9,153 research outputs found
Embedding the Pentagon
The Pentagon Model is an explicit supersymmetric extension of the Standard
Model, which involves a new strongly-interacting SU(5) gauge theory at
TeV-scale energies. We show that the Pentagon can be embedded into an SU(5) x
SU(5) x SU(5) gauge group at the GUT scale. The doublet-triplet splitting
problem, and proton decay compatible with experimental bounds, can be
successfully addressed in this context. The simplest approach fails to provide
masses for the lighter two generations of quarks and leptons; however, this
problem can be solved by the addition of a pair of antisymmetric tensor fields
and an axion.Comment: 39 page
Combined State and Parameter Estimation for a Static Model of the Maypole (Hoop-Column) Antenna Suface
Parameter and state estimation techniques are discussed for an elliptic system arising in a developmental model for the antenna surface of the Maypole Hoop/Column antenna. A computational algorithm based on spline approximations for the state and elastic parameters is given and numerical results obtained using this algorithm are summarized
On the Possibility of Large Axion Decay Constants
The decay constant of the QCD axion is required by observation to be small
compared to the Planck scale. In theories of "natural inflation," and certain
proposed anthropic solutions of the cosmological constant problem, it would be
interesting to obtain a large decay constant for axion-like fields from
microscopic physics. String theory is the only context in which one can
sensibly address this question. Here we survey a number of periodic fields in
string theory in a variety of string vacua. In some examples, the decay
constant can be parameterically larger than the Planck scale but the effective
action then contains appreciable harmonics of order . As a result,
these fields are no better inflaton candidates than Planck scale axions.Comment: 17 pages, no figures, minor change mad
Remarks on the Racetrack Scheme
There are only a small number of ideas for stabilizing the moduli of string
theory. One of the most appealing of these is the racetrack mechanism, in which
a delicate interplay between two strongly interacting gauge groups fixes the
value of the coupling constant. In this note, we explore this scenario. We find
that quite generally, some number of discrete tunings are required in order
that the mechanism yield a small gauge coupling. Even then, there is no sense
in which a weak coupling approximation is valid. On the other hand, certain
holomorphic quantities can be computed, so such a scheme is in principle
predictive. Searching for models which realize this mechanism is thus of great
interest. We also remark on cosmology in these schemes.Comment: 20 pp, latex, discussion of calculability modifie
Simulation of the low earth orbital atomic oxygen interaction with materials by means of an oxygen ion beam
Atomic oxygen is the predominant species in low-Earth orbit between the altitudes of 180 and 650 km. These highly reactive atoms are a result of photodissociation of diatomic oxygen molecules from solar photons having a wavelength less than or equal to 2430A. Spacecraft in low-Earth orbit collide with atomic oxygen in the 3P ground state at impact energies of approximately 4.2 to 4.5 eV. As a consequence, organic materials previously used for high altitude geosynchronous spacecraft are severely oxidized in the low-Earth orbital environment. The evaluation of materials durability to atomic oxygen requires ground simulation of this environment to cost effectively screen materials for durability. Directed broad beam oxygen sources are necessary to evaluate potential spacecraft materials performance before and after exposure to the simulated low-Earth orbital environment. This paper presents a description of a low energy, broad oxygen ion beam source used to simulate the low-Earth orbital atomic oxygen environment. The results of materials interaction with this beam and comparison with actual in-space tests of the same meterials will be discussed. Resulting surface morphologies appear to closely replicate those observed in space tests
Measurement of beam divergence of 30-centimeter dished grids
The beam divergence of a 30-centimeter diameter thruster with dished grids was calculated from current densities measured with a probe rake containing seventeen planar molybdenum probes. The measured data were analyzed as a function of a number of parameters. The most sensitive parameters were the amount of compensation of the accelerator grid and the ratio of net to total accelerating voltage. The thrust losses were reduced by over 5 percent with the use of compensated grids alone, and by variation of other parameters the overall thrust losses due to beam divergence were reduced to less than 2 percent
Reported frequency of physical activity in a large epidemiological study: relationship to specific activities and repeatability over time
BACKGROUND How overall physical activity relates to specific activities and how reported activity changes over time may influence interpretation of observed associations between physical activity and health. We examine the relationships between various physical activities self-reported at different times in a large cohort study of middle-aged UK women. METHODS At recruitment, Million Women Study participants completed a baseline questionnaire including questions on frequency of strenuous and of any physical activity. About 3 years later 589,896 women also completed a follow-up questionnaire reporting the hours they spent on a range of specific activities. Time spent on each activity was used to estimate the associated excess metabolic equivalent hours (MET-hours) and this value was compared across categories of physical activity reported at recruitment. Additionally, 18,655 women completed the baseline questionnaire twice, at intervals of up to 4 years; repeatability over time was assessed using the weighted kappa coefficient (κweighted) and absolute percentage agreement. RESULTS The average number of hours per week women reported doing specific activities was 14.0 for housework, 4.5 for walking, 3.0 for gardening, 0.2 for cycling, and 1.4 for all strenuous activity. Time spent and the estimated excess MET-hours associated with each activity increased with increasing frequency of any or strenuous physical activity reported at baseline (tests for trend, P < 0.003), although the associations for housework were by far the weakest (Spearman correlations, 0.01 and -0.03 respectively for housework, and 0.11-0.37 for all other activities). Repeatability of responses to physical activity questions on the baseline questionnaire declined significantly over time. For strenuous activity, absolute agreement was 64% (κweighted = 0.71) for questionnaires administered less than 6 months apart, and 52% (κweighted = 0.51) for questionnaires more than 2 years apart. Corresponding values for any physical activity were 57% (κweighted = 0.67) and 47% (κweighted = 0.58). CONCLUSIONS In this cohort, responses to simple questions on the frequency of any physical activity and of strenuous activity asked at baseline were associated with hours spent on specific activities and the associated estimated excess MET-hours expended, reported 3 years later. The weakest associations were with housework. Agreement for identical questions asked on two occasions about the frequency of physical activity decreased over time.This work was supported by public funds from Cancer
Research UK and the UK Medical Research Council
Spherically symmetric false vacuum: no-go theorems and global structure
We enumerate all possible types of spacetime causal structures that can
appear in static, spherically symmetric configurations of a self-gravitating,
real, nonlinear, minimally coupled scalar field \phi in general relativity,
with an arbitrary potential V(\phi), not necessarily positive-definite. It is
shown that a variable scalar field adds nothing to the list of possible
structures with a constant \phi field, namely, Minkowski (or AdS),
Schwarzschild, de Sitter and Schwarzschild - de Sitter. It follows, in
particular, that, whatever is V(\phi), this theory does not admit regular black
holes with flat or AdS asymptotics. It is concluded that the only possible
globally regular, asymptotically flat solutions are solitons with a regular
center, without horizons and with at least partly negative potentials V(\phi).
Extension of the results to more general field models is discussed.Comment: Latex2e, 4 pages, 1 bezier figur
de Sitter Vacua, Renormalization and Locality
We analyze the renormalization properties of quantum field theories in de
Sitter space and show that only two of the maximally invariant vacuum states of
free fields lead to consistent perturbation expansions. One is the Euclidean
vacuum, and the other can be viewed as an analytic continuation of Euclidean
functional integrals on . The corresponding Lorentzian manifold is the
future half of global de Sitter space with boundary conditions on fields at the
origin of time. We argue that the perturbation series in this case has
divergences at the origin, which render the future evolution of the system
indeterminate without a better understanding of high energy physics.Comment: JHEP Latex, 13 pages, v2. references adde
Vacuum Bubble in an Inhomogeneous Cosmology
We study the propagation of bubbles of new vacuum in a radially inhomogeneous
Lemaitre-Tolman-Bondi background that includes a cosmological constant. This
exemplifies the classical evolution of a tunneling bubble through a metastable
state with curvature inhomogeneities, and will be relevant in the context of
the Landscape. We demand that the matter profile in the LTB background satisfy
the weak energy condition. For sample profiles that satisfy this restriction,
we find that the evolution of the bubble (in terms of the physically relevant
coordinates intrinsic to the shell) is largely unaffected by the prsence of
local inhomogeneities. Our setup should also be a useful toy model for
capturing the effects of ambient inhomogeneities on an inflating region.Comment: 31 pages, 21(!) figures, v2: minor changes, figures re-sized (might
require zoom on some systems), references adde
- …