216 research outputs found
N=4 Scattering Amplitudes and the Deformed Grassmannian
15 pagesSome time ago the general tree-level scattering amplitudes of N=4 Super Yang-Mills theory were expressed as certain Grassmannian contour integrals. These remarkable formulas allow to clearly expose the super-conformal, dual super-conformal, and Yangian symmetries of the amplitudes. Using ideas from integrability it was recently shown that the building blocks of the amplitudes permit a natural multi-parameter deformation. However, this approach had been criticized by the observation that it seemed impossible to reassemble the building blocks into Yangian-invariant deformed non-MHV amplitudes. In this note we demonstrate that the deformations may be succinctly summarized by a simple modification of the measure of the Grassmannian integrals, leading to a Yangian-invariant deformation of the general tree-level amplitudes. Interestingly, the deformed building-blocks appear as residues of poles in the spectral parameter planes. Given that the contour integrals also contain information on the amplitudes at loop-level, we expect the deformations to be useful there as well. In particular, applying meromorphicity arguments, they may be expected to regulate all notorious infrared divergences. We also point out relations to Gelfand hypergeometric functions and the quantum Knizhnik-Zamolodchikov equations.Peer reviewe
Higher Order Couplings from Heterotic Orbifold Theory
We calculate couplings of arbitrary order from correlation functions among
twisted strings, using conformal field theory. Twisted strings arise in
heterotic string compactified on orbifolds yielding matter fields in the low
energy limit. We calculate completely the classical and the quantum amplitude
including normalization, up to a contribution from Kahler potential. The
classical action has saddle points which are interpreted as worldsheet
instantons described by metastable untwisted strings, formed by twisted strings
distributed at certain fixed points. This understanding generalizes the area
rule, in the case that the locations of twisted strings do not form a polygon,
and provides a general rule for calculating these kinds of instanton
corrections. An interpretation of couplings involving linearly combined states
is given, which commonly appear in non-prime order orbifolds. The quantum part
of the amplitude is given by ratios of gamma functions with order one
arguments.Comment: 32+1 pages, 5 figure
Direct Observation of Propagating Gigahertz Coherent Guided Acoustic Phonons in Free Standing Single Copper Nanowires
We report on gigahertz acoustic phonon waveguiding in free-standing single
copper nanowires studied by femtosecond transient reflectivity measurements.
The results are discussed on the basis of the semianalytical resolution of the
Pochhammer and Chree equation. The spreading of the generated Gaussian wave
packet of two different modes is derived analytically and compared with the
observed oscillations of the sample reflectivity. These experiments provide a
unique way to independently obtain geometrical and material characterization.
This direct observation of coherent guided acoustic phonons in a single
nano-object is also the first step toward nanolateral size acoustic transducer
and comprehensive studies of the thermal properties of nanowires
Three-point functions in superstring theory on AdS3xS3xT4
We consider R and NS spectral flow sectors of type IIB superstring theory on
AdS(3)xS(3)xT(4) in the context of the AdS(3)/CFT(2) correspondence. We present
a derivation of the vertex operators creating spectral flow images of chiral
primary states previously proposed in the literature. We compute spectral flow
conserving three-point functions involving these operators on the sphere. Using
the bulk-to-boundary dictionary, we compare the results with the corresponding
correlators in the dual conformal field theory, the symmetric product orbifold
of T(4). In the limit of small string coupling, agreement is found in all the
cases considered.Comment: 21 pages; v2: New results added. Conclusion on RRNS correlators
revised. Version to appear in JHE
Large scale analytic calculations in quantum field theories
We present a survey on the mathematical structure of zero- and single scale
quantities and the associated calculation methods and function spaces in higher
order perturbative calculations in relativistic renormalizable quantum field
theories.Comment: 25 pages Latex, 1 style fil
A novel unconventional T cell population enriched in Crohn's disease
Objective One of the current hypotheses to explain the proinflammatory immune response in IBD is a dysregulated T cell reaction to yet unknown intestinal antigens. As such, it may be possible to identify disease-associated T cell clonotypes by analysing the peripheral and intestinal T-cell receptor (TCR) repertoire of patients with IBD and controls. Design We performed bulk TCR repertoire profiling of both the TCR alpha and beta chains using high-throughput sequencing in peripheral blood samples of a total of 244 patients with IBD and healthy controls as well as from matched blood and intestinal tissue of 59 patients with IBD and disease controls. We further characterised specific T cell clonotypes via single-cell RNAseq. Results We identified a group of clonotypes, characterised by semi-invariant TCR alpha chains, to be significantly enriched in the blood of patients with Crohn's disease (CD) and particularly expanded in the CD8+ T cell population. Single-cell RNAseq data showed an innate-like phenotype of these cells, with a comparable gene expression to unconventional T cells such as mucosal associated invariant T and natural killer T (NKT) cells, but with distinct TCRs. Conclusions We identified and characterised a subpopulation of unconventional Crohn-associated invariant T (CAIT) cells. Multiple evidence suggests these cells to be part of the NKT type II population. The potential implications of this population for CD or a subset thereof remain to be elucidated, and the immunophenotype and antigen reactivity of CAIT cells need further investigations in future studies
Advances and new applications using the acousto-optic effect in optical fibers
This work presents a short review of the current research on the acousto-optic mechanism applied to optical fibers. The role of the piezoelectric element and the acousto-optic modulator in the excitation of flexural and longitudinal acoustic modes in the frequency range up to 1.2 MHz is highlighted. A combination of the finite elements and the transfer matrix methods is used to simulate the interaction of the waves with Bragg and long period gratings. Results show a very good agreement with experimental data. Recent applications such as the writing of gratings under the acoustic excitation and a novel viscometer sensor based on the acousto-optic mechanism are discussed
Testing With Bars From Dynamic to Quasi-static
International audienceThe numerical calculation of the dynamic loading of a structure includes a great number of steps in which various fundamental or engineering problems are involved. Most of them are addressed in the present course at CISM. In this paper, we discuss the testing of materials in order to model their behaviour
- âŠ