11,873 research outputs found
N=6 Supergravity on and the Superconformal Correspondence
It is argued that N=6 supergravity on , with gauge group corresponds, at the classical level, to a subsector of the ``chiral''
primary operators of N=4 Yang-Mills theories. This projection involves a
``duality transformation'' of N=4 Yang-Mills theory and therefore can be valid
if the coupling is at a self-dual point, or for those amplitudes that do not
depend on the coupling constant.Comment: 9 pages, late
First Stars Contribution to the Near Infrared Background Fluctuations
We show that the emission from the first, metal-free stars inside Population
III objects (PopIIIs) are needed to explain the level of fluctuations in the
Near Infrared Background (NIRB) recently discovered by Kashlinsky et al.
(2002), at least at the shortest wavelengths. Clustering of (unresolved) Pop
IIIs can in fact account for the entire signal at almost all the ~1-30 arcsec
scales probed by observations in the J band. Their contribution fades away at
shorter frequencies and becomes negligible in the K band. ``Normal'', highly
clustered, ~3 galaxies undergoing intense star-formation such as those found
in the Hubble Deep Fields can 'fill in' this gap and provide for the missing
signal. It is in fact found that their contribution to the intensity
fluctuations is the dominant one at lambda=2.17 mum, while it gradually looses
importance in the H and J bands. The joint contribution from these two
populations of cosmic objects is able, within the errors, to reproduce the
observed power spectrum in the whole Near Infrared range on small angular
scales (theta < 200" for Pop III protogalaxies). Signals on larger scales
detected by other experiments instead require the presence of more local
sources.Comment: 6 pages, 4 figures, submitted to MNRA
Background radiation from sterile neutrino decay and reionization
Sterile neutrinos are one of the most promising Warm Dark Matter candidates.
By considering their radiative- and pion-decay channels, we derive the allowed
contribution of sterile neutrinos to the X-ray, optical and near-infrared
cosmic backgrounds. The X-ray background puts a strong constraint on the mass
of radiatively decaying neutrinos (m <= 14 keV), whereas the allowed mass range
for pion-decay neutrinos (for a particle lifetime > 4 X 10^17 s) is 150 <=
m/MeV <= 500. Taking into account these constraints, we find that sterile
neutrinos do not significantly contribute to the optical and near-infrared
background. We further consider the impact of sterile neutrinos on
reionization. We find that the Thomson optical depth due to sterile neutrinos
is tau_e = (0.4-3) X 10^-2 in the case of radiative decays, and it is ~10^-3
for the pion-decay channel. We conclude that these particles must have played
only a minor role in cosmic reionization history.Comment: 13 pages, 9 figures, replaced with revised version, accepted for
publication in MNRA
Scale invariant Volkov-Akulov Supergravity
A scale invariant Goldstino theory coupled to Supergravity is obtained as a
standard supergravity dual of a rigidly scale invariant higher--curvature
Supergravity with a nilpotent chiral scalar curvature. The bosonic part of this
theory describes a massless scalaron and a massive axion in a de Sitter
Universe.Comment: CERN-PH-TH-2015-193. Misprints corrected. Final version to appear in
Phys. Lett.
Extragalactic Background Light: new constraints from the study of the photon-photon absorption on blazar spectra
The study of the Extragalactic Background Light (EBL) is crucial to
understand many astrophysical problems (as the formation of first stars, the
evolution of galaxies and the role of dust emission). At present, one of the
most powerful ways to put constraints on EBL is represented by the study of the
photon-photon absorption on gamma-ray spectra of TeV blazars. Adopting this
method, we found that, if the only contribution to the optical and Near
Infrared (NIR) background is given by galaxies, the spectrum of the blazar
H1426+428 cannot be fitted. To reproduce the observational data of H1426+428 a
Near Infrared excess with respect to galaxy counts is required, with amplitude
consistent with both the Matsumoto et al. (2000) data with Kelsall's model of
zodiacal light (ZL) subtraction and the DIRBE data with Wright's model of ZL
subtraction. The derived constraints on the optical EBL are weaker, because the
experimental errors on blazar data are still bigger than the differences among
various optical EBL models. In the mid-infrared the SPITZER measurement at 24
micron provides the best fit of the blazar spectrum.Comment: 8 pages, 5 figures, to appear in Proceedings of "Baryons in Dark
Matter Halos", 5-9 October 2004, Novigrad, Croati
Flat Symplectic Bundles of N-Extended Supergravities, Central Charges and Black-Hole Entropy
In these lectures we give a geometrical formulation of N-extended
supergravities which generalizes N=2 special geometry of N=2 theories. In all
these theories duality symmetries are related to the notion of "flat symplectic
bundles" and central charges may be defined as "sections" over these bundles.
Attractor points giving rise to "fixed scalars" of the horizon geometry and
Bekenstein-Hawking entropy formula for extremal black-holes are discussed in
some details.Comment: Based on lectures given by S. Ferrara at the 5th Winter School on
Mathematical Physics held at the Asia Pacific Center for Theoretical Physics,
Seul (Korea), February 199
On the Embedding of Space-Time Symmetries into Simple Superalgebras
We explore the embedding of Spin groups of arbitrary dimension and signature
into simple superalgebras in the case of extended supersymmetry. The
R-symmetry, which generically is not compact, can be chosen compact for all the
cases that are congruent mod 8 to the physical conformal algebra so(,2),
. An grading of the superalgebra is found in all cases.
Central extensions of super translation algebras are studied in this framework.Comment: AMS LaTeX, 16 page
Generalized dimensional reduction of supergravity with eight supercharges
We describe some recent investigation about the structure of generic D=4,5
theories obtained by generalized dimensional reduction of D=5,6 theories with
eight supercharges. We relate the Scherk-Schwarz reduction to a special class
of N=2 no-scale gauged supergravities.Comment: Contribution to the proceedings of ``NathFest'' at PASCOS conference,
Northeastern University, Boston, Ma, August 200
Constraining DM through 21 cm observations
Beyond reionization epoch cosmic hydrogen is neutral and can be directly
observed through its 21 cm line signal. If dark matter (DM) decays or
annihilates the corresponding energy input affects the hydrogen kinetic
temperature and ionized fraction, and contributes to the Ly_alpha background.
The changes induced by these processes on the 21 cm signal can then be used to
constrain the proposed DM candidates, among which we select the three most
popular ones: (i) 25-keV decaying sterile neutrinos, (ii) 10-MeV decaying light
dark matter (LDM) and (iii) 10-MeV annihilating LDM. Although we find that the
DM effects are considerably smaller than found by previous studies (due to a
more physical description of the energy transfer from DM to the gas), we
conclude that combined observations of the 21 cm background and of its gradient
should be able to put constrains at least on LDM candidates. In fact, LDM
decays (annihilations) induce differential brightness temperature variations
with respect to the non decaying/annihilating DM case up to Delta_delta T_b=8
(22) mK at about 50 (15) MHz. In principle this signal could be detected both
by current single dish radio telescopes and future facilities as LOFAR;
however, this assumes that ionospheric, interference and foreground issues can
be properly taken care of.Comment: 9 pages, submitted to MNRA
Non-BPS Attractors in 5d and 6d Extended Supergravity
We connect the attractor equations of a certain class of N=2, d=5
supergravities with their (1,0), d=6 counterparts, by relating the moduli space
of non-BPS d=5 black hole/black string attractors to the moduli space of
extremal dyonic black string d=6 non-BPS attractors. For d = 5 real special
symmetric spaces and for N = 4,6,8 theories, we explicitly compute the flat
directions of the black object potential corresponding to vanishing eigenvalues
of its Hessian matrix. In the case N = 4, we study the relation to the (2,0),
d=6 theory. We finally describe the embedding of the N=2, d=5 magic models in
N=8, d=5 supergravity as well as the interconnection among the corresponding
charge orbits.Comment: 1+27 page
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