The Large Hadron Collider is the biggest machine ever built. The particle accelerator is a 17-mile ring that lies inside a tunnel 350 feet beneath the border between France and Switzerland. On September 10, 2008, CERN, the international particle physics research organization that operates the LHC, started up the proton beams amid a media blitz and not-so-latent paranoia that it would produce a black hole that would swallow the Earth. Nine days later, there was an accident.
After a year of repairs and upgrades, CERN is cautiously restarting the accelerator’s massive physics experiments this month. By November’s end, proton beams should once again be slamming. If all goes well, the $9 billion machine will produce its first collisions in early December.
The LHC slams protons together at 99.9999991 percent of the speed of light. The purpose: recreate conditions that existed less than one trillionth of a second after the Big Bang. These high-energy collisions allow physicists to probe the structure of essential matter at incredibly short distances and to observe particles that are no longer stable in our universe.
To achieve its unprecedented speeds, the LHC accelerates beams of protons around a 17-mile-long ring of superconducting magnets. But for the massive electrical currents to flow through the magnets without resistance, the structure has to stay at 1.9 degrees Celcius above zero by continuously circulating 120,000 gallons of liquid helium through the structure. For scale, this is 0.8 degrees Celsius colder than the average temperature of the entire universe, including all the frigid emptiness of deep space.
On September 19, 2008, an electrical connection between two magnets failed, producing a dangerous arc of electric current and rupturing the pipes of liquid helium that were keeping the LHC cool. In a few seconds, 12,000 gallons of liquid helium escaped, a third of which vaporized almost instantly. The resulting explosion completely vaporized several feet of high-strength metal pipe and caused the temperature in the surrounding area to shoot up 100 degrees Celsius.
“There was a significant push to start the machine in 2008 and because of this push, presumably, some corners were cut,” said Brown physicist Greg Landsberg, who has worked at the LHC since 2004. “One of the reasons that the accident happened is that full tests were not accomplished before they started playing with the beams in the machine.”
The LHC is so big and so cold that after the accident it took a month just to warm up the damaged sector enough for technicians to enter the tunnel and survey the destruction. Since most European homes are fitted with electrical heating systems, budgetary restraints force CERN to mothball its accelerators every year as winter falls and electricity prices jump (electricity for the machine is expected to run $25 million per year as it is). As soon as the extent of damage to the LHC was clear, the project’s leaders shut down operations for a full year to repair the magnets and upgrade the machine with new safety equipment.
Despite the LHC team’s enthusiasm to finally start colliding particles, CERN is not taking any chances with the restart. “Nowadays, the accelerator team is much more conservative—I should say careful—in terms of what they are doing,” Landsberg said. “Things are going a little bit more slowly, but much more safely than they were last year.”
When the LHC finally starts crashing proton beams that are expected to exceed the collision energy of Illinois’s Tevatron accelerator—the current champion of the particle physics world—it will be the first time the energy frontier has left the United States. The accomplishment cannot be exaggerated. The peak energy produced in Tevatron’s collisions is 1.96 trillion electron volts; if all goes according to plan, the LHC should reach 14 trillion electron volts.
In the five years that Brown’s High Energy Physics lab has worked on the LHC, Landsberg spent much of his time in the air. With more than 20 people spread over two continents, the lab’s four professors split their time between the particle physics centers at Brown, Fermi National Accelerator Laboratory in Batavia, Illinois (home of the Tevatron), and CERN in Geneva. This requires Landsberg to make the 10 to 14-day trip to Geneva several times each semester and visit Fermilab each week he’s not at CERN.
The Brown lab has long worked at the energy frontier. At Tevatron, Brown was a founding member of the DZero experiment, which explored the fundamental building blocks of matter. As the Brown lab was deciding the role it would play in the Next Big Thing, Landsberg considered both of the LHC’s two main experiments. The decision was between the ATLAS experiment—which looks for the existence of extra dimensions and studies gravity—and the CMS experiment, which Landsberg decided to join. This project hunts for dark matter and the Higgs boson, the fundamental particle that theory suggests gives matter its mass.
These days, particle physics is governed by the Standard Model, a theory of matter that predicts the existence of 17 fundamental particles including electrons, photons, and quarks. Of the 17, only the Higgs boson has eluded observation. So far, at least. Physicists hope to find the Higgs by smashing together the LHC’s high-energy protons and tracking the jets of more fundamental particles that come shooting out.
If the CMS team can identify the Higgs, the existence of all the Standard Model particles will be confirmed. But if not, physicists will have to develop a new theory of everything.
“In the five years since we joined [the CMS team], we have contributed to pretty much every aspect of the experiment,” Landsberg said. The group engineered an algorithm that “selects the interesting interactions from the more than 40 million interactions picked up every second” by the detector. This function is essential, since even CERN’s worldwide, distributed computing Grid cannot handle the full 40 terabytes of unfiltered data CMS will produce every second once collisions begin.
The Brown team also helped design and build the structure of CMS’s silicon particle tracker, in the core of the experiment’s 12,500-ton detector. This tracker allows physicists to identify the paths of particles shooting out from the collisions inside. By analyzing the tracks of these particles, physicists hope to measure their individual energy and momentum to determine if Higgs bosons were produced.
But Landsberg’s most memorable contribution to physics was probably his 2001 paper, “Black Holes at the LHC.” In it, he argues that if the theories predicting the existence of tiny, curled up extra dimensions are right, then the LHC may produce tiny black holes at a prodigious rate. Feasting on hints of impending apocalypse, media pundits spread panic that black holes might grow uncontrollably to swallow the Earth. But as Landsberg told reporters last year, these black holes should quickly disappear in a distinctive burst of energy, and the chances of the LHC completely annihilating the planet are “totally miniscule.”
Rays of hope
Although the famed LHC project stalled, Landsberg insists that the last year was anything but a waste. “The fact that we didn’t have LHC collisions was, of course, disappointing,” he said, “but we got an enormous amount of cosmic ray data, which we used to essentially do many of the things we hoped to do this year with the real beam of particles.”
High-energy cosmic rays—which generate muon particles—continuously bombard the Earth, and might have influenced the LHC detectors, had CERN not buried the accelerator hundreds of feet under Europe. Although the depth keeps most muons from reaching the machine, about 100 muons pass through the CMS detector every second. But this was, in fact, proved useful: the muons provided Landsberg and his team with enough data to calibrate and align equipment and refine their methods. “In fact, we now have [the tracker] aligned with better precision than we expected with the very first collider data,” Landsberg said. This work should allow the CMS team to begin making discoveries almost immediately after collisions begin next month.
Members of Landsberg’s team already have a timeline predicting when it will have enough data to answer some of their biggest questions. The team has practiced its statistical analysis and written the bulk of some papers already. The only thing still needed, of course, is some real data. That should come soon.
Chastened by the embarrassment of last year’s highly publicized accident, CERN is scaling back its second startup celebrations. Last September, Fermilab hosted a champagne-popping pajama party in Batavia where physicists gathered to watch the first beams circulate through the machine. Television cameras packed the LHC control room, providing the footage for pundits to argue over just how black holes would engulf life as we know it.
This year, CERN will communicate by press release.
If the LHC produces a black hole that swallows the Earth,
NICK WERLE B’10 doesn’t have to finish his thesis.