The world's largest particle collider was running again on Friday after an electrical fault forced it to stop just days after being launched to global fanfare, the European Organisation for Nuclear Research (CERN) said.
The problem affected a cooling system for high-powered magnets designed to steer beams of particles around the Large Hadron Collider's 27-kilometre circular tunnel, CERN said.
CERN spokesman James Gillies told AFP a 30-tonne transformer in the cooling system failed on Thursday last week and it had taken about a week to replace the equipment and get temperatures back to their required state.
The steering magnets in the LHC tunnel are chilled to as low as -271 degrees Celsius, which is close to absolute zero and colder than deep outer space.
"In layman's terms, the LHC is a great big fridge, and part of the power supply failed," he said. The LHC took nearly 20 years to complete and at six billion Swiss francs ($5.46 billion) is one of the costliest and most complex scientific experiments ever attempted.
It aims to resolve some of the greatest questions surrounding fundamental matter, such as how particles acquire mass and how they were forged in the "Big Bang" that created the Universe some 13.7 billion years ago.
The September 10 switch-on saw the testing of a clockwise beam, and then an anticlockwise beam. The first collisions are not expected for a number of weeks, given the long process of testing the LHC's equipment.
Gillies said that despite the setback, the LHC was not behind schedule.