425 research outputs found
Seasonal trends in response to inoculation of coast live oak with Phytophthora ramorum
We developed a branch cutting inoculation method to provide a controlled system for studying variation in response to inoculation of coast live oak (Quercus agrifolia) with Phytophthora ramorum. This method has advantages over inoculations of trees in the field, in containing the inoculum and in allowing high levels of replication and the possibility of time series of responses. We previously reported significant tree-to-tree variation, with little population variation in lesion size using this method (Dodd and others 2005). Here we report on a time series in which branch cuttings were collected from the same trees at eight dates through a full year cycle. Branch cuttings were sampled from 33 trees from two sites at China Camp in Marin County, California, including 18 trees from one site that had suffered heavy mortality from this disease (Miwok Meadows) and 15 trees from a second site that has had little infection (Chicken Coop Island)
Strong CP, Flavor, and Twisted Split Fermions
We present a natural solution to the strong CP problem in the context of
split fermions. By assuming CP is spontaneously broken in the bulk, a weak CKM
phase is created in the standard model due to a twisting in flavor space of the
bulk fermion wavefunctions. But the strong CP phase remains zero, being
essentially protected by parity in the bulk and CP on the branes. As always in
models of spontaneous CP breaking, radiative corrections to theta bar from the
standard model are tiny, but even higher dimension operators are not that
dangerous. The twisting phenomenon was recently shown to be generic, and not to
interfere with the way that split fermions naturally weaves small numbers into
the standard model. It follows that out approach to strong CP is compatible
with flavor, and we sketch a comprehensive model. We also look at deconstructed
version of this setup which provides a viable 4D model of spontaneous CP
breaking which is not in the Nelson-Barr class.Comment: 30 pages, 5 Figure
Predicting the Cosmological Constant from the Causal Entropic Principle
We compute the expected value of the cosmological constant in our universe
from the Causal Entropic Principle. Since observers must obey the laws of
thermodynamics and causality, the principle asserts that physical parameters
are most likely to be found in the range of values for which the total entropy
production within a causally connected region is maximized. Despite the absence
of more explicit anthropic criteria, the resulting probability distribution
turns out to be in excellent agreement with observation. In particular, we find
that dust heated by stars dominates the entropy production, demonstrating the
remarkable power of this thermodynamic selection criterion. The alternative
approach - weighting by the number of "observers per baryon" - is less
well-defined, requires problematic assumptions about the nature of observers,
and yet prefers values larger than present experimental bounds.Comment: 38 pages, 9 figures, minor correction in Figure
A Universe Without Weak Interactions
A universe without weak interactions is constructed that undergoes big-bang
nucleosynthesis, matter domination, structure formation, and star formation.
The stars in this universe are able to burn for billions of years, synthesize
elements up to iron, and undergo supernova explosions, dispersing heavy
elements into the interstellar medium. These definitive claims are supported by
a detailed analysis where this hypothetical "Weakless Universe" is matched to
our Universe by simultaneously adjusting Standard Model and cosmological
parameters. For instance, chemistry and nuclear physics are essentially
unchanged. The apparent habitability of the Weakless Universe suggests that the
anthropic principle does not determine the scale of electroweak breaking, or
even require that it be smaller than the Planck scale, so long as technically
natural parameters may be suitably adjusted. Whether the multi-parameter
adjustment is realized or probable is dependent on the ultraviolet completion,
such as the string landscape. Considering a similar analysis for the
cosmological constant, however, we argue that no adjustments of other
parameters are able to allow the cosmological constant to raise up even
remotely close to the Planck scale while obtaining macroscopic structure. The
fine-tuning problems associated with the electroweak breaking scale and the
cosmological constant therefore appear to be qualitatively different from the
perspective of obtaining a habitable universe.Comment: 27 pages; 4 figure
Anomaly mediated supersymmetry breaking and its test in linear colliders
Signatures of anomaly mediated supersymmetry breaking in linear colliders are
briefly reviewed after presenting an outline of the theoretical framework. A
unique and distinct feature of a large class of models of this type is a
winolike chargino which is very closely degenerate in mass with the lightest
neutralino. The very slow decay of this chargino results in a heavily ionizing
charged track and one soft charged pion with a characteristic momentum
distribution, leading to unique signals in linear colliders which are
essentially free of background. The determination of chargino and slepton
masses from such events is a distinctly interesting possibility.Comment: 15 pages, LaTex, 4 PS figures, ws-mpla.cls file included. One
reference added. To appear as a Brief Review in Modern Physics Letters
Color Superconductivity from Supersymmetry
A supersymmetric composite model of color superconductivity is proposed.
Quarks and diquarks are dynamically generated as composite fields by a newly
introduced strong gauge dynamics. It is shown that the condensation of the
scalar component of the diquark supermultiplet occurs when the chemical
potential becomes larger than some critical value. We believe that the model
well captures aspects of the diquark condensate behavior and helps our
understanding of the diquark dynamics in real QCD. The results obtained here
might be useful when we consider a theory composed of quarks and diquarks.Comment: 4 pages, 2 figures, An error in Eq.(10) correcte
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