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

The acoustic cut-off frequency of the Sun and the solar magnetic activity cycle

The acoustic cut-off frequency -the highest frequency for acoustic solar eigenmodes- is an important parameter of the solar atmosphere as it determines the upper boundary of the p-mode resonant cavities. At frequencies beyond this value, acoustic disturbances are no longer trapped but traveling waves. Interference amongst them give rise to higher-frequency peaks -the pseudomodes- in the solar acoustic spectrum. The pseudomodes are shifted slightly in frequency with respect to p modes making possible the use of pseudomodes to determine the acoustic cut-off frequency. Using data from GOLF and VIRGO instruments on board the SOHO spacecraft, we calculate the acoustic cut-off frequency using the coherence function between both the velocity and intensity sets of data. By using data gathered by these instruments during the entire lifetime of the mission (1996 till the present), a variation in the acoustic cut-off frequency with the solar magnetic activity cycle is found.Comment: Paper accepted in ApJ. 26 Pages, 9 figure

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

Polyradical character and spin frustration in fullerene molecules: An ab initio non-collinear Hartree--Fock study

Most {\em ab initio} calculations on fullerene molecules have been carried out based on the paradigm of the H\"uckel model. This is consistent with the restricted nature of the independent-particle model underlying such calculations, even in single-reference-based correlated approaches. On the other hand, previous works on some of these molecules using model Hamiltonians have clearly indicated the importance of short-range inter-atomic spin-spin correlations. In this work, we consider {\em ab initio} non-collinear Hartree--Fock (HF) solutions for representative fullerene systems: the bowl, cage, ring, and pentagon isomers of C$_{20}$, and the larger C$_{30}$, C$_{36}$, C$_{60}$, C$_{70}$, and C$_{84}$ fullerene cages. In all cases but the ring we find that the HF minimum corresponds to a truly non-collinear solution with a torsional spin density wave. Optimized geometries at the generalized HF (GHF) level lead to fully symmetric structures, even in those cases where Jahn-Teller distortions have been previously considered. The nature of the GHF solutions is consistent with the $\pi$-electron space becoming polyradical in nature: each $p$-orbital remains effectively singly occupied. The spin frustration, induced by the pentagon rings in an otherwise anti-ferromagnetic background, is minimized at the HF level by aligning the spins in non-collinear arrangements. The long-range magnetic ordering observed is reminiscent of the character of broken symmetry HF solutions in polyacene systems.Comment: 16 figure

Multi-reference symmetry-projected variational approximation for the ground state of the doped one-dimensional Hubbard model

A multi-reference configuration mixing scheme is used to describe the ground state, characterized by well defined spin and space group symmetry quantum numbers as well as doping fractions $N_{e}/N_{sites}$, of one dimensional Hubbard lattices with nearest-neighbor hopping and periodic boundary conditions. Within this scheme, each ground state is expanded in a given number of nonorthogonal and variationally determined symmetry-projected configurations. The results obtained for the ground state and correlation energies of half-filled and doped lattices with 30, 34 and 50 sites, compare well with the exact Lieb-Wu solutions as well as with the ones obtained with other state-of-the-art approximations. The structure of the intrinsic symmetry-broken determinants resulting from the variational procedure is interpreted in terms of solitons whose translational and breathing motions can be regarded as basic units of quantum fluctuations. It is also shown that in the case of doped 1D lattices, a part of such fluctuations can also be interpreted in terms of polarons. In addition to momentum distributions, both spin-spin and density-density correlation functions are studied as functions of doping. The spectral functions and density of states, computed with an ansatz whose quality can be well-controlled by the number of symmetry-projected configurations used to approximate the $N_{e} \pm 1$ electron systems, display features beyond a simple quasiparticle distribution, as well as spin-charge separation trends.Comment: 16 pages, 11 figure