Nuclear Theory Seminars at Texas A&M: Spring 2013

Location: M108

January, 18, 4:00pm
      Nuclear Seminar
Yunpeng Liu -TAMU -
B_c meson enhancement in relativistic heavy ion collisions at LHC . "


B_c meson production in Pb+Pb collisions at sqrt(s)=2.76 A TeV is surveyed in both a statistical coalescence model and a transport model. The nuclear modification factor R_{AA} is predicted to be between 2 and 18 in the most central collisions, which can help to confirm the regeneration mechanism. Momentum dependence is also investigated in the transport model. A strong suppression of the transverse momentum is found in central collisions accompanying the enhancement in yield. The spectrum and elliptic flow of B_c are also discussed.
January, 25, 3:00pm at Hawking Auditorium       Nuclear Seminar
N. Auerbach -Tel Aviv University -
The Shear Viscosity to Entropy Density Ratio in Nuclei . "


Experiments at the Relativistic Heavy Ion Collider (RHIC) suggest that the state of matter produced in the experiments has a low shear-viscosity to entropy-density ratio eta/s. In some super-symmetric gauge theories one finds that eta/s=1/4pi. It has been conjectured that this value is the lower limit for a large class of physical systems. We address the question, what is the value of this ratio in regular, finite nuclei at low temperature. We use the experimental and theoretical results for the widths of giant vibrational states in nuclei in order to calculate the above ratio. Another type of collective motion is the process of fission. Using a classical macroscopic approach to describe spontaneous and induced fission one can also extract a value for this ratio. We find that the values of eta/s are not very different from the ones found in the RHIC experiments. Using the conjectured lower limit for the ratio we present lower limits for the widths of giant resonances in nuclei.
February, 22, 4:00pm       Nuclear Seminar
Chiho Nonaka- Kobayashi-Maskawa Institute, U of Nagoya -
A new scheme of causal viscous hydrodynamics for relativistic heavy-ion collisions: Riemann solver for quark-gluon plasma. "


In this article, we present a state-of-the-art algorithm for solving the relativistic viscous hydrodynamic equation with QCD equation of state. The numerical method is based on the second-order Godunov method and has less numerical dissipation, which are crucial in describing of quark-gluon plasma in high energy heavy-ion collisions. We apply the algorithm to several numerical test problems such as sound wave propagation, shock tube and blast wave problems. In the sound wave propagation, the intrinsic numerical viscosity is measured and its explicit expression is shown, which is the second-order of spatial resolution both in the presence and absence of physical viscosity. The expression of the numerical viscosity can be used to determine the maximum cell size in order to accurately measure the effect of physical viscosity in the numerical simulation for relativistic heavy-ion collisions.
March, 8, 4:00pm       Nuclear Seminar
Taesoo Song- TAMU -
Quarkonium formation time in quark-gluon plasma. "


The quarkonium formation time in a quark-gluon plasma (QGP) is determined from the space-time correlator of heavy quark vector currents using the quarkonium in-medium mass and wave function obtained from heavy quark potentials extracted from the lattice QCD. It is found that the formation time of a quarkonium increases with the temperature of the QGP and diverges near its dissociation temperature. Also, the quarkonium formation time is longer if the heavy quark potential is taken to be the free energy from lattice calculations for a heavy quark pair, compared to that based on the more negative internal energy.
Thursday(Unusual Time), March 28 10:30am, M108
Adrian Dumitru -CUNY Baruch College -
Magnetic flux loop in high-energy heavy-ion collisions. "


Relativistic collisions of heavy ions generate strong non-Abelian color fields. Right after the impact the longitudinal Ez and Bz components dominate. I discuss the 2D structure of the longitudinal magnetic field as probed by magnetic flux loops in the transverse plane. It appears that over relevant distance scales the fields can be interpreted as uncorrelated Z(N) vortices of size 1/Qs, where Qs is the so-called "gluon saturation momentum" that separates the perturbative from the non-linear regime. The vortex field background also modifies propagation of hard particles.
March, 29, 4:00pm       Nuclear Seminar
Anton Wiranata- LBNL/CCNU -
The Ratio of eta/s of an Interacting Hadron Gas. "


We are going to show a comparison among current available cross sections such as Breit-Wigner/UrQMD (BW), experimental phase shift (EXP) and the so called K-matrix (KM) cross sections. We applied these cross sections in the p - p interacting system. BW cross sections are unable to preserve unitarity of the T -matrix if more than one resonance is included. In the case of EXP cross sections, the number of resonances considered are only two (s & ?-resonances) with an additional repulsive channel. The KM can do both of these things, it preserves unitarity of the T -matrix and it is able to include heavier resonances. As we are going to show in this work, the inclusion of heavier resonances is one of crucial factors in the determination of shear viscosity and entropy density in the hadronic regime. More resonances in the interaction means low shear viscosity and high entropy density hence the ratio of ?/s will be reduced by significant amount for temperatures near the transition temperature..
April, 5, 4:00pm       Nuclear Seminar
Prasanth Jaikumar- California State University at Long Beach -
Using r-mode oscillations to constrain the equation of state for neutron stars. "


Gravitational waves are a fundamental prediction of Einstein's theory of general relativity, but have only been indirectly observed so far. Neutron stars are among the primary sources for these waves, which can be excited in a variety of ways, one example being the r-mode oscillations arising from the Coriolis force. For rotating neutron stars, r-modes probe the interior of the star, making them a penetrating probe of the equation of state for cold and dense matter, much as photons and dileptons are for the quark-gluon plasma. The challenge for theoretical r-mode calculations comes from the uncertainty in sources of viscous damping and friction within the neutron star fluid. We will examine a few of these sources in detail and estimate their relative importance for r-mode damping in different phases of cold and dense matter. Finally, we will compare our theoretical results for r-modes to observational constraints set by the high rotation speeds of some low mass X-ray binaries in order to constrain the equation of state for neutron star matter.
April, 12, 4:00pm       Nuclear Seminar
Yunpeng Liu- TAMU -
Gluon dissociation of charmonium beyond the dipole approximation. "


Gluon dissociation of charmonium is an important mechanism for understanding charmonium suppression in relativistic heavy ion collisions. The cross section for this process is usually calculated in the dipole approximation by assuming the wavelength of the gluon is much larger than the radius of the charmonium. To check the validity of this approximation, we have carried out a non-relativistic potential model calculation that takes into account the full gluon wave function. We have found that the dipole approximation overestimates the cross section, and the difference between our result and that based on the dipole approximation can be as large as a factor of 3. Implication of our results on charmonium suppression in heavy ion collisions will also be discussed.
April, 19, 4:00pm       Nuclear Seminar
Kyong Chol Han- TAMU -
Medium modification of jet fragmentation. "


The hadronization of partonic jets produced in elementary collisions at high energies is usually modeled by fragmentation functions that are fitted to experimentally measured hadron spectra. For jets produced in relativistic heavy ion collisions, the presence of the quark-gluon plasma formed in the collisions not only affects their energies but is also expected to influence their hadronization to hadrons. We have studied the latter by treating the jet as a shower of partons, which are then converted to hadrons via recombination of shower partons among themselves as well as with the thermal partons in the quark-gluon plasma. Using the shower partons obtained from the PYTHIA Monte Carlo generator, we have found that a hybrid approach that includes both the recombination of shower partons and the independent fragmentation of the remnant jet constructed from shower partons left from the recombination reproduces very well the hadron spectra from the fragmentation of original jet. The medium effect on the conversion of jets to hadrons is then studied for heavy ion collisions at both RHIC and LHC by using a blast wave model for the produced quark-gluon plasma. We find that including hadron production from the recombination of shower partons with the thermal partons in the quark-gluon plasma leads to an almost factor of two enhancement in the production of hadrons with intermediate transverse momentum of 3-5 GeV/c at RHIC and 7-10 GeV/c at LHC. Our results thus suggest that medium modification of jet fragmentation provides plausible explanation for the enhanced production of intermediate transverse momentum hadrons observed in experiments.
April, 26, 4:00pm       Nuclear Seminar
Bjoern Schenke- Brookhaven National Laboratory-
Fluctuating Gluon Fields and Flow in High-Energy Nuclear Collisions. "


The fluid dynamic description of the hot and dense matter created in heavy-ion collisions has been extremely successful in describing measured spectra and anisotropic flow of produced particles at low momentum. In recent years it has become clear that both viscous effects and a detailed understanding of the initial state and its fluctuations are essential to consistently describe the wide range of available experimental data and to extract properties of the produced strongly interacting system. I will review progress towards a comprehensive description of the bulk dynamics in heavy-ion collisions and present a new quantum-chromo-dynamics based initial state model that includes sub-nucleonic color-charge fluctuations and Yang-Mills dynamics of the produced gluon fields at early times. Coupled to viscous fluid dynamic evolution it reproduces experimental data from heavy-ion collisions on anisotropic flow coefficients and their event-by-event distributions from RHIC and LHC. I will further discuss the initial state and the role of hydrodynamics in proton-nucleus and deuteron-nucleus collisions.

Previous seminars

Fall 2012
Spring 2012
Fall 2011
Spring 2011
Fall 2010
Spring 2010
Fall 2009
Spring 2009
Fall 2008
Spring 2008
Fall 2007
Spring 2007
Fall 2006
Spring 2006
Fall 2005
Spring 2005
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Spring 2004
Fall 2003

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