Relativistic Coulomb scattering of spinless bosons

The relativistic scattering of spin-0 bosons by spherically symmetric Coulomb fields is analyzed in detail with an arbitrary mixing of vector and scalar couplings. It is shown that the partial wave series reduces the scattering amplitude to the closed Rutherford formula exactly when the vector and scalar potentials have the same magnitude, and as an approximation for weak fields. The behavior of the scattering amplitude near the conditions that furnish its closed form is also discussed. Strong suppressions of the scattering amplitude when the vector and scalar potentials have the same magnitude are observed either for particles or antiparticles with low incident momentum. We point out that such strong suppressions might be relevant in the analysis of the scattering of fermions near the conditions for the spin and pseudospin symmetries. From the complex poles of the partial scattering amplitude the exact closed form of bound-state solutions for both particles and antiparticles with different scenarios for the coupling constants are obtained. Perturbative breaking of the accidental degeneracy appearing in a pair of special cases is related to the nonconservation of the Runge-Lenz vector

Convective instabilities in two superposed horizontal liquid layers heated laterally

This work is devoted to the theoretical study of the stability of two superposed horizontal liquid layers bounded by two solid planes and subjected to a horizontal temperature gradient. The liquids are supposed to be immiscible with a nondeformable interface. The forces acting on the system are buoyancy and interfacial tension. Four different flow patterns and temperature profiles are found for the basic state. A linear perturbative analysis with respect to two and three dimensional perturbations reveals the existence of three kind of patterns. Depending on the relative height of both liquids several situations are predicted: either wave propagation from cold to the hot regions, or waves propagating in the opposite direction or still stationary longitudinal rolls. The behavior of three different pairs of liquids which have been used in experiments on bilayers under vertical gradient by other authors have been examined. The instability mechanisms are discussed and a qualitative interpretation of the different behaviors exhibited by the system is provided. In some configurations it is possible to find a codimension-two point created by the interaction of two Hopf modes with different frequencies and wavenumbers. These results suggest to consider two liquid layers as an interesting prototype for the study of propagation and interaction of waves in the context of the B\'enard-Marangoni problem.Comment: 21 pages, 9 figures, 2 tables;accepted to be published in PR

A new contribution to the nuclear modification factor of non-photonic electrons in Au+Au collisions at sqrt(s) = 200 GeV

We investigate the effect of the so-called anomalous baryon/meson enhancement to the nuclear modification factor of non-photonic electrons in Au+Au collisions at sqrt(s) = 200 GeV. It is demonstrated that an enhancement of the charm baryon/meson ratio, as it is observed for non-strange and strange hadrons, can be responsible for part of the amplitude of the nuclear modification factor of non-photonic electrons. About half of the measured suppression of non-photonic electrons in the 2-4 pt range can be explained by a charm baryon/meson enhancement of 5. This contribution to the non-photonic electron nuclear modification factor has nothing to do with heavy quark energy loss.Comment: 14 pages, 3 figure

Direct determination of the crystal field parameters of Dy, Er and Yb impurities in the skutterudite compound CeFe4_{4}P12_{12} by Electron Spin Resonance

Despite extensive research on the skutterudites for the last decade, their electric crystalline field ground state is still a matter of controversy. We show that Electron Spin Resonance (ESR) measurements can determine the full set of crystal field parameters (CFPs) for the Th cubic symmetry (Im3) of the Ce$_{1-x}$R$_{x}$Fe$_{4}$P$_{12}$ (R = Dy, Er, Yb, $x\lesssim 0.003$) skutterudite compounds. From the analysis of the ESR data the three CFPs, B4c, B6c and B6t were determined for each of these rare-earths at the Ce$^{3+}$ site. The field and temperature dependence of the measured magnetization for the doped crystals are in excellent agreement with the one predicted by the CFPs Bnm derived from ESR.Comment: 7 pages, 5 figures, to appear in PR

Debt management in Brazil : evaluation of the Real Plan and challenges ahead

Brazil's domestic debt has posed two challenges to policymakers: it has grown very fast and, despite progress, remains extremely short in maturity. The authors analyze Brazil's experience with domestic public debt management, searching for policy prescriptions for the next few years. After briefly reviewing the recent history of the country's domestic debt, they decompose the large rise in federal bonded debt in 1995-98, searching for its macroeconomic causes. The main explanations: extremely high interest payments (caused by Brazil's weak fiscal stance and quasi-fixed exchange rate regime) and the accumulation of assets (especially obligations of Brazil's states). Simulations of the net debt path for the near future underscore the importance of a tighter fiscal stance to prevent the debt-to-GDP ratio from growing further. The authors'main policy advice is to foster and rely more on inflation-linked bonds--the least harmful way to lengthen debt maturity.Economic Theory&Research,Banks&Banking Reform,Public Sector Economics&Finance,Payment Systems&Infrastructure,Strategic Debt Management,Economic Theory&Research,Banks&Banking Reform,Strategic Debt Management,Public Sector Economics&Finance,Municipal Financial Management

Sensitivity analysis of the solar rotation to helioseismic data from GONG, GOLF and MDI observations

Accurate determination of the rotation rate in the radiative zone of the sun from helioseismic observations requires rotational frequency splittings of exceptional quality as well as reliable inversion techniques. We present here inferences based on mode parameters calculated from 2088-days long MDI, GONG and GOLF time series that were fitted to estimate very low frequency rotational splittings (nu < 1.7 mHz). These low frequency modes provide data of exceptional quality, since the width of the mode peaks is much smaller than the rotational splitting and hence it is much easier to separate the rotational splittings from the effects caused by the finite lifetime and the stochastic excitation of the modes. We also have implemented a new inversion methodology that allows us to infer the rotation rate of the radiative interior from mode sets that span l=1 to 25. Our results are compatible with the sun rotating like a rigid solid in most of the radiative zone and slowing down in the core (R_sun < 0.2). A resolution analysis of the inversion was carried out for the solar rotation inverse problem. This analysis effectively establishes a direct relationship between the mode set included in the inversion and the sensitivity and information content of the resulting inferences. We show that such an approach allows us to determine the effect of adding low frequency and low degree p-modes, high frequency and low degree p-modes, as well as some g-modes on the derived rotation rate in the solar radiative zone, and in particular the solar core. We conclude that the level of uncertainties that is needed to infer the dynamical conditions in the core when only p-modes are included is unlikely to be reached in the near future, and hence sustained efforts are needed towards the detection and characterization of g-modes.Comment: Accepted for publication in Astrophysical journal. 15 pages, 19 figure