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Once the machine is in full operation, two streams of invisible protons will be whipped up in opposite directions around an underground racetrack to 99.999999 percent of the speed of light. When the two waves of protons slam into each other, scientists expect particles to melt into bits of energy up to 100,000 times hotter than the sun's core a state that should replicate what the entire universe was like just an instant after it came into being.

How can the Large Hadron Collider possibly perform such feats? That's where the wonder begins.

Going down ...
No one was allowed in the underground tunnel for Wednesday's maiden run, but a visit during the final phases of the LHC's construction provided an inside look at the wonder at work.

During the seven-year construction phase, components of the collider and its detectors had to be lowered down piecemeal from CERN's assembly halls, then put together in underground caverns as big as cathedrals.

Although the scale of the project is impressive, these cathedrals are no gleaming shrines to science: Our trip felt more like going into the bowels of a well-worn power plant or subway system. That's because most of the facility was actually carved out in the 1980s for an earlier particle-smasher called the Large Electron Positron collider, or LEP. CERN has spent the past seven years remodeling the space for the Large Hadron Collider.

Steven Nahn, a physicist at the Massachusetts Institute of Technology, conducted research at CERN during the LEP era. "They stole our tunnel, that's the way I see it," Nahn joked as Limon showed us around.

For years, Nahn, Limon and thousands of other researchers have pitched in on the design and assembly of the LHC's instruments, forsaking quiet laboratories for the din of the construction site as well as the occasional industrial mishap.

The LHC tunnel: Misbehaving magnets
Limon is a veteran of Fermilab's Tevatron, which had been the world's most powerful collider but is being dethroned by the LHC. At full power, the proton beams at the LHC will run into each other with the force of two 400-ton bullet trains going 100 mph. That amounts to 14 trillion electron volts, or about seven times the Tevatron's maximum power.

To bend those subatomic bullet trains into a circular path requires a chain of more than 1,800 superconducting magnets that have been chilled so close to absolute zero that they're colder than the average temperature of outer space (1.9 Kelvin, or 456.3 degrees below zero Fahrenheit).

Some of those magnets have to be collimated to focus the beams precisely at the ring's four collision points, like a telescope focusing light onto its mirrors. Drawing on its experience from Tevatron, Fermilab was put in charge of providing many of those magnets. But back in March 2007, a design flaw led to a violent breakdown during a cooldown test. The supports that held the magnet in place came loose with a loud bang and a cloud of dust.

"Everybody ducked about two seconds after it happened," Limon recalled.

The LHC's scheduled startup had to be delayed 10 months to install and test a fix for the faulty magnets. Even with the fix, there's no guarantee that the magnetic field will always hold. A runaway proton beam could blast right through its helium-cooled pipeline and kill anyone who got in its way. That's why the tunnel is sealed off for each run. If anything goes wrong, a computer-controlled system will shut down the collider and send the errant beam down a blind alley within milliseconds.

However, if everything goes right, each pulse of protons will whip around the ring 11,000 times a second, traveling the equivalent of a trip to Neptune and back before they slam into the protons going the other way at four points around the ring. Four main detectors will watch what happens next.

ATLAS and CMS: What the detectors do
For millennia, people have studied how things work by breaking them apart and watching what happens to the pieces. Physicists started doing that with atoms about 90 years ago, confirming that atoms were composed of electrons, protons and neutrons plus a menagerie of other particles they never expected to find. (After the discovery of the muon, physicist Isidor Rabi famously exclaimed, "Who ordered that?")

Physicists determined that protons, neutrons and many of the other particles were built up from even more fundamental constituents known as quarks. The particles built up from quarks are classified as hadrons, and that's where the LHC's name comes from: It's a large collider that smashes hadrons together.

So what will come out of those tiny, trillion-degree smash-ups? The LHC will look for exotic high-energy particles that supposedly came into existence just after the big bang for example, the Higgs boson (which is thought to give other particles their mass) or supersymmetric particles (which may account for much of the universe's dark matter).

These particles can't be detected directly, because they interact so weakly with ordinary matter. Instead, the LHC's detectors will track how those particles decay into more easily detectable particles as they fly out from the collision point.

It's like reconstructing the scene of a crime from forensic evidence: Scientists will try to track down the usual suspects (or, they hope, the extremely unusual suspects) by analyzing the subatomic evidence that the culprits leave behind.
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