Vol. 6, No.1
The future of particle physics—and the men and women who have devoted their lives to it—hinges on what happens in Switzerland over the next 12 months at the Large Hadron Collider. The experiment, which is the largest in history, makes theoretical particle physics look less like hard science…and more like a game of chance.
In 1596, astronomer Johannes Kepler began his obsessive search for the answer to what was easily the most burning and fundamental question in all of physics: Why did the world have five planets? What larger truths about the universe could be gleaned from the number five?
The story, which today makes most people smirk, puts David Kaplan into a cold sweat. Kaplan is a theoretical particle physicist at Johns Hopkins University, and his field is headed for a major paradigm shift over the next few years. On Sept. 10, Geneva's European Center for Nuclear Research (CERN, according to its French acronym) tentatively fired up the biggest physics experiment in history—the 17-mile Large Hadron Collider particle accelerator—to complete a major piece of science's fundamental understanding of particle physics. But according to many physicists, there's a good chance that the LHC won't simply "plug up" some holes and neatly round out incomplete theories: There's an equally good chance that those theories, like Kepler's, could turn out to be so wrong that even the questions are absurd.
The LHC was built to ascertain what gives fundamental particles their mass. The current system of classifying particles and the forces by which they interact is known as the standard model of particle physics, and its description of the fundamental nature of all matter is almost perfect. Almost. What's missing from the standard model is why those particles have mass, and right now, the most likely culprit is a hypothetical particle called the Higgs boson. But it turns out that the further you delve into the Higgs—what it might be, whether or not it exists, and the implications of its discovery—the further you fall down the rabbit hole of extra dimensions and dark matter (see "Where Did the Higgs Go?" p.15).
Theoretical physicists from all over the world have been drawn to Geneva in droves, driven by a mixture of excitement and terror. When the LHC data comes in, says Kaplan, "Everything will change totally."
The ostensible goal of the experiment is the completion of the standard model, but the discoveries at the LHC could pave the road for a vastly more complex landscape of particle physics. And depending on what happens over the next few years, entire careers could soon be the butt of a short, esoteric joke.
On the leafy north side of the Hopkins' Homewood campus, the fourth floor of the Bloomberg Center for Physics and Astronomy houses the labs and offices of the many physicists working on the LHC. There are two groups: the theoretical physicists, who conjure from hair-raising mathematical equations models for how the universe works; and the experimental particle physicists, who figure out how to translate those hypotheses into testable experiments. These groups have very different expectations for what the LHC will reveal, and for each group, the Higgs particle has a different set of potential consequences.
Most of those in the Hopkins LHC group are experi-mental particle physicists. They have spent the past seven years working on a 25-inch barrel filled with tiny camera chips—chips much like the ones in your cell phone—that will track what happens inside the guts of one of the four giant experiments located around the accelerator. Petar Maksimovic and Andrei Gritsan, two of those involved in the work, have devoted most of their careers to this two-foot barrel. "The purpose of the LHC is to test whether there is a Higgs field," explains Gritsan, a young assistant professor. Some theorists predict that the Higgs field will contradict many of the assumptions in the standard model. Others go so far as to suppose it does not exist at all. "That's an even more exciting possibility," Gritsan effuses.
And that's the difference between the experimentalists and the theorists.
Kaplan is a theoretical physicist by training, and he describes the mood in his corner of particle physics less as "exciting" than "nerve-racking." The stakes are incredibly high. "We're all in a heightened state of panic." For a tenured professor, the implications are not as straightforward as losing one's job: "It's worse," Kaplan says bluntly. "It's spending 10, 15, 20 years on one field—and in a single moment, every paper you have ever written on the topic is completely irrelevant." Not only is an entire career's worth of publishing now worthless to the scientific community, he says, "But now you're not trained for anything else!"
Kaplan has spent much of the last year at CERN, where he is filming a documentary about the run-up to the LHC's start, aptly titled Particle Fever. Theoretical physicists from all over the world have been drawn to Geneva in droves, driven by a mixture of excitement and terror. When the LHC data comes in, says Kaplan, "Everything will change totally."
Think of the LHC as a two-way, 17-mile racetrack around which proton bunches (like little race cars filled with trillions of protons) speed toward each other—at just a hair under the speed of light—to die in fiery collisions every 25 nanoseconds. The goal of this extraordinary violence is the creation of the hypothetical Higgs boson, which has variously been called the Higgs particle, the "God particle," or just plain Higgs. Higgs, says Adam Falk, the James B. Knapp Dean of the Zanvyl Krieger School of Arts and Sciences and a theoretical physicist, "is the simplest manifestation of the simplest theory of what the forces are that generate mass."
The standard model is a mainstay of nuclear physics. Like all matter, we are made of atoms, and an atom is made of a cloud of electrons swirling around a core of protons and neutrons, and those subatomic particles are made of quarks, in exotic denominations called "flavors": up, down, strange, charm, top, and bottom. But for some reason, these quarks that give mass to protons and neutrons, which in turn give mass to atoms, which in turn give mass to all matter—act like mass-less particles. Physicists have been frustrated for years at this inability to explain the plumbing.
"We know that they have mass," says Falk, who is also a member of the high-energy theory group in the Department of Physics and Astronomy. "What we don't understand is the underlying dynamics that give those particles their masses."
"We think it's the Higgs field," says Jonathan Bagger, Krieger-Eisenhower Professor of Physics, who is also a vice provost. The Higgs field is believed to have emerged at an early point in the formation of the universe, along with its associated Higgs particle. The field, which is still theorized to pervade the cosmos, supposedly gives mass to any particles that interact with it. Some theoretical physicists, including Hopkins theorist and Alumni Centennial Professor Raman Sundrum, have even postulated that finding the Higgs particle could lead to an understanding of the force of gravity, another conspicuous hole that particle physics needs to plug (at issue: Why is gravity so weak compared to the other forces?). Sundrum's famous collaboration with Harvard theoretical physicist Lisa Randall has led to exotic theories of extra dimensions, which posit our universe as inhabiting a three-dimensional pocket of a many-dimensional universe.
But before the most exotic theories can be investigated, the first task is the simple observation of the Higgs. The problem is, the Higgs can't exist today. The universe is simply too stable, and the Higgs too massive, and any experiment that hopes to coax it out of the ether needs to generate enough energy to recreate the conditions of the early universe.
To do something so outrageous, nearly 10,000 scientists—particle physicists, engineers, and nuclear physicists among them—from hundreds of institutions scattered over 77 countries have worked together for two decades to build the accelerator and its four enormous experiments.
CERN straddles the French and Swiss borders, extending from the Swiss town of Meyrin to the French town of Prévessin. The scenery is deceptively uneventful; clusters of modest apartment complexes occasionally interrupt long, even rows of sunflowers. The above-ground part of the CERN campus is modest to the point of shabbiness. The hallways look like 1960s middle schools, with beige spotted linoleum and chipped paint. It's a jarring disconnect, then, to emerge from the long elevator ride downward into vaulted chambers that glitter with the most sophisticated equipment in human history, capable of recreating the conditions that prevailed an instant after the universe exploded into being.
Buried under the Swiss and French countryside are four gigantic, multi-kiloton instruments, famously created from more metal than the Eiffel Tower. If you think of the LHC as a 17-mile loop of string—with the particle collisions occurring inside that string—these four experiments are colossal beads threaded around that string.
When engineers first began the planning and design of the LHC in 1986, many of the technologies it uses did not yet exist. And it needed a lot of next-generation technologies: The likelihood of actually discovering a new particle depends on the precise understanding of extremely complex detectors and analysis systems, all of them the first of their kind. "The flow of data is tremendous," says Bagger. Most of it will be noise and needs to be thrown out, but, Bagger says, "You don't want to throw out the baby with the bathwater." To capture and record all the data, the Hopkins group is mainly focused on the Compact Muon Solenoid (CMS) detector. One of the four LHC experiments, it has the information-processing power of the entire world's telecommunication network.
The LHC also needed radiation-resistant microchips that could function in an environment more hostile than the unprotected surface of Mars; enough processing power to instantaneously sift through 100 million events per second; enough data storage for 15 petabytes of data a year (a petabyte being equal to a thousand of the biggest hard drives available today); and superconducting magnets powerful enough to control a particle beam stronger than a speeding train. Everything that enables the collisions of trillions of protons, and the gathering and storage of the resultant data, did not exist in 1986. "We counted on progress," Bagger says. The gamble paid off. The construction of the LHC was an international effort on an unprecedented scale: Pakistan and India are collaborating, as are Iran, Israel, and the United States. Since the first blueprints were put on paper, the project has sucked up the equivalent of $8 billion, leading to a running joke on the campus that the only black hole at the LHC is financial.
Apart from answering the Higgs questions, the LHC will likely produce some game-changing technology spin-offs. Predicting what those might be would require a crystal ball, but consider the technologies other accelerators have ushered in. Though superconducting magnets existed before Fermilab built its Tevatron accelerator, the sheer size of this machine necessitated an industrial process for manufacturing the magnets, which eventually led to the proliferation of MRI machines in hospitals. Researchers at CERN invented the World Wide Web (the very first HTML page was written there) to solve the communication problems endemic to an international collaboration among thousands of physicists.
Creating a Higgs particle is not as simple as just smashing the protons together to throw off the shrapnel of their constituent quarks. "The fact is, you are not splitting these protons apart," says Bagger. Instead, when the beams collide, the goal is to smash those quarks into each other with a force enormous enough to annihilate both. Because they cease to exist as matter, their entire being is converted into equivalent amounts of energy. The protons are merely carriers for the massive amounts of energy necessary to make the jump from one side of E=mc2 to the other. That energy, in turn, should yield strange, heavy particles that would be unable to exist in the current configuration of space-time.
The Higgs should be 100 times the mass of a proton, roughly the mass of a gold atom. Its lifetime is extraordinarily short: billionths, perhaps trillionths of a second. You'd think it would be poor payoff; even that tiny lifespan requires enormous amounts of energy. But there's a huge amount of information in how the particle disappears. Then it's a matter of forensics, and that's where the Johns Hopkins team comes in.
Their work involves the CMS experiment, a cylindrical detector on the French side of the LHC, which radiates out around the accelerator shaft in a series of ever-larger concentric shells, each containing sophisticated tracking equipment to detect the escaping particles. "The trackers are layered like Russian nesting dolls," explains Morris Swartz, a professor at Hopkins collaborating with Gritsan and Maksimovic. The Hopkins team's barrel is the innermost "doll." Given the mammoth size of the CMS experiment (the outermost shell is the height of a five-story building), "people are always surprised at how small the pixel tracker is," Swartz laughs. But it is no less monstrously complex than the rest of the machine: Its 66 million sensitive elements do a job analogous to a camera taking 40 million pictures each second. The Hopkins team has spent years developing and perfecting the silicon sensors and control electronics for the little pixel tracker in a collaboration that includes several other research entities, including Fermi National Accelerator Laboratory in Batavia, Illinois.
Gritsan has devoted much of his career to designing, building, and testing the little pixel trackers that will form the heart of CMS. But it was not until two years ago, when a colleague handed him the first prototype, that he understood where his painstaking work would culminate. "It was the first time I could see how, from such small structures, we were about to assemble a 12-kiloton detector," he says. He estimates that the barrel-like inner pixel tracker and its outer strip of pixels contain between 15,000 and 20,000 sensors.
Why so many chips? Even if the Higgs particle is created, there is no way to detect it directly or in real time. Once it has flashed into existence, it decays instantaneously into smaller, much lighter particles—and again, according to Einstein's equation, since they have smaller mass and smaller energy, these particles will be moving "really, really fast," Kaplan says. "You never actually see the Higgs. You see the result of the Higgs." The only way to know what happened during that microscopic explosion is to track exactly where all the debris went. From that data, both theoretical and experimental physicists will reverse-engineer what happened during that tiny fraction of a second…and what it means for the Higgs particle, the origin of mass, and physics itself.
You would think everyone would be eager to find the Higgs and close the books on this problem that has so vexed particle physics for so many decades. You would be wrong.
There are as many theories about what will happen in Geneva as there are physicists working on the Higgs. "This thing is a giant shootout," Bagger says. "Every theorist has his or her own idea of what will be found."
But regardless of the larger theory, it all comes down to one of four scenarios that will determine the future of particle physics: that the Higgs and only the Higgs will be found; that the Higgs will be found in concert with a constellation of other new particles; that many particles will be found, but no Higgs; and the most fascinating (or horrifying, depending on your perspective) of all—that the LHC will find nothing at all.
In some ways, the first option is also the worst. If only the Higgs is found and nothing else, "it would take the air out of the field," Kaplan says. "Physicists won't know what to do next."
Bagger explains: "We won't know where the loose strings are. In physics, we look for the loose strings and pull; we try to unravel a story. If it's all wrapped up beautifully with a bow around it, we won't know what to do next."
Bagger says that although he would welcome the existence of the Higgs alongside a smattering of other particles, he is averse to a complete resolution of the standard model's mysteries. His reluctance can be attributed to the discovery of dark matter. Dark matter and dark energy, which we now know make up 95 percent of the universe, connect the tiny scales of particle physics to the vast reaches of cosmology even while frustrating all efforts at classification. "We know dark matter is not quarks," Bagger says. "But there is no dark matter in the standard model. So if the LHC only finds a standard model Higgs and nothing else, we have a huge problem on our hands." Bagger hopes for the creation of dark matter particles at LHC. If LHC can whip them up out of thin air, he says, then scientists could study the particles and their properties whenever they wanted.
Falk confesses that he, too, would prefer something more interesting and complicated than an open-and-shut standard model. "I'm not so secretly hoping that the simple theory of the Higgs is not confirmed," he says. Echoing Bagger's sentiments, he says the world would be a less interesting place "if that's all there were."
But what if other particles are found—without the Higgs to tie up the standard model? That would make for a complicated situation, says Falk. "If there is no Higgs," he explains, "it doesn't mean the LHC has failed." However, such subtleties are not popular with the governments that control the purse strings of future physics experiments, and that funded the LHC with an implicit goal of finding the resolution to the standard model. The International Linear Collider, the planned next generation of giant particle accelerators, is dependent on politicians who may not be impressed by a perceived "failure" to find the Higgs. "If they fail, people aren't sure what's going to happen to fundamental science," Kaplan warns.
Then there is the possibility of finding nothing—neither the Higgs nor an array of new particles that provide further strings to pull. Kaplan considers that possibility his worst nightmare, a 21st-century variation on Kepler's search for the cosmological meaning of five: "When you go into a field, you make a decision," he says. "What questions am I going to address? What are the things I think are important and answerable? If you discover later that not only are they not answerable but, in fact, they were stupid questions—why do I think there are five planets? Wow—that's the panic some people are feeling right now."
But where the theorists feel panic, the experimentalists find this to be the most tantalizing possibility of all. Gritsan and Maksimovic are all but crossing their fingers for the discovery of "nothing" because, as Gritsan puts it, this would violate some truly fundamental principles. To Maksimovic, it means the answer is "nothing anyone has dreamed about."
For example, Sundrum and Randall have posited that the origin of mass is closely tied to another problem with the standard model—the weakness of gravity. This theory does away with the Higgs altogether in favor of extra dimensions that leech gravity's true strength out of a four-dimensional universe. "If we are the ones to discover new physics," Maksimovic suggests, "that would mean Raman is correct and he goes to Stockholm one year [for a Nobel prize], and we go the following."
To hedge his bets, Kaplan moved away from pure theorizing. He identifies himself as a phenomenologist: Instead of hanging his career on one theory—his own or someone else's—Kaplan will look at the data generated at the LHC and develop theories that fit the findings.
Possibly influenced by Kaplan, some graduate students have taken a similar approach. Ian Tolfree, a Hopkins graduate student on the theory side, says that by the time he had to choose his graduate work, he decided to hedge. "As a graduate student in theory, your entire career is basically spent studying one of these theories," he explains. "If the theory turns out to be the winner, you've hit the jackpot; if it turns out to be a loser, you're plain out of luck. So how do you choose one?" For that reason, Tolfree says, standing on the precipice of the biggest paradigm shift in decades has been "a bit nerve-racking" for the graduate students. And yet the allure of the blank slate has drawn him in as well. "Part of me hopes no theory explains the data," he says.
Though the LHC has been dogged by sensational press about its supposed ability to generate world-eating black holes, the machine's biggest threat is to itself. A mechanism with so many excruciatingly delicate parts that must be kept at optimal conditions (the magnets, for example, must be cooled to temperatures colder than outer space) is simply too expensive and too delicate for a technician to merely flip a switch to turn it on.
On Sept. 10, celebration rippled through the world as the first beam of protons was sent without collisions through the ring. It was a momentous milestone for engineering (at Fermilab, where the LHC turn-on happened at 2 a.m., they celebrated with a pajama party), but the enormous machine worked for barely two weeks before a helium leak in the cryogenics system forced a shut-down that will leave the accelerator out of commission until April.
The mood at CERN is resigned but in good cheer. Rather than being a catastrophe, many scientists see the hiatus as a well-deserved rest from the increasingly frenetic preamble. The whole project has taken almost 25 years. "I guess people here can wait a few more months," joked one CERN physicist. Most of the physicists involved understand that with any project this size, being a one-of-a-kind instrument, bugs will need to be ironed out.
And even after that, the LHC won't be fully operational for its first year or so. The machine is simply too expensive to leave anything to chance, so physicists will spend a full 12 months nervously working the LHC up to full capacity.
Maksimovic says it won't feel real, anyway, until the collisions take place, and even then, more than anything he feels fatigue at the amount of work to be done over the next year. "It's all blood, sweat, and tears," he says, "for at least six months, or nine, or 12."
Bagger says with some longing in his voice that today's undergraduates and graduate students are at an enviable point in human history. "They're at the perfect time to join this adventure," he says. "It's not going to be over in two years: A facility like this has a 20-year lifetime."
But of all the questions the LHC could answer, Kaplan says, the broadest of all is this: What should human beings be doing? "For 400 years we've pushed this envelope," he says. "Can we go deeper? Can we understand what is light, what is matter, what is vacuum, and space, and how did the universe get here?"
Sally Adee is a science writer based in New York City.
In the first millionths of a second after the Big Bang, the newborn universe was extremely hot. In a hot gas, particles move very quickly, and when they collide, they have a great deal of energy. In the extreme conditions of the new universe, the collisions resulted in huge amounts of energy—so much, says Hopkins physicist Jonathan Bagger, that at that point, it was just a grab bag of particles. All that energy allowed elementary particles to separate, combine, and recombine the way steam condenses into water and back again. But as the universe cooled, matter solidified into its various states as in a game of musical chairs: Whatever state you happened to be in when the universe had cooled to a certain point, that was where you stayed. And that, particle physicists theorize, is how the Higgs vanished from our current capacity of detection, even as quarks, leptons, and electrons froze into place as the basic building blocks of matter.
The world's particle physicists are hoping for different outcomes once the LHC begins generating data a year from now. Here's what various players at Hopkins are wishing for…
Jonathan Bagger, Krieger-Eisenhower Professor of physics: Wants to see the Higgs along with a smattering of other particles, ideally dark matter particles. He's rooting against a simple resolution of the standard model. "In physics, we look for the loose strings and pull. If it's all wrapped up beautifully with a bow around it, we won't know what to do next."
Adam Falk, James B. Knapp Dean of the School of Arts and Sciences and professor of physics: "I'm not so secretly hoping that the simple theory of the Higgs is not confirmed." The world would be a less interesting place "if that's all there were."
Petar Maksimovic, associate professor of physics, and Andrei Gritsan, assistant professor: They've crossed their fingers for finding "nothing," which would violate some fundamental principles and open up the possibility for "nothing anyone has dreamed about," Maksimovic says.
Ian Tolfree, graduate student: The allure of the blank slate has drawn him in as well. "Part of me hopes no theory explains the data," he says.
David Kaplan, associate professor of physics: The idea of nothing being found—neither a simple Higgs nor an array of new particles—is his nightmare. He's become a phenomenologist; rather than hanging his career on one theory, he'll focus on the data generated at the LHC and develop theories that fit the findings.