10,041 research outputs found
Supply-side peacekeeping: theories and new evidence from a panel data analysis
Why do nations with heterogeneous economies, geographic positions and institutions agree to dispatch their troops to remote conflict areas? This paper explores the domestic and international determinants of countries' contribution to peacekeeping operations from 1999 to 2009. Individual nations make their decision about where, when and how to send their military personnel as well as the justifications on which they base their involvement in sovereign states. Moral imperative for peacekeeping may be universally accepted but a country decision to participate is also based on self-interest combined to the geo-strategic dimension and finally constrained by political and technical considerations. Empirical results suggest that at the domestic level technical forces, such as the sustainability of multiple missions and military capabilities, all play a role. At the international level peacekeeping contributions are driven by the security threat that a conflict poses and the number of displaced people
Superfluid to Bose-glass transition in a 1D weakly interacting Bose gas
We study the one-dimensional Bose gas in spatially correlated disorder at
zero temperature, using an extended density-phase Bogoliubov method. We analyze
in particular the decay of the one-body density matrix and the behaviour of the
Bogoliubov excitations across the phase boundary. We observe that the
transition to the Bose glass phase is marked by a power-law divergence of the
density of states at low energy. A measure of the localization length displays
a power-law energy dependence in both regions, with the exponent equal to -1 at
the boundary. We draw the phase diagram of the superfluid-insulator transition
in the limit of small interaction strength.Comment: 4 pages, 4 figure
Shining LUX on Isospin-Violating Dark Matter Beyond Leading Order
Isospin-violating dark matter (IVDM) has been proposed as a viable scenario
to reconcile conflicting positive and null results from direct detection dark
matter experiments. We show that the lowest-order dark matter-nucleus
scattering rate can receive large and nucleus-dependent corrections at
next-to-leading order (NLO) in the chiral expansion. The size of these
corrections depends on the specific couplings of dark matter to quark flavors
and gluons. In general the full NLO dark-matter-nucleus cross-section is not
adequately described by just the zero-energy proton and neutron couplings.
These statements are concretely illustrated in a scenario where the dark matter
couples to quarks through scalar operators. We find the canonical IVDM scenario
can reconcile the null XENON and LUX results and the recent CDMS-Si findings
provided its couplings to second and third generation quarks either lie on a
special line or are suppressed. Equally good fits with new values of the
neutron-to-proton coupling ratio are found in the presence of nonzero heavy
quark couplings. CDMS-Si remains in tension with LUX and XENON10/100 but is not
excluded.Comment: 11 pages, 5 figure
Composite Majorana Fermion Wavefunctions in Nanowires
We consider Majorana fermions (MFs) in quasi-one-dimensional nanowire systems
containing normal and superconducting sections where the topological phase
based on Rashba spin orbit interaction can be tuned by magnetic fields. We
derive explicit analytic solutions of the MF wavefunction in the weak and
strong spin orbit interaction regimes. We find that the wavefunction for one
single MF is a composite object formed by superpositions of different MF
wavefunctions which have nearly disjoint supports in momentum space. These
contributions are coming from the extrema of the spectrum, one centered around
zero momentum and the other around the two Fermi points. As a result, the
various MF wavefunctions have different localization lengths in real space and
interference among them leads to pronounced oscillations of the MF probability
density. For a transparent normal-superconducting junction we find that in the
topological phase the MF leaks out from the superconducting into the normal
section of the wire and is delocalized over the entire normal section, in
agreement with recent numerical results by Chevallier et al. (arXiv:1203.2643)
Mean-field phase diagram of the 1-D Bose gas in a disorder potential
We study the quantum phase transition of the 1D weakly interacting Bose gas
in the presence of disorder. We characterize the phase transition as a function
of disorder and interaction strengths, by inspecting the long-range behavior of
the one-body density matrix as well as the drop in the superfluid fraction. We
focus on the properties of the low-energy Bogoliubov excitations that drive the
phase transition, and find that the transition to the insulator state is marked
by a diverging density of states and a localization length that diverges as a
power-law with power 1. We draw the phase diagram and we observe that the
boundary between the superfluid and the Bose glass phase is characterized by
two different algebraic relations. These can be explained analytically by
considering the limiting cases of zero and infinite disorder correlation
length.Comment: 10 pages, 10 figure
One-step synthesis of pyridines and dihydropyridines in a continuous flow microwave reactor
The Bohlmann–Rahtz pyridine synthesis and the Hantzsch dihydropyridine synthesis can be carried out in a microwave flow reactor or using a conductive heating flow platform for the continuous processing of material. In the Bohlmann–Rahtz reaction, the use of a Brønsted acid catalyst allows Michael addition and cyclodehydration to be carried out in a single step without isolation of intermediates to give the corresponding trisubstituted pyridine as a single regioisomer in good yield. Furthermore, 3-substituted propargyl aldehydes undergo Hantzsch dihydropyridine synthesis in preference to Bohlmann–Rahtz reaction in a very high yielding process that is readily transferred to continuous flow processing
Realistic heterointerfaces model for excitonic states in growth-interrupted quantum wells
We present a model for the disorder of the heterointerfaces in GaAs quantum
wells including long-range components like monolayer island formation induced
by the surface diffusion during the epitaxial growth process. Taking into
account both interfaces, a disorder potential for the exciton motion in the
quantum well plane is derived. The excitonic optical properties are calculated
using either a time-propagation of the excitonic polarization with a
phenomenological dephasing, or a full exciton eigenstate model including
microscopic radiative decay and phonon scattering rates. While the results of
the two methods are generally similar, the eigenstate model does predict a
distribution of dephasing rates and a somewhat modified spectral response.
Comparing the results with measured absorption and resonant Rayleigh scattering
in GaAs/AlAs quantum wells subjected to growth interrupts, their specific
disorder parameters like correlation lengths and interface flatness are
determined. We find that the long-range disorder in the two heterointerfaces is
highly correlated, having rather similar average in-plane correlation lengths
of about 60 and 90 nm. The distribution of dephasing rates observed in the
experiment is in agreement with the results of the eigenstate model. Finally,
we simulate highly spatially resolved optical experiments resolving individual
exciton states in the deduced interface structure.Comment: To appear in Physical Review
Neural networks and betting strategies for tennis
Recently, the interest of the academic literature on sports statistics has increased enormously. In such a framework, two of the most significant challenges are developing a model able to beat the existing approaches and, within a betting market framework, guarantee superior returns than the set of competing specifications considered. This contribution attempts to achieve both these results, in the context of male tennis. In tennis, several approaches to predict the winner are available, among which the regression-based, point-based and paired comparison of the competitors’ abilities play a significant role. Contrary to the existing approaches, this contribution employs artificial neural networks (ANNs) to forecast the probability of winning in tennis matches, starting from all the variables used in a large selection of the previous methods. From an out-of-sample perspective, the implemented ANN model outperforms four out of five competing models, independently of the considered period. For what concerns the betting perspective, we propose four different strategies. The resulting returns on investment obtained from the ANN appear to be more broad and robust than those obtained from the best competing model, irrespective of the betting strategy adopted
The Role of Lattice QCD in Searches for Violations of Fundamental Symmetries and Signals for New Physics
This document is one of a series of whitepapers from the USQCD collaboration.
Here, we discuss opportunities for Lattice Quantum Chromodynamics (LQCD) in the
research frontier in fundamental symmetries and signals for new physics. LQCD,
in synergy with effective field theories and nuclear many-body studies,
provides theoretical support to ongoing and planned experimental programs in
searches for electric dipole moments of the nucleon, nuclei and atoms, decay of
the proton, - oscillations, neutrinoless double- decay
of a nucleus, conversion of muon to electron, precision measurements of weak
decays of the nucleon and of nuclei, precision isotope-shift spectroscopy, as
well as direct dark matter detection experiments using nuclear targets. This
whitepaper details the objectives of the LQCD program in the area of
Fundamental Symmetries within the USQCD collaboration, identifies priorities
that can be addressed within the next five years, and elaborates on the areas
that will likely demand a high degree of innovation in both numerical and
analytical frontiers of the LQCD research.Comment: A whitepaper by the USQCD Collaboration, 30 pages, 9 figure
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