188 resultados para radiation mechanisms
Resumo:
In this note we demonstrate the use of top polarization in the study of t (t) over bar resonances at the LHC, in the possible case where the dynamics implies a non-zero top polarization. As a probe of top polarization we construct an asymmetry in the decay-lepton azimuthal angle distribution (corresponding to the sign of cos phi(l)) in the laboratory. The asymmetry is non-vanishing even for a symmetric collider like the LHC, where a positive z axis is not uniquely defined. The angular distribution of the leptons has the advantage of being a faithful top-spin analyzer, unaffected by possible anomalous tbW couplings, to linear order. We study, for purposes of demonstration, the case of a Z' as might exist in the little Higgs models. We identify kinematic cuts which ensure that our asymmetry reflects the polarization in sign and magnitude. We investigate possibilities at the LHC with two energy options: root s = 14TeV and root s = 7TeV, as well as at the Tevatron. At the LHC the model predicts net top quark polarization of the order of a few per cent for M-Z' similar or equal to 1200GeV, being as high as 10% for a smaller mass of the Z' of 700GeV and for the largest allowed coupling in the model, the values being higher for the 7TeV option. These polarizations translate to a deviation from the standard-model value of azimuthal asymmetry of up to about 4% (7%) for 14 (7) TeV LHC, whereas for the Tevatron, values as high as 12% are attained. For the 14TeV LHC with an integrated luminosity of 10 fb(-1), these numbers translate into a 3 sigma sensitivity over a large part of the range 500 less than or similar to M-Z' less than or similar to 1500GeV.
Resumo:
This paper introduces CSP-like communication mechanisms into Backus’ Functional Programming (FP) systems extended by nondeterministic constructs. Several new functionals are used to describe nondeterminism and communication in programs. The functionals union and restriction are introduced into FP systems to develop a simple algebra of programs with nondeterminism. The behaviour of other functionals proposed in this paper are characterized by the properties of union and restriction. The axiomatic semantics of communication constructs are presented. Examples show that it is possible to reason about a communicating program by first transforming it into a non-communicating program by using the axioms of communication, and then reasoning about the resulting non-communicating version of the program. It is also shown that communicating programs can be developed from non-communicating programs given as specifications by using a transformational approach.
Resumo:
In this paper, a finite element analysis of steady-state dynamic crack growth under Mode I, plane strain, small-scale yielding conditions is performed in a rate dependent plastic material characterized by the over-stress model. The main objective of the paper is to obtain theoretically the dependence of dynamic fracture toughness on crack speed. Crack propagation due to a ductile (micro-void) mechanism or a brittle (cleavage) mechanism, as well as transition from one mode to another are considered. The conversion from ductile to brittle has been observed experimentally but has received very little attention using analytical methods. Local fracture criteria based on strains and stresses are used to describe ductile and brittle fracture mechanisms. The results obtained in this paper are in general agreement with micro-structural observations of mode conversion during fracture initiation. Finally, the particular roles played by material rate sensitivity and inertia are examined in some detail.
Resumo:
We report 3d-4d4d Auger spectra of Ce metal with the use of synchrotron radiation to excite the initial core hole. By sweeping the excitation energy through the 3d-->4f threshold, it has been possible to excite different initial states selectively, enabling us to analyze the complex spectrum in terms of different contributions arising from various deca channels.
Resumo:
We report the 4d-XY (X, Y = 5p, 4f, and the conduction band) Auger spectra of clean Gd using a monochromatic photon source with energies above and below the 3d threshold. The spectra with higher hv show the existence of intense spectator-hole Auger transitions. Comparison of these spectra with those obtained with a primary electron source allows detailed interpretation of the various features and explains the unusual spin polarization of the electron-induced spectrum reported earlier.
Resumo:
A conceptual model is proposed to explain the observed aperiodicity in the short term climate fluctuations of the tropical coupled ocean-atmosphere system. This is based on the evidence presented here that the tropical coupled ocean-atmosphere system sustains a low frequency inter-annual mode and a host of higher frequency intra-seasonal unstable modes. At long wavelengths, the low frequency mode is dominant while at short wavelengths, the high frequency modes are dominant resulting in the co-existence of a long wave low frequency mode with some short wave intra-seasonal modes in the tropical coupled system. It is argued that due to its long wavelength, the low frequency mode would behave like a linear oscillator while the higher frequency short wave modes would be nonlinear. The conceptual model envisages that an interaction between the low frequency linear oscillator and the high frequency nonlinear oscillations results in the observed aperiodicity of the tropical coupled system. This is illustrated by representing the higher frequency intra-seasonal oscillations by a nonlinear low order model which is then coupled to a linear oscillator with a periodicity of four years. The physical mechanism resulting in the aperiodicity in the low frequency oscillations and implications of these results on the predictability of the coupled system are discussed.
Resumo:
We study change in the polarization of electromagnetic waves due to the stimulated Raman scattering in a plasma. In this process an electromagnetic wave undergoes coherent scattering off an electron plasma wave. It is found that some of the observed polarization properties such as the rapid temporal variations, sense reversal, rotation of the plane of polarization, and change of nature of polarization in the case of pulsars and quasars could be accounted for through stimulated Raman scattering.
Texture evolution and operative mechanisms during large-strain deformation of nanocrystalline nickel
Resumo:
The large-strain deformation of nanocrystalline nickel was investigated at room temperature and cryogenic (liquid N-2) temperature. Deformation mechanisms ranging from grain boundary sliding to slip, operate due to a wide distribution of grain sizes. These mechanisms leave their finger print in the deformation texture evolution during rolling of nanocrystalline nickel. The occurrence and severance of different mechanisms is understood by a thorough characterization of the deformed samples using X-ray diffraction, X-ray texture measurements, electron back-scattered diffraction and transmission electron microscopy. Crystal plasticity-based viscoplastic self-consistent simulations were used to further substantiate the experimental observations. Thus, a comprehensive understanding of deformation behavior of nanocrystalline nickel, which is characterized by simultaneous operation of dislocation-dominated and grain boundary-mediated mechanisms, has been developed.
Resumo:
The modulational instability of a large-amplitude, linearly polarized electromagnetic wave propagating in an electron-positron plasma is considered, including the combined effect of relativistic mass variation of the plasma particles, harmonic generation, and the non-resonant, finite-frequency electrostatic density perturbations, all caused by the large-amplitude radiation field. The radiation from many strong sources, such as AGN and pulsars, has been observed to vary over a host of time-scales. It is possible that the extremely rapid variations in the non-thermal continuum of AGN, as well as in the non-thermal radio radiation from pulsars, can be accounted for by the modulational instabilities to which radiation may be subjected during its propagation out of the emission region.
Resumo:
Marked ball grinding tests were carried out in the laboratory using high carbon low alloy steel (cast and forged) and high chrome cast iron balls. Relative ball wear as a function of grinding period and milling conditions was evaluated for the different type of ball materials in the grinding of lead-zinc sulphide and phosphate ores. Results indicated that ball wear increased with time and showed a sharp increase for wet grinding over dry grinding. Ball wear under wet grinding conditions was also influenced by the gaseous atmosphere in the mill. The influence of oxygen on the corrosive wear of grinding balls was increasingly felt in case of sulphide ore grinding. The grinding ball materials could be arranged in the following order with respect to their overall wear resistance:
Resumo:
Semiconductor based nanoscale heterostructures are promising candidates for photocatalytic and photovoltaic applications with the sensitization of a wide bandgap semiconductor with a narrow bandgap material being the most viable strategy to maximize the utilization of the solar spectrum. Here, we present a simple wet chemical route to obtain nanoscale heterostructures of ZnO/CdS without using any molecular linker. Our method involves the nucleation of a Cd-precursor on ZnO nanorods with a subsequent sulfidation step leading to the formation of the ZnO/CdS nanoscale heterostructures. Excellent control over the loading of CdS and the microstructure is realized by merely changing the initial concentration of the sulfiding agent. We show that the heterostructures with the lowest CdS loading exhibit an exceptionally high activity for the degradation of methylene blue (MB) under solar irradiation conditions; microstructural and surface analysis reveals that the higher activity in this case is related to the dispersion of the CdS nanoparticles on the ZnO nanorod surface and to the higher concentration of surface hydroxyl species. Detailed analysis of the mechanism of formation of the nanoscale heterostructures reveals that it is possible to obtain deterministic control over the nature of the interfaces. Our synthesis method is general and applicable for other heterostructures where the interfaces need to be engineered for optimal properties. In particular, the absence of any molecular linker at the interface makes our method appealing for photovoltaic applications where faster rates of electron transfer at the heterojunctions are highly desirable.
Resumo:
Hamiltonian constructed in a first principles manner, we explored the origin of magnetism and the T-c trend in Cr-based double perovskite series, Sr2CrB'O-6 (B' = W/Re/Os). Our study shows that the apparently puzzling T-c trend in Sr2CrB'O-6 (B' = W/Re/Os) series can be understood in terms of the interplay of the hybridization driven mechanism and the superexchange mechanism.