LHC switches off for two-year break
The Large Hadron Collider has turned off its particle beams ahead of a shut-down period that will last two years.
The particle accelerator is best known for identifying a particle believed to be the Higgs boson in late 2012.
But following technical faults shortly after it first switched on, the machine has never been run at the full energies for which it was designed.
A programme of repairs and upgrades to the accelerator and its infrastructure should allow that in late 2014.
The LHC's beams were "dumped" early on Thursday morning, but it will take until Saturday morning for the machine's 1,734 magnets to warm up to room temperature.
Then an unprecedented period of upgrade and repair - dubbed "Long Shutdown 1" - will begin.
The machine ran at particle energies of 8 trillion electron-volts (teraelectronvolts; TeV) in 2012, up from the prior high point of 7TeV in 2011.
But when the shutdown concludes, slated for the end of November 2014, it should be set to run at 14TeV - far and away the highest-energy collisions ever attempted by scientists.
"We have been running successfully, but only at half the maximum energy, because we can only safely run the magnets at half the design current," said Tony Weidberg, a University of Oxford physicist who works on the LHC's Atlas detector.
What is an electronvolt?
- Charged particles tend to speed up in an electric field, defined as an electric potential - or voltage - spread over a distance
- One electron volt (eV) is the energy gained by a single electron as it accelerates through a potential of one volt
- It is a convenient unit of measure for particle accelerators, which speed particles up through much higher electric potentials
- The first accelerators only created bunches of particles with an energy of about a million eV
- The LHC can reach particle energies a million times higher: up to several teraelectronvolts (TeV)
- This is still only the energy in the motion of a flying mosquito
- But LHC beams include hundreds of trillions of these particles, each travelling at 99.99999999% of the speed of light
The problem has been the connections between the giant magnets that help steer charged particles around the LHC's 27km-long ring.
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.
"After the incident, the long-term plan was to get some running at intermediate energy and then have a long shut-down when we improve the connections between the magnets," Prof Weidberg told BBC News.
"That's a major operation, because you have to warm up all these superconducting magnets and go in and do repairs."
But the shut-down schedule also includes upgrades to all four of the LHC's detectors, the shielding of electronics, and even an overhaul of the ventilation system of the tunnel that houses the main accelerator ring.
The shut-down is due to conclude in late November 2014. after which the system will be put through its paces and experiments are expected to resume in February or March 2015.
In the meantime, scientists will stay busy analysing plenty of data from the 2012 data run, which thanks to improvements to the focusing of the LHC's beams as well as the slightly higher energies, provided more than twice as much data as the 2011 run.