Sensitivity of nucleon-nucleus scattering to the off-shell behavior of on-shell equivalent NN potentials

The sensitivity of nucleon-nucleus elastic scattering to the off-shell behavior of realistic nucleon-nucleon interactions is investigated when on-shell equivalent nucleon-nucleon potentials are used. The study is based on applications of the full-folding optical model potential for an explicit treatment of the off-shell behavior of the nucleon-nucleon effective interaction. Applications were made at beam energies between 40 and 500 MeV for proton scattering from 40Ca and 208Pb. We use the momentum-dependent Paris potential and its local on-shell equivalent as obtained with the Gelfand-Levitan and Marchenko inversion formalism for the two nucleon Schroedinger equation. Full-folding calculations for nucleon-nucleus scattering show small fluctuations in the corresponding observables. This implies that off-shell features of the NN interaction cannot be unambiguously identified with these processes. Inversion potentials were also constructed directly from NN phase-shift data (SM94) in the 0-1.3 GeV energy range. Their use in proton-nucleus scattering above 200 MeV provide a superior description of the observables relative to those obtained from current realistic NN potentials. Limitations and scope of our findings are presented and discussed.Comment: 17 pages tightened REVTeX, 8 .ps figures, submitted to Phys. Rev.

Surface-peaked medium sensitivity of the optical potential: an exact result

Microscopic optical model potentials for elastic hadron-nucleus scattering usually take the form of a convolution of a two-body effective interaction with the target ground-state mixed density. Within the Brueckner-Bethe-Goldstone gmatrix approach for the effective interaction, nuclear medium effects are made explicit by means spatial integrals throughout the bulk of the nucleus. In this contribution we discuss a novel and exact approach to track down the manifestation of intrinsic nuclear medium effects. After examining the momentumand coordinate-space structure of a two-body effective interaction –spherically symmetric in its mean coordinate– it is demonstrated that the intrinsic medium effects in the optical potential depend solely on the gradient of a reduced interaction. This feature implies the confinement of intrinsic medium effects to regions where the density varies most, i.e. the nuclear surface. This finding may be of special significance in the study of nuclear collisions sensitive to the peripheric structure of nuclei. We illustrate some of its implications in the context of 10Be + p elastic scattering at 39.1A MeV

Ozone exchange within and above an irrigated Californian orchard

In this study, the canopy effects on the vertical ozone exchange within and above Californian orchard are investigated. We examined the comprehensive dataset obtained from the Canopy Horizontal Array Turbulence Study (CHATS). CHATS typifies a rural central Californian site, with O3 mixing ratios of less than 60 ppb and moderate NOx mixing ratios. The CHATS campaign covered a complete irrigation cycle, with our analysis including periods before and after irrigation. Lower O3 mixing ratios were found following irrigation, together with increased wind speeds, decreased air temperatures and increased specific humidity. Friction velocity, sensible heat and gas fluxes above the canopy were estimated using variations on the flux-gradient method, including a method which accounts for the roughness sublayer (RSL). These methods were compared to fluxes derived from observed eddy diffusivities of heat and friction velocity. We found that the use of the RSL parameterization, which accounts for the canopy-induced turbulent mixing above the canopy, resulted in a stronger momentum, heat, and ozone exchange fluxes above this orchard, compared to the method which omits the RSL. This was quantified by the increased friction velocity, heat flux and ozone deposition flux of up to 12, 29, and 35% at 2.5 m above the canopy, respectively. Within the canopy, vertical fluxes, as derived from local gradients and eddy diffusivity of heat, were compared to fluxes calculated using the Lagrangian inverse theory. Both methods showed a presence of vertical flux divergence of friction velocity, heat and ozone, suggesting that turbulent mixing was inefficient in homogenizing the effects driven by local sources and sinks on vertical exchange of those quantities. This weak mixing within the canopy was also corroborated in the eddy diffusivities of friction velocity and heat, which were calculated directly from the observations. Finally, the influence of water stress on the O3 budget was examined by comparing the results prior and after the irrigation. Although the analysis is limited to the local conditions, our in situ measurements indicated differences in the O3 mixing ratio prior and after irrigation during CHATS. We attribute these O3 mixing ratio changes to enhanced biological emission of volatile organic compounds (VOCs), driven by water stress

Fixed Effect Estimation of Large T Panel Data Models

This article reviews recent advances in fixed effect estimation of panel data models for long panels, where the number of time periods is relatively large. We focus on semiparametric models with unobserved individual and time effects, where the distribution of the outcome variable conditional on covariates and unobserved effects is specified parametrically, while the distribution of the unobserved effects is left unrestricted. Compared to existing reviews on long panels (Arellano and Hahn 2007; a section in Arellano and Bonhomme 2011) we discuss models with both individual and time effects, split-panel Jackknife bias corrections, unbalanced panels, distribution and quantile effects, and other extensions. Understanding and correcting the incidental parameter bias caused by the estimation of many fixed effects is our main focus, and the unifying theme is that the order of this bias is given by the simple formula p/n for all models discussed, with p the number of estimated parameters and n the total sample size.Comment: 40 pages, 1 tabl

Evaluación de los aprendizajes logrados, de una propuesta basada en aprendizaje cooperativo para la enseñanza de las disoluciones

El presente trabajo forma parte de un proyecto más amplio sobre “el estudio de las variables de ‘intervención sistemática del profesor’ y ‘organización grupal’ en la eficacia de un método basado en el aprendizaje cooperativo para la asignatura de química en educación media”. En este estudio se presentan los resultados de los instrumentos aplicados a posteriori de una propuesta de aprendizaje de las disoluciones bajo un enfoque cooperativo. Los indicadores de eficacia de la estrategia son obtenidos a partir de instrumentos de evaluación diagnóstica, formativa y sumativa

Annihilation of Dipolar Dark Matter to Photons

In this work we study the annihilation of fermionic dark matter, considering it as a neutral particle with nonvanishing magnetic ($M$) and electric ($D$) dipole moments. Effective cross-section of the process $\chi \overline{\chi} \rightarrow \gamma \gamma$ is computed starting from a general form of coupling $\chi \overline{\chi} \gamma$ in the framework of an extension of the Standard Model. By taking into account annihilation of DM pairs into mono-energetic photons, we found that for small masses, $m_\chi \leq 10\,\textrm{GeV}$, an electric dipole moment $\sim 10^{-16}\, \textrm{e cm}$ is required to satisfy the current residual density inferences. Additionally, in order to pin down models viable to describe the physics of dark matter at the early Universe we also constrain our model according to recent measurements of the temperature anisotropies of the cosmic background radiation, we report constraints to the electric and magnetic dipole moments for a range of masses within our model.Comment: 20 pages, 6 figure