Magnetic and pair correlations of the Hubbard model with next-nearest-neighbor hopping

A combination of analytical approaches and quantum Monte Carlo simulations is used to study both magnetic and pairing correlations for a version of the Hubbard model that includes second-neighbor hopping $t^{\prime }=-0.35t$ as a model for high-temperature superconductors. Magnetic properties are analyzed using the Two-Particle Self-Consistent approach. The maximum in magnetic susceptibility as a function of doping appears both at finite $$ and at $t^{\prime }=0$ but for two totally different physical reasons. When $t^{\prime }=0$, it is induced by antiferromagnetic correlations while at $t^{\prime }=-0.35t$ it is a band structure effect amplified by interactions. Finally, pairing fluctuations are compared with $$-matrix results to disentangle the effects of van Hove singularity and of nesting on superconducting correlations. The addition of antiferromagnetic fluctuations increases slightly the $d$-wave superconducting correlations despite the presence of a van Hove singularity which tends to decrease them in the repulsive model. Some aspects of the phase diagram and some subtleties of finite-size scaling in Monte Carlo simulations, such as inverted finite-size dependence, are also discussed.Comment: Revtex, 8 pages + 15 uuencoded postcript figure

From cosmic deceleration to acceleration: new constraints from SN Ia and BAO/CMB

We use type Ia supernovae (SN Ia) data in combination with recent baryonic acoustic oscillations (BAO) and cosmic microwave background (CMB) observations to constrain a kink-like parametrization of the deceleration parameter ($q$). This $q$-parametrization can be written in terms of the initial ($q_i$) and present ($q_0$) values of the deceleration parameter, the redshift of the cosmic transition from deceleration to acceleration ($z_t$) and the redshift width of such transition ($\tau$). By assuming a flat space geometry, $q_i=1/2$ and adopting a likelihood approach to deal with the SN Ia data we obtain, at the 68% confidence level (C.L.), that: $z_t=0.56^{+0.13}_{-0.10}$, $\tau=0.47^{+0.16}_{-0.20}$ and $q_0=-0.31^{+0.11}_{-0.11}$ when we combine BAO/CMB observations with SN Ia data processed with the MLCS2k2 light-curve fitter. When in this combination we use the SALT2 fitter we get instead, at the same C.L.: $z_t=0.64^{+0.13}_{-0.07}$, $\tau=0.36^{+0.11}_{-0.17}$ and $q_0=-0.53^{+0.17}_{-0.13}$. Our results indicate, with a quite general and model independent approach, that MLCS2k2 favors Dvali-Gabadadze-Porrati-like cosmological models, while SALT2 favors $\Lambda$CDM-like ones. Progress in determining the transition redshift and/or the present value of the deceleration parameter depends crucially on solving the issue of the difference obtained when using these two light-curve fitters.Comment: 25 pages, 9 figure

A mathematical analysis of the evolution of perturbations in a modified Chaplygin gas model

One approach in modern cosmology consists in supposing that dark matter and dark energy are different manifestations of a single `quartessential' fluid. Following such idea, this work presents a study of the evolution of perturbations of density in a flat cosmological model with a modified Chaplygin gas acting as a single component. Our goal is to obtain properties of the model which can be used to distinguish it from another cosmological models which have the same solutions for the general evolution of the scale factor of the universe, without the construction of the power spectrum. Our analytical results, which alone can be used to uniquely characterize the specific model studied in our work, show that the evolution of the density contrast can be seen, at least in one particular case, as composed by a spheroidal wave function. We also present a numerical analysis which clearly indicates as one interesting feature of the model the appearence of peaks in the evolution of the density constrast.Comment: 21 pages, accepted for publication in General Relativity and Gravitatio

A measurement of lifetime differences in the neutral D-meson system

Using a high statistics sample of photoproduced charm particles from the FOCUS experiment at Fermilab, we compare the lifetimes of neutral D mesons decaying via D0 to K- pi+ and K- K+ to measure the lifetime differences between CP even and CP odd final states. These measurements bear on the phenomenology of D0 - D0bar mixing. If the D0 to K-pi+ is an equal mixture of CP even and CP odd eigenstates, we measure yCP = 0.0342 \pm 0.0139 \pm 0.0074.Comment: 15 pages, 5 figure

Search for CP violation in D0 and D+ decays

A high statistics sample of photoproduced charm particles from the FOCUS (E831) experiment at Fermilab has been used to search for CP violation in the Cabibbo suppressed decay modes D+ to K-K+pi+, D0 to K-K+ and D0 to pi-pi+. We have measured the following CP asymmetry parameters: A_CP(K-K+pi+) = +0.006 +/- 0.011 +/- 0.005, A_CP(K-K+) = -0.001 +/- 0.022 +/- 0.015 and A_CP(pi-pi+) = +0.048 +/- 0.039 +/- 0.025 where the first error is statistical and the second error is systematic. These asymmetries are consistent with zero with smaller errors than previous measurements.Comment: 12 pages, 4 figure

Measurements of the Sigma_c^0 and Sigma_c^{++} Mass Splittings

Using a high statistics sample of photoproduced charmed particles from the FOCUS experiment at Fermilab (FNAL-E831), we measure the mass splittings of the charmed baryons Sigma_c^0 and Sigma_c^{++}. We find M(Sigma_c^0 - Lambda_c^+) = 167.38 +/- 0.21 +/- 0.13 MeV/c^2 and M(Sigma_c^++ - Lambda_c^+) = 167.35 +/- 0.19 +/- 0.12 MeV/c^2 with samples of 362 +/- 36 and 461 +/- 39 events, respectively. We measure the isospin mass splitting M(Sigma_c^++ - Sigma_c^0) to be -0.03 +/- 0.28 +/- 0.11 Mev/c^2. The first errors are statistical and the second are systematic.Comment: 10 pages, 2 figure

Consistency analysis of a nonbirefringent Lorentz-violating planar model

In this work analyze the physical consistency of a nonbirefringent Lorentz-violating planar model via the analysis of the pole structure of its Feynman propagators. The nonbirefringent planar model, obtained from the dimensional reduction of the CPT-even gauge sector of the standard model extension, is composed of a gauge and a scalar fields, being affected by Lorentz-violating (LIV) coefficients encoded in the symmetric tensor $\kappa_{\mu\nu}$. The propagator of the gauge field is explicitly evaluated and expressed in terms of linear independent symmetric tensors, presenting only one physical mode. The same holds for the scalar propagator. A consistency analysis is performed based on the poles of the propagators. The isotropic parity-even sector is stable, causal and unitary mode for $0\leq\kappa_{00}<1$. On the other hand, the anisotropic sector is stable and unitary but in general noncausal. Finally, it is shown that this planar model interacting with a $\lambda|\varphi|^{4}-$Higgs field supports compactlike vortex configurations.Comment: 11 pages, revtex style, final revised versio

Passatempo Virus, a Vaccinia Virus Strain, Brazil

Passatempo virus was isolated during a zoonotic outbreak. Biologic features and molecular characterization of hemagglutinin, thymidine kinase, and vaccinia growth factor genes suggested a vaccinia virus infection, which strengthens the idea of the reemergence and circulation of vaccinia virus in Brazil. Molecular polymorphisms indicated that Passatempo virus is a different isolate