14,317 research outputs found
A Simple Explanation for the X(3872) Mass Shift Observed for Decay to D^{*0} {D^0}bar
We propose a simple explanation for the increase of approximately
3 MeV/c^2 in the mass value of the X(3872) obtained from
D^{*0} {D^0}bar decay relative to that obtained from decay to J/psi pi+ pi-.
If the total width of the X(3872) is 2-3 MeV, the peak position in the
D^{*0} {D^0}bar invariant mass distribution is sensitive to the final state
orbital angular momentum because of the proximity of the X(3872) to D^{*0}
{D^0}bar threshold. We show that for total width 3 MeV and one unit of orbital
angular momentum, a mass shift ~3 MeV/c^2 is obtained; experimental mass
resolution should slightly increase this value. A consequence is that
spin-parity 2^- is favored for the X(3872).Comment: 3.5 pages, 4 eps figure
Two-Dimensional Electrons in a Strong Magnetic Field with Disorder: Divergence of the Localization Length
Electrons on a square lattice with half a flux quantum per plaquette are
considered. An effective description for the current loops is given by a
two-dimensional Dirac theory with random mass. It is shown that the
conductivity and the localization length can be calculated from a product of
Dirac Green's functions with the {\it same} frequency. This implies that the
delocalization of electrons in a magnetic field is due to a critical point in a
phase with a spontaneously broken discrete symmetry. The estimation of the
localization length is performed for a generalized model with fermion
levels using a --expansion and the Schwarz inequality. An argument for the
existence of two Hall transition points is given in terms of percolation
theory.Comment: 10 pages, RevTeX, no figure
Visibility of Cold Atomic Gases in Optical Lattices for Finite Temperatures
In nearly all experiments with ultracold atoms time-of-flight pictures are
the only data available. In this paper we present an analytical strong-coupling
calculation for those time-of-flight pictures of bosons in an optical lattice
in the Mott phase. This allows us to determine the visibility, which quantifies
the contrast of peaks in the time-of-flight pictures, and we suggest how to use
it as a thermometer.Comment: Author Information under
http://www.theo-phys.uni-essen.de/tp/ags/pelster_dir
Mixtures of fermionic atoms in an optical lattice
A mixture of light and heavy spin-polarized fermionic atoms in an optical
lattice is considered. Tunneling of the heavy atoms is neglected such that they
are only subject to thermal fluctuations. This results in a complex interplay
between light and heavy atoms caused by quantum tunneling of the light atoms.
The distribution of the heavy atoms is studied. It can be described by an
Ising-like distribution with a first-order transition from homogeneous to
staggered order. The latter is caused by an effective nonlocal interaction due
to quantum tunneling of the light atoms. A second-order transition is also
possible between an ordered and a disordered phase of the heavy atoms
Integer Quantum Hall Effect for Lattice Fermions
A two-dimensional lattice model for non-interacting fermions in a magnetic
field with half a flux quantum per plaquette and levels per site is
considered. This is a model which exhibits the Integer Quantum Hall Effect
(IQHE) in the presence of disorder. It presents an alternative to the
continuous picture for the IQHE with Landau levels. The large limit can be
solved: two Hall transitions appear and there is an interpolating behavior
between the two Hall plateaux. Although this approach to the IQHE is different
from the traditional one with Landau levels because of different symmetries
(continuous for Landau levels and discrete here), some characteristic features
are reproduced. For instance, the slope of the Hall conductivity is infinite at
the transition points and the electronic states are delocalized only at the
transitions.Comment: 9 pages, Plain-Te
Lower Bound for the Fermi Level Density of States of a Disordered D-Wave Superconductor in Two Dimensions
We consider a disordered d--wave superconductor in two dimensions. Recently,
we have shown in an exact calculation that for a lattice model with a
Lorentzian distributed random chemical potential the quasiparticle density of
states at the Fermi level is nonzero. As the exact result holds only for the
special choice of the Lorentzian, we employ different methods to show that for
a large class of distributions, including the Gaussian distribution, one can
establish a nonzero lower bound for the Fermi level density of states. The fact
that the tails of the distributions are unimportant in deriving the lower bound
shows that the exact result obtained before is generic.Comment: 15 preprint pages, no figures, submitted to PR
The effect of ram pressure on the star formation, mass distribution and morphology of galaxies
We investigate the dependence of star formation and the distribution of the
components of galaxies on the strength of ram pressure. Several mock
observations in X-ray, H and HI wavelength for different ram-pressure
scenarios are presented. By applying a combined N-body/hydrodynamic description
(GADGET-2) with radiative cooling and a recipe for star formation and stellar
feedback 12 different ram-pressure stripping scenarios for disc galaxies were
calculated. Special emphasis was put on the gas within the disc and in the
surroundings. All gas particles within the computational domain having the same
mass resolution. The relative velocity was varied from 100 km/s to 1000 km/s in
different surrounding gas densities in the range from to
g/cm. The temperature of the surrounding gas was
initially K. The star formation of a galaxy is enhanced by more
than a magnitude in the simulation with a high ram-pressure (
dyn/cm) in comparison to the same system evolving in isolation. The
enhancement of the star formation depends more on the surrounding gas density
than on the relative velocity. Up to 95% of all newly formed stars can be found
in the wake of the galaxy out to distances of more than 350 kpc behind the
stellar disc. Continuously stars fall back to the old stellar disc, building up
a bulge-like structure. Young stars can be found throughout the stripped wake
with surface densities locally comparable to values in the inner stellar disc.
Ram-pressure stripping can shift the location of star formation from the disc
into the wake on very short timescales. (Abridged)Comment: 19 pages, 25 figures, A&A accepted, high resolution version can be
found at http://astro.uibk.ac.at/~wolfgang/kapferer_rps_galaxies.pd
Internal kinematics of isolated modelled disk galaxies
We present a systematic investigation of rotation curves (RCs) of fully
hydrodynamically simulated galaxies, including cooling, star formation with
associated feedback and galactic winds. Applying two commonly used fitting
formulae to characterize the RCs, we investigate systematic effects on the
shape of RCs both by observational constraints and internal properties of the
galaxies. We mainly focus on effects that occur in measurements of intermediate
and high redshift galaxies. We find that RC parameters are affected by the
observational setup, like slit misalignment or the spatial resolution and also
depend on the evolution of a galaxy. Therefore, a direct comparison of
quantities derived from measured RCs with predictions of semi-analytic models
is difficult. The virial velocity V_c, which is usually calculated and used by
semi-analytic models can differ significantly from fit parameters like V_max or
V_opt inferred from RCs. We find that V_c is usually lower than typical
characteristic velocities derived from RCs. V_max alone is in general not a
robust estimator for the virial mass.Comment: 9 pages, 15 figures, accepted for publication in A&
Quantum phases in mixtures of fermionic atoms
A mixture of spin-polarized light and heavy fermionic atoms on a finite size
2D optical lattice is considered at various temperatures and values of the
coupling between the two atomic species. In the case, where the heavy atoms are
immobile in comparison to the light atoms, this system can be seen as a
correlated binary alloy related to the Falicov-Kimball model. The heavy atoms
represent a scattering environment for the light atoms. The distributions of
the binary alloy are discussed in terms of strong- and weak-coupling
expansions. We further present numerical results for the intermediate
interaction regime and for the density of states of the light particles. The
numerical approach is based on a combination of a Monte-Carlo simulation and an
exact diagonalization method. We find that the scattering by the correlated
heavy atoms can open a gap in the spectrum of the light atoms, either for
strong interaction or small temperatures.Comment: 15 pages, 8 figure
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