Evaluation Trends

published or submitted for publicatio

Enhanced Diffusion of a Needle in a Planar Course of Point Obstacles

The transport of an infinitely thin, hard rod in a random, dense array of point obstacles is investigated by molecular dynamics simulations. Our model mimics the sterically hindered dynamics in dense needle liquids. The center-of-mass diffusion exhibits a minimum, and transport becomes increasingly fast at higher densities. The diffusion coefficient diverges according to a power law in the density with an approximate exponent of 0.8. This observation is connected with a new divergent time scale, reflected in a zig-zag motion of the needle, a two-step decay of the velocity-autocorrelation function, and a negative plateau in the non-Gaussian parameter.Comment: accepted for publication in Phys. Rev. Let

Neutrino masses and mixings in a Minimal S_3-invariant Extension of the Standard Model

The mass matrices of the charged leptons and neutrinos, that had been derived in the framework of a Minimal S_3-invariant Extension of the Standard Model, are here reparametrized in terms of their eigenvalues. The neutrino mixing matrix, V_PMNS, is then computed and exact, explicit analytical expressions for the neutrino mixing angles as functions of the masses of the neutrinos and charged leptons are obtained. The reactor, theta_13, and the atmosferic, theta_23, mixing angles are found to be functions only of the masses of the charged leptons. The numerical values of theta_13{th} and theta_23{th} computed from our theoretical expressions are found to be in excellent agreement with the latest experimental determinations. The solar mixing angle, theta_12{th}, is found to be a function of both, the charged lepton and neutrino masses, as well as of a Majorana phase phi_nu. A comparison of our theoretical expression for the solar angle theta_12{th} with the latest experimental value theta_12{exp} ~ 34 deg allowed us to fix the scale and origin of the neutrino mass spectrum and obtain the mass values |m_nu1|=0.0507 eV, |m_nu2|=0.0499 eV and |m_nu3|=0.0193 eV, in very good agreement with the observations of neutrino oscillations, the bounds extracted from neutrinoless double beta decay and the precision cosmological measurements of the CMB.Comment: To appear in the Proceedings of the XXIX Symposium on Nuclear Physics, Cocoyoc, Mex., January 2006. Some typographical errors on formulae correcte

Integrating out the Dirac sea in the Walecka model

We derive a purely fermionic no-sea effective theory, featuring positive-energy states only for the Walecka model. In contrast to the so-called mean-field theory approach with the no-sea approximation, where the Dirac sea is simply omitted from the outset, we turn to the relativistic Hartree approximation and explicitly construct a no-sea effective theory from the underlying quantum field theory. Several results obtained within these two approaches are confronted with each other. This sheds new light on the reliability of the mean-field theory with the no-sea approximation as well as the role of the Dirac sea. Restricting to 1+1 dimensions, we obtain new analytical insights into nonuniform nuclear matter.Comment: 15 pages, 8 figures, several points clarified, Fig.7 replaced, references adde

Helical liquids and Majorana bound states in quantum wires

We show that the combination of spin-orbit coupling with a Zeeman field or strong interactions may lead to the formation of a helical liquid in single-channel quantum wires. In a helical liquid, electrons with opposite velocities have opposite spin precession. We argue that zero-energy Majorana bound states are formed in various situations when the wire is situated in proximity to a conventional s-wave superconductor. This occurs when the external magnetic field, the superconducting gap, or, in particular, the chemical potential vary along the wire. We discuss experimental consequences of the formation of the helical liquid and the Majorana bound states.Comment: 4+epsilon page