Development and demonstration of a flutter-suppression system using active controls

The application of active control technology to suppress flutter was demonstrated successfully in the transonic dynamics tunnel with a delta-wing model. The model was a simplified version of a proposed supersonic transport wing design. An active flutter suppression method based on an aerodynamic energy criterion was verified by using three different control laws. The first two control laws utilized both leading-edge and trailing-edge active control surfaces, whereas the third control law required only a single trailing-edge active control surface. At a Mach number of 0.9 the experimental results demonstrated increases in the flutter dynamic pressure from 12.5 percent to 30 percent with active controls. Analytical methods were developed to predict both open-loop and closed-loop stability, and the results agreed reasonably well with the experimental results

Cosmic Reionisation by Stellar Sources: Population II Stars

We study the reionisation of the Universe by stellar sources using a numerical approach that combines fast 3D radiative transfer calculations with high resolution hydrodynamical simulations. Ionising fluxes for the sources are derived from intrinsic star formation rates computed in the underlying hydrodynamical simulations. Our mass resolution limit for sources is M~ 4.0 x 10^7 h^-1 M_sol, which is roughly an order of magnitude smaller than in previous studies of this kind. Our calculations reveal that the reionisation process is sensitive to the inclusion of dim sources with masses below ~10^9 h^-1 M_sol. We present the results of our reionisation simulation assuming a range of escape fractions for ionising photons and make statistical comparisons with observational constraints on the neutral fraction of hydrogen at z~6 derived from the z=6.28 SDSS quasar of Becker and coworkers. Our best fitting model has an escape fraction of ~20% and causes reionisation to occur by z~8, although the IGM remains fairly opaque until z~6. In order to simultaneously match the observations from the z=6.28 SDSS quasar and the optical depth measurement from WMAP with the sources modeled here, we require an evolving escape fraction that rises from f_esc=0.20 near z~6 to f_esc>~10 at z~18.Comment: 42 pages, 13 figure

Flat Foldings of Plane Graphs with Prescribed Angles and Edge Lengths

When can a plane graph with prescribed edge lengths and prescribed angles (from among $\{0,180^\circ, 360^\circ$\}) be folded flat to lie in an infinitesimally thin line, without crossings? This problem generalizes the classic theory of single-vertex flat origami with prescribed mountain-valley assignment, which corresponds to the case of a cycle graph. We characterize such flat-foldable plane graphs by two obviously necessary but also sufficient conditions, proving a conjecture made in 2001: the angles at each vertex should sum to $360^\circ$, and every face of the graph must itself be flat foldable. This characterization leads to a linear-time algorithm for testing flat foldability of plane graphs with prescribed edge lengths and angles, and a polynomial-time algorithm for counting the number of distinct folded states.Comment: 21 pages, 10 figure

The scalar sector in the Myers-Pospelov model

We construct a perturbative expansion of the scalar sector in the Myers-Pospelov model, up to second order in the Lorentz violating parameter and taking into account its higher-order time derivative character. This expansion allows us to construct an hermitian positive-definite Hamiltonian which provides a correct basis for quantization. Demanding that the modified normal frequencies remain real requires the introduction of an upper bound in the magnitude |k| of the momentum, which is a manifestation of the effective character of the model. The free scalar propagator, including the corresponding modified dispersion relations, is also calculated to the given order, thus providing the starting point to consider radiative corrections when interactions are introduced.Comment: Published in AIP Conf.Proc.977:214-223,200

On the Thermal History of Calculable Gauge Mediation

Many messenger models with realistic gaugino masses are based on meta-stable vacua. In this work we study the thermal history of some of these models. Analyzing R-symmetric models, we point out that while some of the known messenger models clearly prefer the supersymmetric vacuum, there is a vast class of models where the answer depends on the initial conditions. Along with the vacuum at the origin, the high temperature thermal potential also possesses a local minimum far away from the origin. This vacuum has no analog at zero temperature. The first order phase transition from this vacuum into the supersymmetric vacuum is parametrically suppressed, and the theory, starting from that vacuum, is likely to evolve to the desired gauge-mediation vacuum. We also comment on the thermal evolution of models without R-symmetry.Comment: 22 pages. V2: Comments on the SM effects added. Minor corrections. Reference added. Valuable discussion with S. Abel, J. Jaeckel and V. Khoze acknowledged. V3: Types of EOGM explicitly defined in the introduction. Discussions about the phase transitions expanded. Typo corrected. Journal versio

X-ray Scattering Study of the spin-Peierls transition and soft phonon behavior in TiOCl

We have studied the S=1/2 quasi-one-dimensional antiferromagnet TiOCl using single crystal x-ray diffraction and inelastic x-ray scattering techniques. The Ti ions form staggered spin chains which dimerize below Tc1 = 66 K and have an incommensurate lattice distortion between Tc1 and Tc2 = 92 K. Based on our measurements of the intensities, wave vectors, and harmonics of the incommensurate superlattice peaks, we construct a model for the incommensurate modulation. The results are in good agreement with a soliton lattice model, though some quantitative discrepancies exist near Tc2. The behavior of the phonons has been studied using inelastic x-ray scattering with ~2 meV energy resolution. For the first time, a zone boundary phonon which softens at the spin-Peierls temperature Tsp has been observed. Our results show reasonably good quantitative agreement with the Cross-Fisher theory for the phonon dynamics at wave vectors near the zone boundary and temperatures near Tsp. However, not all aspects of the data can be described, such as the strong overdamping of the soft mode above Tsp. Overall, our results show that TiOCl is a good realization of a spin-Peierls system, where the phonon softening allows us to identify the transition temperature as Tsp=Tc2=92 KComment: 14 pages, 14 figure

Photon Conserving Radiative Transfer around Point Sources in multi-dimensional Numerical Cosmology

Many questions in physical cosmology regarding the thermal and ionization history of the intergalactic medium are now successfully studied with the help of cosmological hydrodynamical simulations. Here we present a numerical method that solves the radiative transfer around point sources within a three dimensional cartesian grid. The method is energy conserving independently of resolution: this ensures the correct propagation speeds of ionization fronts. We describe the details of the algorithm, and compute as first numerical application the ionized region surrounding a mini-quasar in a cosmological density field at z=7.Comment: 5 pages, 4 figures, submitted to ApJ

Fermion Masses and Mixing in Four and More Dimensions

We give an overview of recent progress in the study of fermion mass and flavor mixing phenomena. Mass matrix ansatze are considered within the SM and SUSY GUTs where some predictive frameworks based on SU(5) and SO(10) are reviewed. We describe a variety of schemes to construct quark mass matrices in extra dimensions focusing on four major classes: models with the SM residing on 3-brane, models with universal extra dimensions, models with split fermions and models with warped extra dimensions. We outline how realistic patterns of quark mass matrices could be derived from orbifold models in heterotic superstring theory. Finally, we address the fermion mass problem in intersecting D-branes scenarios, and present models with D6-branes able to give a good quantitatively description of quark masses and mixing. The role of flavor/CP violation problem as a probe of new physics is emphasized.Comment: a review based on seminars presented by S.K. in different places, 34 pages, late

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

Continuous distribution of frequencies and deformed dispersion relations

The possibilities that, in the realm of the detection of the so--called deformed dispersion relation, a light source with a continuous distribution of frequencies offers is discussed. It will be proved that the presence of finite coherence length entails the emergence of a new term in the interference pattern. This is a novel trait, which renders a new possibility in the quest for bounds associated with these deformed dispersion relations.Comment: Accepted in Classical and Quantum Gravit