The Large Hadron Collider, a powerful particle accelerator buried beneath the Swiss-French border near Geneva, restarted on Sunday, after two years of "intense maintenance and consolidation" to upgrade the machine, scientists said.
The European Organization for Nuclear Research, known as CERN, said proton beams began circling the machine's 16.7-mile tunnel on Sunday morning.
"The return of beams to the LHC rewards a lot of intense, hard work from many teams of people," Paul Collier, head of CERN's Beam Department, said in a statement. "It's very satisfying for our operators to be back in the driver's seat, with what's effectively a new accelerator to bring on-stream, carefully, step by step."
Scientists hope to use the particle accelerator to revolutionize their understanding of physics by slamming protons together at a speed close to that of light.
After discovering the Higgs Boson particle in 2012, the machine was shut down in 2013 to undergo an extensive upgrade that included consolidating some 10,000 electrical interconnections between the magnets that push the protons along the accelerator.
"Furthermore, the beams will be set up in such a way that they will produce more collisions by bunching protons closer together, with the time separating bunches being reduced from 50 nanoseconds to 25 nanoseconds," CERN said.
"After two years of effort, the LHC is in great shape," CERN Director for Accelerators and Technology Frédérick Bordry said. "But the most important step is still to come when we increase the energy of the beams to new record levels."
Scientists will began collisions of particles over the coming months at speeds almost double what was seen during the machine's first operating run.
They hope to use "Season 2" of the collider to search for dark matter, invisible material which makes up most of the universe.