LHC sees hint of high-speed particle pancake

Particles behaving oddly at the Large Hadron Collider seem to be the strongest signs yet of an unusual "subatomic pancake" called a colour-glass condensate. Theory suggests that matter takes on this guise when it is travelling near the speed of light ? relativistic speeds ? but the effect has not yet been officially observed.

The particles in atomic nuclei are made up of quarks held together by gluons. Gluons are elementary particles responsible for the strong force, also known as the colour force, which is the fundamental force that holds subatomic particles together.

Theory says that at relativistic speeds, particles become flattened and gain additional gluons, creating what is called a colour-glass condensate. Discovering whether relativistic matter actually behaves this way will help physicists better understand the strong force.

The hints of a colour-glass condensate come from CMS, one of the main detectors in the LHC at CERN, near Geneva, Switzerland. Researchers there smashed together beams of protons with beams of lead ions, producing showers of subatomic particles that flew away in all directions at high speed.

The latest smash-up was only meant to be a trial run in preparation for further collisions in January. But analysis of the data revealed something odd: the paths of certain pairs of particles flung out after the collisions seemed to be linked in unexpected ways.

Added ingredients

For example, two particles produced by the same collision may be heading in opposite directions, but they may also be curving slightly upwards in synch. This coordinated motion is odd, since the particles should fly away randomly.

"Most common models assume these particles would be uncorrelated," says Gunther Roland, a physicist at the Massachusetts Institute of Technology and a CMS team member. The data, however, say otherwise. "Such correlations are not so easy to create," says Roland.

Similar effects have previously been seen from high-speed collisions of heavy ions, which can create what is known as a quark-gluon plasma. This is the soupy mix of basic particles thought to have existed millionths of a second after the big bang.

Ripples in the plasma can affect particle pairs emitted from collisions by nudging them in the same direction as each other, producing a distinctive swerve that looks similar to the patterns in the CMS data. But collisions between protons and lead ions should not produce enough of the particles to create quark-gluon plasma, meaning there might be an alternative explanation.

In a colour-glass condensate, the extra gluons in the flattened particles would exist as both particles and waves. Their wave functions might become linked in ways that can influence the directions of the particle pairs, akin to the linked behaviours in quantum entanglement.

Collecting clues

"It is a possible explanation, but it really hasn't been confirmed," says Roland. The CMS test experiment ran for just 4 hours. The full-scale runs early next year should provide 10,000 to 100,000 times more data. "I'm very excited to see what we're going to learn about this in January," he says.

The initial results are very interesting, says David Evans of the University of Birmingham, UK, and a member of the team working with ALICE, another LHC detector. But Evans agrees there is not enough evidence to point the finger at colour-glass condensate just yet.

ALICE's results on proton-lead collisions so far do not indicate they are producing quark-gluon plasma, Evans adds, and his team is currently analysing data that will show whether ALICE has also detected hints of a colour-glass condensate.

Journal reference: arxiv.org/abs/1210.5482

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