8,579,250 research outputs found
Realistic Neutrino Masses from Multi-brane Extensions of the Randall-Sundrum Model?
Scenarios based on the existence of large or warped (Randall-Sundrum model)
extra dimensions have been proposed for addressing the long standing puzzle of
gauge hierarchy problem. Within the contexts of both those scenarios, a novel
and original type of mechanism generating small (Dirac) neutrino masses, which
relies on the presence of additional right-handed neutrinos that propagate in
the bulk, has arisen. The main objective of the present study is to determine
whether this geometrical mechanism can produce reasonable neutrino masses also
in the interesting multi-brane extensions of the Randall-Sundrum model. We
demonstrate that, in some multi-brane extensions, neutrino masses in agreement
with all relevant experimental bounds can indeed be generated but at the price
of a constraint (stronger than the existing ones) on the bulk geometry, and
that the other multi-brane models even conflict with those experimental bounds.Comment: 29 pages, 3 figures, Latex file. References added, study extende
Bright solitons and soliton trains in a fermion-fermion mixture
We use a time-dependent dynamical mean-field-hydrodynamic model to predict
and study bright solitons in a degenerate fermion-fermion mixture in a
quasi-one-dimensional cigar-shaped geometry using variational and numerical
methods. Due to a strong Pauli-blocking repulsion among identical
spin-polarized fermions at short distances there cannot be bright solitons for
repulsive interspecies fermion-fermion interactions. However, stable bright
solitons can be formed for a sufficiently attractive interspecies interaction.
We perform a numerical stability analysis of these solitons and also
demonstrate the formation of soliton trains. These fermionic solitons can be
formed and studied in laboratory with present technology.Comment: 5 pages, 7 figure
Strong Coulomb effects in hole-doped Heisenberg chains
Substances such as the ``telephone number compound''
Sr14Cu24O41 are intrinsically hole-doped. The involved interplay of spin and
charge dynamics is a challenge for theory. In this article we propose to
describe hole-doped Heisenberg spin rings by means of complete numerical
diagonalization of a Heisenberg Hamiltonian that depends parametrically on hole
positions and includes the screened Coulomb interaction among the holes. It is
demonstrated that key observables like magnetic susceptibility, specific heat,
and inelastic neutron scattering cross section depend sensitively on the
dielectric constant of the screened Coulomb potential.Comment: 5 pages, 6 figures, to be published in Eur. Phys. J.
Modelling nucleon-nucleon scattering above 1 GeV
Motivated by the recent measurement of proton-proton spin-correlation
parameters up to 2.5 GeV laboratory energy, we investigate models for
nucleon-nucleon (NN) scattering above 1 GeV. Signatures for a gradual failure
of the traditional meson model with increasing energy can be clearly
identified. Since spin effects are large up to tens of GeV, perturbative QCD
cannot be invoked to fix the problems. We discuss various theoretical scenarios
and come to the conclusion that we do not have a clear phenomenological
understanding of the spin-dependence of the NN interaction above 1 GeV.Comment: 36 pages, 8 figure
High pT leading hadron suppression in nuclear collisions at sqrt(s_NN) = 20 -- 200 GeV: data versus parton energy loss models
Experimental results on high transverse momentum (leading) hadron spectra in
nucleus-nucleus collisions in the range sqrt(s_NN) = 20 -- 200 GeV are reviewed
with an emphasis on the observed suppression compared to free space production
in proton-proton collisions at the corresponding center-of-mass energies. The
transverse-momentum and collision-energy (but seemingly not the in-medium path
length) dependence of the experimental suppression factors measured in central
collisions is consistent with the expectations of final-state non-Abelian
parton energy loss in a dense QCD medium.Comment: Two typos correcte
The statistical parton distributions: status and prospects
New experimental results on polarized structure functions, cross sections for
neutral and charge current reactions and () charge
current on isoscalar targets are compared with predictions using the
statistical parton distributions, which were previously determined. New data on
cross sections for Drell-Yan processes, single jet in collisions and
inclusive production in collisions are also compared with
predictions from this theoretical approach. The good agreement which we find
with all these tests against experiment, strenghtens our opinion on the
relevance of the role of quantum statistics for parton distributions. We will
also discuss the prospects of this physical framework.Comment: 34 pages, 23 figures, references and figures adde
: a complete model for the chain sub-system
A second neighbor model for the chain subsystem of the
has been extracted from ab-initio calculations. This
model does not use periodic approximation but describes the entire chain
through the use of the four-dimensional crystallographic description. Second
neighbors interactions are found to be of same order than the first neighbors
ones. The computed values of the second neighbors magnetic interaction are
coherent with experimental estimations of the intra-dimer magnetic
interactions, even if slightly smaller. The reasons of this underestimation are
detailed. The computed model allowed us to understand the origin of the chain
dimerisation and predicts correctly the relative occurrence of dimers and free
spins. The orbitals respectively supporting the magnetic electrons and the
holes have been found to be essentially supported by the copper 3d orbitals
(spins) and the surrounding oxygen orbitals (holes), thus giving a strong
footing to the existence of Zhang-Rice singlets
Effect of topology on the transport properties of two interacting dots
The transport properties of a system of two interacting dots, one of them
directly connected to the leads constituting a side-coupled configuration
(SCD), are studied in the weak and strong tunnel-coupling limits. The
conductance behavior of the SCD structure has new and richer physics than the
better studied system of two dots aligned with the leads (ACD). In the weak
coupling regime and in the case of one electron per dot, the ACD configuration
gives rise to two mostly independent Kondo states. In the SCD topology, the
inserted dot is in a Kondo state while the side-connected one presents Coulomb
blockade properties. Moreover, the dot spins change their behavior, from an
antiferromagnetic coupling to a ferromagnetic correlation, as a consequence of
the interaction with the conduction electrons. The system is governed by the
Kondo effect related to the dot that is embedded into the leads. The role of
the side-connected dot is to introduce, when at resonance, a new path for the
electrons to go through giving rise to the interferences responsible for the
suppression of the conductance. These results depend on the values of the
intra-dot Coulomb interactions. In the case where the many-body interaction is
restricted to the side-connected dot, its Kondo correlation is responsible for
the scattering of the conduction electrons giving rise to the conductance
suppression
Prospects for measurement and control of the scattering length of metastable helium using photoassociation techniques
A numerical investigation of two-laser photoassociation (PA) spectroscopy on
spin-polarized metastable helium (He*) atoms is presented within the context of
experimental observation of the least-bound energy level in the scattering
potential and subsequent determination of the s-wave scattering length.
Starting out from the model developed by Bohn and Julienne [Phys. Rev. A
\textbf{60}, (1999) 414], PA rate coefficients are obtained as a function of
the parameters of the two lasers. The rate coefficients are used to simulate
one- and two-laser PA spectra. The results demonstrate the feasibility of a
spectroscopic determination of the binding energy of the least-bound level. The
simulated spectra may be used as a guideline when designing such an experiment,
whereas the model may also be employed for fitting experimentally obtained PA
spectra. In addition, the prospects for substantial modification of the He*
scattering length by means of optical Feshbach resonances are considered.
Several experimental issues relating to the numerical investigation presented
here are discussed.Comment: 9 pages, 7 figure
Accumulation of chromium metastable atoms into an Optical Trap
We report the fast accumulation of a large number of metastable 52Cr atoms in
a mixed trap, formed by the superposition of a strongly confining optical trap
and a quadrupolar magnetic trap. The steady state is reached after about 400
ms, providing a cloud of more than one million metastable atoms at a
temperature of about 100 microK, with a peak density of 10^{18} atoms.m^{-3}.
We have optimized the loading procedure, and measured the light shift of the
5D4 state by analyzing how the trapped atoms respond to a parametric
excitation. We compare this result to a theoretical evaluation based on the
available spectroscopic data for chromium atoms.Comment: 7 pages, 5 Figure
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