Particles will circulate in opposite directions in each beam line—clockwise in one, counterclockwise in the other. The individual beam lines will keep the racing particle streams separated—except at four points around the ring where physicists will deliberately allow the streams to cross. At those spots, the LHC physicists will observe the resulting mayhem with detectors of staggering scale and complexity.
Standing at one of these collision points, I try to imagine the energy involved. “If I were down here when the beam was operating, would it be highly radioactive and dangerous?” I ask. “If you were down here when the beam is operating,” Limon replies, “it would be highly radioactive and fatal.” There will be 600 million particle collisions per second, and although the particles themselves are mere specks—less than a million millionth the size of a gnat—their collective energy will be that of an express train. Once set in motion, a stream of particles might circulate for 10 hours before needing to be refreshed. During that time, it would travel more than 6 billion miles, enough to get to the planet Neptune and back.
“I think this is the most complicated thing that humans have ever built,” Limon says, proudly.
The beam pipe (blue) in the LHC's main tunnel; houses two particle streams that zoom in
opposite directions at near light speed.
Image courtesy of © CERN
The LHC’s subatomic fireballs will be the highest-energy particle collisions ever seen on Earth. This is uncharted territory: The collisions at LHC could spray out strange new kinds of matter, unfurl hidden dimensions of space, even generate tiny glowing reenactments of the birth of the universe. In short, there is more than just the search for the Higgs going on at the LHC. “We don’t even know what to expect,” says French physicist Yves Schutz. “We’re now in a domain of energy that nobody has ever explored.”
Schutz is focusing on one of the other projects here. His experiment is A Large Ion Collider Experiment, or ALICE (whimsical acronyms are a way of life here), which will smash ultraheavy lead ions together to create a miniature fireball to mimic the first split second after the Big Bang. In spite of its name, ALICE is one of the two smaller experiments on the ring. The other, LHCb, will seek to understand why the universe contains matter rather than antimatter or, worse, nothing at all (to find out more about these other experiments, see the online version of this article at www.discovermagazine.com).
But the stars of the LHC are the two rival detectors, set diametrically opposite each other on the ring. In one corner is ATLAS; squaring off a little over five miles away is the CMS. Together, these two detectors cost a cool $850 million, and although their designs are quite different, they are looking for exactly the same things.
Touring these vast experiments, one wonders why CERN decided to double its efforts and costs. Why not pour all its resources into one detector to ensure CERN’s place at the top of particle physics as quickly as possible?
The reason is a fundamental principle of science: Experimental results must always be confirmed through duplication. In earlier decades, there was more or less a parity of atom-smashing capabilities between the United States and Europe, each leapfrogging and confirming the results of the other in turn. But when America abandoned its plans to build the Superconducting Super Collider in 1993 (with $2 billion spent and 14 miles of tunnel already dug in Texas), it left the LHC without a peer. So to prevent any embarrassing excursions into the scientific wilderness, CERN decided to build two detectors with independent teams, each to check the results of the other. As the exact properties of the Higgs are unknown, two different designs also allows CERN to hedge its bets.
As I arrive above the surface of the CMS, British physicist Dave Barney explains that the name of his experiment stands for Compact Muon Solenoid. A solenoid is basically a cylindrical electromagnet that generates a very uniform magnetic field inside the cylinder; the uniform field makes it easier to calculate the momentum of particles produced from collisions. The CMS electromagnet is “compact” only in the sense that it is incredibly dense. At 40 feet long, it is the biggest superconducting solenoid ever made, costing $65 million, weighing about 485,000 pounds, and containing as much iron as the Eiffel Tower. From the outside it looks like a huge steel bullet protruding from the center of a steel cylinder some 50 feet tall, covered in cables and instruments and surrounded by scaffolding. “The magnetic field is immense; if they switched it on now and you had steel-capped shoes, you’d fly over there,” Barney says.
The magnet will deflect the spray of new particles created by the colliding streams, while other instruments around it will detect the paths of those particles, soak up and register their energies, divining what they are and where they came from. Many of the particles will survive only a trillionth or less of a second before decaying, but that will be long enough to leave a telltale trail. The vast size of the CMS is a function of the immense energies involved. The bigger the energy, the stronger the magnet needed to deflect the particles and the more space required to register their properties. “If you want to build the biggest bangs in the world,” says Barney, “you have to give them space to breathe.”
