15 August 2012
Scientists taking advantage of the versatility and new capabilities of the Relativistic Heavy Ion Collider (RHIC), an atom smasher at the U.S. Department of Energy’s Brookhaven National Laboratory, have observed first glimpses of a possible boundary separating ordinary nuclear matter, composed of protons and neutrons, from the seething soup of their constituent quarks and gluons that permeated the early universe some 14 billion years ago. Though RHIC physicists have been creating and studying this primordial quark-gluon plasma (QGP) for some time, the latest preliminary data—presented at the Quark Matter 2012 international conference—come from systematic studies varying the energy and types of colliding ions to create this new form of matter under a broad range of initial conditions, allowing the experimenters to unravel its intriguing properties.
“2012 has been a banner year for RHIC, with record-breaking collision rates, first collisions of uranium ions, and first asymmetric collisions of gold ions with copper ions,” said Samuel Aronson, Director of Brookhaven National Laboratory. “These unique capabilities demonstrate the flexibility and outstanding performance of this machine as we seek to explore the subtle interplay of particles and forces that transformed the QGP of the early universe into the matter that makes up our world today.”
The nuclei of today’s ordinary atoms and QGP represent two different phases of matter whose constituents interact through the strongest of Nature’s forces. These interactions are described by a theory known as quantum chromodynamics, or QCD, so scientists sometimes refer to the exploration of QGP and this transition as the study of QCD matter.
As in other forms of matter, the different phases exist under different conditions of temperature and density, which can be mapped out on a “phase diagram,” where the regions are separated by a phase boundary akin to those that separate liquid water from ice and from steam. But in the case of nuclear matter, scientists still are not sure where to draw those boundary lines. RHIC is providing the first clues.
“RHIC is well positioned to explore QCD phase structure because we can vary the collision energy over a wide range, and in so doing, change the temperature and net quark density with which QCD matter is formed,” said Steven Vigdor, Brookhaven’s Associate Laboratory Director for Nuclear and Particle Physics, who leads the RHIC research program.
For example, physicists from RHIC’s STAR and PHENIX collaborations have analyzed results from gold ion collisions taking place at energies of 200 billion electron volts (GeV) per pair of colliding particles, all the way down to 7.7 GeV.
While at the highest energies evidence for QGP formation is widely accepted, “many of the signatures of the QGP developed at 200 GeV disappear as the energy decreases,” said STAR spokesperson Nu Xu, a physicist at Lawrence Berkeley National Laboratory.
In particular, the STAR findings analyzed so far indicate that interactions among “free” quarks and gluons—those characteristic of the “perfect” liquid QGP discovered at RHIC—appear to dominate at energies above 39 GeV, while at energies below 11.5 GeV, the interactions of bound states of quarks and gluons known as hadrons (such as the protons and neutrons of ordinary matter) appear to be the dominant feature observed.
“As you get below 39 GeV, several key observables begin to change,” Xu said.
The PHENIX experiment has observed similar behavior. They have found that quarks passing through the matter produced at collision energies from 39 GeV upward lose energy rapidly, as anticipated for interactions within QGP. Previous PHENIX results from copper-copper collisions at 22 GeV, in contrast, are consistent with no significant energy loss.
These measurements are helping scientists plot definitive points, or signposts, which tell them they may be approaching the boundary between ordinary nuclear matter and the QGP that dominated the early universe. But they haven’t yet proven that a sharp boundary line exists, or found the “critical endpoint” at the termination of that line.
“The critical endpoint, if it exists, occurs at a unique value of temperature and density beyond which QGP and ordinary matter can co-exist,” said Vigdor. It is analogous to a critical point beyond which liquid water and water vapor can co-exist in thermal equilibrium, he said.
Because of the complexity of QCD calculations, there is as yet no consensus among theorists where the QCD critical point should lie or even if it exists. But RHIC experimentalists say they see hints in the data around 20 GeV that resemble signatures predicted to be observed near such a QCD critical point. However, much more data from future experiment runs at RHIC is required to turn these hints into conclusive evidence.
Apparent symmetry violations disappear at low energy
One signal that disappears in gold-gold collisions at RHIC energies below 11.5 GeV is the indication of a small separation of positive from negative electric charge within the matter produced in each individual collision. Ordinarily, such a charge separation would be forbidden by the “mirror symmetry” that is a fundamental feature of QCD. But at the u