When Matter Melts

When Matter MeltsBy comparing theory with data from STAR, Berkeley Lab scientists and theircolleagues map phase changes in the quark-gluon plasmaIn 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 ofquarks and gluons. As the universe expanded, this quark–gluon plasmaquickly 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 quantumchromodynamics, or QCD," says Nu Xu of the U.S. Department of Energy'sLawrence Berkeley National Laboratory (Berkeley Lab), the spokesperson forthe STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at DOE’sBrookhaven National Laboratory. "QCD has been extremely successful atexplaining interactions of quarks and gluons at short distances, such ashigh-energy proton and antiproton collisions at Fermi National AcceleratorLaboratory. But in bulk collections of matter - including the quark-gluonplasma - at longer distances or smaller momentum transfer, an approachcalled lattice gauge theory has to be used."Until recently, lattice QCD calculations of hot, dense, bulk matter couldnot be tested against experiment. Beginning in 2000, however, RHIC wasable to recreate the extreme conditions of the early universe inminiature, by colliding massive gold nuclei (heavy ions) at high energies.Experimentalists at RHIC, working with theorist Sourendu Gupta of India'sTata Institute of Fundamental Research, have recently comparedlattice-theory predictions about the nature of the quark-gluon plasma withcertain STAR experimental results for the first time. In so doing theyhave established the temperature boundary where ordinary matter and quarkmatter cross over and change phase. Their results appear in the journalScience.Phase diagramsThe aim of both the theoretical and experimental work is to explore andfix key points in the phase diagram for quantum chromodynamics. Phasediagrams are maps, showing, for example, how changes in pressure andtemperature 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 possiblephases of QCD such as color superconductivity."Plotting a QCD phase diagram requires both theory calculations andexperimental effort with heavy-ion collisions," says Xu, who is a memberof Berkeley Lab's Nuclear Science Division and an author of the Sciencepaper. Experimental studies require powerful accelerators like RHIC onLong Island or the Large Hadron Collider at CERN in Geneva, whilecalculations of QCD using lattice gauge theory require the world's biggestand fastest supercomputers. Direct comparisons can achieve more thaneither approach alone.One of the basic requirements of any phase diagram is to establish itsscale. A phase diagram of water might be based on the Celsius temperaturescale, defined by the boiling point of water under normal pressure (i.e.,at sea level). Although the boiling point changes with pressure - athigher altitudes water boils at lower temperatures - these changes aremeasured against a fixed value.The scale of the QCD phase diagram is defined by a transition temperatureat the zero value of "baryon chemical potential." Baryon chemicalpotential measures the imbalance between matter and antimatter, and zeroindicates 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 equallysignificant result, showing that the highly dynamical systems of RHIC'sgold-gold collisions, in which the quark-gluon plasma winks in and out ofexistence, in fact achieve thermal equilibrium. Here’s where theory andexperiment 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'sNuclear Science Division and an author of the Science paper. Yet becausedifferences in values of the kind observed by STAR are related tofluctuations in thermodynamic values predicted by lattice gauge theory,says Ritter, "by comparing our results to the predictions of theory, wehave shown that what we measure is in fact consistent with the fireballsreaching thermal equilibrium. This is an important achievement."The scientists were now able to proceed with confidence in establishingthe scale of the QCD phase diagram. After a careful comparison betweenexperimental data and the results from the lattice gauge theorycalculations, 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 theboundary between the low-temperature hadronic phase of ordinary matter andthe high-temperature quark-gluon phase.In search of the critical pointLattice QCD also predicts the existence of a "critical point." In a QCDphase diagram the critical point marks the end of a line showing where thetwo phases cross over, one into the other. By changing the energy, forexample, 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 ofthe collisions through the region of the QCD phase diagram where thecritical point is most likely to be found - from an energy of 200 billionelectrons volts per pair of nucleons (protons or neutrons) down to 5billion electron volts per nucleon pair.Says Ritter, "Establishing the existence of a QCD critical point would bemuch more significant than setting the scale." In 2010, RHIC started aprogram to search for the QCD critical point.Xu says, "In this paper, we compared experimental data with latticecalculations directly, something never done before. This is a real stepforward and allows us to establish the scale of the QCD phase diagram.Thus begins an era of precision measurements for heavy-ion physics.""Scale for the phase diagram of quantum chromodynamics," by SourenduGupta, Xiaofeng Luo, Bedangadas Mohanty, Hans Georg Ritter, and Nu Xu,appears in the 24 June 2011 issue of Science magazine. Gupta is with theTata Institute of Fundamental Research in Mumbai, India, where thetheoretical calculations for this paper were carried out. Mohanty is withthe Variable Energy Cyclotron Centre in Kolkata, India, and was formerly apostdoctoral fellow at Berkeley Lab. Luo, Ritter, and Xu are with BerkeleyLab’s Nuclear Science Division. Luo is also with the University of Scienceand Technology of China in Hefei, and Xu is also with the Central ChinaNormal University in Wuhan. This work was supported by the Indian LatticeGauge Theory Initiative, by India's Department of Atomic Energy-Board ofResearch in Nuclear Sciences, the National Science Foundation of China,the Chinese Ministry of Education, and by DOE’s Office of Science.Lawrence Berkeley National Laboratory addresses the world's most urgentscientific challenges by advancing sustainable energy, protecting humanhealth, creating new materials, and revealing the origin and fate of theuniverse. Founded in 1931, Berkeley Lab's scientific expertise has beenrecognized with 12 Nobel prizes. The University of California managesBerkeley Lab for the U.S. Department of Energy’s Office of Science. Formore, visit www.lbl.gov.An html version of this release, with downloadable images, is athttp://newscenter.lbl.gov/news-releases/2011/06/23/when-matter-melts/