Quasi-Fixed Points and Charge and Colour Breaking in Low Scale Models

We show that the current LEP2 lower bound upon the minimal supersymmetric standard model (MSSM) lightest Higgs mass rules out quasi-fixed scenarios for string scales between 10^6 and 10^{11} GeV unless the heaviest stop mass is more than 2 TeV. We consider the implications of the low string scale for charge and colour breaking (CCB) bounds in the MSSM, and demonstrate that CCB bounds from F and D-flat directions are significantly weakened. For scales less than 10^{10} GeV these bounds become merely that degenerate scalar mass squared values are positive at the string scale.Comment: 17 pages, 4 figures. Replacement has added discussion on errors due to alpha_s(MZ) errors, as well as deviations from the quasi-fixed point. Text has been clarifie

Brane Gases on K3 and Calabi-Yau Manifolds

We initiate the study of Brane Gas Cosmology (BGC) on manifolds with non-trivial holonomy. Such compactifications are required within the context of superstring theory in order to make connections with realistic particle physics. We study the dynamics of brane gases constructed from various string theories on background spaces having a K3 submanifold. The K3 compactifications provide a stepping stone for generalising the model to the case of a full Calabi-Yau three-fold. Duality symmetries are discussed within a cosmological context. Using a duality, we arrive at an N=2 theory in four-dimensions compactified on a Calabi-Yau manifold with SU(3) holonomy. We argue that the Brane Gas model compactified on such spaces maintains the successes of the trivial toroidal compactification while greatly enhancing its connection to particle physics. The initial state of the universe is taken to be a small, hot and dense gas of p-branes near thermal equilibrium. The universe has no initial singularity and the dynamics of string winding modes allow three spatial dimensions to grow large, providing a possible solution to the dimensionality problem of string theory.Comment: 26 pages; Significant revisions: review material truncated; presentation improve

Meta-stable SUSY Breaking Model in Supergravity

We analyze a supersymmetry (SUSY) breaking model proposed by Intriligator, Seiberg and Shih in a supergravity (SUGRA) framework. This is a simple and natural setup which demands neither extra superpotential interactions nor an additional gauge symmetry. In the SUGRA setup, the U(1)R symmetry is explicitly broken by the constant term in the superpotential, and pseudo-moduli field naturally takes non-zero vacuum expectation value through a vanishing cosmological constant condition. Sfermions tend to be heavier than gauginos, and the strong-coupling scale is determined once a ratio of sfermion to gaugino masses is fixed.Comment: 13 page

Noncommutativity from the string perspective: modification of gravity at a mm without mm sized extra dimensions

We explore how the IR pathologies of noncommutative field theory are resolved when the theory is realized as open strings in background B-fields: essentially, since the IR singularities are induced by UV/IR mixing, string theory brings them under control in much the same way as it does the UV singularities. We show that at intermediate scales (where the Seiberg-Witten limit is a good approximation) the theory reproduces the noncommutative field theory with all the (un)usual features such as UV/IR mixing, but that outside this regime, in the deep infra-red, the theory flows continuously to the commutative theory and normal Wilsonian behaviour is restored. The resulting low energy physics resembles normal commutative physics, but with additional suppressed Lorentz violating operators. We also show that the phenomenon of UV/IR mixing occurs for the graviton as well, with the result that, in configurations where Planck's constant receives a significant one-loop correction (for example brane-induced gravity), the distance scale below which gravity becomes non-Newtonian can be much greater than any compact dimensions.Comment: 30 pages. Slight revision: clarified some points and added a referenc

Quasar H II Regions During Cosmic Reionization

Cosmic reionization progresses as HII regions form around sources of ionizing radiation. Their average size grows continuously until they percolate and complete reionization. We demonstrate how this typical growth can be calculated around the largest, biased sources of UV emission, such as quasars, by further developing an analytical model based on the excursion set formalism. This approach allows us to calculate the sizes and growth of the HII regions created by the progenitors of any dark matter halo of given mass and redshift with a minimum of free parameters. Statistical variations in the size of these pre-existing HII regions are an additional source of uncertainty in the determination of very high redshift quasar properties from their observed HII region sizes. We use this model to demonstrate that the transmission gaps seen in very high redshift quasars can be understood from the radiation of only their progenitors and associated clustered small galaxies. The fit sets a lower limit on the redshift of overlap at z = 5.8 +/- 0.1. This interpretation makes the transmission gaps independent of the age of the quasars observed. If this interpretation were correct it would raise the prospects of using radio interferometers currently under construction to detect the epoch of reionization.Comment: 6 pages, 3 figures, accepted by MNRAS, revised to match published versio

Front propagation in laminar flows

The problem of front propagation in flowing media is addressed for laminar velocity fields in two dimensions. Three representative cases are discussed: stationary cellular flow, stationary shear flow, and percolating flow. Production terms of Fisher-Kolmogorov-Petrovskii-Piskunov type and of Arrhenius type are considered under the assumption of no feedback of the concentration on the velocity. Numerical simulations of advection-reaction-diffusion equations have been performed by an algorithm based on discrete-time maps. The results show a generic enhancement of the speed of front propagation by the underlying flow. For small molecular diffusivity, the front speed $V_f$ depends on the typical flow velocity $U$ as a power law with an exponent depending on the topological properties of the flow, and on the ratio of reactive and advective time-scales. For open-streamline flows we find always $V_f \sim U$, whereas for cellular flows we observe $V_f \sim U^{1/4}$ for fast advection, and $V_f \sim U^{3/4}$ for slow advection.Comment: Enlarged, revised version, 37 pages, 14 figure