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LHC CMS yields unexpected 'new stuff'

What’s the ‘matter’? A color-glass condensate - maybe

By Richard Chirgwin, 27 Nov 2012

Is there any phrase in science more exciting than “that’s odd”? MIT researchers right now would probably say “no”, since they suspect that LHC collisions may have yielded a previously-unobserved state of matter.

The unusual particle patterns turned up in what was meant to be a “reference run” of the Compact Muon Solenoid experiment.

The stuff in question, a “color-glass condensate”, is suggested by the behaviour of particles after collisions between lead and protons in the CMS. The “that’s odd” aspect of the collisions is a correlation in the direction that pairs of particles fly away from the collision, even though there’s no way that particles can communicate their direction to each other.

As this paper on Arxiv, shortly to be published in Physical Review B, describes, some pairs of particles somehow “fly in the same direction even though it’s not clear how they can communicate their direction with each other”, as MIT physics professor Gunther Roland says in this release.

The result, identified in an analysis of two million lead-proton collisions, suggests that something else must be imparting direction to the particle pairs – and here’s where the color-glass condensate comes into the picture.

The theory of this exotic stuff was proposed by researchers at Brookhaven National Laboratory to explain similar correlations from the Relativistic Heavy Ion Collider presented in 2004.

At that time, the RHIC theory was disputed, so its proponents, Brookhaven senior scientist Raju Venugopalan (quoted by MIT) and his then student Kevin Dusling, will be watching this development with interest.

At relativistic speeds, matter compresses along its length; and in very high-energy states (such as in the LHC), an accelerated nucleus might spawn large numbers of gluons (the particles that hold quarks together). The effect of this could be to create a “wall” of flattened gluons, and entanglement between the gluons explains how particles created by the collision can “share” direction information.

“It was supposed to be sort of a reference run,” Roland said, “a run in which you can study background effects and then subtract them from the effects that you see in lead-lead collisions.”

About that name. “Color” refers to the property of ‘color’ in quarks and gluons – the type of charge they carry due to the strong force; “glass” is analogous with everyday glass, in which silica looks solid in short timescales, but in very long timescales can be observed to flow; and “condensate” refers to the very high density of gluons. ®

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