Particle theory

Abstract:

The ratio of shear viscosity to volume density of entropy can be used to characterize how close a given fluid is to being perfect. Using string theory methods, we show that this ratio is equal to a universal value of ℏ/4πkB for a large class of strongly interacting quantum field theories whose dual description involves black holes in anti-de Sitter space. We provide evidence that this value may serve as a lower bound for a wide class of systems, thus suggesting that black hole horizons are dual to the most ideal fluids. © 2005 The American Physical Society.

Abstract:

We consider a U(1) gauge theory on the five dimensional orbifold $\mathcal{M}_4\times S^1/Z_2$, where $A_5$ has even $Z_2$ parity. This leads to a light pseudoscalar degree of freedom $W(x)$ in the effective 4D theory below the compactification scale arising from a gauge-invariant brane-to-brane Wilson line. As noted by Arkani-Hamed et al in the non-supersymmetric $S^1$ case the 5D bulk gauge-invariance of the underlying theory together with the non-local nature of the Wilson line field leads to the protection of the 4D theory of $W(x)$ from possible large global-symmetry violating quantum gravitational effects. We study the $S^1/Z_2$ theory in detail, in particular developing the supersymmetric generalization of this construction, involving a pseudoscalar Goldstone field (the `axion') and its scalar and fermion superpartners (`saxion' and `axino'). The global nature of $W(x)$ implies the absence of independent Kaluza-Klein excitations of its component fields. The non-derivative interactions of the (supersymmetric) Wilson line in the effective 4D theory arising from U(1) charged 5D fields $\Phi$ propagating between the boundary branes are studied. We show that, similar to the non-supersymmetric $S^1$ case, these interactions are suppressed by $\exp(-\pi R m_{\Phi})$ where $\pi R$ is the size of the extra dimension.

Abstract:

We study four-dimensional low energy effective actions for conifold transitions of Calabi-Yau spaces in the context of IIB supergravity. The actions are constructed by examining the mass of D3-branes wrapped on collapsing/expanding three-cycles. We then study the cosmology of the conifold transition, including consequences for moduli stabilization, taking into account the effect of the additional states which become light at the transition. We find, the degree to which the additional states are excited is essential for whether the transition is dynamically realized. © 2005 The American Physical Society.