21,613 research outputs found
Accelerator Constraints on Neutralino Dark Matter
The constraints on neutralino dark matter \chi obtained from accelerator
searches at LEP, the Fermilab Tevatron and elsewhere are reviewed, with
particular emphasis on results from LEP 1.5. These imply within the context of
the minimal supersymmetric extension of the Standard Model that m_\chi \ge 21.4
GeV if universality is assumed, and yield for large tan\beta a significantly
stronger bound than is obtained indirectly from Tevatron limits on the gluino
mass. We update this analysis with preliminary results from the first LEP 2W
run, and also preview the prospects for future sparticle searches at the LHC.Comment: Presented by J. Ellis at the Workshop on the Identification of Dark
Matter, Sheffield, September, 1996. 14 pages; Latex; 12 Fig
Moments of the latitudinal dependence of the sunspot cycle : a new diagnostic of dynamo models
All A&A articles are in open access ONE year after their publication date (August 2013)Aims: The latitude-distribution of solar activity as represented by sunspots is studied.
Methods: We first determined the latitudinal distribution of a sunspot cycle by integrating the butterfly diagram at each latitude over the length of each cycle. We then formed the five lowest moments of the latitudinal distribution of all complete sunspot cycles since 1874 and compared these moments with each other.
Results: The three lowest moments correlate remarkably well with each other. For example, the mean latitude of the sunspots during a cycle and the latitude range are correlated at the 0.96 level. A clear asymmetry is seen between the two hemispheres, with the southern solar hemisphere showing consistently stronger and more positive correlations than the northern hemisphere. When applied to different simple dynamo models, the same analysis reveals significant differences between the models and demonstrates that such moments are a useful diagnostic in distinguishing between dynamo models. Remarkably, dynamos without a meridional flow provide results closer to those of the Sunâs northern hemisphere, while a dynamo with a meridional flow produces fields more like those in the Sunâs southern hemisphere. This may provide a clue to the cause of the well-known north-south asymmetry of solar activity
Large N limit of SO(N) gauge theory of fermions and bosons
In this paper we study the large N_c limit of SO(N_c) gauge theory coupled to
a Majorana field and a real scalar field in 1+1 dimensions extending ideas of
Rajeev. We show that the phase space of the resulting classical theory of
bilinears, which are the mesonic operators of this theory, is OSp_1(H|H
)/U(H_+|H_+), where H|H refers to the underlying complex graded space of
combined one-particle states of fermions and bosons and H_+|H_+ corresponds to
the positive frequency subspace. In the begining to simplify our presentation
we discuss in detail the case with Majorana fermions only (the purely bosonic
case is treated in our earlier work). In the Majorana fermion case the phase
space is given by O_1(H)/U(H_+), where H refers to the complex one-particle
states and H_+ to its positive frequency subspace. The meson spectrum in the
linear approximation again obeys a variant of the 't Hooft equation. The linear
approximation to the boson/fermion coupled case brings an additonal bound state
equation for mesons, which consists of one fermion and one boson, again of the
same form as the well-known 't Hooft equation.Comment: 27 pages, no figure
Heat capacity of -GaN: Isotope Effects
Until recently, the heat capacity of GaN had only been measured for
polycrystalline powder samples. Semiempirical as well as
\textit{first-principles} calculations have appeared within the past few years.
We present in this article measurements of the heat capacity of hexagonal
single crystals of GaN in the 20-1400K temperature range. We find that our data
deviate significantly from the literature values for polycrystalline materials.
The dependence of the heat capacity on the isotopic mass has also been
investigated recently for monatomic crystals such as diamond, silicon, and
germanium. Multi-atomic crystals are expected to exhibit a different dependence
of these heat capacities on the masses of each of the isotopes present. These
effects have not been investigated in the past. We also present
\textit{first-principles} calculations of the dependence of the heat capacities
of GaN, as a canonical binary material, on each of the Ga and N masses. We show
that they are indeed different, as expected from the fact that the Ga mass
affects mainly the acoustic, that of N the optic phonons. It is hoped that
these calculations will encourage experimental measurements of the dependence
of the heat capacity on isotopic masses in binary and more complex
semiconductors.Comment: 12 pages, 5 Figures, submitted to PR
Large-eddy simulation and experimental study of heat transfer, nitric oxide emissions and combustion instability in a swirled turbulent high-pressure burner
Nitric oxide formation in gas turbine combustion depends on four key factors: flame stabilization, heat transfer, fuel-air mixing and combustion instability. The design of modern gas turbine burners requires delicate compromises between fuel efficiency, emissions of oxides of nitrogen (NOx) and combustion stability. Burner designs allowing substantial NOx reduction are often prone to combustion oscillations. These oscillations also change the NOx fields. Being able to predict not only the main species field in a burner but also the pollutant and the oscillation levels is now a major challenge for combustion modelling. This must include a realistic treatment of unsteady acoustic phenomena (which create instabilities) and also of heat transfer mechanisms (convection and radiation) which control NOx generation. In this work, large-eddy simulation (LES) is applied to a realistic gas turbine combustion chamber configuration where pure methane is injected through multiple holes in a cone-shaped burner. In addition to a non-reactive simulation, this article presents three reactive simulations and compares them to experimental results. The first reactive simulation neglects effects of cooling air on flame stabilization and heat losses by radiation and convection. The second reactive simulation shows how cooling air and heat transfer affect nitric oxide emissions. Finally, the third reactive simulation shows the effects of combustion instability on nitric oxide emissions. Additionally, the combustion instability is analysed in detail, including the evaluation of the terms in the acoustic energy equation and the identification of the mechanism driving the oscillation. Results confirm that LES of gas turbine combustion requires not only an accurate chemical scheme and realistic heat transfer models but also a proper description of the acoustics in order to predict nitric oxide emissions and pressure oscillation levels simultaneousl
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