Various topics in field theory

title: Time-dependent open string solutions in (2+1)-dimensional gravity.
abstract: We find general, time-dependent solutions produced by open string sources carrying no momentum flow in 2+1 dimensional gravity. The local Poincar\'e group elements associated with these solutions and the coordinate transformations that transform these solutions into Minkowski metric are obtained. We also find the relation between these solutions and the planar wall solutions in 3+1 dimensions.

title: Radiation from a moving scalar source.
abstract: We study classical radiation and the quantum bremsstrahlung effect of a moving point scalar source. Our classical analysis provides another example of resolving a well-known apparent paradox, that of whether or not a constantly accelerating source radiates. Quantum mechanically, we show that for a scalar source with arbitrary motion the tree level emission rate of scalar particles in the inertial frame equals the sum of emission and absorption rates of zero-energy Rindler particles in the Rindler frame. We then explicitly verify this result for a source undergoing constant proper acceleration.

title: WZW model based on the extended de Sitter group
abstract: We study the WZW model based on the centrally extended 2D de Sitter algebra. We obtain the spacetime metric and its explicitly conformally flat expression. The symmetries of the spacetime are found by identifying the Killing vectors with the group generators. The energy-momentum tensor obtained from the affine-Sugawara construction agrees with that from the more conventional approach. The exact center charge agrees to one-loop order with the one-loop beta function equations. We have also studied the representations of the corresponding enveloping Virasoro algebra.

title: Fermions in a global monopole background
abstract: We study the scattering of fermions in a “global monopole” background metric. This is the four-dimensional analogue of the scattering on a cone in three dimensions. The scattering amplitude is exactly obtained. We then study massless fermion-dyon systems in such a background metric. The density of the S -wave fermion condensate is found to be given by a constant times the flat space value of Callan and Rubakov.

title: The momentum analyticity of two point correlators from perturbation theory and AdS/CFT
abstract: The momentum plane analyticity of two point function of a relativistic thermal field theory at zero chemical potential is explored with both perturbative and nonperturbative method. A general principle regarding the location of t he singularities is extracted. In the case of the N = 4 supersymmetric Yang-Mills theory at large Nc, the nonperturbative nature of the singularity (simple poles) is observed in the strong coupling limit via the AdS/CFT correspondence.

title: A correction to Schafroth's superconductivity solution of an ideal charged boson system☆
abstract: The Schafroth superconductivity solution of an ideal gas of charged bosons (with an external uniform background charge density so that the whole system is electrically neutral) gives for the critical magnetic field Hc(T) = [2eλL2(T)]−1, in units h = c = 1, where T is the temperature (assumed to be less than Tc), λL(T) is the London length and e the boson charge. We show that the formula is invalid because the electrostatic exchange energy Eex between bosons has been completely left out in the Schafroth solution. Based on the Schafroth solution, Eex is found to be + ∞ in the normal phase, but 0 in the condensed phase (at T = 0). Of course, the correct solution has to give a finite Eex. At low density the ideal charged boson system turns out not to be a superconductor, but becomes a type II superconductor at high density, with a critical field Hc much larger than the Schafroth result.

title: Low shear viscosity due to Anderson localization
abstract: We study the Anderson Localization effect on the shear viscosity in a system with random medium by Kubo formula. We show that this effect can reduce the shear viscosity nonperturbatively. Then we discuss its possible implementation in heavy-ion collisions, where the created heavy bound states or other collective modes may play the role of the random scatterer underlying Anderson Localization effect.

title: Field theory on a computationally constructed random lattice
abstract: We describe a computational procedure by which random lattices in two, three and four dimensions are constructed in accordance with a prescription given earlier. We report numerical results for a scalar field in two dimensions and for a gauge field - U(1), SU(2), SU(3) - in four dimensions.

title: Linearized gravity in isotropic coordinates in the brane world
abstract: We solve the Einstein equations in the Randall-Sundrum framework using an isotropic ansatz for the metric and obtain an exact expression to first order in the gravitational coupling. The solution is free from metric singularities away from the source and it satisfies the Israel matching condition on a straight brane. At distances far away from the source and on the physical brane this solution coincides with the 4D Schwarzschild metric in isotropic coordinates. Furthermore, we show that the extension of the standard Schwarzschild horizon in the bulk is tubular for any diagonal form of the metric while there is no restriction for the extension of the Schwarzschild horizon in isotropic coordinates.