November 28, 2012
Large Hadron Collider Collisions May Have Produced A New Type Of Matter
John P. Millis, Ph.D. for redOrbit.com — Your Universe Online
In September 2012, the Large Hadron Collider (LHC) conducted a short run of collisions between protons and lead nuclei. The roughly two million events recorded were set to serve as a baseline for lead-lead collisions anticipated for next year. However, these events produced an unexpected result.
Very high-energy particle collisions produce thousands of new particles which stream away from the collision point at nearly the speed of light.
Creating a three-dimensional plot (shown below) showing the fraction of particles arising from the collision against the transverse emission angle and the relative angle of the beam axis produces a distinct “ridge”.
Image Caption: These three plots show the correlation between pairs of particles seen in the CMS detector. (a) shows proton—proton collisions, and the arrow points to the ridge; (b) shows the lead—lead collisions where a similar ridge emerged once more; and (c) denotes the most recent proton—lead collisions where the ridge is seen once more. ÎÎ· is the angle in that plane measured between the two particles in the longitudinal plane. ÎÎ¦ represents the difference between the angles of the two particles in question in the transverse plane. R is a function of both ÎÎ· and ÎÎ¦. (Courtesy: CERN/CMS collaboration)
In other words, the particles created are streaming away from the collision at very small angles relative to the beam. This is somewhat surprising because the light crossing time for the particle pairs is too great to explain their correlated direction. Therefore, a physical mechanism must be responsible for focusing the particle flow.
The Quark-Gluon Plasma
The existence of these ridge-correlations is not new; previous works on Lead-Lead and Proton-Proton collisions have produced similar phenomena, however there are differences.
In the case of collisions of heavy nuclei, such as Lead-Lead or Gold-Gold, the effect can be attributed to the existence of what is known as the quark-gluon plasma. Nucleons, such as protons and neutrons, are made up of particles called quarks, which are bound together by gluons — the carriers of the strong force.
When nuclei collide at very high-energy, a hot, dense quark-gluon plasma is produced. It is theorized that the plasma sweeps up the newly created particles and channels them in the same direction, producing the ridge. [Note: The quark-gluon plasma is the state of matter thought to have existed microseconds after the Big Bang. The plasma described in these experiments has very similar properties, but are not as extreme as that which would have existed in the early Universe. It has never been confirmed that this, or any prior experiments, have reproduced this state of matter; but the hot, dense, highly-viscous plasma that has been produced is thought to be at least similar.]
However, in the case of proton-proton collisions, quark-gluon plasma is not expected to arise. So a different mechanism is needed to explain the observed correlation in these data. This assumption is quite reasonable since, while the ridge correlation appears similar, it is distinct in form and is subtler — having a considerably weaker correlation.
The Color-Glass Condensate and Glasma States of Matter
One proposal argues that, at the highest energies, protons can enter quantum states where the number of gluons increases dramatically. Some theorists believe that particles in this state essentially become sheets of matter — due to relativistic length contraction — known as color-glass condensate. [Note: The term “color” has nothing to do with actual visual color, rather the term references the color-charge of the quarks and gluons. Color-charge is similar to electric-charge and produces fields analogous to the electric and magnetic fields, though with different properties. It is these fields that act as the strong nuclear force that binds nuclei together and is responsible for the structure of protons and neutrons.]
After the collision, a new state of matter - known as a Glasma (gluon-plasma) - could possibly form. Under the proper conditions the interactions of the strong color-fields within this matter could cause it to thermalize into a quark-gluon plasma.
While this interpretation was not widely accepted when it was proposed in 2010, the authors, Venugopalan and Dusling, made a follow-up prediction on the two-particle correlations in proton-lead collisions; specifically that they would have a form similar to the ridge seen in proton-proton collisions, but with a stronger correlation. They argue that these recent results match their prediction.
The Next Step
Has the LHC created a new form of matter? It is certainly too early to tell. Theorists are working on interpretations of this first run of ridge correlation data, including those that would suggest a new form of matter such as the color-glass condensate has formed. However, there are other possibilities that are less exotic.
Also, in early 2013 the LHC will do an extended run of proton-lead collisions, increasing the data a thousand fold over this initial study. With the surprisingly strong correlation seen from the initial data set, researchers believe they will be able to probe some of the most pressing questions about how strongly interacting systems behave.
An MIT paper describing the unexpected findings will appear in an upcoming issue of the journal Physical Review B and is now available on arXiv.