The Large Hadron Collider will run at only half its maximum energy when it restarts in November after a serious fault forced it to be shut down for more than a year.

Officials from the CERN particle physics laboratory in Geneva announced last night that it will be 2011 before the world’s most powerful atom-smasher reaches its full capacity.

While the £4 billion “big bang machine” should eventually be capable of running at an energy of 7 teraelectronvolts (TeV), it will operate initially at just 3.5 TeV when it starts smashing protons together in mid-November. The first science results are expected a few weeks later.

It will move up to higher energies only once engineers are confident that it is safe to do so, and it will reach maximum power only after it is shut down for a refit in the winter of 2010-11.

The lower-energy first run will still allow scientists to use the collider to search for the Higgs boson — the elusive “God particle” that is proposed to give matter its mass — and to investigate other new aspects of physics.

It will take longer to collect the data needed for these experiments, however, increasing the chances that evidence for the Higgs boson might first be found by the less powerful Tevatron accelerator at Fermilab in the United States.

“We’ve selected 3.5 TeV to start because it allows the LHC operators to gain experience of running the machine safely while opening up a new discovery region for the experiments,” said Rolf Heuer, CERN’s director-general, said.

James Gillies, CERN’s head of communications, said an energy of 3.5 TeV would allow operators to gain experience with the machine at lower energies, without seriously compromising the LHC’s ability to investigate new physics.

He said: “The decision had already been taken not to run at over 5 TeV, and the question was, do you lose anything in terms of physics by running at 3.5 TeV rather than 5? It still gives you the same access to new physics, but it will just take longer to get there. We don’t lose sensitivity to the Higgs, or to supersymmetry and dark matter.”

Even at 3.5 TeV, the LHC will still be significantly more powerful than the Tevatron, which operates at a maximum of 1 TeV, and is still in pole position to find the Higgs boson in spite of the delays.

While the Tevatron has established a mass range for the Higgs boson, it will be capable of finding it only if it its mass lies in a narrow band. The LHC should find it whatever its mass, even running at half its maximum capacity.

The first beams were injected into the LHC on September 10 last year, but nine days later a connection between two magnets failed. This caused a huge leak of the helium that cools the 17-mile (27km) ring around which protons will be fired against one another at 99.9999991 per cent of the speed of light.

The leak inflicted further damage and the accelerator was mothballed so that 53 magnets could be replaced. Engineers have since found and replaced other magnet connections that could have been at risk of causing a repeat of the fault, and installed other safety features to prevent another fault.

CERN announced earlier in the year that it would run the LHC throughout the winter at an extra cost of about £13 million, to make up for the delay. The accelerator would normally be shut down over the winter for maintenance and to save on peak electricity.

Once engineers have more experience with running the LHC at 3.5 TeV, the energy will be increased towards a maximum of 5 TeV next year.

At the end of 2010, it will also be used to collide lead ions for the first time, so that the Alice experiment can study quark-gluon plasma, a state of matter thought to have existed immediately after the Big Bang. The LHC will then shut down at the end of 2010, so that further modifications can be made to allow the machine to run at 7 TeV.

Dr Heuer said: “The LHC is a much better understood machine than it was a year ago. We can look forward with confidence and excitement to a good run through the winter and into next year.”

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