14,020 research outputs found
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
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