Infinite impulse response modal filtering in visible adaptive optics

Diffraction limited resolution adaptive optics (AO) correction in visible wavelengths requires a high performance control. In this paper we investigate infinite impulse response filters that optimize the wavefront correction: we tested these algorithms through full numerical simulations of a single-conjugate AO system comprising an adaptive secondary mirror with 1127 actuators and a pyramid wavefront sensor (WFS). The actual practicability of the algorithms depends on both robustness and knowledge of the real system: errors in the system model may even worsen the performance. In particular we checked the robustness of the algorithms in different conditions, proving that the proposed method can reject both disturbance and calibration errors

Soft Masses in Theories with Supersymmetry Breaking by TeV-Compactification

We study the sparticle spectroscopy and electroweak breaking of theories where supersymmetry is broken by compactification (Scherk-Schwarz mechanism) at a TeV. The evolution of the soft terms above the compactification scale and the resulting sparticle spectrum are very different from those of the usual MSSM and gauge mediated theories. This is traced to the softness of the Scherk-Schwarz mechanism which leads to scalar sparticle masses that are only logarithmically sensitive to the cutoff starting at two loops. As a result, squarks and sleptons are naturally an order of magnitude lighter than gauginos. In addition, the mechanism is very predictive and the sparticle spectrum depends on just two new parameters. A significant advantage of this mechanism relative to gauge mediation is that a Higgsino mass $\mu\sim M_susy$ is automatically generated when supersymmetry is broken. Our analysis applies equally well to theories where the cutoff is near a TeV or $M_{Pl}$ or some intermediate scale. We also use these observations to show how we may obtain compactification radii which are hierarchically larger than the fundamental cutoff scale.Comment: 26 pages, 1 figure, Late

Semi-Analytic Calculation of the Gravitational Wave Signal From the Electroweak Phase Transition for General Quartic Scalar Effective Potentials

Upcoming gravitational wave (GW) detectors might detect a stochastic background of GWs potentially arising from many possible sources, including bubble collisions from a strongly first-order electroweak phase transition. We investigate whether it is possible to connect, via a semi-analytical approximation to the tunneling rate of scalar fields with quartic potentials, the GW signal through detonations with the parameters entering the potential that drives the electroweak phase transition. To this end, we consider a finite temperature effective potential similar in form to the Higgs potential in the Standard Model (SM). In the context of a semi-analytic approximation to the three dimensional Euclidean action, we derive a general approximate form for the tunneling temperature and the relevant GW parameters. We explore the GW signal across the parameter space describing the potential which drives the phase transition. We comment on the potential detectability of a GW signal with future experiments, and physical relevance of the associated potential parameters in the context of theories which have effective potentials similar in form to that of the SM. In particular we consider singlet, triplet, higher dimensional operators, and top-flavor extensions to the Higgs sector of the SM. We find that the addition of a temperature independent cubic term in the potential, arising from a gauge singlet for instance, can greatly enhance the GW power. The other parameters have milder, but potentially noticeable, effects.Comment: accepted by JCAP, revisions: removed turbulence contribution, minor changes to experimental sensitivity, fixed various minor typos and text revisions, added references, made it clear we consider only detonations; 17 pages, 4 figures, revtex

Thermally-induced vacuum instability in a single plane wave

Ever since Schwinger published his influential paper [J. Schwinger, Phys. Rev. \textbf{82}, 664 (1951)], it has been unanimously accepted that the vacuum is stable in the presence of an electromagnetic plane wave. However, we advance an analysis that indicates this statement is not rigorously valid in a real situation, where thermal effects are present. We show that the thermal vacuum, in the presence of a single plane-wave field, even in the limit of zero frequency (a constant crossed field), decays into electron-positron pairs. Interestingly, the pair-production rate is found to depend nonperturbatively on both the amplitude of the constant crossed field and on the temperature.Comment: 5 pages, 3 figure

Cycloidal Domains in the Magnetization Reversal Process of Ni80Fe20/Nd16Co84/Gd12Co88 Trilayers

The magnetization reversal of each individual layer in magnetic trilayers ( permalloy / Nd Co / Gd Co ) is investigated in detail with x-ray microscopy and micromagnetic calculations. Two sequential inversion mechanisms are identified. First, magnetic vortex-antivortex pairs move along the field direction while inverting the magnetization of magnetic stripes until they are pinned by defects. The vortex-antivortex displacements are reversible within a field interval which allows their controlled motion. Second, as the reversed magnetic field increases, cycloidal domains appear in the permalloy layer as a consequence of the dissociation of vortex-antivortex pairs due to pinning. The field range where magnetic vortices and antivortices are effectively guided by the stripe pattern is of the order of tens of mT for the Ni Fe layer, as estimated from the stability of cycloid domains in the sample

Soft-Wall Stabilization

We propose a general class of five-dimensional soft-wall models with AdS metric near the ultraviolet brane and four-dimensional Poincar\'e invariance, where the infrared scale is determined dynamically. A large UV/IR hierarchy can be generated without any fine-tuning, thus solving the electroweak/Planck scale hierarchy problem. Generically, the spectrum of fluctuations is discrete with a level spacing (mass gap) provided by the inverse length of the wall, similar to RS1 models with Standard Model fields propagating in the bulk. Moreover two particularly interesting cases arise. They can describe: (a) a theory with a continuous spectrum above the mass gap which can model unparticles corresponding to operators of a CFT where the conformal symmetry is broken by a mass gap, and; (b) a theory with a discrete spectrum provided by linear Regge trajectories as in AdS/QCD models.Comment: 27 pages, 6 figures, 1 table. v2: references added, version to appear in NJP Focus Issue on Extra Dimension

Brane Effects on Extra Dimensional Scenarios: A Tale of Two Gravitons

We analyze the propagation of a scalar field in multidimensional theories which include kinetic corrections in the brane, as a prototype for gravitational interactions in a four dimensional brane located in a (nearly) flat extra dimensional bulk. We regularize the theory by introducing an infrared cutoff given by the size of the extra dimensions and a physical ultraviolet cutoff of the order of the fundamental Planck scale in the higher dimensional theory. We show that, contrary to recent suggestions, the radius of the extra dimensions cannot be arbitrarily large. Moreover, for finite radii, the gravitational effects localized on the brane can substantially alter the phenomenology of collider and/or table-top gravitational experiments. This phenomenology is dictated by the presence of a massless graviton, with standard couplings to the matter fields, and a massive graviton which couples to matter in a much stronger way. While graviton KK modes lighter than the massive graviton couple to matter in a standard way, the couplings to matter of the heavier KK modes are strongly suppressed.Comment: 21 pages, latex2e, axodraw.sty, 2 figure

Structure of self-organized Fe clusters grown on Au(111) analyzed by Grazing Incidence X-Ray Diffraction

We report a detailed investigation of the first stages of the growth of self-organized Fe clusters on the reconstructed Au(111) surface by grazing incidence X-ray diffraction. Below one monolayer coverage, the Fe clusters are in "local epitaxy" whereas the subsequent layers adopt first a strained fcc lattice and then a partly relaxed bcc(110) phase in a Kurdjumov-Sachs epitaxial relationship. The structural evolution is discussed in relation with the magnetic properties of the Fe clusters.Comment: 7 pages, 6 figures, submitted to Physical Review B September 200

The Mixmaster Spacetime, Geroch's Transformation and Constants of Motion

We show that for $U(1)$-symmetric spacetimes on $S^3 \times R$ a constant of motion associated with the well known Geroch transformation, a functional $K[h_{ij},\pi^{ij}]$, quadratic in gravitational momenta, is strictly positive in an open subset of the set of all $U(1)$-symmetric initial data, and therefore not weakly zero. The Mixmaster initial data appear to be on the boundary of that set. We calculate the constant of motion perturbatively for the Mixmaster spacetime and find it to be proportional to the minisuperspace Hamiltonian to the first order in the Misner anisotropy variables, i.e. weakly zero. Assuming that $K$ is exactly zero for the Mixmaster spacetime, we show that Geroch's transformation, when applied to the Mixmaster spacetime, gives a new \mbox{$U(1)$-symmetric} solution of the vacuum Einstein equations, globally defined on \mbox{$S^2 \times S^1 \times R$},which is non-homogeneous and presumably exhibits Mixmaster-like complicated dynamical behavior.Comment: 25 pages, preprint YCTP-20-93, Revte