When matter melts: Scientists map phase changes in quark-gluon plasma

In its infancy, when the universe was a few millionths of a second old, the elemental constituents of matter moved freely in a hot, dense soup of quarks and gluons. As the universe expanded, this quark-gluon plasma quickly cooled, and protons and neutrons and other forms of normal matter "froze out": the quarks became bound together by the exchange of gluons, the carriers of the color force.

"The theory that describes the color force is called quantum chromodynamics, or QCD," says Nu Xu of the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), the spokesperson for the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at DOE's Brookhaven National Laboratory. "QCD has been extremely successful at explaining interactions of quarks and gluons at short distances, such as high-energy proton and antiproton collisions at Fermi National Accelerator Laboratory. But in bulk collections of matter -- including the quark-gluon plasma -- at longer distances or smaller momentum transfer, an approach called lattice gauge theory has to be used."

Until recently, lattice QCD calculations of hot, dense, bulk matter could not be tested against experiment. Beginning in 2000, however, RHIC was able to recreate the extreme conditions of the early universe in miniature, by colliding massive gold nuclei (heavy ions) at high energies.

Experimentalists at RHIC, working with theorist Sourendu Gupta of India's Tata Institute of Fundamental Research, have recently compared lattice-theory predictions about the nature of the quark-gluon plasma with certain STAR experimental results for the first time. In so doing they have established the temperature boundary where ordinary matter and quark matter cross over and change phase. Their results appear in the journal Science.

Phase diagrams

The aim of both the theoretical and experimental work is to explore and fix key points in the phase diagram for quantum chromodynamics. Phase diagrams are maps, showing, for example, how changes in pressure and temperature determine the phases of water, whether ice, liquid, or vapor. A phase diagram of QCD would map the distribution of ordinary matter (known as hadronic matter), the quark-gluon plasma, and other possible phases of QCD such as color superconductivity.

"Plotting a QCD phase diagram requires both theory calculations and experimental effort with heavy-ion collisions," says Xu, who is a member of Berkeley Lab's Nuclear Science Division and an author of the Science paper. Experimental studies require powerful accelerators like RHIC on Long Island or the Large Hadron Collider at CERN in Geneva, while calculations of QCD using lattice gauge theory require the world's biggest and fastest supercomputers. Direct comparisons can achieve more than either approach alone.

One of the basic requirements of any phase diagram is to establish its scale. A phase diagram of water might be based on the Celsius temperature scale, defined by the boiling point of water under normal pressure (i.e., at sea level). Although the boiling point changes with pressure -- at higher altitudes water boils at lower temperatures -- these changes are measured against a fixed value.

The scale of the QCD phase diagram is defined by a transition temperature at the zero value of "baryon chemical potential." Baryon chemical potential measures the imbalance between matter and antimatter, and zero indicates perfect balance.

Through extensive calculations and actual data from the STAR experiment, the team was indeed able to establish the QCD transition temperature. Before they could do so, however, they first had to realize an equally significant result, showing that the highly dynamical systems of RHIC's gold-gold collisions, in which the quark-gluon plasma winks in and out of existence, in fact achieve thermal equilibrium. Here's where theory and experiment worked hand in hand.

"The fireballs that result when gold nuclei collide are all different, highly dynamic, and last an extremely short time," says Hans Georg Ritter, head of the Relativistic Nuclear Collisions program in Berkeley Lab's Nuclear Science Division and an author of the Science paper. Yet because differences in values of the kind observed by STAR are related to fluctuations in thermodynamic values predicted by lattice gauge theory, says Ritter, "by comparing our results to the predictions of theory, we have shown that what we measure is in fact consistent with the fireballs reaching thermal equilibrium. This is an important achievement."

The scientists were now able to proceed with confidence in establishing the scale of the QCD phase diagram. After a careful comparison between experimental data and the results from the lattice gauge theory calculations, the scientists concluded that the transition temperature (expressed in units of energy) is 175 MeV (175 million electron volts).

Thus the team could develop a "conjectural" phase diagram that showed the boundary between the low-temperature hadronic phase of ordinary matter and the high-temperature quark-gluon phase.

In search of the critical point

Lattice QCD also predicts the existence of a "critical point." In a QCD phase diagram the critical point marks the end of a line showing where the two phases cross over, one into the other. By changing the energy, for example, the baryon chemical potential (balance of matter and antimatter) can be adjusted.

Among the world's heavy-ion colliders, only RHIC can tune the energy of the collisions through the region of the QCD phase diagram where the critical point is most likely to be found -- from an energy of 200 billion electrons volts per pair of nucleons (protons or neutrons) down to 5 billion electron volts per nucleon pair.

Says Ritter, "Establishing the existence of a QCD critical point would be much more significant than setting the scale." In 2010, RHIC started a program to search for the QCD critical point.

Xu says, "In this paper, we compared experimental data with lattice calculations directly, something never done before. This is a real step forward and allows us to establish the scale of the QCD phase diagram. Thus begins an era of precision measurements for heavy-ion physics."

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by DOE/Lawrence Berkeley National Laboratory.

Journal Reference:

Sourendu Gupta, Xiaofeng Luo, Bedangadas Mohanty, Hans Georg Ritter and Nu Xu. Scale for the phase diagram of quantum chromodynamics. Science, 24 June 2011 DOI: 10.1126/science.1204621

Physicist's discovery alters conventional understanding of sight

A discovery by a team of researchers led by a Syracuse University physicist sheds new light on how the visual process is initiated. For almost 50 years, scientists have believed that light signals could not be initiated unless special light-receptor molecules in the retinal cells first changed their shape in a process called isomerization. However, the SU research team, which includes researchers from Columbia University, has demonstrated that visual signals can be initiated in the absence of isomerization.

"We have demonstrated that chromophores (light-absorbing substances in retinal photoreceptor molecules), do not have to change shape in order to trigger the visual signal," says Kenneth Foster, professor of physics in SU's College of Arts and Sciences. "The shape-change that results from isomerization is actually the second step in the process. Historically, scientists have focused on isomerization without realizing there is an earlier and more crucial first step."

The research was published online June 23 in the journal Chemistry and Biology and is the cover article for the print version to appear June 24. The work was done in collaboration with Juree Saranak, research assistant professor in the Department of Physics; and Koji Nakanishi, professor of chemistry at Columbia University. Nakanishi's group was responsible for the synthetic chemistry that went into the compounds tested at SU. The National Institutes of Health funded the research.

Chromophores absorb light after it enters the eye, setting off an extremely rapid series of complex molecular changes that enable light signals to be transmitted to, and interpreted by, the brain so that we can visually perceive the world around us. Visual chromophores are composed of retinal (a type of vitamin A), which attaches to a protein (opsin) to form rhodopsin.

Foster's team of researchers discovered that the visual process is initiated by the redistribution of electrons on the chromophores, which occurs during the first few femptoseconds (one-quadrillionth of a second) after light enters the eye. Their experiments showed that when a chromophore absorbs a photon of light, electrons move from the chromophore's "free" end to the place where it attaches to opsin. The movement of the electrons causes a change in the electrical field surrounding the chromophore. That change is detected by nearby amino acids that are highly sensitive to changes in the electrical field. These amino acids, in turn, signal the rest of the rhodopsin molecule to initiate the visual process.

"We found that the complete blocking of isomerization of the chromophore does not preclude vision in our model organism," Foster says. "The signal is triggered as a result of an electronic coupling instead of a geometric change in the chromophore's structure as previously hypothesized. We believe this is a universal mechanism that activates all rhodopsins present in organisms from bacteria to mammals."

Foster attributes his findings to new technologies and scientific information not available 50 years ago when scientists first tried to understand how people see. "Fifty years ago, scientists had little knowledge of the structure of rhodopsins," he says. "Advances in technology have enabled scientists to determine the rhodopsin structure at the level of the amino acids, which enables us to design sensitive experiments to test our hypotheses."

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Syracuse University, via EurekAlert!, a service of AAAS.

Journal Reference:

Kenneth W. Foster, Jureepan Saranak, Sonja Krane, Randy L. Johnson, Koji Nakanishi. Evidence from Chlamydomonas on the Photoactivation of Rhodopsins without Isomerization of Their Chromophore. Chemistry and Biology, Volume 18, Issue 6, 733-742, 24 June 2011 DOI: 10.1016/j.chembiol.2011.04.009

Properties of 'confined' water within single-walled carbon nanotube pores clarified

Water and ice may not be among the first things that come to mind when you think about single-walled carbon nanotubes (SWCNTs), but a Japan-based research team hoping to get a clearer understanding of the phase behavior of confined water in the cylindrical pores of carbon nanotubes zeroed in on confined water's properties and made some surprising discoveries.

The team, from Tokyo Metropolitan University, Nagoya University, Japan Science and Technology Agency, and National Institute of Advanced Industrial Science and Technology, describes their findings in the American Institute of Physics' Journal of Chemical Physics.

Although carbon nanotubes consist of hydrophobic (water repelling) graphene sheets, experimental studies on SWCNTs show that water can indeed be confined in open-ended carbon nanotubes.

This discovery gives us a deeper understanding of the properties of nanoconfined water within the pores of SWCNTs, which is a key to the future of nanoscience. It's anticipated that nanoconfined water within carbon nanotubes can open the door to the development of a variety of nifty new nanothings -- nanofiltration systems, molecular nanovalves, molecular water pumps, nanoscale power cells, and even nanoscale ferroelectric devices.

"When materials are confined at the atomic scale they exhibit unusual properties not otherwise observed, due to the so-called 'nanoconfinement effect.' In geology, for example, nanoconfined water provides the driving force for frost heaves in soil, and also for the swelling of clay minerals," explains Yutaka Maniwa, a professor in the Department of Physics at Tokyo Metropolitan University. "We experimentally studied this type of effect for water using SWCNTs."

Water within SWCNTs in the range of 1.68 to 2.40 nanometers undergoes a wet-dry type of transition when temperature is decreased. And the team discovered that when SWCNTs are extremely narrow, the water inside forms tubule ices that are quite different from any bulk ices known so far. Strikingly, their melting point rises as the SWCNT diameter decreases -- contrary to that of bulk water inside a large-diameter capillary. In fact, tubule ice occurred even at room temperature inside SWCNTs.

"We extended our studies to the larger diameter SWCNTs up to 2.40 nanometers and successfully proposed a global phase behavior of water," says Maniwa. "This phase diagram (see image) covers a crossover from microscopic to macroscopic regions. In the macroscopic region, a novel wet-dry transition was newly explored at low temperature."

Results such as these contribute to a greater understanding of fundamental science because nanoconfined water exists and plays a vital role everywhere on Earth -- including our bodies. "Understanding the nanoconfined effect on the properties of materials is also crucial to develop new devices, such as proton-conducting membranes and nanofiltration," Maniwa notes.

Next up, the team plans to investigate the physical properties of confined water discovered so far inside SWCNTs (such as dielectricity and proton conduction). They will pursue this to obtain a better understanding of the molecular structure and transport properties in biological systems.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by American Institute of Physics, via EurekAlert!, a service of AAAS.