Showing papers in "Brazilian Journal of Physics in 2007"

Landau gauge gluon and ghost propagators from lattice QCD

Abstract: We report on recent numerical computations of the Landau gauge gluon and ghost propagators as well as of the ghost-gluon-vertex function in pure SU(3) Yang-Mills theory and in full QCD on the lattice. Special emphasis is paid to the low momentum region. In particular, we present new data for the gluon propagator at momenta below 300 MeV. We also discuss different systematic effects as there are finite-size, lattice discretization and Gribov copy but also unquenching effects. A MOM-scheme running coupling as(q2) based on the ghost-gluon vertex is calculated and found to decrease for momenta below 550 MeV, even though the renormalization constant of the vertex deviates only weakly from being constant.

Different schematic eyes and their accuracy to the in vivo eye: a quantitative comparison study

Abstract: Current ophthalmic technology allows the manipulation of eye components, such as anterior cornea and lens, of the human eye with a considerable precision and customization. This technology opens up the possibility of exploiting some characteristics of the eye in order to improve the methods of correcting optical aberrations. Moreover, product development and research for the eye-care professional has reached very high standards, since there is nowadays software available to design and simulate practically any mechanical or optical characteristic of the product, even before it is thrown into production line. Although quite similar in the general form, different human eye models simulate the image formation by considering different property combinations in the constitutive elements of the eye structure (such as refraction index and surface curvatures), producing retinal images that resemble very closely those of the biological eye. Using optical design software, we have implemented a simulation of 5 well-known schematic eyes available in the literature. These models were the Helmholtz-Laurance, Gullstrand, Emsley, Greivenkamp and Liou & Brennan. The optical performance of these different models was compared using different quantitative optical quality parameters. The model of Liou and Brennan, contains features of the biological eye that were not considered in previous models, as the distribution of a gradient refraction index and a decentered pupil. Furthermore, it has great reliability since it takes into account the mean value of empirical measurements of the in vivo eye in order to define size and parameters such as anterior and posterior curvature of cornea, lens, axial length, etc. Comparisons between the MTF (Modulation Transfer Function), spot diagrams and ray fan showed the difference in image quality between eye models, and the Strehl Ratio was also used as a parameter of comparison. A careful comparison between the different models showed that the first four schematic eyes have better optical quality than what is expected for the general and healthy emmetropic in vivo eye. Liou and Brennan schematic eye is the one that most closely resembles the in vivo biological eye. Therefore, in applications, such as research or product development for customized vision correction, which must consider optical properties intrinsic to the biological eye, we recommend this latter model; for applications that do not require refraction-limited performance, most of the other models should be a good approximation.

Effect of Nd3 + concentration on the physical and absorption properties of sodium-lead-borate glasses

Abstract: The effect of increasing the rare earth ion concentration on the physical and spectroscopic properties of Nd3 + doped sodium-lead-borate glasses have been studied for the compositions (10-x) Na2O-30PbO-60B2O3-xNd2O3, where x = 1.00, 1.25, 1.50, 1.75 and 2.00 mol %. Optical band gaps, cut-off wavelengths and various spectroscopic parameters (E1, E2, E3, F2, F4, F6 and x4f) have been determined from the room temperature absorption spectra. Judd-Ofelt theory has been employed to determine the intensity parameters W2, W4 and W6 which in turn are used to evaluate radiative transition probability (A), branching ratio (b) and radiative lifetime (tR) for the fluorescent level 4F3 / 2. The W2 parameter and hence the non-symmetric component of electric field acting on Nd3 + ion is found to be highest for glass with 1.75 mol% of Nd2O3. Because of the poor resolution of hypersensitive transition, the covalency of the Nd-O bond has been characterized by the relative intensity of 4I9 / 2 ®4F7 / 2, 4S3 / 2. The highest covalency has been predicted for glass with 2 mol% Nd2O3. The radiative properties are found to improve with an increase in concentration of Nd2O3 for the present study.

Anomalous diffusion of pions at RHIC

Abstract: After pointing out the difference between normal and anomalous diffusion, we consider a hadron resonance cascade (HRC) model simulation for particle emission at RHIC and point out that rescattering in an expanding hadron resonance gas leads to a heavy tail in the source distribution. The results are compared to recent PHENIX measurements of the tail of the particle emitting source in Au+Au collisions at RHIC. In this context, we show how can one distinguish experimentally the anomalous diffusion of hadrons from a second order QCD phase transition.

Temperature-dependent photoluminescence spectra of GaAsSb/AlGaAs and GaAsSbN/GaAs single quantum wells under different excitation intensities

Abstract: The mechanism for low-temperature photoluminescence (PL) emissions in GaAsSb/AlGaAs and GaAsSbN/GaAs strained-layer single quantum wells (SQWs), grown by molecular-beam epitaxy, is studied in detail, using PL spectroscopy as a function of temperature and excitation intensity. In all samples, the PL peak energy as well as the full width at half maximum (FWHM), as a function of temperature, present anomalous behaviors, i:e., the PL peak energy shows a successive red/blue/redshift (S-shaped behavior) and the FWHM shows a successive blue/red/blueshift (“inverted S-shaped curve”) with increasing temperature. At sufficiently low excitation intensity and in a narrow temperature interval (50 ‐ 80 K), the nitrogen-containing samples present two clear competitive PL peaks. The low-energy PL mechanism (8 ‐ 80 K) is dominated by localized PL transitions, while the high-energy PL mechanism is dominated by the ground state (e1-hh1) PL transition. Additionally, these PL peaks show different temperature dependence with the low-energy PL peak, showing a stronger redshift than the high-energy PL peak. A competition process between localized and delocalized excitons is used to discuss these PL properties.

Jet conversions in a quark-gluon plasma

Abstract: We report our recent work on conversions between gluon and quark jets as they traverse through a quark-gluon plasma (QGP) and their effects on the nuclear modification factors for quark and gluon jets as well as the ratios of p/p+ and $\bar{p}$/p- at high transverse momentum in ultra-relativistic heavy ion collisions.

Born-Infeld corrections to the entropy function of heterotic black holes

Abstract: We use the black hole entropy function to study the effect of Born-Infeld terms on the entropy of extremal black holes in heterotic string theory in four dimensions. We find that after adding a set of higher curvature terms to the effective action, attractor mechanism works and Born-Infeld terms contribute to the stretching of near horizon geometry. In the a' ® 0 limit, the solutions of attractor equations for moduli fields and the resulting entropy, are in conformity with the ones for standard two charge black holes.

Propagators, running coupling and condensates in lattice QCD

Abstract: We present a review of our numerical studies of the running coupling constant, gluon and ghost propagators, ghost-gluon vertex and ghost condensate for the case of pure SU(2) lattice gauge theory in the minimal Landau gauge. Emphasis is given to the infrared regime, in order to investigate the confinement mechanisms of QCD. We compare our results to other theoretical and phenomenological studies.

Fluctuations, correlations and non-extensivity

Abstract: The present status of investigations on fluctuations and correlations seen in high energy multiparticle production processes made using the notion of nonextensivity is reviewed.

Electronic Structure of the A3B Compounds: A=Nb; B=Al,Ga,Ge,Sn and In

Abstract: The electronic structure of the A15 compounds A3B A=Nb; B=Al,Ga,Ge,Sn and In is studied by means of fully relativistic full-potential linearized augmented-plane wave (FP-LAPW) calculations within the formalism of the density functional theory. Band structures and densities of states are determined for all the studied compounds. From the calculations are obtained the theoretical lattice constants, the Fermi energy EF , the density of states at the Fermi level N(EF ) which is used to estimate the electronic specific heat coefficient g and the electron-phonon coupling parameter l for the Nb3In compound. Generally, a good agreement between results of the calculations and experiments is found.

HBT and initial conditions for hydrodynamic expansion in A+A collisions

Abstract: The behavior of interferometry radii in central A+A collisions at different energies and also for different nuclei or impact parameters indicates initial transverse flow at a very early stage of the matter evolution. Development of such flow at the pre-thermal partonic stage is considered.

Ferrous Xylenol Gel measurements for 6 and 10 MV photons in small field sizes

Abstract: The Fricke dosimeter is a ferrous sulfate aqueous solution that, when irradiated, oxidizes the Fe2 + ions to Fe3 + . This new concentration, generally determined through spectrophotometry, is directly proportional to the ionizing radiation absorbed energy. The Fricke Xylenol Gel dosimeter (FXG) was developed through the incorporation of swine skin gelatin and xylenol orange. These modifications provided better signal stability and sensitivity for lower absorbed dose measurements, such as those used in radiotherapy. In this work FXG samples were irradiated with absorbed doses of 2 Gy, from 6 MV and 10 MV photons, using small field sizes geometry for dosimetric parameters determination. All the FXG dosimeter readings were accomplished with our specially developed spectrophotometer, using a narrow light beam at the wavelength of 585 nm, where the highest absorbance sensitivity occurs. From our results, we can confirm not only that the FXG dosimetric system (FXG plus a high lateral spatial resolution spectrophotometer) can be used for general dosimetry, but as well for small field size dosimetry of interest in radiosurgery.

Plane symmetric bulk viscous domain wall in Lyra geometry

Abstract: In this paper general solutions are found for domain walls in Lyra geometry in the plane symmetric spacetime metric given by Taub. Expressions for the energy density and pressure of domain walls are derived in both cases of uniform and time varying displacement field b. Some physical consequences of the models are also given. Finally, the geodesic equations and acceleration of the test particle are discussed.

Gribov ambiguities in the maximal abelian gauge

Abstract: The effects of the Gribov copies on the gluon and ghost propagators are investigated in SU(2) Euclidean Yang-Mills theory quantized in the maximal Abelian gauge. By following Gribov's original approach, extended to the maximal Abelian gauge, we are able to show that the diagonal component of the gluon propagator displays the characteristic Gribov type behavior. The off-diagonal component is found to be of the Yukawa type, with a dynamical mass originating from the dimension two gluon condensate, which is also taken into account. Furthermore, the off-diagonal ghost propagator exhibits infrared enhancement. Finally, we make a comparison with available lattice data.

Effective masses for donor binding energies in non-magnetic and magnetic quantum well systems: effect of magnetic field

Abstract: Donor binding energies in quantum well systems using position dependent effective masses are obtained. Results are provided for GaAs-Ga1ixAlxAs and the semimagnetic CdTe-Cd1ix MnxTe systems. In the latter the barrier height reduces in a magnetic field. Using the available experimental data the variations of the barrier height with magnetic field has been obtained using a simple model. Our results are in good agreement with similar works in the literature in the case of constant effective mass for the donor electron. However, in the case of the semi magnetic system and in the case of the position dependent effective mass, the results obtained are shown to be appreciably different for narrow well dimensions. The validity of the effective mass approximation in the case of applied magnetic field is also critically examined.

PT Distribution of Hyperons in 200A GeV Au-Au in Smoothed Particle Hydrodynamics

Abstract: The transverse momentum distributions of hadrons in 200GeV Au-Au collisions at RHIC are calculated using the smoothed particle hydrodynamics code SPheRIO They are compared with data from STAR and PHOBOS Collaborations By employing an equation of state which explicitly incorporates the strangeness conservation and introducing strangeness chemical potential into the code, the transverse spectra give a reasonable description of experimental data, except the multiplicities of hyperons

Global model simulations of low-pressure oxygen discharges

Abstract: We use a global model (volume averaged) to study plasma discharges in molecular oxygen gas in the 1-100 mTorr pressure range. This model determines densities of positive ions O2+ and O + , negative ion O - , electrons, ground state O2 and O atoms, and metastables O2(a1Dg) and O(1D), and electron temperature as function of gas pressure and input power, for a cylindrical discharge. We apply the model to O2 discharges and the results are compared to a particle-in-cell simulation (PIC), experimental data and a volume-averaged global model developed at the University of California at Berkeley. We find that the total positive ion density increases with pressure at low pressures (up to approximately 30 mTorr), and decreases at higher pressures. The electronegativity decreases with increased power and increased pressure as predicted by the global models presented in the literature. The predictions for electron temperature are also in agreement with these models. However, there is a discrepancy betweeen these global models and PIC simulations and experimental data, for 20 and 40 mTorr cases, concerning electronegativity calculations. PIC simulations yield much higher electronegativities. There are strong indications that this is due to the assumption of Maxwellian electron energy distribution functions in the global model, while in the PIC simulations this is clearly not the case.

Derivative dispersion relations above the physical threshold

Abstract: We discuss some formal and practical aspects related to the replacement of Integral Dispersion Relations (IDR) by derivative forms, without high-energy approximations. We first demonstrate that, for a class of functions with physical interest as forward scattering amplitudes, this replacement can be analytically performed, leading to novel Extended Derivative Dispersion Relations (EDDR), which, in principle, are valid for any energy above the physical threshold. We then verify the equivalence between the IDR and EDDR by means of a popular parametrization for total cross sections from proton-proton and antiproton-proton scattering and compare the results with those obtained through other representations for the derivative relations. Critical aspects on the limitations of the whole analysis, from both formal and practical points of view, are also discussed in some detail.

Gas-surface interaction effect on round leading edge aerothermodynamics

Abstract: Effects of incomplete surface accommodation in rarefied gas flow have been studied by using the Direct Simulation Monte Carlo (DSMC) method in conjunction with the Cercignani-Lampis-Lord gas surface interaction model. The DSMC calculations examine differences in predictions of aerodynamic forces and heat transfer between full and partial surface accommodation for hypersonic flow past round leading edges at zero incidence. The aerodynamic performance of round leading edges is assessed by using the heat transfer rate, the total drag and the shock wave standoff distance. Twenty-five combinations of nose radius, normal and tangential accommodation coefficients were used in the simulation. For the flow conditions considered, the analysis showed that stagnation point heating, total drag and shock standoff distance are sensitive to changes on either the normal or tangential accommodation coefficient. The results substantiate that it becomes imperative to take surface accommodation into account in order to make accurate predictions of the aerodynamic forces on, and heat transfer rates to, bodies in rarefied hypersonic flow.

Gluon dominance model and cluster production

Abstract: Multiparticle production in lepton and hadron processes is studied by means of the Gluon Dominance Model (GDM) which is based on the Quantum Chromodynamics (QCD) and phenomenological scheme of hadronization. The model describes the multiplicity distributions and their moments very well. It has revealed an active role of gluons in multiparticle production and confirmed the fragmentation mechanism of hadronization in e+e- annihilation and its change to a recombination mechanism in hadron and nucleus interactions. The GDM explains the shoulder structure of multiplicity distributions. The hadron-pion ratio obtained by GDM has turned out to be in agreement with the experimental RHIC data of Au+Au peripheral collisions. Besides, development of GDM allows one to study the multiplicity behavior of p$p\bar{p}$ annihilation at tens of GeV. The mechanism of soft-photon production and estimates of their emission region have been offered in the framework of this model. The experimental data (project "Thermalization", U-70, IHEP) indicated the cluster nature of multiparticle production revealed by using GDM.

Extending SUSY reach at the CERN large hadron collider using b-tagging

Abstract: We analyze the potential of the CERN Large Hadron Collider on the reach of the focus point (FP) region in the mSUGRA parameter space. This region, consistent with WMAP results, is characterized by multi-TeV masses for the superpartners of quarks and leptons and relatively light charginos and neutralinos. Moreover, since the LSP has a substantial higgsino component, it is expected that the gluino decays predominantly to third generation quarks, producing a final state with multiple hard b jets. Analyzing events with 6T + n jets + tagged b-jets, we show that the LHC reach can improve as much as 20% from current projections. Although we performed the analysis specifically for the FP region, the b-tagging should be important to enhance the SUSY signal in a variety of models where a relatively light gluino decays mostly to third generation quarks.

From Confining Fields on the Lattice to Higher Dimensions in the Continuum

Abstract: We discuss the relation between lattice phenomenology of confining fields in the vacuum state of Yang-Mills theories (mostly SU(2) case) and continuum theories. In the continuum, understanding of the confinement is most straightforward in the dual formulation which involves higher dimensions. We try to bridge these two approaches to the confinement, let it be on a rudimentary level. We review lattice data on low-dimensional vacuum defects, like monopoles, center vortices. There is certain resemblance to dual strings, domain walls considered in the continuum version of Yang-Mills theories.

Electron and positron capture rates on 55Co in stellar matter

Abstract: 55Co is not only present in abundance in presupernova phase but is also advocated to play a decisive role in the core collapse of massive stars. The spectroscopy of electron capture and emitted neutrinos yields useful information on the physical conditions and stellar core composition. B(GT) values to low-lying states are calculated microscopically using the pn-QRPA theory. Our rates are enhanced compared to the reported shell model rates. The enhancement is attributed partly to the liberty of selecting a huge model space, allowing consideration of many more excited states in our rate calculations. Unlike previous calculations the so-called Brink’s hypothesis is not assumed leading to a more realistic estimate of the rates. The electron and positron capture rates are calculated over a wide temperature (0.01×109 - 30×109K) and density (10 - 1011 gcm-3) grid.

Rotating and counterrotating relativistic thin disks as sources of stationary electrovacuum spacetimes

Abstract: A detailed study is presented of the counterrotating model (CRM) for electrovacuum stationary axially symmetric relativistic thin disks of infinite extension without radial stress, in the case when the eigenvalues of the energy-momentum tensor of the disk are real quantities, so that there is not heat flow. We find a general constraint over the counterrotating tangential velocities needed to cast the surface energy-momentum tensor of the disk as the superposition of two counterrotating charged dust fluids. We then show that, in some cases, this constraint can be satisfied if we take the two counterrotating tangential velocities as equal and opposite or by taking the two counterrotating streams as circulating along electro-geodesics. However, we show that, in general, it is not possible to take the two counterrotating fluids as circulating along electro-geodesics nor take the two counterrotating tangential velocities as equal and opposite. A simple family of models of counterrotating charged disks based on the Kerr-Newman solution are considered where we obtain some disks with a CRM well behaved. We also show that the disks constructed from the Kerr-Newman solution can be interpreted, for all the values of parameters, as a matter distribution with currents and purely azimuthal pressure without heat flow. The models are constructed using the well-known "displace, cut and reflect" method extended to solutions of vacuum Einstein-Maxwell equations. We obtain, in all the cases, counterrotating Kerr-Newman disks that are in agreement with all the energy conditions.

Measurement of DIS cross section at HERA

Abstract: This paper presents recent measurements of the inclusive DIS cross section performed by the H1 and ZEUS collaborations at the HERA collider. We discuss relations of the HERA results with the upcoming experiments at the LHC. Importance of the planed measurement of the longitudinal proton structure function FL is commented.

Diffraction in QCD

Abstract: This lecture presents a short review of the main features of diffractive processes and QCD inspired models. It includes the following topics: (1) Quantum mechanics of diffraction: general properties; (2) Color dipole description of diffraction; (3) Color transparency; (4) Soft diffraction in hard reactions: DIS, Drell-Yan, Higgs production; (5) Why Pomerons interact weakly; (6) Small gluonic spots in the proton; (7) Diffraction near the unitarity bound: the Goulianos-Schlein "puzzle"; (8) Diffraction on nuclei: diffractive Color Glass; (9) CGC and gluon shadowing.

New Solutions to the Hierarchy Problem

Abstract: After summarizing the status of the Standard Model, we focus on the Hierarchy Problem and why we believe this strongly suggests the need for new physics at the TeV scale. We then concentrate on theories with extra dimensions and their possible manifestations at this scale.

Femtoscopic correlations in multiparticle production and Beta-Decay

Abstract: The basics of formalism of femtoscopic and spectroscopic correlations are given, the orthogonal character of these correlations is stressed. The similarity and difference of femtoscopic correlations in multiparticle production and beta-decay is discussed.

Compact charged stars

Abstract: We investigate the possibility that charged compact objects could be the accelerators of high energy cosmic rays. In order to do so, we choose to first solve numerically a system of differential equations describing the structure of charged compact objects, including the generalization of the Tolman-Oppenheimer-Volkoff equation for this class of objects. We assume a polytropic equation of state for the fluid and, for simplicity, a linear relation between charge density and the fluid energy density. We obtain upper limits for the charge such objects can acquire and study the stability of these equilibrium configurations.

An improved gauge unfixing formalism and the abelian pure chern simons theory

Abstract: We propose a new scheme of embedding constrained systems based on the Gauge Unfixing formalism. Our aim is to modify directly the original phase space variables of a system in order to be gauge invariant quantities. We apply our procedure in a nontrivial constrained model that is the Abelian Pure Chern Simons Theory where new results are obtained. Among them we can cite the development of a systematic procedure in order to separate the first and the second class constraints, and the obtainment of the same initial Abelian Pure Chern Simons Lagrangian as the gauge invariant Lagrangian. This last result shows that the gauge symmetry of the action is certainly preserved.