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Scientists have hailed a successful switch-on for an enormous experiment which will recreate the conditions a few moments after the Big Bang.

They have fired a beam of particles called protons around the 27km-long tunnel which houses the Large Hadron Collider (LHC).

The £5bn machine on the Swiss-French border is designed to smash particles together with cataclysmic force. Scientists hope it will shed light on fundamental questions in physics.

The beam completed its first circuit of the underground tunnel at just before 0930 BST. "There it is," project leader Lyn Evans said when the beam completed its lap. There were cheers in the control room when engineers heard of the successful test.

He added later: "We had a very good start-up."

The LHC is arguably the most complicated and ambitious experiment ever built; the project has been hit by cost overruns, equipment trouble and construction problems. The switch-on itself is two years late.

The collider is operated by the European Organization for Nuclear Research - better known by its French acronym Cern.

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 beam - made up of particles called protons - around this 27km-long ring.

Eventually, two proton beams will be steered in opposite directions around the LHC at close to the speed of light, completing about 11,000 laps each second. At allotted points around the tunnel, the beams will cross paths, smashing together near four massive "detectors" that monitor the collisions for interesting events. Scientists are hoping that new sub-atomic particles will emerge, revealing fundamental insights into the nature of the cosmos. "We will be able to see deeper into matter than ever before," said Dr Tara Shears, a particle physicist at the University of Liverpool.

"We will be looking at what the Universe was made of billionths of a second after the Big Bang. That is amazing, that really is fantastic."

The LHC should answer one very simple question: What is mass?

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

"We know the answer will be found at the LHC," said Jim Virdee, a particle physicist at Imperial College London.

The currently favoured model involves a particle called the Higgs boson - dubbed the "God Particle". According to the theory, particles acquire their mass through interactions with an all-pervading field carried by the Higgs.