LHC, RHIC at the "Ridge" of Nuclear Matter

This guest post is written by BNL theoretical physicist Raju Venugopalan. After earning his Ph.D. from Stony Brook University in 1992, Venugopalan worked at several universities in the United States and at the Niels Bohr Institute in Copenhagen, Denmark, before joining Brookhaven in 1998. He is the leader of the nuclear theory group in Brookhaven's physics department.

Raju Venogopalan.jpg

Raju Venogopalan

Last week, members of the Compact Muon Solenoid (CMS) collaboration at CERN's Large Hadron Collider (LHC) announced that they've found a phenomenon that's similar to one observed by physicists at Brookhaven's Relativistic Heavy Ion Collider (RHIC) called the "ridge effect."

This effect refers to a correlation that RHIC's experiments see in the particle debris created when two beams of gold ions (atoms stripped of electrons) collide with one another at nearly the speed of light. Some of these particles are correlated -- or associated together -- when they are created at the point of collision, which creates a ridge-like structure in scientists' data plots.

Because the ridge may carry information about the very early stages of the particle collision, it has become an important element in understanding the formation of the quark-gluon plasma (QGP), an extremely hot and dense form of matter that existed in the first few microseconds after the Big Bang.

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Image of a 7 TeV proton-proton collision in CMS producing more than 100 charged particles. Credit: CERN/CMS Collaboration

A group of theorists centered at Brookhaven have proposed a common explanation for the ridge structure in both the gold-gold collisions at RHIC and the proton-proton collisions at LHC, attributing its origin to extremely dense gluon states that are universal to protons and nuclei. These states are typically very short-lived. But at very high energies, due to Einstein's special relativity, their lifetimes get enhanced. Brookhaven scientists have described these dense gluon states as "color glass condensates."

Although gluons are the fundamental partners of quarks, and are responsible for much of the mass of visible matter, their collective properties are only now beginning to be understood.

Theorists at the RIKEN-BNL Research Center, Brookhaven's nuclear theory group, the City University of New York, Institut de Physique Théorique, Saclay (France), and the University of Jyväskylä and University of Helsinki (Finland), predicted that color glass condensate interacts strongly in collisions at high energies, stretching out like tiny rubber bands between the quarks, which go through nearly transparently. The tension along the rubber band (sometimes called a "flux tube") is uniform, which explains why particles produced are uniformly correlated along that band. The ridge is a "visual" image of these correlations.

Correlation plot.png

From CMS data: The correlation functions for "minimum bias" collisions (left) and for collisions that produced at least 110 charged particle tracks in the detector (right); the new ridge is indicated with an arrow. Credit: CERN/CMS Collaboration

In nuclei, it is the decay of these flux tubes that later forms the perfect fluid known as QGP. At RHIC, the ridge signal is greatly enhanced by the flow of QGP fluid in later stages of the collision, and some explanations interpreted it purely as a result of this QGP flow. While flow cannot be ruled out as a component of the CERN signal, the data structure points to the universal dynamical origin predicted by BNL theorists. My colleagues and I now have an article in the physics archive arXiv.org detailing how our prior predictions are consistent with CERN's findings.

Further significant tests of theory are now feasible with the suite of RHIC upgrades under way and the imminent running of lead beams at LHC. These tests will clarify the description of protons and nuclei as color glass condensates. A better understanding of these proton and nuclear states will also help quantify the properties of the nearly perfect fluid discovered at RHIC. Even more sensitive tests of color glass condensate properties are feasible at a future collider in which electron beams will collide with protons and nuclei.

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