LHC hopes for collisions by Christmas

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But particle physicists will have to scale back the energies of their experiments for years.

LHC experiments such as the Compact Muon Solenoid (above) are still waiting to see particle collisions after the accelerator was left crippled by an accident in September 2008 (inset). Credit: M. BRICE, CERN; CERN

The Large Hadron Collider (LHC) should yield its first data by Christmas, smashing protons at energies high enough to begin pushing back the boundaries of particle physics. But the world's largest particle accelerator will only be operating at half the energy that it was originally designed for, and may not reach that peak until 2011 — if at all.

The new schedule was announced by CERN, Europe's particle-physics laboratory near Geneva, Switzerland, on 6 August. The 6.5-billion Swiss franc (US$6-billion) LHC has been offline for nearly a year, following an accident on 19 September 2008 just nine days after the facility circulated its first particle beams to worldwide fanfare. When a faulty connection burnt out between two of the superconducting magnets used to accelerate particles, 8.7 kilo-amps of current arced outwards in a massive short-circuit. The current ripped a hole in the pipe carrying liquid helium that surrounds the superconducting wire, spewing out soot and a blast of boiling helium, which damaged a total of 53 magnets.

During repairs this spring, a new set of problems related to faulty magnet connections was discovered. Although CERN officials believe they now have the problem in hand, they are being cautious in ramping up the beams' energies. In mid-November, protons will be injected into the 27-kilometre-long accelerator ring at low energies to show that two particle beams can circulate well in opposite directions. About four weeks later, the beams will be accelerated to about 3.5 teraelectronvolts (TeV) each, halfway to the LHC's maximum beam energy. Smashing these beams together will create showers of exotic particles, and experimenters will begin gathering the data they need to calibrate their instruments.

"We could have a Christmas present for the experimenters if we're lucky," says Steve Myers, head of CERN's accelerator department. If the initial collisions go well, Myers says, engineers will ramp up to energies between 4 and 5 TeV per beam sometime in 2010. The beams will then shut down in October or November 2010 for six months in order to make further repairs and install more helium-release safety valves, before attempting to reach higher beam energies in 2011.

Theorists say that LHC collisions with a total energy of 7 TeV should start to reveal previously unseen particles, although it might take a little longer than hoped to accumulate evidence for their existence.

The standard model of particle physics, describing the zoo of subatomic particles and the forces that control them, begins to break down at energies above 1 TeV. Experiments at higher energies could reveal a whole new menagerie of particles predicted by a popular successor to the standard model called supersymmetry. Also within reach are particles from a hypothesized form of dark matter, believed to make up a quarter of the mass of the Universe. Finally, one of the most sought-after targets is the Higgs boson, the only particle predicted by the standard model that has not yet been found. The elusive boson is a marker of the Higgs mechanism, which could explain how particles have mass.

Bad welds

The 2008 accident was caused by a faulty weld in a section of superconducting wire connecting two magnets. The subsequent months of inspections of the rest of the LHC found just four more bad welds in this type of connection — but also revealed a far more widespread problem.

The superconducting wires are surrounded by copper wires, which act as 'safety valves' to carry any sudden surge in current. The copper wire will only come into play if the superconducting wires warm up and lose their ability to conduct electricity without resistance.

Tests over the past few weeks have revealed 80 bad welds between sections of copper wire, but with roughly 10,000 copper-wire welds scattered around the LHC, not all have been inspected. This element of uncertainty means that CERN must increase the operating energy of the machine cautiously to avoid further accidents.

The cost of repairs so far is 40 million Swiss francs. Myers acknowledges there was a quality-control problem with the welds, and that the systems for detecting current surges and mitigating a catastrophic release of helium could have been improved. "It's usually the simple things that cause you problems," he says.

Peter Limon, a physicist at Fermi National Accelerator Laboratory in Batavia, Illinois, says these birth pains are "typical". Limon points out that various problems meant that Fermilab's Tevatron — currently the world's highest-energy collider — took years to reach its maximum collision energy of 1.8 TeV. With a year or two of data collection needed for the LHC to make a definitive Higgs discovery, the Tevatron still has a chance at bagging it first. Tevatron physicists plan to present the latest update on their Higgs hunt on 18 August at the Lepton-Photon conference in Hamburg, Germany.

Gordon Kane, a theorist at the University of Michigan in Ann Arbor, says that the physics community is frustrated by delays at the LHC, but adds that an extra year is not so long to hold on for the Higgs particle, given that its existence was proposed in 1964. "I'm going to be there when it's discovered," says the 72-year-old Kane, "no matter how long it takes."

Additional information

For more on the LHC, see www.nature.com/lhc

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Hand, E. LHC hopes for collisions by Christmas. Nature 460, 784–785 (2009) doi:10.1038/460784a

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