Strangeness on the nucleon

Observables from parity violation in elastic electron-nucleon scattering and neutral current quasi-elastic neutrino-nucleus scattering are employed as tools to improve the current knowledge on the strangeness content in the nucleon.Comment: Proceedings of International Scientific Meeting on Nuclear Physics, 9-13th September 2012. La R\'abida, Huelva, Spai

Parity violation in quasielastic electron-nucleus scattering within the relativistic impulse approximation

We study parity violation in quasielastic (QE) electron-nucleus scattering using the relativistic impulse approximation. Different fully relativistic approaches have been considered to estimate the effects associated with the final-state interactions. We have computed the parity-violating quasielastic (PVQE) asymmetry and have analyzed its sensitivity to the different ingredients that enter in the description of the reaction mechanism: final-state interactions, nucleon off-shellness effects, current gauge ambiguities. Particular attention has been paid to the description of the weak neutral current form factors. The PVQE asymmetry is proven to be an excellent observable when the goal is to get precise information on the axial-vector sector of the weak neutral current. Specifically, from measurements of the asymmetry at backward scattering angles good knowledge of the radiative corrections entering in the isovector axial-vector sector can be gained. Finally, scaling properties shown by the interference $\gamma-Z$ nuclear responses are also analyzed.Comment: 15 pages, 11 figure

Parity violation and dynamical relativistic effects in (e⃗,e′N)(\vec{e},e'N) reactions

It is well known that coincidence quasielastic $(\vec{e},e'N)$ reactions are not appropriate to analyze effects linked to parity violation due the presence of the fifth electromagnetic (EM) response $R^{TL'}$. Nevertheless, in this work we develop a fully relativistic approach to be applied to parity-violating (PV) quasielastic $(\vec{e},e'N)$ processes. This is of importance as a preliminary step in the subsequent study of inclusive quasielastic PV $(\vec{e},e')$ reactions. Moreover, our present analysis allows us to disentangle effects associated with the off-shell character of nucleons in nuclei, gauge ambiguities and the role played by the lower components in the nucleon wave functions, i.e., dynamical relativistic effects. This study can help in getting clear information on PV effects. Particular attention is paid to the relativistic plane-wave impulse approximation where the explicit expressions for the PV single-nucleon responses are shown for the first time.Comment: 39 pages, 9 figure

Global analysis of parity-violating asymmetry data for elastic electron scattering

We perform a statistical analysis of the full set of parity-violating asymmetry data for elastic electron scattering including the most recent high precision measurement from $Q$-weak. Given the basis of the present analysis, our estimates appear to favor non-zero vector strangeness, specifically, positive (negative) values for the electric (magnetic) strange form factors. We also provide an accurate estimate of the axial-vector nucleon form factor at zero momentum transfer, $G_A^{ep}(0)$. Our study shows $G_A^{ep}(0)$ to be importantly reduced with respect to the currently accepted value. We also find our analysis of data to be compatible with the Standard Model values for the weak charges of the proton and neutron.Comment: 6 pages, 4 figures, 2 tables. Accepted for publication in PR

Cool dwarfs in wide multiple systems. Paper 6: A curious quintuple system of a compact Sun-like triple and a close pair of an M dwarf and a very cool white dwarf at a wide separation

The system WDS 16329+0315 is an old, nearby quintuple physical system in the thick Galactic disc formed by a close-resolved, triple primary of solar metallicity, namely HD 149162, and a very wide, common proper motion, secondary pair, formed by the mid-M dwarf G-17-23 and the white dwarf LSPM J1633+0311S. We present an exhaustive astrometric and photometric data compilation of the system, including Gaia DR2 parallaxes and proper motions, and the first analysis of the nature of the faintest component. LSPM J1633+0311S (HD 149162 C) is a very cool white dwarf with an effective temperature of only about 5500 K, near the coolest end of the grid of theoretical models.Comment: The Observatory, in press, to appear in December 201

Un procedimiento para obtener clusters utilizando la D.V.S. de una matriz. Comparaciones con el biplot y con el modelo Q-factorial

Durante las últimas décadas, el análisis de un conjunto de n individuos medidos en p variables, proporcionando una matriz de datos Xn,p, mediante técnicas de representación que utilizan la Descomposición en Valores Singulares de la matriz Xn,p (o alguna derivada), han permitido resumir la información que aportan los datos en alguna forma óptima, siendo muy útil para indicar la presencia de clusters entre los n individuos y/o para prevenir ante posibles clasificaciones erróneas producidas por técnicas de agrupamiento más complejas. En este artículo estudiaremos un procedimiento que puede utilizarse en ocasiones para obtener clasificaciones naturales de un conjunto de datos, basado en la representación biplot y en el modelo Q-factorial que puede obtenerse a partir de la DVS

Chemical abundances of late-type pre-main sequence stars in the σ\sigma-Orionis cluster

The young $\sigma$-Orionis cluster is an important location for understanding the formation and evolution of stars, brown dwarfs, and planetary-mass objects. Its metallicity, although being a fundamental parameter, has not been well determined yet. We present the first determination of the metallicity of nine young late-type stars in $\sigma$-Orionis. Using the optical and near-infrared broadband photometry available in the literature we derive the effective temperatures for these nine cluster stars, which lie in the interval 4300--6500 K (1--3 \Msuno). These parameters are employed to compute a grid of synthetic spectra based on the code MOOG and Kurucz model atmospheres. We employ a $\chi^2$-minimization procedure to derive the stellar surface gravity and atmospheric abundances of Al, Ca, Si, Fe, Ni and Li, using multi-object optical spectroscopy taken with WYFFOS+AF2 at at the William Herschel Telescope ($\lambda/\delta\lambda\sim7500$). The average metallicity of the $\sigma$-Orionis cluster is [Fe/H] $= -0.02\pm0.09\pm0.13$ (random and systematic errors). The abundances of the other elements, except lithium, seem to be consistent with solar values. Lithium abundances are in agreement with the "cosmic" $^7$Li abundance, except for two stars which show a $\log \epsilon(\mathrm{Li})$ in the range 3.6--3.7 (although almost consistent within the error bars). There are also other two stars with $\log \epsilon(\mathrm{Li})\sim 2.75$. We derived an average radial velocity of the $\sigma$-Orionis cluster of $28\pm4$km/s. The $\sigma$-Orionis metallicity is roughly solar.Comment: Accepted for publication in Astronomy and Astrophysic

Un procedimiento para obtener clusters utilizando la D.V.S. de una matriz. Comparaciones con el biplot y con el modelo Q-factorial

Durante las últimas décadas, el análisis de un conjunto de n individuos medidos en p variables, proporcionando una matriz de datos Xn,p, mediante técnicas de representación que utilizan la Descomposición en Valores Singulares de la matriz Xn,p (o alguna derivada), han permitido resumir la información que aportan los datos en alguna forma óptima, siendo muy útil para indicar la presencia de clusters entre los n individuos y/o para prevenir ante posibles clasificaciones erróneas producidas por técnicas de agrupamiento más complejas. En este artículo estudiaremos un procedimiento que puede utilizarse en ocasiones para obtener clasificaciones naturales de un conjunto de datos, basado en la representación biplot y en el modelo Q-factorial que puede obtenerse a partir de la DVS