3,518 research outputs found
On the arcmin structure of the X-ray Universe
We present the angular correlation function of the X-ray population of 1063
XMM-Newton observations at high Galactic latitudes, comprising up to ~30000
sources over a sky area of ~125 sq. degrees in the energy bands: soft (0.5-2
keV) and hard (2-10 keV). This is the largest sample of serendipitous X-ray
sources ever used for clustering analysis purposes to date and the results have
been determined with unprecedented accuracy. We detect significant clustering
signals in the soft and hard bands (~10 sigma and ~5 sigma, respectively). We
deproject the angular correlation function via Limber's equation and calculate
the typical spatial lengths. We infer that AGN at redshifts ~1 are embedded in
dark matter halos with typical masses of log M ~ 12.6/h Msol and lifetimes in
the range ~3-5 x 10^8 years, which indicates that AGN activity is a transient
phase in the life of galaxies.Comment: 4 pages, 1 figure. Proc. of the conference "X-ray Astronomy 2009:
Present status, multiwavelength approach and future perspectives", September
2009, Bologna. To appear in AIP Conf. Proc. (editors: A. Comastri, M. Cappi,
L. Angelini)
The Holographic Life of the eta'
In the string holographic dual of large-N_c QCD with N_f flavours of
Kruczenski et al, the eta' meson is massless at infinite N_c and dual to a
collective fluctuation of N_f D6-brane probes in a supergravity background.
Here we identify the string diagrams responsible for the generation of a mass
of order N_f/N_c, consistent with the Witten-Veneziano formula, and show that
the supregravity limit of these diagrams corresponds to mixings with
pseudoscalar glueballs. We argue that the dependence on the theta-angle in the
supergravity description occurs only through the combination theta + 2
\sqrt{N_f} eta' / f_pi, as dictated by the U(1) anomaly. We provide a
quantitative test by computing the linear term in the eta' potential in two
independent ways, with perfect agreement.Comment: 1+26 pages, 8 figures; V4: Appendix added, version published in JHE
Protection of the Superconducting Corrector Magnets for the LHC
n the LHC about 6500 superconducting corrector magnets will be powered either in stand-alone mode or in electrical circuits of up to 154 magnets. Single corrector magnets are designed to be self-protected in case of a quench. The protection scheme of magnets powered in series depends on the energy stored in the magnet and on the number of magnets in the circuit. A quench is detected by measuring the resistive voltage of the circuit. The power converter is switched off, and for most circuits part of the energy is extracted with a resistor. Some magnets may require a resistor or possibly a diode parallel to the magnet in order to avoid overheating of the superconducting wire or an unacceptable voltage level. Experiments have been performed to understand quenching of prototype corrector magnets. In order to determine the adequate protection schemes for the magnet circuits the results have been used as input for simulations to extrapolate to the LHC conditions
Integrated design of superconducting accelerator magnets
This chapter introduces the main features of the ROXIE program which has been developed for the design of the superconducting magnets for the Large Hadron Collider (LHC) at CERN. The program combines numerical field calculation with a reduced vector-potential formulation, the application of vector-optimization methods, and the use of genetic as well as deterministic minimization algorithms. Together with the applied concept of features, the software is used as an approach towards integrated design of superconducting magnets. The main quadrupole magnet for the LHC, was chosen as an example for the integrated design process. (17 refs)
String Theory and Quantum Chromodynamics
I review recent progress on the connection between string theory and quantum
chromodynamics in the context of the gauge/gravity duality. Emphasis is placed
on conciseness and conceptual aspects rather than on technical details. Topics
covered include the large-Nc limit of gauge theories, the gravitational
description of gauge theory thermodynamics and hydrodynamics, and
confinement/deconfinement thermal phase transitions.Comment: 38 pages, 24 figures. Lectures given at the RTN Winter School on
"Strings, Supergravity and Gauge Theories" at CERN on January 15-19, 200
Role of QseG membrane protein in beneficial enterobacterial interactions with plants and Mesorhizobia
Homologs of qseG gene (coding for the membrane protein QseG), along with the qseEF genes, are present in many Enterobacteriaceae; however, its role in non-pathogenic strains is still unknown. To fill this knowledge gap, we investigated the role of QseG protein of a plant-associated enterobacterium in the interactions with its legume host and in the benefits induced by this enterobacterium in the Mesorhizobium–chickpea symbiosis. Here, we showed that QseG of Kosakonia sp. MH5 is involved in the following processes: (i) the evasion of the plant immune system and (ii) the efficient colonization of chickpea root cells. Furthermore, these features are essential for the beneficial effects of this strain on the Mesorhizobium–chickpea symbiosis. This study demonstrates that the role of QseG is transversal to pathogenic and non-pathogenic enterobacteria and is a step forward to better understanding the molecular bases of plant–bacteria interactions established between legume and beneficial endophytic enterobacteria.ME
Probes on D3-D7 Quark-Gluon Plasmas
We study the holographic dual model of quenched flavors immersed in a
quark-gluon plasma with massless dynamical quarks in the Veneziano limit. This
is modeled by embedding a probe D7 brane in a background where the backreaction
of massless D7 branes has been taken into account. The background, and hence
the effects, are perturbative in the Veneziano parameter N_f/N_c, therefore
giving small shifts of all magnitudes like the constituent mass, the quark
condensate, and several transport coefficients. We provide qualitative results
for the effect of flavor degrees of freedom on the probes. For example, the
meson melting temperature is enhanced, while the screening length is
diminished. The drag force is also enhanced.Comment: 31 pages, 17 figure
The properties of the Malin 1 galaxy giant disk: A panchromatic view from the NGVS and GUViCS surveys
Low surface brightness galaxies (LSBGs) represent a significant percentage of
local galaxies but their formation and evolution remain elusive. They may hold
crucial information for our understanding of many key issues (i.e., census of
baryonic and dark matter, star formation in the low density regime, mass
function). The most massive examples - the so called giant LSBGs - can be as
massive as the Milky Way, but with this mass being distributed in a much larger
disk. Malin 1 is an iconic giant LSBG, perhaps the largest disk galaxy known.
We attempt to bring new insights on its structure and evolution on the basis of
new images covering a wide range in wavelength. We have computed surface
brightness profiles (and average surface brightnesses in 16 regions of
interest), in six photometric bands (FUV, NUV, u, g, i, z). We compared these
data to various models, testing a variety of assumptions concerning the
formation and evolution of Malin 1. We find that the surface brightness and
color profiles can be reproduced by a long and quiet star-formation history due
to the low surface density; no significant event, such as a collision, is
necessary. Such quiet star formation across the giant disk is obtained in a
disk model calibrated for the Milky Way, but with an angular momentum
approximately 20 times larger. Signs of small variations of the star-formation
history are indicated by the diversity of ages found when different regions
within the galaxy are intercompared.For the first time, panchromatic images of
Malin 1 are used to constrain the stellar populations and the history of this
iconic example among giant LSBGs. Based on our model, the extreme disk of Malin
1 is found to have a long history of relatively low star formation (about 2
Msun/yr). Our model allows us to make predictions on its stellar mass and
metallicity.Comment: Accepted in Astronomy and Astrophysic
Supersymmetric black rings and three-charge supertubes
We present supergravity solutions for 1/8-supersymmetric black supertubes
with three charges and three dipoles. Their reduction to five dimensions yields
supersymmetric black rings with regular horizons and two independent angular
momenta. The general solution contains seven independent parameters and
provides the first example of non-uniqueness of supersymmetric black holes. In
ten dimensions, the solutions can be realized as D1-D5-P black supertubes. We
also present a worldvolume construction of a supertube that exhibits three
dipoles explicitly. This description allows an arbitrary cross-section but
captures only one of the angular momenta.Comment: 59 pages, 6 figures; v2: minor correction
Quarkonium dissociation by anisotropy
We compute the screening length for quarkonium mesons moving through an
anisotropic, strongly coupled N=4 super Yang-Mills plasma by means of its
gravity dual. We present the results for arbitrary velocities and orientations
of the mesons, as well as for arbitrary values of the anisotropy. The
anisotropic screening length can be larger or smaller than the isotropic one,
and this depends on whether the comparison is made at equal temperatures or at
equal entropy densities. For generic motion we find that: (i) mesons dissociate
above a certain critical value of the anisotropy, even at zero temperature;
(ii) there is a limiting velocity for mesons in the plasma, even at zero
temperature; (iii) in the ultra-relativistic limit the screening length scales
as with \epsilon =1/2, in contrast with the isotropic result
\epsilon =1/4.Comment: 39 pages, 26 figures; v2: minor changes, added reference
- …