389 research outputs found
Semileptonic Hyperon Decays on the Lattice: an Exploratory Study
We present preliminary results of an exploratory lattice study of the vector
form factor relevant for the semileptonic hyperon decay
. This study is based on the same method used for the
extraction of for the decay . The main purpose
of this study is to test the method for hyperon form factors in order to
estimate the precision that can be reached and the importance of SU(3)-breaking
effects.Comment: 3 pages, 5 figures, talk presented at Lattice2004(weak), Fermilab,
Batavia, Illinois, 21-26 June 200
Radiative corrections to the Dalitz plot of K_{l3}^\pm decays
We calculate the model-independent radiative corrections to the Dalitz plot
of K_{l3}^\pm decays to order (\alpha/\pi)(q/M_1), where q is the momentum
transfer and M_1 is the mass of the kaon. The final results are presented,
first, with the triple integration over the variables of the bremsstrahlung
photon ready to be performed numerically and, second, in an analytical form.
These two forms are useful to crosscheck on one another and with other
calculations. This paper is organized to make it accessible and reliable in the
analysis of the Dalitz plot of precision experiments and is not compromised to
fixing the form factors at predetermined values. It is assumed that the real
photons are kinematically discriminated. Otherwise, our results have a general
model-independent applicability.Comment: RevTex4, 38 pages, 5 figures, 5 tables; some typos corrected;
discussion extended to compare with other result
MRI-based mechanical analysis of carotid atherosclerotic plaque using a material-property-mapping approach: A material-property-mapping method for plaque stress analysis
Background and objective
Atherosclerosis is a major underlying cause of cardiovascular conditions. In order to understand the biomechanics involved in the generation and rupture of atherosclerotic plaques, numerical analysis methods have been widely used. However, several factors limit the practical use of this information in a clinical setting. One of the key challenges in finite element analysis (FEA) is the reconstruction of the structure and the generation of a mesh. The complexity of the shapes associated with carotid plaques, including multiple components, makes the generation of meshes for biomechanical computation a difficult and in some cases, an impossible task. To address these challenges, in this study, we propose a novel material-property-mapping method for carotid atherosclerotic plaque stress analysis that aims to simplify the process.
Methods
The different carotid plaque components were identified and segmented using magnetic resonance imaging (MRI). For the mapping method, this information was used in conjunction with an in-house code, which provided the coordinates for each pixel/voxel and tissue type within a predetermined region of interest. These coordinates were utilized to assign specific material properties to each element in the volume mesh which provides a region of transition. The proposed method was subsequently compared to the traditional method, which involves creating a composed mesh for the arterial wall and plaque components, based on its location and size.
Results
The comparison between the proposed material-property-mapping method and the traditional method was performed in 2D, 3D structural-only, and fluid-structure interaction (FSI) simulations in terms of stress, wall shear stress (WSS), time-averaged WSS (TAWSS), and oscillatory shear index (OSI). The stress contours from both methods were found to be similar, although the proposed method tended to produce lower local maximum stress values. The WSS contours were also in agreement between the two methods. The velocity contours generated by the proposed method were verified against phase-contrast magnetic resonance imaging (MRI) measurements, for a higher level of confidence.
Conclusion
This study shows that a material-property-mapping method can effectively be used for analyzing the biomechanics of carotid plaques in a patient-specific manner. This approach has the potential to streamline the process of creating volume meshes for complex biological structures, such as carotid plaques, and to provide a more efficient and less labor-intensive method
The importance of blood rheology in patient-specific computational fluid dynamics simulation of stenotic carotid arteries
The initiation and progression of atherosclerosis, which is the main cause of cardiovascular diseases, correlate with local haemodynamic factors such as wall shear stress (WSS). Numerical simulations such as computational fluid dynamics (CFD) based on medical imaging have been employed to analyse blood flow in different arteries with and without luminal stenosis. Patient-specific CFD models, however, have assumptions on blood rheology. The differences in the calculated haemodynamic factors between different rheological models have not been fully evaluated. In this study, carotid magnetic resonance imaging (MRI) was performed on six patients with different degrees of carotid stenosis and two healthy volunteers. Using the 3D reconstructed carotid geometries and the patient-specific boundary conditions, CFD simulations were performed by applying a Newtonian and four non-Newtonian models (Carreau, Cross, Quemada and Power-law). WSS descriptors and pressure gradient were analysed and compared between the models. The differences in the maximum and the average oscillatory shear index between the Newtonian and the non-Newtonian models were lower than 12.7% and 12%, respectively. The differences in pressure gradient were also within 15%. The differences in the mean time-averaged WSS (TAWSS) between the Newtonian and Cross, Carreau and Power-law models were lower than 6%. In contrast, a higher difference (26%) was found in Quemada. For the low TAWSS, the differences from the Newtonian to the non-Newtonian models were much larger, in the range of 0.4–31% for Carreau, 3–22% for Cross, 5–51% for Quemada and 10–41% for Power-law. The study suggests that the assumption of a Newtonian model is reasonable when the overall flow pattern or the mean values of the WSS descriptors are investigated. However, the non-Newtonian model is necessary when the low TAWSS region is the focus, especially for arteries with severe stenosis
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Financial crises and economic recovery: cross-country heterogeneity and cross-sectional dependence
This paper estimates the impulse responses of output to financial crises using a local projections panel estimator that accounts for cross-country heterogeneity and cross-sectional dependence. Using a long span of data (1870–2016) for a group of 17 advanced economies we show that once we control for unobserved common factors and parameter heterogeneity in the panel, there is strong evidence of economic recovery within the first 10 years after a financial crisis
Can the polarization of the strange quarks in the proton be positive ?
Recently, the HERMES Collaboration at DESY, using a leading order QCD
analysis of their data on semi-inclusive deep inelastic production of charged
hadrons, reported a marginally positive polarization for the strange quarks in
the proton. We argue that a non-negative polarization is almost impossible.Comment: 6 pages, latex, minor changes in the discussion after Eq. (9
Two-step ATP-driven opening of cohesin head
The cohesin ring is a protein complex composed of four core subunits: Smc1A, Smc3, Rad21 and Stag1/2. It is involved in chromosome segregation, DNA repair, chromatin organization and transcription regulation. Opening of the ring occurs at the "head" structure, formed of the ATPase domains of Smc1A and Smc3 and Rad21. We investigate the mechanisms of the cohesin ring opening using techniques of free molecular dynamics (MD), steered MD and quantum mechanics/molecular mechanics MD (QM/MM MD). The study allows the thorough analysis of the opening events at the atomic scale: I) ATP hydrolysis at the Smc1A site, evaluating the role of the carboxy-terminal domain of Rad21 in the process; ii) the activation of the Smc3 site potentially mediated by the movement of specific amino acids; and iii) opening of the head domains after the two ATP hydrolysis events. Our study suggests that the cohesin ring opening is triggered by a sequential activation of the ATP sites in which ATP hydrolysis at the Smc1A site induces ATPase activity at the Smc3 site. Our analysis also provides an explanation for the effect of pathogenic variants related to cohesinopathies and cancer
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