the answer. Similarly, we might ask, Why does the electron have the mass that it does? Why is it lighter than, say, the Higgs boson? Again: on this, the Standard Model is silent.
Theoretical particle physicists spend a lot of time thinking about such questions. They build models that explain why the Standard Model looks the way it does. String theory, for example, is one effort to get down to a deeper level of reality. Other examples abound.
All these additional theories have a problem, however. Any theory (like string theory) that involves new physics necessarily implies the existence of new hypothetical particles. These particles might have an extremely high mass, which would explain why we have not already spotted them in accelerators like the LHC, as high-mass particles are difficult to create. But even high-mass particles would still affect ordinary particles like the Higgs boson. Why? The answer lies in quantum weirdness.
In quantum mechanics, particles interact with one another via the exchange of so-called virtual particles that pop into and out of existence. For example, the repulsive electric force between two electrons is described, to first approximation, by the electrons exchanging a virtual photon. Richard Feynman derived elegant rules to describe quantum effects in terms of stable particles interacting with additional virtual particles.
In quantum theory, however, anything that is not strictly forbidden will in fact happen, at least occasionally. Electrons will not just interact with one another via the exchange of virtual particles, they will also interact with all other particles—including our new, hypothetical particles suggested by extensions of the Standard Model. And these interactions would create problems—unless, that is, we have something like supersymmetry.
Consider the Higgs boson, which in the Standard Model gives elementary particles mass. If you had a Higgs but also had some superheavy particles, they would talk to one another via virtual quantum interactions. The Higgs would itself become superheavy. And the instant after that, everything in the universe would transform into superheavy particles. You and I would collapse into black holes. The best explanation for why we do not is supersymmetry.
The Promise of Supersymmetry
The basic idea of supersymmetry, generally known by the nickname “SUSY” (pronounced “Suzy”), was developed by physicists in the 1970s who were interested in the relation between symmetries and particle physics. Supersymmetry is not one particular theory but rather a framework for theories. Many individual models of the universe can be “supersymmetric” if they share certain properties.
Many ordinary symmetries are built into the physical laws for particles and forces. These laws do not care about where you are, when you do the measurement, what direction you are facing, or whether you are moving or at rest with respect to the objects that you are observing. These spacetime symmetries mathematically imply conservation laws for energy, momentum and angular momentum; from symmetries themselves, we can derive the relation between energy, momentum and mass famously exemplified by E = mc
2. All of this has been pretty well understood since 1905, when Albert Einstein developed special relativity.
Quantum physics seems to respect these symmetries. Scientists have even used the symmetries to predict new phenomena. For example, Paul Dirac showed in 1930 that when you combine quantum mechanics with relativity, spacetime symmetries imply that every particle has to have a related antiparticle—a particle with opposite charge. This idea seemed crazy at the time because no one had ever seen an antiparticle. But Dirac was proved right. His theoretical symmetry arguments led to the bold but correct prediction that there are about twice as many elementary particles as everyone expected.
Supersymmetry relies on an argument that is similar to Dirac's. It postulates that there exists a quantum extension of spacetime called superspace and that particles are symmetric in this superspace.
Superspace does not have ordinary spatial dimensions like left-right and up-down but rather extra fermionic dimensions. Motion in a fermionic dimension is very limited. In an ordinary spatial dimension, you can move as far as you want in any direction, with no restriction on the size or number of steps that you take. In contrast, in a fermionic dimension your steps are quantized, and once you take one step that fermionic dimension is “full.” If you want to take any more steps, you must either switch to a different fermionic dimension, or you must go back one step.
If you are a boson, taking one step in a fermionic dimension turns you into a fermion; if you are a fermion, one step in a fermionic dimension turns you into a boson. Furthermore, if you take one step in a fermionic dimension and then step back again, you will find that you have also moved in ordinary space or time by some minimum amount. Thus, motion in the fermionic dimensions is tied up, in a complicated way, with ordinary motion.
Why does all of this matter? Because in a supersymmetric world, the symmetries across fermionic dimensions restrict how particles can interact. In particular, so-called natural supersymmetries greatly suppress the effects of virtual particles. Natural supersymmetries prevent Higgs bosons from interacting with high-energy particles in such a way that we all turn into black holes. (Theories that are supersymmetric but not natural require us to come up with additional mechanisms to suppress virtual particles.) Natural supersymmetry clears the way for physicists to develop new ideas to make sense of the Standard Model.
The Search for Supersymmetry
All supersymmetric theories imply that every boson particle has a fermion partner particle, a superpartner, and vice versa. Because none of the known boson and fermion particles seem to be superpartners of one another, supersymmetry can be correct only if the universe contains a large number of superpartner particles that have eluded detection.
Therein lies the rub. In the simplest, most powerful versions of supersymmetry—natural supersymmetry—the superpartners should not be that much heavier than the Higgs boson. That means that we should be able to find them at the LHC. Indeed, if you would have asked physicists 10 years ago, most would have guessed that by now we should have already found evidence of superpartners.
And yet we have not. One of us (Spiropulu) remembers the night in 2009 that I went to work as a shift leader at the CMS detector just before midnight. The control room was crowded with physicists, each monitoring a different subsystem of the massively complex, 14,000-metric-ton detector. At 2 a.m., I got a call from the CERN Control Center on the opposite side of the 27-kilometer-long LHC ring: tonight was the night; they were going for the highest-energy proton collisions ever attempted.
I gave the signals to carefully bring up each portion of the CMS, keeping the more fragile parts of the detector for last. At 4:11 a.m., the full detector went live. A wall of monitors went wild, with ultrafast electronics flashing displays of the collisions happening 20 million times a second 100 meters below. After chasing supersymmetry for a decade at Fermilab's Tevatron collider in Batavia, Ill., my heart leapt in anticipation of recognizing certain patterns. Calm, I told myself, this is only the beginning—it is seductive to analyze collisions by visual inspection, but it is impossible to make a discovery like that.
Indeed, you don't build a $10-billion collider with its giant detectors, turn it on and expect discoveries on the first night—or even during the first year. Yet our expectations were high from the very start. At CMS (and at ATLAS), we had laid out an elaborate plan to discover supersymmetry with the first LHC data. We had geared up to find dark matter particles in supersymmetry signals, not directly but as “missing energy”: a telltale imbalance of visible particles recoiling from something unseen. We even went so far as to write a template for the discovery paper with a title and a date.
That paper remains unwritten. The experiments have left only a few unexplored windows in which superpartners might be hiding. They can't be too light, or we would have found them already, and they can't be too heavy, because then they wouldn't satisfy the needs of natural supersymmetry, which is the type of supersymmetry that is effective at suppressing virtual particles. If the LHC does not find them during its next run—and does not do so quickly—the crisis in physics will mount.
Life after Supersymmetry
Theorists are not ready to give up on a more general idea of supersymmetry, though—even if it cannot do all the work that we were hoping natural supersymmetry would do. Recall that supersymmetry is a framework for making models of the world, not a model itself, so future data may vindicate the idea of supersymmetry even if all current models are excluded.
During a talk at the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara, Nima Arkani-Hamed, a physicist at the Institute for Advanced Study in Princeton, N.J., paced to and fro in front of the blackboard, addressing a packed room about the future of supersymmetry. What if supersymmetry is not found at the LHC, he asked, before answering his own question: then we will make new supersymmetry models that put the superpartners just beyond the reach of the experiments. But wouldn't that mean that we would be changing our story? That's okay; theorists don't need to be consistent—only their theories do.
This unshakable fidelity to supersymmetry is widely shared. Particle theorists do admit, however, that the idea of natural supersymmetry is already in trouble and is headed for the dustbin of history unless superpartners are discovered soon. This is the kind of conundrum that has in the past led to paradigm shifts in science. For example, more than a century ago the failure to find the “luminiferous ether” led to the invention of special relativity.
If supersymmetry is not a true description of the world, what might take its place? Here are three different speculative answers. All of them imply profoundly new directions for thinking about basic physics and cosmology:
The multiverse: The strengths of the fundamental forces and the relative size of particle masses involve numbers, the origins of which are a mystery. We don't like to think that the numbers are random, because if they were slightly different, the universe would be a much different place. Atoms would have trouble forming, for example, and life would fail to evolve. In the parlance of theoretical physics, the universe appears to be “finely tuned.” Supersymmetry attempts to provide an answer for why these parameters take the values they do. It carves out a doorway to a deeper level of physics. But what if that doorway doesn't exist?
In that case, we are left to consider the possibility that this fine-tuning is just a random accident—a notion that becomes more appealing if one postulates a multiverse. In the multiverse scenario, the big bang produced not just the universe that we see but also a very large number of variations on our universe that we do not see. In this case, the answer to questions such as “Why does the electron have the mass that it does?” takes an answer in the form of: “That's just the random luck of the draw—other parts of the multiverse have different electrons with different masses.” The seemingly precise tunings that we puzzle over are mere accidents of cosmic history. Only the universes with parameters finely tuned to allow life to develop will have physicists in them wondering why they did not find natural supersymmetry at the LHC.
To many physicists, however, the multiverse bears an uneasy resemblance to asserting that anomalies in particle physics are caused by armies of invisible angels. As Nobel laureate David Gross has said, appealing to unknowable initial conditions sounds like giving up.
Extra dimensions: Physicists Lisa Randall of Harvard University and Raman Sundrum of the University of Maryland have shown that an extra dimension with a “warped” geometry can explain gravity's weakness in comparison with the other known forces. If these extra dimensions are microscopic, we might not have noticed them yet, but their size and shape could have a dramatic effect on high-energy particle physics. In such models, rather than finding superpartners at the LHC, we may discover Kaluza-Klein modes, exotic heavy particles whose mass is actually their energy of motion in the extra dimensions.
Dimensional transmutation: Instead of invoking supersymmetry to suppress virtual particle effects, a new idea is to embrace such effects to explain where mass comes from. Consider for a moment the proton. The proton is not an elementary particle. It is made up of an assembly of three quarks, which have a minuscule mass, and gluons, which have no mass at all. The proton is much heavier than the sum total of the quarks and gluons inside of it. Where does this mass come from? It comes from the energy fields generated by the “strong” force that holds the proton together. Our understanding of these fields allows us to accurately predict the proton's mass based on just ordinary numbers such as pi.
It's an odd situation in particle physics. Usually we can compute masses only by starting with other masses. For example, the Standard Model gives us no way to predict the mass of the Higgs boson—we have to measure it. This seems like an obvious mistake, given how cleverly we can predict the mass of the proton. Building on seminal work by William A. Bardeen, a physicist at Fermilab, a few radical theorists are now suggesting that the Higgs mass scale is generated through a similar process called dimensional transmutation.
If this approach is to keep the useful virtual particle effects while avoiding the disastrous ones—a role otherwise played by supersymmetry—we will have to abandon popular speculations about how the laws of physics may become unified at superhigh energies. It also makes the long-sought connection between quantum mechanics and general relativity even more mysterious. Yet the approach has other advantages. Such models can generate mass for dark matter particles. They also predict that dark matter interacts with ordinary matter via a force mediated by the Higgs boson. This dramatic prediction will be tested over the next few years both at the LHC and in underground dark matter detection experiments.
The Higgs may hold other clues. The discovery of the Higgs boson shows that there is a Higgs energy field turned on everywhere in the universe that gives mass to elementary particles. This means that the vacuum of “empty” space is a busy place, with both Higgs energy and virtual particles producing complicated dynamics. One might then wonder if the vacuum is really stable or if some unlucky quantum event could one day trigger a catastrophic transition from our universe to a clean slate. Supersymmetry acts to stabilize the vacuum and prevent such mishaps. But without supersymmetry, the stability of the vacuum depends sensitively on the mass of the Higgs: a heavier Higgs implies a stable universe, whereas a lighter one implies eventual doom. Remarkably, the measured Higgs mass is right on the edge, implying a long-lived but ultimately unstable vacuum [see box on opposite page]. Nature is trying to tell us something, but we don't know what.
The Future
If superpartners are discovered in the next run of the LHC, the current angst of particle physicists will be replaced by enormous excitement over finally breaching the threshold of the superworld. A wild intellectual adventure will begin.
Yet if superpartners are not found, we face a paradigm rupture in our basic grasp of quantum physics. Already this prospect is inspiring a radical rethinking of basic phenomena that underlie the fabric of the universe. A better understanding of the properties of the Higgs boson will be central to building a new paradigm. Experimental signals of dark matter, that lonely but persistent outlier of particle physics, may ultimately be a beacon showing the way forward.