3,362 research outputs found
The role of dissipation in biasing the vacuum selection in quantum field theory at finite temperature
We study the symmetry breaking pattern of an O(4) symmetric model of scalar
fields, with both charged and neutral fields, interacting with a photon bath.
Nagasawa and Brandenberger argued that in favourable circumstances the vacuum
manifold would be reduced from S^3 to S^1. Here it is shown that a selective
condensation of the neutral fields, that are not directly coupled to photons,
can be achieved in the presence of a minimal ``external'' dissipation, i.e. not
related to interactions with a bath. This should be relevant in the early
universe or in heavy-ion collisions where dissipation occurs due to expansion.Comment: Final version to appear in Phys. Rev. D, 2 figures added, 2 new
sub-section
Estimate of convection-diffusion coefficients from modulated perturbative experiments as an inverse problem
The estimate of coefficients of the Convection-Diffusion Equation (CDE) from
experimental measurements belongs in the category of inverse problems, which
are known to come with issues of ill-conditioning or singularity. Here we
concentrate on a particular class that can be reduced to a linear algebraic
problem, with explicit solution. Ill-conditioning of the problem corresponds to
the vanishing of one eigenvalue of the matrix to be inverted. The comparison
with algorithms based upon matching experimental data against numerical
integration of the CDE sheds light on the accuracy of the parameter estimation
procedures, and suggests a path for a more precise assessment of the profiles
and of the related uncertainty. Several instances of the implementation of the
algorithm to real data are presented.Comment: Extended version of an invited talk presented at the 2012 EPS
Conference. To appear in Plasma Physics and Controlled Fusio
Heavy Quark Parameters and Vcb from Spectral Moments in Semileptonic B Decays
We extract the heavy quark masses and non-perturbative parameters from the
Delphi preliminary measurements of the first three moments of the charged
lepton energy and hadronic mass distributions in semileptonic B decays, using a
multi-parameter fit. We adopt two formalisms, one of which does not rely on a
1/mc expansion and makes use of running quark masses. The data are consistent
and the level of accuracy of the experimental inputs largely determines the
present sensitivity. The results allow to improve on the uncertainty in the
extraction of Vcb.Comment: 13 pages, 2 figure
Kinematic Constraints on Formation of Bound States of Cosmic Strings - Field Theoretical Approach
Superstring theory predicts the potential formation of string networks with
bound states ending in junctions. Kinematic constraints for junction formation
have been derived within the Nambu-Goto thin string approximation. Here we test
these constraints numerically in the framework of the Abelian-Higgs model in
the Type-I regime and report on good agreement with the analytical predictions.
We also demonstrate that strings can effectively pass through each other when
they meet at speeds slightly above the critical velocity permitting bound state
formation. This is due to reconnection effects that are beyond the scope of the
Nambu-Goto approximation.Comment: 6 pages, 12 eps figures - matches the published versio
Numerical Investigations of Oscillons in 2 Dimensions
Oscillons, extremely long-living localized oscillations of a scalar field,
are studied in theories with quartic and sine-Gordon potentials in two spatial
dimensions. We present qualitative results concentrating largely on a study in
frequency space via Fourier analysis of oscillations. Oscillations take place
at a fundamental frequency just below the threshold for the production of
radiation, with exponentially suppressed harmonics. The time evolution of the
oscillation frequency points indirectly to a life time of at least 10 million
oscillations. We study also elliptical perturbations of the oscillon, which are
shown to decay. We finish by presenting results for boosted and collided
oscillons, which point to a surprising persistence and soliton-like behaviour.Comment: Matches the published version (12 pages, 34 figures
Influence of High-Productivity Process Parameters on the Surface Quality and Residual Stress State of AISI 316L Components Produced by Directed Energy Deposition
The production of large components is one of the most powerful applications of laser powder-directed energy deposition (LP-DED) processes. High productivity could be achieved, when focusing on industrial applications, by selecting the proper process parameters. However, it is of crucial importance to understand the strategies that are necessary to increase productivity while maintaining the overall part quality and minimizing the need for post-processing. In this paper, an analysis of the dimensional deviations, surface roughness and subsurface residual stresses of samples produced by LP-DED is described as a function of the applied energy input. The aim of this work is to analyze the effects of high-productivity process parameters on the surface quality and the mechanical characteristics of the samples. The obtained results show that the analyzed process parameters affect the dimensional deviations and the residual stresses, but have a very little influence on surface roughness, which is instead dominated by the presence of unmelted particles
EuroSpine Task Force on Research: support for spine researchers
In recognition of the value of research to the practice of spine care, Federico Balagué and Ferran Pellisé, at the time President and Secretary for EuroSpine, asked Margareta Nordin to set up a Task Force on Research (TFR) for EuroSpine during summer 2011. The concept was to stimulate and facilitate a research community within the society, through two main functions: (1) distribution of EuroSpine funds to researchers; (2) develop and deliver research training/education courses. What has the EuroSpine TFR accomplished since its inception
Three-dimensional printing for restoration of the donor face : A new digital technique tested and used in the first facial allotransplantation patient in Finland
Background and aims: Prosthetic mask restoration of the donor face is essential in current facial transplant protocols. The aim was to develop a new three-dimensional (3D) printing (additive manufacturing; AM) process for the production of a donor face mask that fulfilled the requirements for facial restoration after facial harvest. Materials and methods: A digital image of a single test person's face was obtained in a standardized setting and subjected to three different image processing techniques. These data were used for the 3D modeling and printing of a donor face mask. The process was also tested in a cadaver setting and ultimately used clinically in a donor patient after facial allograft harvest. Results: and Conclusions: All the three developed and tested techniques enabled the 3D printing of a custom-made face mask in a timely manner that is almost an exact replica of the donor patient's face. This technique was successfully used in a facial allotransplantation donor patient. (C) 2016 British Association of Plastic, Reconstructive and Aesthetic Surgeons. Published by Elsevier Ltd. All rights reserved.Peer reviewe
On the quality of unsupported overhangs produced by laser powder bed fusion
One of the main design constraints for additive manufacturing is the definition of downward-facing surfaces, which can lead to problems, like part failing or warping, during construction and poor surface quality. In this paper, a specific index has been defined to represent the surface quality of the downward-facing surfaces induced by the laser powder bed fusion (L-PBF) process. In order to validate the quality index, a design of experiment (DoE) that considers geometric parameters of the overhangs has been defined and carried out, and the quality of resulting surfaces has been evaluated using an optical scanning system. The statistical analysis (ANOVA) has allowed identifying the relationships between significant geometrical parameters and the quality index here proposed
Abrasive fluidized bed finishing to improve the fatigue behaviour of Ti6Al4V parts fabricated by electron beam melting
A study of the abrasive fluidized bed (AFB) finishing process was conducted to quantify the obtainable improvement of the fatigue behaviour of Ti6Al4V parts produced by electron beam melting (EBM). Axial-symmetric EBM samples were rotated at high speed inside a fluidized bed of stainless-steel media. The effects of the treatment time and the rotational speed on morphological features and fatigue life of the EBM samples were investigated. Outcomes showed that the improvement in surface properties induced by the AFB finishing process determined an increase up to 50% in fatigue life and a shift of the S-N curve
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