The Large Hadron Collider experiment is on

The first - clockwise - proton beam completed its first circuit of the underground tunnel at just before 0930 GMT. The second - anti-clockwise - proton beam successfully circled the ring after 1400 GMT. The beams have not yet been run continuously. So far, they have been stopped, or "dumped", after just a few circuits.Cern has not yet announced when it plans to carry out the first collisions, but some low-energy collisions could happen in the next few days. This will allow engineers to calibrate instruments, but will not produce data of scientific interest.
The vast circular tunnel - the "ring" - which runs under the French-Swiss border contains more than 1,000 cylindrical magnets arranged end-to-end.
The magnets are there to steer the proton beam around this 27km-long ring.
In order to get both beams to circulate continuously around the LHC, engineers have to "close the orbit". The beams themselves are made up of several "packets" - each about a metre long - containing billions of protons.
The protons would disperse if left to their own devices, so engineers use electrical forces to "grab" them, keeping the particles tightly huddled in packets.
Once the beams are captured, the same system of electrical forces is used to give the particles an energetic kick, accelerating them to greater and greater speeds.
At allotted points around the tunnel, the beams will cross paths, smashing together near four massive "detectors" that monitor the collisions for interesting events.
LHC DETECTORS
ATLAS - one of two so-called general purpose detectors. Atlas will be used to look for signs of new physics, including the origins of mass and extra dimensions
CMS - the second general purpose detector will, like ATLAS, hunt for the Higgs boson and look for clues to the nature of dark matter
ALICE - will study a "liquid" form of matter called quark-gluon plasma that existed shortly after the Big Bang
LHCb - Equal amounts of matter and anti-matter were created in the Big Bang. LHCb will try to investigate what happened to the "missing" anti-matter