RHIC Atom Smasher Reveals First Hints Of Phase Boundary
August 15, 2012

Atom Smasher Closes In Border Between Primordial Plasma And Ordinary Matter

April Flowers for redOrbit.com - Your Universe Online

Scientists at the U.S. Department of Energy's Brookhaven National Laboratory have observed the first glimpses of a possible boundary separating ordinary nuclear matter, made up of protons and neutrons, from the seething soup of their constituent quarks and gluons using the versatility and new capabilities of the Relativistic Heavy Ion Collider (RHIC).

A quark-gluon plasma (QGP) exists at extremely high temperatures or density, consisting of asymptotically free quarks and gluons, which are several of the basic building blocks of baryonic matter.

Presented at the Quark Matter 2012 international conference, the latest preliminary data coming from the RHIC physicists comes from the systematic studies varying the energy and types of colliding ions to create this primordial QGP under a broad range of initial conditions, allowing researchers 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 theory of quantum chromodynamics (QCD) is the theory that describes the interactions between the constituents of nuclei of today's ordinary atoms and QGP, two different phases of matter.  Scientists sometimes refer to the exploration of QGP and this transition as the study of QCD matter.

The different phases exist under different conditions of temperature and density, which can be mapped out on a "phase diagram."  The diagram shows where the regions are separated by a phase boundary akin to those that separate liquid water from ice or steam.  In the case of nuclear matter, scientists still are not sure where to draw those boundary lines, but 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.

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.  (STAR and PHENIX are detectors which search for signatures of the GCP and hadrons, respectively.)

Evidence for the formation of QGP is widely accepted at the highest energies, however many of the signatures of QGP developed at 200 GeV disappear as the energy decreases.

STAR findings indicate that interactions among "free" quarks and gluons 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 at hadrons, appear to be the dominant feature observed.

A hadron is a composite particle made up of quarks held together by the strong force, one of the four fundamental interactions of nature. Hadrons are categorized into two families: baryon (made of three quarks) and mesons (made of one quark and one antiquark).

PHENIX has observed similar behavior.  The physicists on this project found that quarks passing through the matter produced at collision energies from 39 GeV upward lose energy rapidly.  Previous PHENIX results from copper-copper collisions at 22 GeV, in contrast, are consistent with no significant energy loss.

These measurements tell scientists they may be approaching the boundary between ordinary nuclear matter and the QGP that dominated the early universe. But they haven't 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.

The QGP calculations are so complex that there is no consensus among theorists where the QCD critical point should lie or even if it exists.  RHIC researchers, however, say they see hints in the data around 20 GeV that resemble signatures predicted to be observed near such a QCD critical point.  Much more data is going to be required from future RHIC experiments to turn these hints into conclusive evidence.

In gold-gold collisions at RHIC, one signal that disappears at 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.  Normally, such a charge separation would be forbidden by the "mirror symmetry" that is a fundamental part of QCD. At the ultra-high temperatures of QGP, the theory allows such symmetry violations to occur in localized bubbles as long as they average out to zero when bubbles from all collision events are looked at collectively.

“Such symmetry-violating bubbles are of crucial interest in the early, high-temperature history of the universe, where analogous bubbles are speculated to have played a central role in producing the preponderance of matter over antimatter in today´s universe, enabling our existence,” Vigdor said.

There are two schools of thought here.  The disappearing hints may be another signal that the lower-energy RHIC collisions are no longer producing QGP, or it could be that the hints arise instead from a "background" phenomenon that is related to the almond-like shape of the overlap region formed when two spherical gold ions collide in a not quite head-on fashion.

By using head on collisions of football shaped uranium ions aligned in upright positions, scientists are able to study the effects of this almond-like interaction region without the strong surrounding magnetic field produced from off-center gold-gold collisions.

Results from STAR physicists seem to rule out the role of the background effect.  If this is confirmed, the uranium-uranium collisions will provide further evidence for the symmetry violating bubble interpretation of the gold-gold data and the disappearance of QGP at lower RHIC energies.

Plasma formation is affected by the way quarks and gluons are arranged in ordinary matter.  This arrangement also modifies production of experimental probes of the plasma's properties.  Understanding the effects of the probes requires good knowledge of the probe before it encounters QGP.  To gain this knowledge, RHIC experimenters have collected a large data set from collisions of gold ions with deuterons (the nuclei of heavy hydrogen).

PHENIX physicists report that there are fewer high-momentum single hadrons and collections of hadrons called "jets" produced in dead-on central deuteron-gold collisions than more glancing deuteron-gold collisions.

“We expect jet suppression in quark-gluon plasma, because jets lose energy in dense matter such as the plasma,” said PHENIX spokesperson Barbara Jacak, a physicist at Stony Brook University. “But this result shows that we have to correct for this initial state effect when figuring out how much the plasma suppresses the production of jets.”

The initial state is related to the arrangement of quarks and gluons deep inside the gold nucleus, which some theories predict could be a condensed form of gluons called color-glass condensate, as hinted at in earlier results published by PHENIX.

Other new measurements were reported at Quark 2012 as well.  These concern the probability of heavy quarks (bottom and charm) and their anti-matter counterparts pairing up to form bound states called "quarkonia" within the QGP and in the "cold" nuclear matter probed in the deuteron-gold collisions.

QCD tells us that the force between a quark and an antiquark increases in strength as they are pulled apart, much like being connected by an invisible rubber band.  The strength of this force, however, should be reduced in QGP, leading physicists to expect the formation of quarkonia to also be reduced in QGP, with the probability of finding such species decreasing with larger-size bound states.

The STAR experiment reported new results consistent with this expectation by studying different size bound states of bottom quarks and antiquarks. PHENIX has studied suppression of bound states of charm and anti-charm quarks in various beam combinations, both with and without plasma formation. New results indicate that their formation is already suppressed in collisions of deuterons with gold nuclei, when no QGP is formed.

“This reflects both the reduced production rates for heavy quarks and the fact that the bound state sometimes breaks up as it passes through normal (cold) nuclear matter,” said Jacak. “It is crucial to quantify this if we are to understand QGP effects on the binding,” she said.

“These new results on the phase boundary, symmetry-violating bubbles, initial state effects, and the production of quark-antiquark bound states illustrate how scientists are exploiting RHIC´s unique versatility for precision determinations of the properties of quark-gluon plasma,” Vigdor said. “It is this versatility, in combination with dramatic advances we´ve made in the rate of collisions provided at RHIC, that will allow our scientists in the coming decade to answer the pointed questions raised by RHIC´s exciting discoveries about this early universe matter.”

Image 2 (below): The nuclear phase diagram: RHIC sits in the energy "sweet spot" for exploring the transition between ordinary matter made of hadrons and the early universe matter known as quark-gluon plasma. Credit: Brookhaven National Laboratory