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The largest, most complex scientific instrument in the world will begin its long-delayed hunt for new particles, forces and extra dimensions today.

Operators of the Large Hadron Collider planned for the machine to try to steer speeding particles into head-on collisions this morning, creating microscopic bursts of energy that mimic conditions that existed a fraction of a second after the Big Bang.



The start of high-energy operations at the collider, at CERN, the European Nuclear Research Organisation, on the outskirts of Geneva, marks the end of a frustrating 18-month delay for physicists who saw the machine shut down for essential repair work after an explosion in September 2008, just nine days after it was first switched on.

The £6 billion ($9.9 billion) collider, which occupies a 27 kilometre circular tunnel 100 metres beneath the French-Swiss border, accelerated two counter-rotating beams of protons to an unprecedented energy of 3.5 trillion electron volts (TeV) last week.

Today, the beams were due to be crossed for the first time in the highest-energy, man-made collisions ever.

The machine was designed to collide two 7 TeV beams of protons, but laboratory managers decided in January to operate the machine at half power until the end of next year.

The machine will then close for a year of more engineering work to ensure it can run at full power in 2013 without breaking down again.

The Large Hadron Collider is expected to make new discoveries about the laws of physics at the highest energies and smallest scales ever probed.

Physicists hope these will help them decide which of their theories of nature are right and which are wrong.

Many physicists at the research organisation are almost certain they will find the Higgs boson, aka the ''God particle'', which underpins a theory that explains where fundamental particles get their masses from.

CERN researchers will sift through the subatomic debris of proton collisions for signs of extra dimensions and up to now invisible particles that will bolster belief in ''supersymmetry'', a theory that doubles the number of particle species in the universe. Other results may point to ''hidden worlds'' of particles and forces.