Scientists at the European Organisation for Nuclear Research (Cern) have smashed together proton beams for the first time, following more than a year of repairs.
The results are a big step forward for the world's largest atom smasher, housed under the Swiss-French border, which has been built to reproduce "Big Bang" conditions.
It is hoped that the experiments, which run in a 27-kilometre tunnel, will already start giving clues about the origins of the universe in the next few months as the particle collider moves towards full power.
The collisions, seen by massive detectors, were a side effect of the quick advances being made by the Large Hadron Collider during its startup phase, which began Friday night, said Rolf Heuer, director-general of Cern.
"It's a great achievement to have come this far in so short a time," Heuer said at a press conference on Monday.
The collision was achieved by sending two bunches of subatomic particles around the tunnel ring in opposite directions.
Last Friday the LHC was switched back on after being halted by an accident 14 months ago, only ten days after it first started.
swissinfo.ch and agencies