11,124 research outputs found
N-flation: Non-Gaussianity in the horizon-crossing approximation
We analyze the cosmic non-gaussianity produced in inflation models with multiple uncoupled fields with monomial potentials, such as Nflation. Using the horizon-crossing approximation to compute the non-gaussianity, we show that when each field has the same form of potential, the prediction is independent the number of fields, their initial conditions, and the spectrum of masses/couplings. It depends only on the number of e-foldings after the horizon crossing of observable perturbations. We also provide a further generalization to the case where the fields can have monomial potentials with different powers. Unless the horizon-crossing approximation is substantially violated, the predicted non-gaussianity is too small to ever be observed
Nflation: observable predictions from the random matrix mass spectrum
We carry out numerical investigations of the perturbations in Nflation models
where the mass spectrum is generated by random matrix theory. The
tensor-to-scalar ratio and non-gaussianity are already known to take the
single-field values, and so the density perturbation spectral index is the main
parameter of interest. We study several types of random field initial
conditions, and compute the spectral index as a function of mass spectrum
parameters. Comparison with microwave anisotropy data from the Wilkinson
Microwave Anisotropy Probe shows that the model is currently viable in the
majority of its parameter space.Comment: 5 pages RevTeX with 4 figures. Minor corrections to match version to
appear in Physical Review
High H2 Storage of Hexagonal Metal−Organic Frameworks from First-Principles-Based Grand Canonical Monte Carlo Simulations
Stimulated by the recent report by Yaghi and co-workers of hexagonal metal−organic frameworks (MOF) exhibiting reversible binding of up to 7.5 wt % at 77 K and 70 bar for MOF-177 (called here IRMOF-2-24), we have predicted additional trigonal organic linkers, including IRMOF-2-60, which we calculate to bind 9.7 wt % H2 storage at 77 K and 70 bar, the highest known value for 77 K. These calculations are based on grand canonical Monte Carlo (GCMC) simulations using force fields that match accurate quantum mechanical calculations on the binding of H2 to prototypical systems. These calculations were validated by comparison to the experimental loading curve for IRMOF-2-24 at 77K. We then used the theory to predict the effect of doping Li into the hexagonal MOFs, which leads to substantial H2 density even at ambient temperatures. For example, IRMOF-2-96-Li leads to 6.0 wt % H2 storage at 273 K and 100 bar, the first material to attain the 2010 DOE target
Dynamics of assisted quintessence
We explore the dynamics of assisted quintessence, where more than one scalar field is present with the same potential. For potentials with tracking solutions, the fields naturally approach the same values—in the context of inflation this leads to the assisted inflation phenomenon where several fields can cooperate to drive a period of inflation though none is able to individually. For exponential potentials, we study the fixed points and their stability confirming results already in the literature, and then carry out a numerical analysis to show how assisted quintessence is realized. For inverse power-law potentials, we find by contrast that there is no assisted behavior—indeed those are the unique (monotonic) potentials where several fields together behave just as a single field in the same potential. More generally, we provide an algorithm for generating a single-field potential giving equivalent dynamics to multifield assisted quintessence
Mass Spectrum Dependence of Higgs-mediated mu-e Transition in the MSSM
In this paper, we study non-decoupling - transition effects by
Higgs-mediated contribution in the MSSM, when some SUSY mass parameters are
much greater than TeV. In order to treat CP-odd Higgs mass as a
free parameter, we consider the non-universal Higgs mass model (NUHM), and
assume the only left- or right-handed sleptons had flavor-mixing mass terms. If
both Higgs-mediated and ordinary SUSY contribution are significant, the ratio
of branching ratios \BR(\meg) / \BR(\maleal) becomes sensitive to SUSY mass
parameters. We investigated these mass-sensitive regions and the behavior of
the ratio \BR(\meg) / \BR(\maleal) in some mass spectrum of the NUHM, and
found that this ratio drastically depends on the mass spectrum structure and
chirality of flavor violation. Log factor from two split mass scale influences
the way of interference between gaugino- and Higgs-mediated contributions
significantly.Comment: 19 pages, 8 figures, it will appear in PR
A String Theory Which Isn't About Strings
Quantization of closed string proceeds with a suitable choice of worldsheet
vacuum. A priori, the vacuum may be chosen independently for left-moving and
right-moving sectors. We construct {\sl ab initio} quantized bosonic string
theory with left-right asymmetric worldsheet vacuum and explore its
consequences and implications. We critically examine the validity of new vacuum
and carry out first-quantization using standard operator formalism. Remarkably,
the string spectrum consists only of a finite number of degrees of freedom:
string gravity (massless spin-two, Kalb-Ramond and dilaton fields) and two
massive spin-two Fierz-Pauli fields. The massive spin-two fields have negative
norm, opposite mass-squared, and provides a Lee-Wick type extension of string
gravity. We compute two physical observables: tree-level scattering amplitudes
and one-loop cosmological constant. Scattering amplitude of four dilatons is
shown to be a rational function of kinematic invariants, and in
factorizes into contributions of massless spin-two and a pair of massive
spin-two fields. The string one loop partition function is shown to perfectly
agree with one loop Feynman diagram of string gravity and two massive spin-two
fields. In particular, it does not exhibit modular invariance. We critically
compare our construction with recent studies and contrast differences.Comment: 42 pages, 1 figure, minor corrections, references added; v3: minor
corrections, references added, published version in JHE
High-temperature deformation and microstructural analysis for Si3N4-Sc2O3
It was indicated that Si3N4 doped with Sc2O3 may exhibit high temperature mechanical properties superior to Si3N4 systems with various other oxide sintered additives. High temperature deformation of samples was studied by characterizing the microstructures before and after deformation. It was found that elements of the additive, such as Sc and O, exist in small amounts at very thin grain boundary layers and most of them stay in secondary phases at triple and multiple grain boundary junctions. These secondary phases are devitrified as crystalline Sc2Si2O7. Deformation of the samples was dominated by cavitational processes rather than movements of dislocations. Thus the excellent deformation resistance of the samples at high temperature can be attributed to the very small thickness of the grain boundary layers and the crystalline secondary phase
Nflation: multi-field inflationary dynamics and perturbations
We carry out numerical investigations of the dynamics and perturbations in
the Nflation model of Dimopoulos et al. (2005). This model features large
numbers of scalar fields with different masses, which can cooperate to drive
inflation according to the assisted inflation mechanism. We extend previous
work to include random initial conditions for the scalar fields, and explore
the predictions for density perturbations and the tensor-to-scalar ratio. The
tensor-to-scalar ratio depends only on the number of e-foldings and is
independent of the number of fields, their masses, and their initial
conditions. It therefore always has the same value as for a single massive
field. By contrast, the scalar spectral index has significant dependence on
model parameters. While normally multi-field inflation models make predictions
for observable quantities which depend also on the unknown field initial
conditions, we find evidence of a `thermodynamic' regime whereby the predicted
spectral index becomes independent of initial conditions if there are enough
fields. Only in parts of parameter space where the mass spectrum of the fields
is extremely densely packed is the model capable of satisfying the tight
observational constraints from WMAP3 observations.Comment: 6 pages RevTeX4, 4 figures included. Updated to match PRD accepted
version. Analysis and conclusions unchanged. New references, especially
astro-ph/0510441 which was first to give the general r=8/N resul
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