One beam circulates clockwise, the other anticlockwise.
"It's great to see beam circulating in the LHC again," said Cern Director General Rolf Heuer.
CERN's director, Rolf Heuer, said in a statement, "It's great to see beam circulating in the LHC again," but he and others cautioned that there was a long way to go before the collider started producing the physics it was designed for.
In early April, after a slight delay, twin proton beams circulated the LHC's 27km ring, 30 storeys below the Swiss-French border, for the first time in two years.
Sunday's restart saw the beams circulating at low energy, but over the coming days the accelerator team will steadily turn them up, until the protons are whizzing around the machine at 13TeV or teraelectron volts, or nearly twice as much energy as before.
The Relativistic Heavy Ion Collider ran for the first time in the summer of 2000 with Au beams circulating at SNN=130 GeV and at approximately 10% of the machine's final design luminosity.
The duct consists of two channels, i.e., a beam channel where a beam circulates and a Synchrotron Radiation (SR) channel (antechamber) aside where the SR passes through.
An impressive number of elements, spread over the large volume of the spectrometer, were commissioned for many months with cosmic rays and were ready to take data when the first beam was circulated in the LHC.
A fault in 2008, just nine days after particle beams first circulated at the LHC, caused what is known as a "quench" in a number of the magnets, in turn resulting in a leak of liquid helium and sparking a repair operation that took more than a year.
In the LHC's twin rings, beams will circulate in opposite directions, and the protons will collide head-on in the hearts of four gigantic particle detectors spaced around the ring.
