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LHC switching gears

European collider shifts from protons to lead

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Physicists at the Large Hadron Collider in Geneva have said a temporary au revoir to energetic protons and are settling in for a more leaden experience. On November 4, the collider ended a seven-month run in which twin beams of protons collided head on at record-setting energies of 3.5 trillion electron-volts per beam. Now researchers are readying the atom smasher for a month of collisions in which lead atoms stripped of their electrons will crash into each other.

Collisions between the lead ions are expected to generate a primordial soup known as a quark-gluon plasma. (Quarks are the fundamental building blocks of such subatomic particles as protons and neutrons; gluons are the messenger particles of the strong force, which binds quarks together.) Such a plasma is believed to have existed a few millionths of a second after the Big Bang.

The center of the quark-gluon plasma produced by the LHC may reach temperatures 500,000 times greater than that at the sun’s core. Studying the material may reveal how quarks and gluons cooled following the Big Bang, forming the complex particles and structures seen in the universe today.
 
The lead collisions will test the capabilities of the collider’s ALICE experiment, which was designed to record data on interactions between heavy ions. After completing the collisions between lead ions, the collider will shut down for two months before resuming high-energy collisions between protons in early 2011.

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