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Intelligent Machines

The Making of a New Collider

The biggest physics experiment ever, CERN’s new particle accelerator, goes live this summer.

  • by Katherine Bourzac
  • April 22, 2008
  • The biggest physics experiment ever, CERN’s Large Hadron Collider (LHC), goes live this summer. The international project, whose design was approved in 1994, cost over $6 billion. Thousands of powerful magnets, cooled by tons of liquid helium to 1.9 Kelvin (just above absolute zero), will guide two beams of protons as they travel in opposite directions around a 27-kilometer tunnel at close to the speed of light; then magnets at two locations will pull the beams together for the highest-energy particle collisions ever achieved. By identifying the products of these collisions, physicists hope to test the standard model of physics and discover new subatomic particles. (Read Nobel laureate Jerome ­Friedman’s thoughts on the LHC.)





    Precision Before Collision
    The compact muon solenoid (CMS), one of the LHC’s main detectors, is made up of 11 conjoined pieces; each has layers of detectors tuned to find different particles that might result from proton collisions, including muons and electrons. This piece weighs 1,270 metric tons and was lowered from above ground to the bottom of the tunnel, 90 meters below, with mere centimeters of clearance. It took 11 hours.
27 kilometers of magnets
The two beams of protons will speed through their underground tunnels at a site straddling the border between France and Switzerland.
The tubes here house the high-power magnets that guide the beams.
A side view shows the channels through which the beams will accelerate (openings at center of image, one with a cable dangling from it). These two pipes are surrounded by superconducting metal cables (not ­visible in this image) through which a tremendous electric current flows, creating strong magnetic fields that guide the protons around the LHC and then toward each other for the high-energy collisions. Two tubes for the liquid helium that cools the magnets are visible at the bottom of the cross section.
This story is part of our May/June 2008 Issue
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CMS detectors
The CMS is named for its compact magnet, called a solenoid because of its coiled shape, and for one of the particles it specializes in detecting, the muon. When protons collide inside the CMS, the magnet at its heart (metal collar) deflects the resulting subatomic particles so that their paths intersect with many layers of detectors.
Layers of silicon tiles inside the inner tracker barrel, which nests inside the magnet, pinpoint the location of charged particles and measure their momentum.
The protruding barrel of the piece below also fits inside the magnet’s hollow. The rings of gold-­colored boxes are muon chambers that will detect the particles.
Picturing Higgs and Z Prime
Data gathered inside the CMS and other detectors will be reconstructed as event visualizations like the hypothetical ones pictured here. In this image (and the one on the next page), the dots represent ionization signals left by particles traversing a detector. Software picks through the data to trace particles’ paths, represented as lines. The existence of newly observed particles is inferred if the products they decay into are detected. One of the particles likely to be detected by the CMS in its early days is called Z prime, says MIT particle physicist Steven Nahn; the evidence it leaves behind is thought to include two easy-to-detect particles, muons and electrons. The visualization is of a Z prime decaying into jets of energetic particles, represented by the rectangular beams.
The image is a visualization of a Higgs boson decaying into four muons. The curly lines represent particles with low momentum that don’t reach the farthest detectors. Providing evidence of the Higgs boson, a hypothe­sized particle that is thought to explain why particles have mass, would be a major coup for the LHC.

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