Europe’s Large Hadron Collider tests the bounds of physics – and budgets
Scientists look for technologies to push particles faster, better, and cheaper.
If all goes well, this weekend a handful of protons will make their first, tentative entrance into the main rings of the world’s most powerful time machine.
It’s an important step toward the full-scale start-up of the Large Hadron Collider (LHC), a mammoth particle accelerator spanning the French-Swiss border. That start is expected in early September.
Physicists will accelerate beams of protons in opposite directions – each along its own nearly 17-mile circular path – to nearly the speed of light. Then, they will steer the hair-thin beams of protons into head-on collisions.
From the subatomic mayhem that ensues, physicists say they anticipate discoveries that will fill out the picture they have drawn during the past century of matter and the basic forces of nature – the so-called standard model. They also expect to see evidence of new physics beyond the standard model, including insights into the nature of dark matter and dark energy, which make up the vast majority of the energy and matter in the universe.
Yet even as scientists and engineers put the LHC through its final tests, researchers worldwide are exploring ways to build more-powerful, less-expensive accelerators. Results from the LHC will play a key role in determining how much more powerful they need to be. But one thing is clear, several physicists say: Attempts to use today’s technologies for tomorrow’s collider frontiers are likely to face virtually insurmountable cost and technical barriers.
“As you go up to higher energies, these facilities become more and more expensive,” says Dennis Kovar, associate director for high-energy physics in the US Department of Energy’s Office of Science. “If there are going to be next-generation colliders, one is going to have to have some breakthroughs in technology ... to be able to do it at an affordable price.”
The estimated price tag for the LHC is $5 billion to $10 billion.
How colliders work
At the high-energy frontier, physicists have been working with two groups of crash dummies: protons and their antimatter counterparts, antiprotons; and electrons and their mirror opposites, positrons. Each group has its own set of advantages and disadvantages, explains Harry Weerts, director of high-energy physics at Argonne National Laboratory in Argonne, Ill.
With the LHC, for instance, protons are the collision particles of choice. They carry an electrical charge (positive), which allows scientists to use magnets to focus the protons into tight beams and steer them around their circular racetrack. In the LHC’s case, the collider boasts more than 1,600 superconducting steering and focusing magnets. And protons have a respectable amount of mass, so scientists can whisk them around a bent path without the particles losing much energy.
But protons also have a drawback: They are made up of other particles – three quarks – that are bound to each other with yet more particles, called gluons. When protons collide, “it’s like colliding a bag of billiard balls with another bag of billiard balls,” Dr. Weerts says. The bags slam together, but the meaningful collision action is taking place among the billiard balls, not the sacks holding them. Indeed, he says, in reality, the LHC is really a quark collider.
By whatever name, the upshot is: The collisions are a mess. Scientists must sift through a lot of collision debris to spot the signatures of the particles they are trying to find. That requires long periods of operation to amass enough statistics on the collisions to convince themselves and their colleagues they have a genuine “eureka!” result.
Faster particles, accelerating costs
The collision energy in a proton collider must be substantially higher than might otherwise be the case because the energy is parceled out to varying degrees among all the quarks and gluons in the mix, not just among two protons.
Researchers at the European Organization for Nuclear Research (CERN) are laying plans to upgrade the LHC’s power in about 10 years. Still, scientists say they are fast approaching the limits for affordable proton colliders, even when an international collaboration is sharing the cost. At some point on the climb up the collision-energy ladder, the protons’ heft won’t prevent them from losing increasing amounts of travel energy as they constantly shift directions in a circular ring, rendering them less practical. And the magnetic fields needed to keep a rein on the protons would grow so high that no known, or at least affordable, material could withstand the physical stresses the magnetic fields would set up.
This has prompted many physicists looking harder at electrons, and their heavier cousins, muons, to literally get more bang for the buck. Electrons have been getting a workout for years at the Stanford Linear Accelerator Center (SLAC) in Menlo Park, Calif., and at the LHC’s predecessor at CERN, the Large Electron-Positron Collider.
Unlike protons, electrons are fundamental particles – they have no internal components. So every hit is clean; all of the energy of the collision goes into forming the heavier particles scientists are interested in. Scientists can accomplish the same physics with fewer collisions than a proton collider requires.
The proposed International Linear Collider (ILC), for instance, would smack electrons into an onrushing beam of positrons to create copious numbers of particles called the Higgs boson. It’s a particle thought to impart mass to other particles, and the LHC stands an excellent chance of finding it. But, researchers say, that’s like discovering the shoreline of a new continent. It’s an important advance, but takes repeat visits to truly explore.
The LHC uses a specific energy level to ensure its collisions are in the predicted range for detecting Higgs, as well as other phenomenon. The ILC initially would operate with a fraction of that energy, some 500 billion electron-volts instead the 14 trillion used by the LHC.
But electrons are not the perfect projectiles either. In effect, they chafe at flitting around in circles. They have so little mass that as they speed around an accelerator ring, they lose energy through a form of radiation known as synchrotron radiation. So accelerators that use electrons for high-energy physics experiments typically are straight-line, or linear, colliders. Here, too, the size of the accelerator has limits – largely economic. SLAC’s linear collider is two miles long. The ILC’s would initially span 22 miles. It would need to grow by another nine miles to achieve the highest collision energy physicists envision for it.
Ways to push particles even faster
To rein in the size of such machines, researchers are exploring several unconventional approaches to giving electrons and positrons a series of swift kicks.
Last year, for example, researchers using facilities at SLAC reported a significant proof-of-concept advance using an approach a Tour de France cyclist might appreciate.
The team sent two closely spaced pulses of electrons through a heated tube filled with lithium ions. The first pulse turned the gas into a plasma and set up a wake as it passed. Electrons from the second pulse that hit the wake got kicked to far larger energies. Over a length of just under three feet, the team accelerated a small fraction of electrons to energies they ordinarily would reach if they traveled the full length of SLAC’s two-mile tunnel.
The small number of affected electrons, and the one-instance nature of the experiment show this approach clearly is a work in progress, researchers say. Linear colliders require multiple shots of particles to build up enough collision statistics to yield a meaningful interpretation. Within two years, the research team expects to have a two-shot experiment ready to fire. If they can pull off a rapid-fire repeat, “then we’ve got something,” says Thomas Katsouleas, dean of Duke University’s Pratt School of Engineering and a member of the team working on the approach.
Other teams are using lasers, instead of an initial pulse of electrons, to set up the wake in the plasma. In 2006, a group a the Lawrence Berkeley National Laboratory and Oxford University reported kicking an electron beam to more than 1 billion electron volts over a distance of less than two inches.
But there may be hope yet for ensuring rings don’t fall out of fashion. Researchers also are exploring “muon colliders.” Muons are heavier cousins to electrons. With far more mass than an electron, muons can be used in circular accelerators, notes Harold Kirk, a physicist at Brookhaven National Laboratory in New York. They are still fundamental particles and so would yield the same clean hits that electrons provide. But they decay into other particles in 2.2 millionths of a second, he adds.
That sets up a significant challenge, he says, “to get the muons created, captured, collected, and accelerated” all within a muon’s lifetime. Einstein’s theory of special relativity, however, comes to the rescue. Once the muons are accelerated to nearly the speed of light, 2.2 microseconds from the particles’ perspective stretches out to many milliseconds as seen by a technician monitoring the particles. And that’s plenty of time to get the collision work done, he adds.
Last year at CERN, a team Mr. Kirk led put a prototype muon factory through its paces and showed that it could generate copious number of muons. The process also generated another type of particle physicists are keenly interested in – neutrinos. Thus a muon collider could pull double duty as a tool for neutrino physics, in addition to it high-energy-physics duties, Kirk says.
The next step, he adds, is to collect and cool the diffuse cloud of muons so that they can be focused and sent as a beam through an accelerator. The team is working on a concept to achieve that goal in an experiment in Britain slated for sometime in the next two years.
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[Editor's Note: As a commenter pointed out, the original version of this story had Einstein's theory flipped around. The faster a muon travels, the longer it takes to decay from the technician's point of view.]