2,194 research outputs found
On Black Attractors in 8D and Heterotic/Type IIA Duality
Motivated by the study of black attractors in 8D supergravity with 16
supersymmetries, we use the field theory approach and 8D supersymmetry with non
trivial central charges to shed light on the exact duality between heterotic
string on T^2 and type IIA on real connected and compact surfaces {\Sigma}2. We
investigate the two constraints that should be obeyed by {\Sigma}2 and give
their solutions in terms of intersecting 2-cycles as well their classification
using Dynkin diagrams of affine Kac-Moody algebras. It is shown as well that
the moduli space of these dual theories is given by
SO(1,1)x((SO(2,r+2))/(SO(2)xSO(r+2))) where r stands for the rank of the gauge
symmetry G_{r} of the 10D heterotic string on T^2. The remarkable cases
r=-2,-1,0 as well as other features are also investigated.Comment: LaTex, 18 pages, 2 figures, To appear in JHE
Ethephon as a potential abscission agent for table grapes: Effects on pre-harvest abscission, fruit quality, and residue
Some plant growth regulators, including ethephon, can stimulate abscission of mature grape berries. The stimulation of grape berry abscission reduces fruit detachment force (FDF) and promotes the development of a dry stem scar, both of which could facilitate the production of high quality stemless fresh-cut table grapes. The objective of this research was to determine how two potential abscission treatments, 1445 and 2890 mg/L ethephon, affected FDF, pre-harvest abscission, fruit quality, and ethephon residue of Thompson Seedless and Crimson Seedless grapes. Both ethephon treatments strongly induced abscission of Thompson Seedless berries causing >90% pre-harvest abscission. Lower ethephon rates, a shorter post-harvest interval, or berry retention systems such as nets, would be needed to prevent excessive pre-harvest losses. The treatments also slightly affected Thompson Seedless berry skin color, with treated fruit being darker, less uniform in color, and with a more yellow hue than non-treated fruit. Ethephon residues on Thompson Seedless grapes treated with the lower concentration of ethephon were below legal limits at harvest. Ethephon treatments also promoted abscission of Crimson Seedless berries, but pre-harvest abscission was much lower (≅49%) in Crimson Seedless compared to Thompson Seedless. Treated fruits were slightly darker than non-treated fruits, but ethephon did not affect SSC, acidity, or firmness of Crimson Seedless, and ethephon residues were below legal limits
Conformal Blocks for Arbitrary Spins in Two Dimensions
Conformal blocks for four point functions for fields with arbitrary spins in
two dimensions are obtained by evaluating an appropriate integral. The results
are just products of hypergeometric functions of the conformally invariant
cross ratios formed from the four complex coordinates. Results for scalars
previously obtained are a special case. Applications to four point functions
involving the energy momentum tensor are discussed.Comment: 7 pages, version 2 extra references and small changes, version 3
small corrections and extra material, version to be publishe
Seed conformal blocks in 4D CFT
We compute in closed analytical form the minimal set of \u201cseed\u201d conformal blocks associated to the exchange of generic mixed symmetry spinor/tensor operators in an arbitrary representation (\u2113, \u2113) of the Lorentz group in four dimensional conformal field theories. These blocks arise from 4-point functions involving two scalars, one (0, |\u2113 12 \u2113|) and one (|\u2113 12 \u2113|, 0) spinors or tensors. We directly solve the set of Casimir equations, that can elegantly be written in a compact form for any (\u2113, \u2113), by using an educated ansatz and reducing the problem to an algebraic linear system. Various details on the form of the ansatz have been deduced by using the so called shadow formalism. The complexity of the conformal blocks depends on the value of p = |\u2113 12 \u2113| and grows with p, in analogy to what happens to scalar conformal blocks in d even space-time dimensions as d increases. These results open the way to bootstrap 4-point functions involving arbitrary spinor/tensor operators in four dimensional conformal field theories
Operator Product Expansion of Inflationary Correlators and Conformal Symmetry of de Sitter
We study the multifield inflationary models where the cosmological
perturbation is sourced by light scalar fields other than the inflaton. The
corresponding perturbations are both scale invariant and special conformally
invariant. We exploit the operator product expansion technique of conformal
field theories to study the inflationary correlators enjoying the symmetries
present during the de Sitter epoch. The operator product expansion is
particularly powerful in characterizing inflationary correlation functions in
two observationally interesting limits, the squeezed limit of the three-point
correlator and the collapsed limit of the four-point correlator. Despite the
fact that the shape of the four-point correlators is not fixed by the
symmetries of de Sitter, its exact shape can be found in the collapsed limit
making use of the operator product expansion. By employing the fact that
conformal invariance imposes the two-point cross-correlations of the light
fields to vanish unless the fields have the same conformal weights, we are able
to show that the Suyama-Yamaguchi inequality relating the coefficients of the bispectrum in the squeezed limit and of the
trispectrum in the collapsed limit also holds when the light fields are
intrinsically non-Gaussian. In fact, we show that the inequality is valid
irrespectively of the conformal symmetry, being just a consequence of
fundamental physical principles, such as the short-distance expansion of
operator products. The observation of a strong violation of the inequality will
then have profound implications for inflationary models as it will imply either
that multifield inflation cannot be responsible for generating the observed
fluctuations independently of the details of the model or that some new
non-trivial degrees of freedom play a role during inflation.Comment: 47 pages 3 figure
First stars, very massive black holes and metals
Recent studies suggest that the initial mass function (IMF) of the first
stars was likely to be extremely top-heavy, unlike what is observed at present.
We propose a scenario to generate fragmentation to lower masses once the first
massive stars have formed and derive constraints on the primordial IMF. We
estimate the mass fraction of pair-unstable supernovae, shown to be the
dominant sources of the first heavy elements. These metals enrich the gas up to
about solar metallicity, when a transition to efficient
cooling-driven fragmentation occurs producing 1 solar mass clumps. We argue
that the remaining fraction of the first stars ends up in 100 solar mass VMBHs
(Very Massive Black Holes). We obtain constraints on the fraction of first
stars that contribute to the initial metal enrichment and the transition
redshift for primordial IMF away from a top-heavy one, by making various
assumptions about the fate of these VMBHs at late times. We conclude with a
discussion of several hitherto unexplored implications of a high-mass dominated
star formation mode in the early Universe.Comment: revised version, Fig. 4 modified, abbreviated abstract, matches
accepted version in ApJ, ApJ in pres
Classical behavior of deformed sine-Gordon models
In this work we deform the phi^4 model with distinct deformation functions,
to propose a diversity of sine-Gordon-like models. We investigate the proposed
models and we obtain all the topological solutions they engender. In
particular, we introduce non-polynomial potentials which support some exotic
two-kink solutions.Comment: 12 pages, 12 figures; version to appear in Physica
On the properties of massive Population III stars and metal-free stellar populations
We present realistic models for massive Population III stars and stellar
populations based on non-LTE model atmospheres, recent stellar evolution tracks
and up-to-date evolutionary synthesis models, to study their spectral
properties, including their dependence on age, star formation history, and IMF.
(..) The main results regarding integrated stellar populations are: * For young
bursts and the case of a constant SFR, nebular continuous emission - neglected
in previous studies - dominates the spectrum redward of Lyman-alpha (...).
Therefore predicted emission line equivalent widths are considerably smaller
than found in earlier studies, whereas the detection of the continuum is eased.
Nebular line and continuous emission strongly affect the broad band photometric
properties of Pop III objects. * Due to stellar evolution, the hardness of the
ionising spectrum decreases rapidly, leading to the disappearance of the
characteristic HeII 1640 recombination lines after ~ 3 Myr in instantaneous
bursts. * The relative efficiency of ionising photon energy to heavy element
rest mass production, eta, of metal-poor and metal-free populations is
increased by factors of ~ 4 to 18 with respect to solar metallicity and for
``standard'' IMFs. * The lowest values of eta ~ 1.6 - 2.2 % are obtained for
IMFs exclusively populated with high mass stars (M_low >~ 50 Msun). If correct,
the yields dominated by pair creation SNae then predict large overabundances of
O/C and Si/C compared to solar abundance ratios. Detailed results are given in
tabular form and as fit formulae for implementation in other calculations.
(abridged abstract)Comment: Accepted for publication in A&A. LaTeX, 15 pages including 8 figures.
Related models are available at http://webast.ast.obs-mip.fr/people/schaerer/
. SEDs will be available by request from the author
(mailto:[email protected]). Y-axis of Figs. 5 correcte
On Black Hole Effective Potential in 6D/7D N=2 Supergravity
Using the harmonic superspace method and the duality between real and complex
representations of hypermultiplets, we compute the explicit scalar field
expression of the quaternionic metric of the moduli space
SO(1,1)x[SO(4,k)/(SO(4)xSO(k))] of 6D N=2 supergravity with generic k Maxwell
supermultiplets. The obtained metric includes the particular case k=20
associated with 10D type IIA superstring on K3. Uplifting to 7D N=2
supergravity is described and aspects of 6D/7D black attractor effective
potentials are studied.Comment: 49 pages, To appear in NP
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