LHC to probe early universe in best detail yet

The Large Hadron Collider will spend four weeks probing the conditions of the early universe in better detail than ever before, as it takes a break from the hunt for the Higgs boson.

The LHC's main activity for 2011, colliding pairs of protons, came to an end as scheduled on 30 October. The experiment has now produced about 6 inverse femtobarns of collision data, about three times the total used in the last major analysis searching for the Higgs boson, thought to endow other particles with mass, which was reported in August.

As researchers start analysing the new data, the LHC is switching to colliding lead ions for four weeks, starting on 5 November. These collisions produce pockets of very dense and hot matter, recreating the conditions in the first moments after the big bang.

Lead ion collisions at the LHC last year showed hints of producing a quark-gluon plasma, an exotic state of matter in which quarks ? normally bound in pairs or triplets ? are able to wander freely. The phenomenon has been observed previously at the Relativistic Heavy Ion Collider in Brookhaven, New York, but the LHC's higher-energy collisions allows higher temperatures to be obtained.

Finer detail

"Basically you get closer to the moment of big bang," says Greg Landsberg of Brown University in Providence, Rhode Island, physics coordinator for CMS, one of the LHC's two main detectors.

The experimenters hope to obtain about 10 times as much data this year compared to the LHC's 2010 lead collisions, allowing them to investigate quark-gluon plasma in more detail. They want to better probe how the behaviour of matter changes with temperature near the quark-gluon plasma state.

They will also try colliding protons with lead ions, something never done at the LHC before. These collisions are cleaner than those involving pairs of lead ions. The data could be useful as a reference when trying to interpret the messier lead-lead collisions.

The LHC will shut down in early December, then switch back on in March 2012 for proton-proton collisions, possibly at a slightly higher energy of 8 TeV, compared to 7 TeV this year, says Landsberg. The higher energy would improve sensitivity to the Higgs by about 30 per cent, and could also help reveal signs of supersymmetry, an extension of the standard model of particle physics.

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