239 research outputs found
O(a) Perturbative improvement for Wilson fermions
The coefficient of the O(a)-improved
Sheikholeslami-Wohlert action for Wilson fermions are perturbatively
determined at one-loop level and estimated at two-loop level.Comment: 8-pages + (two pages of Fig.s) MPI-ph/93-2
Monopole condensation in the ground state of gauge theories: a disorder parameter
We construct a disorder parameter for dual superconductivity of the ground
state of gauge theory.Comment: 3 pages, contribution to the Lattice '94 conference, shell archive
containing uuencoded LATEX file + 2 figure
Universality class of the mode-locked glassy random laser
By means of enhanced Monte Carlo numerical simulations parallelized on GPU's
we study the critical properties of the spin-glass-like model for the
mode-locked glassy random laser, a -spin model with complex spins with a
global spherical constraint and quenched random interactions. Using two
different boundary conditions for the mode frequencies we identify the critical
points and the critical indices of the random lasing phase transition using ,
with finite size scaling techniques. The outcome of the scaling analysis is
that the mode-locked random laser is in a mean-field universality class, though
different from the mean-field class of the Random Energy Model and the glassy
random laser in the narrow band approximation, that is, the fully connected
version of the present model. The low temperature (high pumping) phase is
finally characterized by means of the overlap distribution and evidence for the
onset of replica symmetry breaking in the lasing regime is provided.Comment: 15 pages, 11 figure
Dual superconductivity and vacuum properties in Yang--Mills theories
We address, within the dual superconductivity model for color confinement,
the question whether the Yang-Mills vacuum behaves as a superconductor of type
I or type II. In order to do that we compare, for the theory with gauge group
SU(2), the determination of the field penetration depth with that of
the superconductor correlation length . The latter is obtained by
measuring the temporal correlator of a disorder parameter developed by the Pisa
group to detect dual superconductivity. The comparison places the vacuum close
to the border between type I and type II and marginally on the type II side. We
also check our results against the study of directly measurable effects such as
the interaction between two parallel flux tubes, obtaining consistent
indications for a weak repulsive behaviour. Future strategies to improve our
investigation are discussed.Comment: 23 pages, 15 figures. Simulations on finer lattices and with
different monopole charges added. Final version to be published in Nuclear
Physics
RENORMALONS FROM EIGHT LOOP EXPANSION OF THE GLUON CONDENSATE IN LATTICE GAUGE THEORY,
We use a numerical method to obtain the weak coupling perturbative
coefficients of local operators with lattice regularization. Such a method
allows us to extend the perturbative expansions obtained so far by analytical
Feynman diagrams calculations. In SU(3) lattice gauge theory in four dimensions
we compute the first eight coefficients of the expectation value of the Wilson
loop on the elementary plaquette which is related to the gluon condensate. The
computed eight coefficients grow with the order much faster than predicted by
the presence of the infrared renormalon associated to the dimension of the
gluon condensate. However the renormalon behaviour for large order is quite
well reproduced if one considers the expansion coefficients in a new coupling
related to the lattice coupling by large perturbative corrections. This is
expected since the lattice and continuum Lambda scales differ by almost two
orders of magnitude.Comment: 11 pages, LaTe
Numerical determination of the exponents controlling the relationship between time, length and temperature in glass-forming liquids
There is a certain consensus that the very fast growth of the relaxation time
occurring in glass-forming liquids on lowering the temperature must be
due to the thermally activated rearrangement of correlated regions of growing
size. Even though measuring the size of these regions has defied scientists for
a while, there is indeed recent evidence of a growing correlation length
in glass-formers. If we use Arrhenius law and make the mild assumption that the
free-energy barrier to rearrangement scales as some power of the size of
the correlated regions, we obtain a relationship between time and length,
. According to both the Adam-Gibbs and the Random
First Order theory the correlation length grows as , even though the two theories disagree on the value of
. Therefore, the super-Arrhenius growth of the relaxation time with the
temperature is regulated by the two exponents and through the
relationship . Despite a few
theoretical speculations, up to now there has been no experimental
determination of these two exponents. Here we measure them numerically in a
model glass-former, finding and . Surprisingly, even though
the values we found disagree with most previous theoretical suggestions, they
give back the well-known VFT law for the relaxation time, .Comment: 9 pages, 8 figure
Two Loop Computation of a Running Coupling in Lattice Yang-Mills Theory
We compute the two loop coefficient in the relation between the lattice bare
coupling and the running coupling defined through the Schroedinger functional
for the case of pure SU(2) gauge theory. This result is needed as one
computational component to relate the latter to the MSbar-coupling, and it
allows us to implement O(a) improvement of the Schroedinger functional to
two-loop order. In addition, the two-loop beta-function is verified in a
perturbative computation on the lattice, and the behavior of an improved bare
coupling is investigated beyond one loop.Comment: 26 pages, uuencoded compressed tar file, new: acknowledgement adde
Yellow laser performance of Dy in co-doped Dy,Tb:LiLuF
We present laser results obtained from a Dy-Tb co-doped
LiLuF crystal, pumped by a blue emitting InGaN laser diode, aiming for
the generation of a compact 578 nm source. We exploit the yellow Dy
transition F H to generate
yellow laser emission. The lifetime of the lower laser level is quenched via
energy transfer to co-doped Tb ions in the fluoride crystal. We report
the growth technique, spectroscopic study and room temperature continuous wave
(cw) laser results in a hemispherical cavity at 574 nm and with a highly
reflective output coupler at 578 nm. A yellow laser at 578 nm is very relevant
for metrological applications, in particular for pumping of the forbidden
SP Ytterbium clock transition, which is
recommended as a secondary representation of the second in the international
system (SI) of units. This paper was published in Optics Letters and is made
available as an electronic reprint with the permission of OSA. The paper can be
found at the following URL on the OSA website:
http://dx.doi.org/10.1364/OL.39.006628. Systematic or multiple reproduction or
distribution to multiple locations via electronic or other means is prohibited
and is subject to penalties under law.Comment: 8 pages, 5 figure
Proximal and distal control for ligand binding in neuroglobin: role of the CD loop and evidence for His64 gating
Neuroglobin (Ngb) is predominantly expressed in neurons of the central and peripheral nervous systems and it clearly seems to be involved in neuroprotection. Engineering Ngb to observe structural and dynamic alterations associated with perturbation in ligand binding might reveal important structural determinants, and could shed light on key features related to its mechanism of action. Our results highlight the relevance of the CD loop and of Phe106 as distal and proximal controls involved in ligand binding in murine neuroglobin. We observed the effects of individual and combined mutations of the CD loop and Phe106 that conferred to Ngb higher CO binding velocities, which we correlate with the following structural observations: the mutant F106A shows, upon CO binding, a reduced heme sliding hindrance, with the heme present in a peculiar double conformation, whereas in the CD loop mutant "Gly-loop", the original network of interactions between the loop and the heme was abolished, enhancing binding via facilitated gating out of the distal His64. Finally, the double mutant, combining both mutations, showed a synergistic effect on CO binding rates. Resonance Raman spectroscopy and MD simulations support our findings on structural dynamics and heme interactions in wild type and mutated Ngbs
A cohort study on acute ocular motility disorders in pediatric emergency department
Background: Acute ocular motility disorders (OMDs) in children admitted to Emergency Department (ED) represents a not so rare condition with a wide spectrum of different etiologies. The emergency physician must be skilled in rapidly identifying patients with potentially life threatening (LT) forms, requiring further diagnostic procedures. The aim of the study was to assess characteristics of children with acute Ocular Motility Disorders (OMDs), and to identify "red flags" for recognition of underlying life-threatening (LT) conditions. Methods: A retrospective cohort study evaluated children (2 months-17 years) admitted to a tertiary Emergency Department in 2009-2014. A subgroup analysis was performed comparing children with and without LT conditions. Results: Of 192 visits for OMDs, the isolated strabismus occurred most frequently (55.6%), followed by pupil disorders (31.8%), ptosis (5.2%) and combined OMDs (11.5%). The majority of acute OMDs involved no underlying LT conditions (n = 136) and most of them were infants or toddlers (50%). In a multivariable analysis, LT conditions included especially children over 6 years of age, increasing the odds ratio by 2% for each months of age (p = 0.009). LT etiologies were 16 times more likely in combined OMDs (p = 0.018), were over 13 times more likely to report associated extra-ocular signs/symptoms (p = 0.017) and over 50 times more likely to report co-morbidity (p = 0.017). Conclusion: OMDs are not an uncommon presentation at ED. Although most of them involve non-LT conditions, the ED physician should consider potential "red flags" for appropriate management of children such as age > 6 years, combined OMDs, extra-ocular symptoms and co-morbidity
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