773 research outputs found
On Critical Velocities in Exciton Superfluidity
The presence of exciton phonon interactions is shown to play a key role in
the exciton superfluidity. We apply the Landau criterion for an exciton-phonon
condensate moving uniformly at zero temperature. It turns out that there are
essentially two critical velocities in the theory. Within the range of these
velocities the condensate can exist only as a bright soliton. The excitation
spectrum and differential equations for the wave function of this condensate
are derived.Comment: 7 pages, Latex; to be published in Phys.Rev.Lett (1997
F-term uplifting via consistent D-terms
The issue of fine-tuning necessary to achieve satisfactory degree of
hierarchy between moduli masses, the gravitino mass and the scale of the
cosmological constant has been revisited in the context of supergravities with
consistent D-terms. We have studied (extended) racetrack models where
supersymmetry breaking and moduli stabilisation cannot be separated from each
other. We show that even in such cases the realistic hierarchy can be achieved
on the expense of a single fine-tuning. The presence of two condensates changes
the role of the constant term in the superpotential, W_0, and solutions with
small vacuum energy and large gravitino mass can be found even for very small
values of W_0. Models where D-terms are allowed to vanish at finite vevs of
moduli fields - denoted `cancellable' D-terms - and the ones where D-terms may
vanish only at infinite vevs of some moduli - denoted `non-cancellable' -
differ markedly in their properties. It turns out that the tuning with respect
to the Planck scale required in the case of cancellable D-terms is much weaker
than in the case of non-cancellable ones. We have shown that, against
intuition, a vanishing D-term can trigger F-term uplifting of the vacuum energy
due to the stringent constraint it imposes on vacuum expectation values of
charged fields. Finally we note that our models only rely on two dimensionful
parameters: M_P and W_0.Comment: 10 pages, 2 figures, plain Latex, references adde
Clinical ROC Studies of Digital Stereo Mammography
The objective of this study was to explore and document the diagnostic utility of digital stereo mammography for the detection of localized breast cancer in women. In it we characterized the ability of experienced mammographers, general radiologists, and non-radiologists to detect three types of tumor masses embedded within a heterogeneous background of normal tissue elements in numerically simulated digital mammograms. The simulated mammograms were displayed to the subjects on a high resolution video display, both in stereo mode and in mono mode. Half of the mammograms contained a single tumor, ranging from 0.3 to 0.8 cm in maximal diameter. Each reader rated 120 images (60 in stereo and 60 in mono) as to the probability of abnormality on scale of 1-5. Observer responses were evaluated using receiver operating characteristic (ROC) analysis to characterize any difference in diagnostic performance between the two viewing modes. The synthesized mammograms and the digital display were highly rated by the participant radiologists as promising tools for future research. The results of ROC analysis, however, indicated no significant difference in tumor detection when the same readers utilized the stereo mode versus the mono mode (Az mono = 0.833 versus, Az stereo = 0.826). The results were similar for readers of all 3 experience levels--mammographers, general radiologists, and non-radiologists
Cosmological vacuum selection and metastable susy breaking
We study gauge mediation in a wide class of O'Raifeartaigh type models where
supersymmetry breaking metastable vacuum is created by gravity and/or quantum
corrections. We examine their thermal evolution in the early universe and the
conditions under which the susy breaking vacuum can be selected. It is
demonstrated that thermalization typically makes the metastable supersymmetry
breaking cosmologically disfavoured but this is not always the case. Initial
conditions with the spurion displaced from the symmetric thermal minimum and a
small coupling to the messenger sector can result in the realization of the
susy breaking vacuum even if the reheating temperature is high. We show that
this can be achieved without jeopardizing the low energy phenomenology. In
addition, we have found that deforming the models by a supersymmetric mass term
for messengers in such a way that the susy breaking minimum and the susy
preserving minima are all far away from the origin does not change the
conclusions. The basic observations are expected to hold also in the case of
models with an anomalous U(1) group.Comment: 28 pages, 4 figures, plain Latex, journal versio
The Messenger Sector of SUSY Flavour Models and Radiative Breaking of Flavour Universality
The flavour messenger sectors and their impact on the soft SUSY breaking
terms are investigated in SUSY flavour models. In the case when the flavour
scale M is below the SUSY breaking mediation scale M_S, the universality of
soft terms, even if assumed at M_S, is radiatively broken. We estimate this
effect in a broad class of models. In the CKM basis that effect gives flavour
off-diagonal soft masses comparable to the tree-level estimate based on the
flavour symmetry.Comment: 24 pages, 3 figures. v3: minor changes in the text, typos corrected,
version accepted for publication in JHE
Use of Surrogate end points in HTA
The different actors involved in health system decision-making and regulation have to deal with the question which are valid parameters to assess the health value of health technologies
Beyond MFV in family symmetry theories of fermion masses
Minimal Flavour Violation (MFV) postulates that the only source of flavour
changing neutral currents and CP violation, as in the Standard Model, is the
CKM matrix. However it does not address the origin of fermion masses and mixing
and models that do usually have a structure that goes well beyond the MFV
framework. In this paper we compare the MFV predictions with those obtained in
models based on spontaneously broken (horizontal) family symmetries, both
Abelian and non-Abelian. The generic suppression of flavour changing processes
in these models turns out to be weaker than in the MFV hypothesis. Despite
this, in the supersymmetric case, the suppression may still be consistent with
a solution to the hierarchy problem, with masses of superpartners below 1 TeV.
A comparison of FCNC and CP violation in processes involving a variety of
different family quantum numbers should be able to distinguish between various
family symmetry models and models satisfying the MFV hypothesis.Comment: 34 pages, no figure
Brucella spp. of amphibians comprise genomically diverse motile strains competent for replication in macrophages and survival in mammalian hosts
Twenty-one small Gram-negative motile coccobacilli were isolated from 15 systemically diseased African bullfrogs (Pyxicephalus edulis), and were initially identified as Ochrobactrum anthropi by standard microbiological identification systems. Phylogenetic reconstructions using combined molecular analyses and comparative whole genome analysis of the most diverse of the bullfrog strains verified affiliation with the genus Brucella and placed the isolates in a cluster containing B. inopinata and the other non-classical Brucella species but also revealed significant genetic differences within the group. Four representative but molecularly and phenotypically diverse strains were used for in vitro and in vivo infection experiments. All readily multiplied in macrophage-like murine J774-cells, and their overall intramacrophagic growth rate was comparable to that of B. inopinata BO1 and slightly higher than that of B. microti CCM 4915. In the BALB/c murine model of infection these strains replicated in both spleen and liver, but were less efficient than B. suis 1330. Some strains survived in the mammalian host for up to 12 weeks. The heterogeneity of these novel strains hampers a single species description but their phenotypic and genetic features suggest that they represent an evolutionary link between a soil-associated ancestor and the mammalian host-adapted pathogenic Brucella species
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