Scientists turn on biggest ‘Big Bang Machine’

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.

Video   Supercollider conducts first tests Sept. 10:  The first tests of the world's largest particle collider went off without a hitch, as scientists fired a beam of protons through the 17 mile-long structure. NBC's Dawna Freisen reports. MSNBC

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.

To solve their mysteries, the LHC's scientific sleuths will use the latest and greatest tools of the trade, built at a cost of billions of dollars. The two main detectors — ATLAS (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid) — are structured like the layers of an onion to spot different kinds of particles:

• Trackers: Both detectors have tracking devices at the center to follow the paths of short-lived particles.
• Calorimeters: The next layers are two different types of calorimeters that measure the energies of the particles given off. One captures electromagnetic energy, while the other captures the energy from particles such as protons, neutrons and pions.
• Magnets: Huge magnets are built into each detector to bend the paths of the particles so they can be identified by their charge.
• Muon detectors: The outer layers of the detector track the paths of muons, particles that can't be stopped by any of the inner layers.

Probing the smallest scales of matter requires some of the biggest machines ever devised. ATLAS is the largest of all detectors, measuring 151 feet long and 82 feet high — bigger than your typical apartment building.

"It has an awful lot of free space inside," CERN theoretical physicist John Ellis explained. "The reason for that is, they want to be able to measure particles which come out of the collision ... even if the interior of the detector is so clogged with collision products they can't measure them properly there."

Over on the other side of the LHC's ring, CMS takes up less than half as much space as ATLAS but weighs almost twice as much. It contains more iron than the Eiffel Tower, built into alternating magnetized layers with particle detectors like a metallic jelly roll. CMS' built-in magnets and its expensive fine-resolution silicon tracker are part of a different strategy to do the same things that ATLAS does.

"You get big arguments between the ATLAS guys and the CMS guys as to which is the best way to measure these particles," Ellis said. "ATLAS is going to bend them that way, CMS is going to bend them this way, and we'll see in a few years' time which is the better idea."

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