4,934 research outputs found
The Dirac operator spectrum: a perturbative approach
By computing the Dirac operator spectrum by means of Numerical Stochastic
Perturbation Theory, we aim at throwing some light on the widely accepted
picture for the mechanism which is behind the Bank-Casher relation. The latter
relates the chiral condensate to an accumulation of eigenvalues in the low end
of the spectrum. This can be in turn ascribed to the usual mechanism of
repulsion among eigenvalues which is typical of quantum interactions. First
results appear to confirm that NSPT can indeed enable us to inspect a huge
reshuffling of eigenvalues due to quantum repulsion.Comment: 8 pages, 6 figures; talk presented at the 27th International
Symposium on Lattice Field Theory (Lattice 2009), Beijing, China, 26-31 Jul
200
High loop renormalization constants for Wilson fermions/Symanzik improved gauge action
We present the current status of our computation of quark bilinear
renormalization constants for Wilson fermions and Symanzik improved gauge
action. Computations are performed in Numerical Stochastic Perturbation Theory.
Volumes range from 10^4 to 32^4. Renormalization conditions are those of the
RI'-MOM scheme, imposed at different values of the physical scale. Having
measurements available at several momenta, irrelevant effects are taken into
account by means of hypercubic symmetric Taylor expansions. Finite volumes
effects are assessed repeating the computations at different lattice sizes. In
this way we can extrapolate our results to the continuum limit, in infinite
volume.Comment: 8 pages, 3 figures, talk presented at the 27th International
Symposium on Lattice Field Theory (Lattice 2009), Beijing, China, 26-31 Jul
200
High-loop perturbative renormalization constants for Lattice QCD (III): three-loop quark currents for Iwasaki gauge action and n_f=4 Wilson fermions
This is the third of a series of papers on three-loop computation of
renormalization constants for Lattice QCD. Our main point of interest are
results for the regularization defined by Iwasaki gauge action and n_f=4 Wilson
fermions. Our results for quark bilinears renormalized according to the RI'-MOM
scheme can be compared to non-perturbative results. The latter are available
for Twisted Mass QCD: being defined in the chiral limit, renormalization
constants must be the same. We also address more general problems. In
particular, we discuss a few methodological issues connected to summing the
perturbative series such as the effectiveness of Boosted Perturbation Theory
and the disentanglement of irrelevant and finite volume contributions.
Discussing these issues we consider ont only the new results of this paper, but
also those for the regularization defined by tree-level Symanzik improved gauge
action and n_f=2 Wilson fermions, which we presented in a recent paper of ours.
We finally comment to which extent the techniques we put at work in the NSPT
context can provide a fresher look into the lattice version of the RI'-MOM
scheme.Comment: 20 pages, 4 figures, pdflatex The Section on different ways of
summing the series has been updated: a few extra informations have been
provided and a clearer notation has been introduce
SEM characterization of mm-long nanowires
The fabrication of optical fibre nanowires has recently attracted much attention [1-5]. Nanowires longer than 110mm [2] and with diameters smaller than 20 nm [5] have now been fabricated using a top-down approach. Because of the extraordinarily large ratio between length and diameter (>100000), the characterization of optical fibre nanowires requires instrumentation capable of measuring lengths over a range spanning more than five orders of magnitude. In our experiments dimensional characterization along the nanowire has been performed using an SEM and calibrated references. The samples are first attached to conductive carbon pads to avoid electrostatic build-up. Charging makes accurate metrology difficult because the electron beam can be deflected by the induced electric field on the sample. Fig. 1 illustrates a nanowire with a radius r=30nm wrapped around a microfibre with r=2”m. Variations in radius of another nanowire along its length are shown in fig. 2
Novel applications of fibre tapers
In this talk we will discuss new applications for fibre tapers ranging from femtosecond pulse manipulation to optical clock generation in micro-coil resonators. Fundamental to these interactions is the enhanced nonlinearity arising from the smaller cores making compact optical devices possible
Highly nonlinear dynamics in a slowly sedimenting colloidal gel
We use a combination of original light scattering techniques and particles
with unique optical properties to investigate the behavior of suspensions of
attractive colloids under gravitational stress, following over time the
concentration profile, the velocity profile, and the microscopic dynamics.
During the compression regime, the sedimentation velocity grows nearly linearly
with height, implying that the gel settling may be fully described by a
(time-dependent) strain rate. We find that the microscopic dynamics exhibit
remarkable scaling properties when time is normalized by strain rate, showing
that the gel microscopic restructuring is dominated by its macroscopic
deformation.Comment: Physical Review Letters (2011) xxx
Bethe--Salpeter equation in QCD
We extend to regular QCD the derivation of a confining
Bethe--Salpeter equation previously given for the simplest model of scalar QCD
in which quarks are treated as spinless particles. We start from the same
assumptions on the Wilson loop integral already adopted in the derivation of a
semirelativistic heavy quark potential. We show that, by standard
approximations, an effective meson squared mass operator can be obtained from
our BS kernel and that, from this, by expansion the
corresponding Wilson loop potential can be reobtained, spin--dependent and
velocity--dependent terms included. We also show that, on the contrary,
neglecting spin--dependent terms, relativistic flux tube model is reproduced.Comment: 23 pages, revte
Experimental evidence of high-resolution ghost imaging and ghost diffraction with classical thermal light
High-resolution ghost image and ghost diffraction experiments are performed
by using a single source of thermal-like speckle light divided by a beam
splitter. Passing from the image to the diffraction result solely relies on
changing the optical setup in the reference arm, while leaving untouched the
object arm. The product of spatial resolutions of the ghost image and ghost
diffraction experiments is shown to overcome a limit which was formerly thought
to be achievable only with entangled photons.Comment: 5 pages, 4 figure
Long-term results of iliac aneurysm repair with iliac branched endograft. A 5-year experience on 100 consecutive cases
Background: Iliac branch device (IBD) technique has been introduced as an appealing and effective solution to avoid complications occurring during repair of aorto-iliac aneurysm with extensive iliac involvement. Nevertheless, no large series with long-term follow-up of IBD are available. The aim of this study was to analyse safety and long-term efficacy of IBD in a consecutive series of patients.Methods: Between 2006 and 2011, 100 consecutive patients were enrolled in a prospective database on IBD. Indications included unilateral or bilateral common iliac artery aneurysms combined or not with abdominal aneurysms. Patients were routinely followed up with computed tomography. Data were reported according to the Kaplan-Meier method.Results: There were 96 males, mean age 74.1 years. Preoperative median common iliac aneurysm diameter was 40 mm (interquartile range (IQR): 35-44 mm). Sixty-seven patients had abdominal aortic aneurysm >35 mm (IQR: 40-57 mm) associated with iliac aneurysm. Eleven patients presented hypogastric aneurysm. Twelve patients underwent isolated iliac repair with IBD and 88 patients received associated endovascular aortic repair. Periprocedural technical success rate was 95%, with no mortality. Two patients experienced external iliac occlusion in the first month. At a median follow-up of 21 months (range 1-60) aneurysm growth >3 mm was detected in four iliac (4%) arteries. Iliac endoleak (one type III and two distal type I) developed in three patients and buttock claudication in four patients. Estimated patency rate of internal iliac branch was 91.4% at 1 and 5 years. Freedom from any reintervention rate was 90% at 1 year and 81.4% at 5 years. No late ruptures occurred.Conclusions: Long-term results show that IBD use can ensure persistent iliac aneurysm exclusion at 5 years, with low risk of reintervention. This technique can be considered as a first endovascular option in patients with extensive iliac aneurysm disease and favourable anatomy. (C) 2011 European Society for Vascular Surgery. Published by Elsevier Ltd. All rights reserved
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