By Paul Rincon, Science reporter, BBC News
Engineers operating the Large Hadron Collider (LHC) have smashed together proton beams in the machine for the very first time.
The low-energy collisions came after researchers circulated two beams simultaneously in the LHC's 27km-long tunnel earlier on Monday.
Cern's director of communications, Dr James Gillies, said the first collisions had taken place just as a news conference was under way on Monday to discuss progress following the machine's restart at the weekend.
"We didn't have time to analyse them then. We waited until all four of the (detectors) had seen good candidates (for collisions)," he told BBC News.
The spokesperson for the Alice experiment, Jurgen Schukraft, said cheers erupted with the first collisions.
"This is simply tremendous," he said.
Engineers restarted the LHC on Friday evening after a 14-month hiatus while the machine was being repaired.
It had to be shut down shortly after its inauguration when an electrical fault led to magnets being damaged and to one tonne of liquid helium leaking into the tunnel.
