170 research outputs found
The Neutralino Sector of the Next-to-Minimal Supersymmetric Standard Model
The Next-to-Minimal Supersymmetric Standard Model (NMSSM) includes a Higgs
iso-singlet superfield in addition to the two Higgs doublet superfields of the
minimal extension. If the Higgs fields remain weakly coupled up to the GUT
scale, as naturally motivated by the concept of supersymmetry, the mixing
between singlet and doublet fields is small and can be treated perturbatively.
The mass spectrum and mixing matrix of the neutralino sector can be analyzed
analytically and the structure of this 5-state system is under good theoretical
control. We also determine decay modes and production channels in sfermion
cascade decays to these particles at the LHC and pair production in e+e-
colliders.Comment: 27 pages, 8 figure
Topology of the conceptual network of language
We define two words in a language to be connected if they express similar
concepts. The network of connections among the many thousands of words that
make up a language is important not only for the study of the structure and
evolution of languages, but also for cognitive science. We study this issue
quantitatively, by mapping out the conceptual network of the English language,
with the connections being defined by the entries in a Thesaurus dictionary. We
find that this network presents a small-world structure, with an amazingly
small average shortest path, and appears to exhibit an asymptotic scale-free
feature with algebraic connectivity distribution.Comment: 4 pages, 2 figures, Revte
Theoretical upper bound on the mass of the LSP in the MNSSM
We study the neutralino sector of the Minimal Non-minimal Supersymmetric
Standard Model (MNSSM) where the problem of the Minimal Supersymmetric
Standard Model (MSSM) is solved without accompanying problems related with the
appearance of domain walls. In the MNSSM as in the MSSM the lightest neutralino
can be the absolutely stable lightest supersymmetric particle (LSP) providing a
good candidate for the cold dark matter component of the Universe. In contrast
with the MSSM the allowed range of the mass of the lightest neutralino in the
MNSSM is limited. We establish the theoretical upper bound on the lightest
neutralino mass in the framework of this model and obtain an approximate
solution for this mass.Comment: 15 pages, 2 figures, references adde
Anderson localization of polaron states
Using the vanishing of the typical polaron tunneling rate as an indicator of
the breakdown of itinerancy, we study the localization of polaron states in a
generic model for a disordered polaronic material. We find that extremely small
disorder causes an Anderson localization of small polaron states. However, the
ratio between the critical disorder strength needed to localize all states in
the polaron band and the renormalized bandwidth is not necessarily smaller than
for a bare electron.Comment: 4 pages, 3 figure
Metal-insulator transition and charge ordering in the extended Hubbard model at one-quarter filling
We study with exact diagonalization the zero temperature properties of the
quarter-filled extended Hubbard model on a square lattice. We find that
increasing the ratio of the intersite Coulomb repulsion, , to the band width
drives the system from a metal to a charge ordered insulator. The evolution of
the optical conductivity spectrum with increasing is compared to the
observed optical conductivity of several layered molecular crystals with the
theta and beta'' crystal structures.Comment: 5 pages, 3 figure
Signatures of small-world and scale-free properties in large computer programs
A large computer program is typically divided into many hundreds or even
thousands of smaller units, whose logical connections define a network in a
natural way. This network reflects the internal structure of the program, and
defines the ``information flow'' within the program. We show that, (1) due to
its growth in time this network displays a scale-free feature in that the
probability of the number of links at a node obeys a power-law distribution,
and (2) as a result of performance optimization of the program the network has
a small-world structure. We believe that these features are generic for large
computer programs. Our work extends the previous studies on growing networks,
which have mostly been for physical networks, to the domain of computer
software.Comment: 4 pages, 1 figure, to appear in Phys. Rev.
Ground state of the three-band Hubbard model
The ground state of the two-dimensional three-band Hubbard model in oxide
superconductors is investigated by using the variational Monte Carlo method.
The Gutzwiller-projected BCS and spin- density wave (SDW) functions are
employed in the search for a possible ground state with respect to dependences
on electron density. Antiferromagnetic correlations are considerably enhanced
near half-filling. It is shown that the d-wave state may exist away from
half-filling for both the hole and electron doping cases. The overall structure
of the phase diagram obtained by the calculations qualitatively agrees with
experimental indications. The superconducting condensation energy is in
reasonable agreement with the experimental value obtained from specific heat
and critical magnetic field measurements for optimally doped samples. The
inhomogeneous SDW state is also examined near 1/8-hole doping.Comment: 10 pages, 17 figure
Application of non-HDL cholesterol for population-based cardiovascular risk stratification: results from the Multinational Cardiovascular Risk Consortium.
BACKGROUND: The relevance of blood lipid concentrations to long-term incidence of cardiovascular disease and the relevance of lipid-lowering therapy for cardiovascular disease outcomes is unclear. We investigated the cardiovascular disease risk associated with the full spectrum of bloodstream non-HDL cholesterol concentrations. We also created an easy-to-use tool to estimate the long-term probabilities for a cardiovascular disease event associated with non-HDL cholesterol and modelled its risk reduction by lipid-lowering treatment. METHODS: In this risk-evaluation and risk-modelling study, we used Multinational Cardiovascular Risk Consortium data from 19 countries across Europe, Australia, and North America. Individuals without prevalent cardiovascular disease at baseline and with robust available data on cardiovascular disease outcomes were included. The primary composite endpoint of atherosclerotic cardiovascular disease was defined as the occurrence of the coronary heart disease event or ischaemic stroke. Sex-specific multivariable analyses were computed using non-HDL cholesterol categories according to the European guideline thresholds, adjusted for age, sex, cohort, and classical modifiable cardiovascular risk factors. In a derivation and validation design, we created a tool to estimate the probabilities of a cardiovascular disease event by the age of 75 years, dependent on age, sex, and risk factors, and the associated modelled risk reduction, assuming a 50% reduction of non-HDL cholesterol. FINDINGS: Of the 524 444 individuals in the 44 cohorts in the Consortium database, we identified 398 846 individuals belonging to 38 cohorts (184 055 [48·7%] women; median age 51·0 years [IQR 40·7-59·7]). 199 415 individuals were included in the derivation cohort (91 786 [48·4%] women) and 199 431 (92 269 [49·1%] women) in the validation cohort. During a maximum follow-up of 43·6 years (median 13·5 years, IQR 7·0-20·1), 54 542 cardiovascular endpoints occurred. Incidence curve analyses showed progressively higher 30-year cardiovascular disease event-rates for increasing non-HDL cholesterol categories (from 7·7% for non-HDL cholesterol <2·6 mmol/L to 33·7% for ≥5·7 mmol/L in women and from 12·8% to 43·6% in men; p<0·0001). Multivariable adjusted Cox models with non-HDL cholesterol lower than 2·6 mmol/L as reference showed an increase in the association between non-HDL cholesterol concentration and cardiovascular disease for both sexes (from hazard ratio 1·1, 95% CI 1·0-1·3 for non-HDL cholesterol 2·6 to <3·7 mmol/L to 1·9, 1·6-2·2 for ≥5·7 mmol/L in women and from 1·1, 1·0-1·3 to 2·3, 2·0-2·5 in men). The derived tool allowed the estimation of cardiovascular disease event probabilities specific for non-HDL cholesterol with high comparability between the derivation and validation cohorts as reflected by smooth calibration curves analyses and a root mean square error lower than 1% for the estimated probabilities of cardiovascular disease. A 50% reduction of non-HDL cholesterol concentrations was associated with reduced risk of a cardiovascular disease event by the age of 75 years, and this risk reduction was greater the earlier cholesterol concentrations were reduced. INTERPRETATION: Non-HDL cholesterol concentrations in blood are strongly associated with long-term risk of atherosclerotic cardiovascular disease. We provide a simple tool for individual long-term risk assessment and the potential benefit of early lipid-lowering intervention. These data could be useful for physician-patient communication about primary prevention strategies. FUNDING: EU Framework Programme, UK Medical Research Council, and German Centre for Cardiovascular Research
Nonperturbative renormalization group approach to frustrated magnets
This article is devoted to the study of the critical properties of classical
XY and Heisenberg frustrated magnets in three dimensions. We first analyze the
experimental and numerical situations. We show that the unusual behaviors
encountered in these systems, typically nonuniversal scaling, are hardly
compatible with the hypothesis of a second order phase transition. We then
review the various perturbative and early nonperturbative approaches used to
investigate these systems. We argue that none of them provides a completely
satisfactory description of the three-dimensional critical behavior. We then
recall the principles of the nonperturbative approach - the effective average
action method - that we have used to investigate the physics of frustrated
magnets. First, we recall the treatment of the unfrustrated - O(N) - case with
this method. This allows to introduce its technical aspects. Then, we show how
this method unables to clarify most of the problems encountered in the previous
theoretical descriptions of frustrated magnets. Firstly, we get an explanation
of the long-standing mismatch between different perturbative approaches which
consists in a nonperturbative mechanism of annihilation of fixed points between
two and three dimensions. Secondly, we get a coherent picture of the physics of
frustrated magnets in qualitative and (semi-) quantitative agreement with the
numerical and experimental results. The central feature that emerges from our
approach is the existence of scaling behaviors without fixed or pseudo-fixed
point and that relies on a slowing-down of the renormalization group flow in a
whole region in the coupling constants space. This phenomenon allows to explain
the occurence of generic weak first order behaviors and to understand the
absence of universality in the critical behavior of frustrated magnets.Comment: 58 pages, 15 PS figure
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