4,441 research outputs found
Mordell-Weil Torsion and the Global Structure of Gauge Groups in F-theory
We study the global structure of the gauge group of F-theory compactified
on an elliptic fibration . The global properties of are encoded in the
torsion subgroup of the Mordell-Weil group of rational sections of .
Generalising the Shioda map to torsional sections we construct a specific
integer divisor class on as a fractional linear combination of the
resolution divisors associated with the Cartan subalgebra of . This divisor
class can be interpreted as an element of the refined coweight lattice of the
gauge group. As a result, the spectrum of admissible matter representations is
strongly constrained and the gauge group is non-simply connected. We exemplify
our results by a detailed analysis of the general elliptic fibration with
Mordell-Weil group and as well as a further
specialization to . Our analysis exploits the
representation of these fibrations as hypersurfaces in toric geometry.Comment: 42 pages, 10 figures; v2: references adde
Stochastic vortex dynamics in two-dimensional easy-plane ferromagnets: Multiplicative versus additive noise
We study how thermal fluctuations affect the dynamics of vortices in the
two-dimensional classical, ferromagnetic, anisotropic Heisenberg model
depending on their additive or multiplicative character. Using a collective
coordinate theory, we analytically show that multiplicative noise, arising from
fluctuations in the local field term of the Landau-Lifshitz equations, and
Langevin-like additive noise both have the same effect on vortex dynamics
(within a very plausible assumption consistent with the collective coordinate
approach). This is a non-trivial result, as multiplicative and additive noises
usually modify the dynamics quite differently. We also carry out numerical
simulations of both versions of the model finding that they indeed give rise to
very similar vortex dynamics.Comment: 10 pages, 6 figure
Intrauterine repair of gastroschisis in fetal rabbits
Objective: Infants with gastroschisis (GS) still face severe morbidity. Prenatal closure may prevent gastrointestinal organ damage, but intrauterine GS repair (GSR) has not been established yet. Methods: In New Zealand White rabbits we developed and compared GS versus GSR: creation of GS was achieved by hysterotomy, right-sided laparotomy of the fetus and pressure on the abdominal wall to provoke evisceration. GSR was accomplished by careful reposition of eviscerated organs and a running suture of the fetal abdominal wall. For study purposes, 18 animals were divided equally into 3 groups: GS, GS with GSR after 2 h, and unmanipulated controls (C). Vitality was assessed by echocardiography. After 5 h all animals were sacrificed. Results: GSR inflicted no increased mortality, because all fetuses survived GS or GS with GSR. All fetuses with GS demonstrated significant evisceration of abdominal organs. In contrast, the abdominal wall of the fetuses from GSR was intact. Conclusion:The present animal model demonstrated the technical feasibility and success of an intrauterine repair of GS for the first time. However, further long-term studies (leaving GS and GSR in utero for several days) will be necessary to compare survival rates and intestinal injury, motility or absorption. The clinical application of GSR in utero remains a vision so far. Copyright (C) 2003 S. Karger AG, Basel
A study for active control research and validation using the Total In-Flight Simulator (TIFS) aircraft
The results of a feasibility study and preliminary design for active control research and validation using the Total In-Flight Simulator (TIFS) aircraft are documented. Active control functions which can be demonstrated on the TIFS aircraft and the cost of preparing, equipping, and operating the TIFS aircraft for active control technology development are determined. It is shown that the TIFS aircraft is as a suitable test bed for inflight research and validation of many ACT concepts
Finite temperature dynamics of vortices in the two dimensional anisotropic Heisenberg model
We study the effects of finite temperature on the dynamics of non-planar
vortices in the classical, two-dimensional anisotropic Heisenberg model with
XY- or easy-plane symmetry. To this end, we analyze a generalized
Landau-Lifshitz equation including additive white noise and Gilbert damping.
Using a collective variable theory with no adjustable parameters we derive an
equation of motion for the vortices with stochastic forces which are shown to
represent white noise with an effective diffusion constant linearly dependent
on temperature. We solve these stochastic equations of motion by means of a
Green's function formalism and obtain the mean vortex trajectory and its
variance. We find a non-standard time dependence for the variance of the
components perpendicular to the driving force. We compare the analytical
results with Langevin dynamics simulations and find a good agreement up to
temperatures of the order of 25% of the Kosterlitz-Thouless transition
temperature. Finally, we discuss the reasons why our approach is not
appropriate for higher temperatures as well as the discreteness effects
observed in the numerical simulations.Comment: 12 pages, 8 figures, accepted for publication in European Physical
Journal B (uses EPJ LaTeX
Expression of functional sphingosine-1 phosphate receptor-1 is reduced by B cell receptor signaling and increased by inhibition of PI3 kinase ÎŽ but not SYK or BTK in chronic lymphocytic leukaemia cells
BCR signaling pathway inhibitors such as ibrutinib, idelalisib, and fostamatinib (respective inhibitors of Brutonâs tyrosine kinase,
PI3KÎŽ, and spleen tyrosine kinase) represent a significant therapeutic advance in B cell malignancies, including chronic lymphocytic
leukemia (CLL). These drugs are distinctive in increasing blood lymphocytes while simultaneously shrinking enlarged lymph
nodes, suggesting anatomical redistribution of CLL cells from lymph nodes into the blood. However, the mechanisms underlying
this phenomenon are incompletely understood. In this study, we showed that the egress receptor, sphingosine-1-phosphate (S1P)
receptor 1 (S1PR1), was expressed at low levels in normal germinal centers and CLL lymph nodes in vivo but became upregulated
on normal B cells and, to a variable and lesser extent, CLL cells following in vitro incubation in S1P-free medium. Spontaneous
recovery of S1PR1 expression on normal B and CLL cells was prevented by BCR cross-linking, whereas treatment of CLL cells
with idelalisib increased S1PR1 expression and migration toward S1P, the greatest increase occurring in cases with unmutated
IgH V region genes. Intriguingly, ibrutinib and fostamatinib had no effect on S1PR1 expression or function. Conversely,
chemokine-induced migration, which requires integrin activation and is essential for the entry of lymphocytes into lymph nodes as
well as their retention, was blocked by ibrutinib and fostamatinib, but not idelalisib. In summary, our results suggest that different
BCR signaling inhibitors redistribute CLL cells from lymph nodes into the blood through distinct mechanisms: idelalisib actively
promotes egress by upregulating S1PR1, whereas fostamatinib and ibrutinib may reduce CLL cell entry and retention by
suppressing chemokine-induced integrin activation
Sound archaeology: terminology, Palaeolithic cave art and the soundscape
This article is focused on the ways that terminology describing the study of music and sound within archaeology has changed over time, and how this reflects developing methodologies, exploring the expectations and issues raised by the use of differing kinds of language to define and describe such work. It begins with a discussion of music archaeology, addressing the problems of using the term âmusicâ in an archaeological context. It continues with an examination of archaeoacoustics and acoustics, and an emphasis on sound rather than music. This leads on to a study of sound archaeology and soundscapes, pointing out that it is important to consider the complete acoustic ecology of an archaeological site, in order to identify its affordances, those possibilities offered by invariant acoustic properties. Using a case study from northern Spain, the paper suggests that all of these methodological approaches have merit, and that a project benefits from their integration
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