The world's largest science experiment, the Large Hadron Collider began activity again this month after more than a year of repairs. Last week, scientists circulated two beams of particles simultaneously around the collider for the first time, testing the equipment's ability to synchronise the beams and look for proton-to-proton collisions.

The collider is a particle accelerator including a 27, kilometre loop of tunnel 100 metres under the border between Switzerland and France at Geneva. Atomic particles are collided at high speeds in the tunnel and scientists will measure the results.

The equipment has been under repairs since a malfunction in September last year and its restart was delayed earlier this month after a power failure. Though media reports blamed the latter problem on bread dropped by a bird, the official line is that feathers and breadcrumbs were found at the site but there was no evidence they had caused the power failure.

Director General of CERN, the European Organisation for Nuclear Research, Rolf Heuer, said last week's test was a great achievement. "But we need to keep a sense of perspective - there's still much to do before we can start the LHC physics programme."

Scientists involved with the project said they welcomed the first particle collisions with cheers and the resulting information was beautiful. "We're all ready for serious data taking in a few days time," said a spokesperson, Andrei Golutvin.

The collider was brought to its operating temperature of -271 Celsius in early October and particles were injected later that month. This month, the equipment was tested by circulating beams around the ring alternately in one direction and then the other and gradually increasing the lifetime of the beams to 10 hours.

The next step is described as an "intense commissioning phase" in which the intensity and speed of the beams will be increased. It is expected that by Christmas, the LHC will have provided good quantities of collision data for the experiments' calibrations.

Despite the delays that have dogged the project's first year, CERN's director for accelerators Steve Myers, said the researchers involved had learned a lot. "The LHC is a far better understood machine than it was a year ago," he said. "We've learned from our experience, and engineered the technology that allows us to move on. That's how progress is made."

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