Minimal string-scale unification of gauge couplings

We look for the minimal particle content which is necessary to add to the standard model in order to have a complete unification of gauge couplings and gravity at the weakly coupled heterotic string scale. Using the current precision electroweak data, we find that the presence of a vector-like fermion at an intermediate scale and a non-standard hypercharge normalization are in general sufficient to achieve this goal at two-loop level. If one requires the extra matter scale to be below the TeV scale, then it is found that the addition of three vector-like fermion doublets with a mass around 700 GeV yields a perfect string-scale unification, provided that the affine levels are kY=13/3, k2=1 and k3=2, as in the SU(5) X SU(5) string-GUT. Furthermore, if supersymmetry is broken at the unification scale, the Higgs mass is predicted in the range 125 GeV - 170 GeV, depending on the precise values of the top quark mass and tan(beta) parameter.Comment: 12 pages, 2 figures, references updated and comments added, final version to appear in Phys. Lett.

The Impacts of Cash and In-Kind Transfers on Consumption and Labor Supply: Experimental Evidence from Rural Mexico

The authors use the unique experimental design of the Food Support Program (Programa Apoyo Alimentario) to analyze in-kind and cash transfers in the poor rural areas of southern states of Mexico. They compare the impacts of monthly in-kind and cash transfers of equivalent value (mean share 11.5 percent of pre-program consumption) on household welfare as measured by food and total consumption, adult labor supply, and poverty. The results show that approximately two years later the transfer has a large and positive impact on total and food consumption. There are no differences in the size of the effect of transfer in cash versus transfers in-kind on consumption. The transfer, irrespective of type, does not affect overall participation in labor market activities but induces beneficiary households to switch their labor allocation from agricultural to nonagricultural activities. The analysis finds that the program leads to a significant reduction in poverty. Overall, the findings suggest that the Food Support Program intervention is able to relax the binding liquidity constraints faced by poor agricultural households, and thus increases both equity and efficiency.Adult Work Incentives; Cash Transfers; Consumption; Difference-in-Differences; In-Kind Transfers; Mexico; Poverty Measures; PAL; Randomized design.

Natural Gauge and Gravitational Coupling Unification and the Superpartner Masses

The possibility to achieve unification at the string scale in the context of the simplest supersymmetric grand unified theory is investigated. We find conservative upper bounds on the superpartner masses consistent with the unification of gauge and gravitational couplings, M_{\tilde G} < 5 TeV and M_{\tilde f} < 3 \times 10^7 GeV, for the superparticles with spin one-half and zero, respectively. These bounds hint towards the possibility that this supersymmetric scenario could be tested at future colliders, and in particular, at the forthcoming LHC.Comment: 8 pages, 4 figures, version accepted in Physics Letters

Anomaly-free U(1) gauge symmetries in neutrino seesaw flavor models

Adding right-handed neutrino singlets and/or fermion triplets to the particle content of the Standard Model allows for the implementation of the seesaw mechanism to give mass to neutrinos and, simultaneously, for the construction of anomaly-free gauge group extensions of the theory. We consider Abelian extensions based on an extra U(1)_X gauge symmetry, where X is an arbitrary linear combination of the baryon number B and the individual lepton numbers L_{e,mu,tau}. By requiring cancellation of gauge anomalies, we perform a detailed analysis in order to identify the charge assignments under the new gauge symmetry that lead to neutrino phenomenology compatible with current experiments. In particular, we study how the new symmetry can constrain the flavor structure of the Majorana neutrino mass matrix, leading to two-zero textures with a minimal extra fermion and scalar content. The possibility of distinguishing different gauge symmetries and seesaw realizations at colliders is also briefly discussed.Comment: 12 pages, 2 figures, 7 tables; comments and references added, a new subsection with nonstandard interactions of neutrinos included; final version to appear in Phys. Rev.

Texture Zeros and Weak Basis Transformations

We investigate the physical meaning of some of the "texture zeros" which appear in most of the Ansatze on quark masses and mixings. It is shown that starting from arbitrary quark mass matrices and making a suitable weak basis transformation one can obtain some of these sets of zeros which therefore have no physical content. We then analyse the physical implications of a four-texture zero Ansatz which is in agreement with all present experimental data.Comment: 11 pages, typeset using revte

Preparation and characterization of α-Fe nanowires located inside double wall carbon nanotubes

Capillary effect was used to fill double wall carbon nanotubes (DWCNT) with iron. The samples are characterized by Mössbauer and Raman spectroscopies, TEM, SAED, and magnetization. The experimental results indicate the presence of a-Fe nanowires inside the DWCNTs. The samples are ferromagnetic at room temperature. There are three striking results due to the confinement effects on the physical behavior of a-Fe: the hyperfine fields increase, the Debye temperature decreases and Raman modes are observed

Contribution Ceilings and the Incidence of Payroll Taxes

Social security contributions (SSCs) are typically formally split between employers and employees as payroll taxes, levied on earnings at a constant tax rate that applies only up to a ceiling, above which the marginal tax rate falls to a reduced rate, often 0. Such contribution ceilings create a concave kink point in the budget set of workers and hence should generate a dip in the distribution of earnings around the ceiling through labour supply responses (the reverse of bunching expected at convex kink points) but such a dip is not observed empirically. This paper sets out a new approach to infer the incidence of SSCs that exploits the absence of this dip and the fact that (mechanically) the distributions of labour cost (earnings inclusive of all payroll taxes), gross earnings (net of employer payroll taxes) and net earnings (net of both employer and employee payroll taxes) cannot all be smooth around a kink. The other papers in this special issue apply the method to data for Germany, France, the Netherlands and the UK and all find that distribution of gross earnings is smooth around kinks (implying that the distributions of labour costs and net-of-tax earnings are not) even though the concept of gross earnings is irrelevant in the standard static model of labour supply and demand that dominates the public economics literature. This suggests that other features of the labour market, such as wage bargaining based on the gross earnings concept, are relevant for determining the incidence of SSCs.Fil: Gonzalez Alvaredo, Facundo. Paris School of Economics; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Breda, Thomas. Paris School of Economics; FranciaFil: Roantree, Barra. Institute for Fiscal Studies; Reino UnidoFil: Saez, Emmanuel. University of California at Berkeley; Estados Unido

Boundary Integral Equation Methods for Simulation and Design of Photonic Devices

This thesis presents novel boundary integral equation (BIE) and associated optimization methodologies for photonic devices. The simulation and optimization of such structures is a vast and rapidly growing engineering area, which impacts on design of optical devices such as waveguide splitters, tapers, grating couplers, and metamaterial structures, all of which are commonly used as elements in the field of integrated photonics. The design process has been significantly facilitated in recent years on the basis of a variety of methods in computational electromagnetic (EM) simulation and design. Unfortunately, however, the expense required by previous simulation tools has limited the extent and complexity of the structures that can be treated. The methods presented in this thesis represent the results of our efforts towards accomplishing the dual goals of 1) Accurate and efficient EM simulation for general, highly-complex three-dimensional problems, and 2) Development of effective optimization methods leading to an improved state of the art in EM design. One of the main proposed elements utilizes BIE in conjunction with a modified-search algorithm to obtain the modes of uniform waveguides with arbitrary cross sections. This method avoids spurious solutions by means of a certain normalization procedure for the fields within the waveguides. In order to handle problems including nonuniform waveguide structures, we introduce the windowed Green function (WGF) method, which used in conjunction with auxiliary integral representations for bound mode excitations, has enabled accurate simulation of a wide variety of waveguide problems on the basis of highly accurate and efficient BIE, in two and three spatial dimensions. The "rectangular-polar" method provides the basic high-order singular-integration engine. Based on non-overlapping Chebyshev-discretized patches, the rectangular-polar method underlies the accuracy and efficiency of the proposed general-geometry three-dimensional BIE approach. Finally, we introduce a three-dimensional BIE framework for the efficient computation of sensitivities — i.e. gradients with respect to design parameters — via adjoint techniques. This methodology is then applied to the design of metalenses including up to a thousand parameters, where the overall optimization process takes in the order of three hours using five hundred computing cores. Forthcoming work along the lines of this effort seeks to extend and apply these methodologies to some of the most challenging and exciting design problems in electromagnetics in general, and photonics in particular.</p

Le commerce international et l’investissement direct étranger en Amérique latine: complémentarité ou substituabilité? Le cas du Mexique, de l’Argentine et du Brésil

Numéro de référence interne originel : a1.1 g 111

Exact charged black-hole solutions in D-dimensional f(T) gravity: torsion vs curvature analysis

We extract exact charged black-hole solutions with flat transverse sections in the framework of D-dimensional Maxwell-f(T) gravity, and we analyze the singularities and horizons based on both torsion and curvature invariants. Interestingly enough, we find that in some particular solution subclasses there appear more singularities in the curvature scalars than in the torsion ones. This difference disappears in the uncharged case, or in the case where f(T) gravity becomes the usual linear-in-T teleparallel gravity, that is General Relativity. Curvature and torsion invariants behave very differently when matter fields are present, and thus f(R) gravity and f(T) gravity exhibit different features and cannot be directly re-casted each other.Comment: 24 pages, 3 figures. arXiv admin note: text overlap with arXiv:1110.402