Research Description Nasser Soliman Demir

In the first few microseconds following the Big Bang, the very hot and dense Universe was thought to be in a state of matter known as the quark gluon plasma (QGP). The QGP is rather different from the normal matter we observe everyday; protons and neutrons are examples of particles in which quarks and gluons are bound. It is said that quarks and gluons are confined in Nature; we do not observe them individually as free particles, but only in composite objects which are composed of quarks and gluons bound together tightly. For example, while we observe the proton (a particle consisting of two up quarks and one down quark), we do not observe those three quarks individually in Nature. However, it is thought that at sufficiently hot and dense conditions, one can “liberate” quarks and gluons from the confined state. The state in which quarks and gluons are not confined, but quasi-free or deconfined , is known as the QGP. Why is the QGP useful to study? It will not only give us insight into how the medium of the Early Universe behaved, but will help us understand how nuclear matter behaves under extreme conditions. Scientists at many high energy accelerators, including the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory on Long Island, New York attempt to recreate the QGP by colliding heavy nuclei (such as Copper, Lead, and Gold) at speeds very close to the speed of light. The hope is that colliding sufficiently heavy particles at very fast speeds will compress and heat the matter to the degree necessary to produce the QGP. However, one of the biggest challenges involved in analyzing the QGP is that the state is transient; it is so short lived (lifetime is on the order of ∼ 10−23 s), that the quarks and gluons recombine to from bound states (known as hadrons), and the particle detectors in such experiments measure properties of the final-state hadrons rather than the free quarks themselves.[2] As a result, there is a great deal of detective work in phenomenology and theoretical modeling involved in identifying possible signatures for the existence of a QGP, and investigating such properties. One of the top science stories of 2005 was the discovery of a “near perfect fluid” at RHIC.[1, 2, 3] It was previously expected that if a QGP were to be created at RHIC energies, the resulting matter would be a gas. However, a great deal of experimental evidence exists suggesting that the state created at RHIC is not only not a gas, but a nearly ideal fluid.[4, 5, 6, 7] The resistance to flow in a fluid is characterized by a quantity known as the shear viscosity (η). Traditionally, an ideal fluid has been defined as having η = 0. However, such a fluid is unphysical, and there has been a paradigm shift in defining an “ideal fluid.” The related quantity proposed for this definition is the viscosity to entropy ratio ηs , where s is the entropy density . A crude way to think about the entropy is that it characterizes the amount of disorder in the system; how many possible configurations a system can take at a given time. In particular, a revolutionary technique in string theory, which has long been criticized in the scientific community as failing to produce any concrete, falsifiable results, has predicted a possible minimum bound for a class of fluids: ηs ≥ h̄ 4πkB , where h̄ is Planck’s constant divided by 2π, and kB is Boltzmann’s constant.[8] This famous result, known as the Kovtun-Son-Starinets (KSS) bound, conjectures that the most “ideal fluid” should have a value corresponding to η=s ( h̄ 4πkB ) . Thus, understanding the QGP created at RHIC will also help us better understand the nature of near perfect fluidity. It should also be noted that a program for performing such collisions of heavy ions at very high speeds is also planned for the Large Hadron Collider (LHC), which has recently opened near Geneva, Switzerland. There further exists speculation that a QGP created at the LHC would actually be a fluid with a higher viscosity, and the current relativistic heavy ion community is anxiously awaiting the heavy ion experiments due to begin in 2010.[9] A cartoon of a relativistic heavy ion reaction is shown in Figure 1. The two pancake-shaped objects in the “initial state” panel represent heavy nuclei (such as gold nuclei) colliding at very fast speeds. The “pre-equilibrium” phase refers to the process where the nuclei collide and “mesh”, and scattering among the quarks and gluons takes place. The “QGP and hydrodynamic expansion” phase is where the QGP thermalizes, and then expands like an ideal fluid. “Hadronization” refers to when the quarks and gluons of the QGP reorganize into bound states called hadrons (protons and neutrons are examples of hadrons). “Freezeout” refers to when the hadrons stop colliding with