The Large Hadron Collider (LHC) experiment could be re-started on Saturday morning at the earliest, officials have said.
Engineers have sent proton particles all the way round the Large Hadron Collider (LHC) machine for the first time in more than a year.
Engineers are preparing to send a beam of sub-atomic particles all the way round the 27km-long circular tunnel which houses the LHC.
But they still do not have a stable circulating beam; this step is expected to happen after 0600 GMT on Saturday.
The £6bn machine on the French-Swiss border is designed to shed light on fundamental questions about the cosmos.
The LHC is housed in a 27km-long circular tunnel some 100m beneath the French-Swiss border.
The LHC has been shut down for repairs since an accident in September 2008.
It will smash together beams of protons in a bid to shed light on the nature of the Universe.
Operated by the European Organization for Nuclear Research (Cern), the LHC will create similar conditions to those which were present moments after the Big Bang.
There are some 1,200 "superconducting" magnets arranged end-to-end in the underground tunnel.
These magnets bend proton beams in opposite directions around the main "ring" at close to the speed of light.
At allotted points around the tunnel, the proton beams cross paths, smashing into one another. Physicists hope to extend our knowledge of physics, looking for new sub-atomic particles in the debris of these collisions.
On Wednesday, the vast physics lab was handed over from the hardware commissioning team to the operations team. This handover takes place every time a particle beam is injected into the LHC "ring".
But James Gillies, director of communications at Cern, told BBC News: "There won't be any circulating beam before Saturday morning.
"They are scheduled to start putting particles in the LHC (on Friday) evening. We have done that already several times this year."
The LHC's tunnel runs for 27km under the Franco-Swiss border
The beams themselves are made up of "packets" - each about a metre long - containing billions of protons.
The protons would disperse if left to their own devices. So once beams are circulating around the LHC, they have to be stabilised.
This involves using electrical forces to "capture" the protons, keeping them tightly huddled in packets.
Ruediger Schmidt, head of hardware commissioning for the LHC, said the first particle collisions could happen within the next few weeks.
"We first want both beams 'stable at injection'. We have to do many tests and only then would we look at colliding beams," he told BBC News.
The LHC was designed to run at energies of seven trillion electron volts. But the machine will clash together protons at energies of just 3.5 trillion electron volts (TeV) in its first few months of operation.
1 - 14 quadrupole magnets replaced2 - 39 dipole magnets replaced3 - More than 200 electrical connections repaired4 - Over 4km of beam pipe cleaned5 - New restraining system installed for some magnets6 - Hundreds of new helium ports being installed around machine7 - Thousands of detectors added to early warning system
"To get to 3.5 (TeV) is fantastic, because that is further than we have ever looked before," said Dr Tara Shears, a physicist at the University of Liverpool, UK, who will be studying data from the LHC.
"We have no idea where the chinks in our understanding of the Universe are going to become apparent. Anything which lets us see further than we have already is immensely valuable."
To much fanfare, engineers first circulated a beam all the way around the LHC on 10 September 2008.
But just nine days later, an electrical fault in one of the connections between superconducting magnets caused a tonne of liquid helium to leak into the tunnel.
Engineers carried out a "beam test" last month
Liquid helium is used to cool the LHC to its operating temperature of 1.9 kelvin (-271C; -456F).
The machine has been shut down ever since the accident, to allow repairs to take place.
The damage caused to the collider meant 53 superconducting magnets had to be replaced and about 200 electrical connections repaired.
Engineers have also been installing a new early warning system which could prevent incidents of the kind which shut down the experiment.
Cern has spent some 40m Swiss Francs (£24m) on repairs to the collider.
