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Collaborating for a "Perfect" Scan of Nuclear Matter

superconducting magnets

Superconducting magnets of the Large Hadron Collider (left) and Brookhaven's Relativistic Heavy Ion Collider (right).

As the finishing touches are put on the world's most powerful particle accelerator in Switzerland, and plans for others pop up across the globe, Brookhaven's Relativistic Heavy Ion Collider (RHIC) continues to exploit its unique ability to explore the surprising features of matter bound by the strongest of Nature&'s forces. Although RHIC's overall mission is quite different from other machines on the horizon, new scientific facilities are incorporating heavy ion capabilities similar to RHIC. This healthy competition and collaboration with facilities worldwide will greatly enhance the exploration of nuclear matter — the inner cogs that make up the nucleus, and really, everything around us.

On the most basic level, scientists know that the nucleus is made of particles called protons and neutrons, which are made of smaller particles called quarks and gluons — the most fundamental constituents of matter. They know that the quarks are grouped into triplets held together by gluons (named for their Elmer's-like properties). But, they also know that these elementary particles weren't always glued together. Go back about 13.7 billion years, a hundred-millionth of a second after the Big Bang, and you'd find quarks and gluons floating freely. And that's where it gets sticky.

Using RHIC, researchers have revealed surprising results in their quest to recreate this moment in history, in microcosm. By colliding beams of heavy gold nuclei at very high energies, RHIC provides a small-scale replication of the ultra-hot, dense conditions thought to have existed immediately following the birth of the universe. However, instead of producing a gas of free quarks and gluons, RHIC's energetic collisions appear to produce something more like a liquid — a "perfect" liquid with almost no viscosity, or frictional resistance to flow.

solenoidal tracker at RHIC

The Solenoidal Tracker at RHIC (STAR), a massive detector that specializes in tracking the thousands of particles produced by each ion collision at RHIC.

To continue exploring the nature of this perfect liquid, RHIC physicists are building on their remarkable early discoveries to mount precision studies with newly refined experimental and theoretical techniques. A near-term upgrade, expected to increase the machine's collision rate and improve the sensitivity of detectors, would make critical measurements at RHIC more quantitative, allowing scientists to learn more from theory-experiment comparisons of the properties of the perfect liquid. For example, scientists can examine the applicability of "string theory," which has suggested completely unanticipated connections between the strongly interacting matter produced at RHIC and gravitational systems such as black holes.

Healthy competition and collaboration with facilities worldwide will enhance the exploration of the inner cogs that make up the nucleus and everything around us.

But physicists will need the assistance of multiple scientific facilities to determine the so-called phase diagram of nuclear matter. A phase diagram shows the boundaries between different types of the same substance as external conditions, such as pressure and temperature, change. One of the best-known phase diagrams illustrates what happens to water as it freezes in an ice cube tray or turns to steam while boiling in a pot on the stove. For nuclear matter, the phase diagram aims to show the boundary between normal matter, composed of compact neutrons and protons, and a system of liberated quarks and gluons.

Exploring the phase diagram of nuclear matter requires much more than kitchen-sink experiments. To reach the extreme conditions required to "melt" normal matter into a soup of quarks and gluons, which happens at temperatures more than 100,000 times hotter than the center of the sun, scientists need an array of accelerators capable of creating collisions of varying energies.

The Large Hadron Collider (LHC), a powerful accelerator now coming online in Switzerland, is expected to reveal how the perfect liquid evolves at even higher temperatures than are produced at RHIC. Although the machine's primary focus is to create new, massive particles by colliding beams of protons, the LHC also will collide lead ions and devote a short amount of its run time — about four weeks per year — to nuclear physics.

"We're curious to find out if matter analogous to the perfect liquid can be created at even higher temperatures, and if so, what it looks like," said Brookhaven's Physics Department Chair Tom Ludlam. "Will it become even more perfect? Or will it actually behave like a gas for a fraction of its tiny lifetime? We may be able to determine that from studies at the LHC."

nuclear matter phase diagram

The LHC, RHIC, and FAIR will each explore a different section of the nuclear matter phase diagram, which is depicted in this graphic. As temperature and density change, so does the boundary between normal hadronic matter, composed of neutrons and protons, and a system of unconfined quarks and gluons. The critical point is an as-yet-undiscovered landmark; the transition from hadron gas to quark-gluon plasma is predicted to be smooth at densities to the left of the critical point, but sharp on the other side, at higher densities.

To further explore the nuclear matter phase diagram, future RHIC experiments will demand lower energy collisions than produced in the past. Researchers are particularly interested in using RHIC to pinpoint the location of the "critical point," a threshold of temperature and density above which there is no sharp transition between two phases. On one side of the critical point associated with nuclear matter, there is an obvious difference between normal matter and a substance of unconfined quarks and gluons, and the transition from one phase to the other is sharp. But on the other side, the two phases can coexist and the transition from one to the other is a smooth crossover. So far, collisions at RHIC result in temperatures well above the critical point. With versatility in beam species and energies, and long running times for collisions of heavy nuclei at lower temperatures, RHIC is well positioned to uncover this telltale transition point, Ludlam said.

"Pinning down a critical point in the phase diagram is the best way to understand how quarks and gluons work together over large volumes to form the perfect liquid," Ludlam said. "If we find it, experimental data on both sides of the point will reveal a great deal about the fundamental processes that have produced the matter we see in our universe today."

In addition to the LHC and RHIC, one more facility will soon enter the energy-scanning mission. GSI, the German research center for heavy ion physics, is currently building the Facility for Antiproton and Ion Research (FAIR), a series of synchrotrons, storage rings, and detectors meant to study numerous aspects of physics. FAIR, scheduled to begin operating in 2016, will use extremely high nuclear densities in a low-energy scan that could map out the nuclear matter phase diagram well to the right of the critical point.

"We need specialized information from each of these facilities to paint a complete picture of nuclear matter," Ludlam said, adding that "to make sense of what will be seen at very high energies at the LHC and very low energies at FAIR, physicists need to know what has happened and will happen at RHIC. As data emerge from all three of these world-class facilities, we will enter a golden age of heavy ion physics."