World and Nation

Proton Beams Are Back on Track at Collider

Physicists returned to their future on Friday. About 10 p.m. outside Geneva, scientists at CERN, the European Center for Nuclear Research, succeeded in sending beams of protons clockwise around the 17-mile underground magnetic racetrack known as the Large Hadron Collider, the world’s biggest and most expensive physics experiment.

For physicists, the event was a milestone on the way back from disaster and the resumption of a 15-year, $9 billion quest to investigate laws and forces that prevailed when the universe was less than a trillionth of a second old.

The collider was designed to accelerate protons to energies of seven trillion electron volts apiece and smash them together in tiny fireballs in an effort to replicate and study the conditions of the Big Bang.

The first time protons circled the collider, on Sept. 10, 2008, the event was celebrated with Champagne and midnight pajama parties around the world. But the festivities were cut short a few days later when an electrical connection between a pair of the collider’s giant superconducting electromagnets vaporized.

Subsequent work revealed that the machine was riddled with thousands of connections unable to handle the high currents required to run the collider at its intended energy.

Physicists and engineers have spent the past year testing and making repairs. While they have not replaced all the faulty connections, they have patched things up enough to allow the collider to run at less than full speed.

Calling the past year’s work a “Herculean … effort,” CERN’s director for accelerators, Steve Myers, said the engineers had learned from painful experience and understood the collider far better than they had before.

CERN’s director, Rolf Heuer, said in a statement, “It’s great to see beam circulating in the LHC again,” but he and others cautioned that there was a long way to go before the collider started producing the physics it was designed for.

When the collider begins to do real physics next year, it will run at half its original design energy, with protons of 3.5 trillion electron volts. The energy will be increased gradually during the year, but it could be years, physicists say, before the machine reaches its full potential.

Thousands of the troublesome junctions will have to be rebuilt during a yearlong shutdown in 2011, and engineers have to figure out why several dozen of the superconducting magnets seem to have lost their ability to operate at high intensities.

The delay has given new life to the collider’s main rival, the Tevatron at the Fermi National Accelerator Laboratory in Illinois.

If all goes well, CERN says, the protons will start colliding at low energies in about a week.

Those first collisions will occur at the so-called injection energy of 450 billion electron volts. The machine will then quickly step up to 1.1 trillion electron volts, which is just above the energy of the Tevatron.

CERN is hoping to achieve that landmark as a symbolic Christmas present before a short holiday shutdown.