5,031 research outputs found
Three-dimensional super Yang-Mills with unquenched flavor
We construct analytically the gravity duals of three-dimensional, super
Yang-Mills-type theories with supersymmetry coupled to
quark flavors. The backreaction of the quarks on the color degrees of freedom
is included, and corresponds on the gravity side to the backreaction of
D6-branes on the background of D2-branes. The D6-branes are smeared over
the compact part of the geometry, which must be a six-dimensional nearly
K\"ahler manifold in order to preserve supersymmetry. For massless quarks, the
solutions flow in the IR to an fixed point dual to a
Chern-Simons-matter theory. For light quarks the theories exhibit
quasi-conformal dynamics (walking) at energy scales , with the 't Hooft coupling.Comment: 21 pages, 1 figure. v2 Minor editing, refs. added. Tallies with
published versio
X-ray sources as tracers of the large-scale structure in the Universe
We review the current status of studies of large-scale structure in the X-ray
Universe. After motivating the use X-rays for cosmological purposes, we discuss
the various approaches used on different angular scales including X-ray
background multipoles, cross-correlations of the X-ray background with galaxy
catalogues, clustering of X-ray selected sources and small-scale fluctuations
and anisotropies in the X-ray background. We discuss the implications of the
above studies for the bias parameter of X-ray sources, which is likely to be
moderate for X-ray selected AGN and the X-ray background (~1-2). We finally
outline how all-sky X-ray maps at hard X-rays and medium surveys with large sky
coverage could provide important tests for the cosmological models.Comment: Invited review presented at the Workshop X-ray Astronomy'99: Stellar
endpoints, AGN and the diffuse X-ray background (Astrophys Lett and Comm
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
A ClpB Chaperone Knockout Mutant of Mesorhizobium ciceri Shows a Delay in the Root Nodulation of Chickpea Plants
Several molecular chaperones are known to be involved in bacteria stress response. To investigate the role of chaperone ClpB in rhizobia stress tolerance as well as in the rhizobiaplant symbiosis process, the clpB gene from a chickpea microsymbiont, strain Mesorhizobium ciceri LMS-1, was identified and a knockout mutant was obtained. The ClpB knockout mutant was tested to several abiotic stresses, showing that it was unable to grow after a heat shock and it was more sensitive to acid shock than the wild-type strain. A plant-growth assay performed to evaluate the symbiotic performance of the clpB mutant showed a higher proportion of ineffective root nodules obtained with the mutant than with the wild-type strain. Nodulation kinetics analysis showed a 6- to 8-day delay in nodule appearance in plants inoculated with the Delta clpB mutant. Analysis of nodC gene expression showed lower levels of transcript in the Delta clpB mutant strain. Analysis of histological sections of nodules formed by the clpB mutant showed that most of the nodules presented a low number of bacteroids. No differences in the root infection abilities of green fluorescent protein tagged clpB mutant and wild-type strains were detected. To our knowledge, this is the first study that presents evidence of the involvement of the chaperone ClpB from rhizobia in the symbiotic nodulation process
Quench Heater Studies for the LHC Magnets
About 2000 LHC (CERN's Large Hadron Collider) superconducting magnets will be protected with quench heaters against development of excessive voltage and overheating after a resistive transition. The quench heater strips are powered by capacitor bank discharge power supplies. The strips are made of stainless steel partially plated with copper to reduce their resistance and to allow for the connection of quench heaters in series. The strips are embedded in between two polyimide foils. The initial power density and the current decay time determine the quench heater effectiveness. Since only one type of heater power supply will be available, the copper plating cycle is adapted for the various magnet types to keep the resistance of the heater circuit constant. Different quench heater designs have been tested on various prototype magnets to optimise the copper-plating cycle and the electric insulation of the heater strip. This paper summarises the experimental results and computations that allowed to finalise the heater strip layout for all LHC magnets
Risk factors for incidence and persistence of disability in chronic major depression and alcohol use disorders: longitudinal analyses of a population-based study
BackgroundMajor depression and alcohol use disorders are risk factors for incidence of disability. However, it is still unclear whether a chronic course of these health conditions is also prospectively associated with incidence of disability. The aim of the present study was, first, to confirm whether chronic major depression (MD) and alcohol use disorders (AUD) are, respectively, risk factors for persistence and incidence of disability in the general population; and then to analyze the role of help-seeking behavior in the course of disability among respondents with chronic MD and chronic AUD. MethodData from two assessments in the National Epidemiologic Survey on Alcohol and Related Conditions were analyzed. Disability was measured by eight domains of the Short Form 12 Health Survey version 2 (SF-12). Generalized estimating equations and logistic regression models were run to estimate risk factors for persistence and incidence of disability, respectively. ResultsAnalyses conducted on data from the US general population showed that chronic MD was the strongest risk factor for incidence and persistence of disability in the social functioning, emotional role and mental health domains. Chronic AUD were risk factors for incidence and persistence of disability in the vitality, social functioning, and emotional role domains. Within the group of chronic MD, physical comorbidity and help-seeking were associated with persistent disability in most of the SF-12 domains. Help-seeking behavior was also associated with incidence of problems in the mental health domain for the depression group. Regarding the AUD group, comorbidity with physical health problems was a strong risk factor for persistence of disability in all SF-12 domains. Help-seeking behavior was not related to either persistence or incidence of disability in the chronic alcohol group. ConclusionsChronic MD and chronic AUD are independent risk factors for persistence and incidence of disability in the US general population. People with chronic MD seek help for their problems when they experience persistent disability, whereas people with chronic AUD might not seek any help even if they are suffering from persistent disability.<br/
- …