This award will support the study of collisions between ultra-relativistic nuclei, ultimately leading to a determination of the properties and of the equation of state of nuclear matter over a wide range of temperatures and densities. High energy density physics is a rapidly growing field that spans a broad range of physics sub-disciplines including nuclear physics, plasma physics, laser physics, fluid dynamics, and magnetohydrodynamics. The astrophysics results from new terrestrial and orbital observatories have enabled study of high energy density physics on the stellar, galactic, and universal scales through the study of giant planets, brown dwarfs, white dwarfs, neutron stars, supernovae, gamma-ray bursters, and the big bang. The studies supported by this award of relativistic nucleus-nucleus collisions probe the high-temperature region of high energy density physics phase space, where there is a transition from hadron to partonic matter, the quark-gluon plasma (QGP). Although it is clear that dense quark matter is created in the early stages of relativistic heavy-ion collisions, identification of the phase transition boundary will require a comprehensive and correlated set of measured collision observables, as well as, realistic and extensive calculations which relate the observables to nuclear matter parameters.
For the next three years, efforts will be focused on the acquisition, analysis, and interpretation of data taken with the Solenoidal Tracker At RHIC (STAR) experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) on Long Island, and the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) at the European Center for Nuclear Research (CERN) in Geneva, Switzerland. Junior scientists will also participate in an essential way in this research.
The two largest colliders in the world are the Large Hadron Collider (LHC) in Geneva, Switzerland and the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York. Both facilities are used to create collisions using the nuclei of heavy elements at relativistic speeds. In the course of these collisions, the nuclear matter is compressed and heated reaching temperatures almost a million times hotter than those achieved during the detonation of a thermo-nuclear device. At a high enough energy density, the nuclear matter is compressed enough to melt the neutrons and protons that make-up the nucleus into their constituent quarks and gluons. This plasma of quarks and gluons that is created briefly in the laboratory resembles the state of the universe roughly one microsecond after the big bang. Study of the properties of the plasma of quarks and qluons and of the nature of the transition from a phase of discrete hadrons (made of quarks, i.e. neutrons, protons, and pions) to a plasma phase sheds light on the nature of the early universe and on the cores of extremely dense astrophysical objects like neutron stars. The Davis group has led efforts to explore the properties of quark-gluon plasma matter across a broad range of temperatures and densities. This requires using both the RHIC and LHC facilities. The LHC provides the highest energy collisions in the world, 2.76 TeV for symmetric collisions of heavy ions. The temperatures in these collisions reach several hundred MeV (2×1012 K), and the density of quarks and gluons is so high that the matter is almost opaque to objects which interact using the strong nuclear or color force. The density and viscosity of the matter can be probed by studying rare events in which an extremely heavy particle is created. Examples of such rare heavy particles are the Z0 gauge boson and the Upsilon meson. The Z0 does not interact through the strong force, therefore it should not interact with the plasma medium. Indeed it is found that regardless of the size or shape of the plasma medium created during the collisions, the production of Z0 bosons remains unaffected. The upsilon meson, which is made of a bottom and an anti-bottom quark, does interact through the strong force, and therefore it is expected to be stopped or at least slowed down by the plasma medium. Indeed in head-on collisions between two heavy ions, it is found that only 40% of the expected production is observed. Hadronic jets, which are created by lighter quarks or gluons also are seen to be suppressed by the plasma. At RHIC, there is greater flexibility about the collision energy and the colliding systems. In order to better understand the nature of the transition between the dense gas of discrete hadrons and the plasma phase, a scan of collision energies has been performed. Au+Au collisions were studied at 200, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV as well as some fixed-target Au+Al data at lower energies. The goal of this beam energy scan was to find the onset energy at which the nuclear matter was first broken down into a plasma of the constituent quarks and gluons. It is found that the energies above 27 GeV all show similar plasma-like behavior. At the higher energies, there is a clear suppression of the high transverse momentum hadrons; these high transverse momentum hadrons are usually the leading particle in a hadronic jet. This suppression goes away for the lowest two energies of the scan (11.5 and 7.7 GeV). Another plasma signature can be found in the hydrodynamic flow patterns in the events. From these flow patterns, the nature of the medium can be inferred. It is found that for energies above 27 GeV the flow is established at the level of the quarks, indicating the medium was in the plasma phase during the compression. At the lowest energies, this pattern breaks down and it is found that the flow is established in a gas of hadrons. More precise studies will be needed to refine the transition from plasma dominated systems to hadron dominated and to determine the nature of the phase transition.
