930 research outputs found
COBE ground segment attitude determination
The Cosmic Background Explorer (COBE) spacecraft was launched in November 1989 by NASA to survey the sky for primordial radiation left from the Big Bang explosion. The success of the mission requires an accurate determination of the spacecraft attitude. While the accuracy of the attitude obtained from the attitude sensors is adequate for two of the experiments, the higher accuracy required by the Diffuse Infrared Background Experiment (DIRBE) is obtained by using the DIRBE instrument as a special type of star sensor. Presented here is an overview of the attitude processing algorithms used at the Cosmology Data Analysis Center (CDAC) and the results obtained from the flight data
COBE ground segment gyro calibration
Discussed here is the calibration of the scale factors and rate biases for the Cosmic Background Explorer (COBE) spacecraft gyroscopes, with the emphasis on the adaptation for COBE of an algorithm previously developed for the Solar Maximum Mission. Detailed choice of parameters, convergence, verification, and use of the algorithm in an environment where the reference attitudes are determined form the Sun, Earth, and star observations (via the Diffuse Infrared Background Experiment (DIRBE) are considered. Results of some recent experiments are given. These include tests where the gyro rate data are corrected for the effect of the gyro baseplate temperature on the spacecraft electronics
Hiding the Higgs at the LHC
We study a simple extension of the standard model where scalar singlets that
mix with the Higgs doublet are added. This modification to the standard model
could have a significant impact on Higgs searches at the LHC. The Higgs doublet
is not a mass eigenstate and therefore the expected nice peak of the standard
model Higgs disappears. We analyze this scenario finding the required
properties of the singlets in order to make the Higgs "invisible" at the LHC.
In some part of the parameter space even one singlet could make the discovery
of the SM Higgs problematic. In other parts, the Higgs can be discovered even
in the presence of many singlets.Comment: 9 pages, 1 figure. V2- References added. V3- Several examples and one
fig. adde
Consistent Anisotropic Repulsions for Simple Molecules
We extract atom-atom potentials from the effective spherical potentials that
suc cessfully model Hugoniot experiments on molecular fluids, e.g., and
. In the case of the resulting potentials compare very well with the
atom-atom potentials used in studies of solid-state propertie s, while for
they are considerably softer at short distances. Ground state (T=0K) and
room temperatu re calculations performed with the new potential resolve
the previous discrepancy between experimental and theoretical results.Comment: RevTeX, 5 figure
MODIS Science Algorithms and Data Systems Lessons Learned
For almost 10 years, standard global products from NASA's Earth Observing System s (EOS) two Moderate Resolution Imaging Spectroradiometer (MODIS) sensors are being used world-wide for earth science research and applications. This paper discusses the lessons learned in developing the science algorithms and the data systems needed to produce these high quality data products for the earth sciences community. Strong science team leadership and communication, an evolvable and scalable data system, and central coordination of QA and validation activities enabled the data system to grow by two orders of magnitude from the initial at-launch system to the current system able to reprocess data from both the Terra and Aqua missions in less than a year. Many of the lessons learned from MODIS are already being applied to follow-on missions
Quantum Codes for Controlling Coherent Evolution
Control over spin dynamics has been obtained in NMR via coherent averaging,
which is implemented through a sequence of RF pulses, and via quantum codes
which can protect against incoherent evolution. Here, we discuss the design and
implementation of quantum codes to protect against coherent evolution. A
detailed example is given of a quantum code for protecting two data qubits from
evolution under a weak coupling (Ising) term in the Hamiltonian, using an
``isolated'' ancilla which does not evolve on the experimental time scale. The
code is realized in a three-spin system by liquid-state NMR spectroscopy on
13C-labelled alanine, and tested for two initial states. It is also shown that
for coherent evolution and isolated ancillae, codes exist that do not require
the ancillae to initially be in a (pseudo-)pure state. Finally, it is shown
that even with non-isolated ancillae quantum codes exist which can protect
against evolution under weak coupling. An example is presented for a six qubit
code that protects two data spins to first order.Comment: Reformatted single spaced with figures incorporated into text (18
pages, 6 figures, PDF only, submitted to J. Chem. Phys.
A Study of Quantum Error Correction by Geometric Algebra and Liquid-State NMR Spectroscopy
Quantum error correcting codes enable the information contained in a quantum
state to be protected from decoherence due to external perturbations. Applied
to NMR, quantum coding does not alter normal relaxation, but rather converts
the state of a ``data'' spin into multiple quantum coherences involving
additional ancilla spins. These multiple quantum coherences relax at differing
rates, thus permitting the original state of the data to be approximately
reconstructed by mixing them together in an appropriate fashion. This paper
describes the operation of a simple, three-bit quantum code in the product
operator formalism, and uses geometric algebra methods to obtain the
error-corrected decay curve in the presence of arbitrary correlations in the
external random fields. These predictions are confirmed in both the totally
correlated and uncorrelated cases by liquid-state NMR experiments on
13C-labeled alanine, using gradient-diffusion methods to implement these
idealized decoherence models. Quantum error correction in weakly polarized
systems requires that the ancilla spins be prepared in a pseudo-pure state
relative to the data spin, which entails a loss of signal that exceeds any
potential gain through error correction. Nevertheless, this study shows that
quantum coding can be used to validate theoretical decoherence mechanisms, and
to provide detailed information on correlations in the underlying NMR
relaxation dynamics.Comment: 33 pages plus 6 figures, LaTeX article class with amsmath & graphicx
package
A Seesaw Mechanism in the Higgs Sector
In this letter we revisit the seesaw Higgs mechanism. We show how a seesaw
mechanism in a two Higgs doublets model can trigger the electroweak symmetry
breaking if at least one of the eigenvalues of the squared mass matrix is
negative. We then consider two special cases of interest. In the decoupling
scenario, there is only one scalar degree of freedom in the low energy regime.
In the degenerate scenario, all five degrees of freedom are in the low energy
regime and will lead to observables effects at the LHC. Furthermore, in that
scenario, it is possible to impose a discrete symmetry between the doublets
that makes the extra neutral degrees of freedom stable. These are thus viable
dark matter candidates. We find an interesting relation between the electroweak
symmetry breaking mechanism and dark matter.Comment: 10 page
Z(2)-Singlino Dark Matter in a Portal-Like Extension of the Minimal Supersymmetric Standard Model.
We propose a Z2-stabilized singlino () as a dark matter candidate in extended and R-parity violating versions of the supersymmetric standard model. interacts with visible matter via a heavy messenger field S, which results in a supersymmetric version of the Higgs portal interaction. The relic abundance of can account for cold dark matter if the messenger mass satisfies GeV. Our model can be implemented in many realistic supersymmetric models such as the next-to-minimal supersymmetric (SUSY) standard model and nearly minimal SUSY standard model
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