3,564 research outputs found
On Heterotic Orbifolds, M Theory and Type I' Brane Engineering
Horava--Witten M theory -- heterotic string duality poses special problems
for the twisted sectors of heterotic orbifolds. In [1] we explained how in M
theory the twisted states couple to gauge fields apparently living on M9 branes
at both ends of the eleventh dimension at the same time. The resolution
involves 7D gauge fields which live on fixed planes of the (T^4/Z_N) x
(S^1/Z_2) x R^{5,1} orbifold and lock onto the 10D gauge fields along the
intersection planes. The physics of such intersection planes does not follow
directly from the M theory but there are stringent kinematic constraints due to
duality and local consistency, which allowed us to deduce the local fields and
the boundary conditions at each intersection. In this paper we explain various
phenomena at the intersection planes in terms of duality between HW and type I'
superstring theories. The orbifold fixed planes are dual to stacks of D6
branes, the M9 planes are dual to O8 orientifold planes accompanied by D8
branes, and the intersections are dual to brane junctions. We engineer several
junction types which lead to distinct patterns of 7D/10D gauge field locking,
7D symmetry breaking and/or local 6D fields. Another aspect of brane
engineering is putting the junctions together; sometimes, the combined effect
is rather spectacular from the HW point of view and the quantum numbers of some
twisted states have to `bounce' off both ends of the eleventh dimension before
their heterotic identity becomes clear. Some models involve D6/O8 junctions
where the string coupling diverges towards the orientifold plane. We use the
heterotic-HW-I' duality to predict what should happen at such junctions.Comment: 118 pages, uses phyzzx, color printer advice
The String Tension in Two Dimensional Gauge Theories
We review and elaborate on properties of the string tension in
two-dimensional gauge theories. The first model we consider is massive QED in
the limit. We evaluate the leading string tension both in the
fermionic and bosonized descriptions. We discuss the next to leading
corrections in . The next-to-leading terms in the long distance behavior
of the quark-antiquark potential, are evaluated in a certain region of external
versus dynamical charges. The finite temperature behavior is also determined.
In we review the results for the string tension of quarks in cases with
dynamical quarks in the fundamental, adjoint, symmetric and antisymmetric
representations. The screening nature of is re-derived.Comment: 25 pages, Latex. v2: several changes, mainly in section
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The Microenvironment Determines the Breast Cancer Cells' Phenotype: Organization of MCF7 Cells in 3D Cultures
Background: Stromal-epithelial interactions mediate breast development, and the initiation and progression of breast cancer. In the present study, we developed 3-dimensional (3D) in vitro models to study breast cancer tissue organization and the role of the microenvironment in phenotypic determination. Methods: The human breast cancer MCF7 cells were grown alone or co-cultured with primary human breast fibroblasts. Cells were embedded in matrices containing either type I collagen or a combination of reconstituted basement membrane proteins and type I collagen. The cultures were carried out for up to 6 weeks. For every time point (1-6 weeks), the gels were fixed and processed for histology, and whole-mounted for confocal microscopy evaluation. The epithelial structures were characterized utilizing immunohistochemical techniques; their area and proliferation index were measured using computerized morphometric analysis. Statistical differences between groups were analyzed by ANOVA, Dunnett's T3 post-hoc test and chi-square. Results: Most of the MCF7 cells grown alone within a collagen matrix died during the first two weeks; those that survived organized into large, round and solid clusters. The presence of fibroblasts in collagen gels reduced MCF7 cell death, induced cell polarity, and the formation of round and elongated epithelial structures containing a lumen. The addition of reconstituted basement membrane to collagen gels by itself had also survival and organizational effects on the MCF7 cells. Regardless of the presence of fibroblasts, the MCF7 cells both polarized and formed a lumen. The addition of fibroblasts to the gel containing reconstituted basement membrane and collagen induced the formation of elongated structures. Conclusions: Our results indicate that a matrix containing both type I collagen and reconstituted basement membrane, and the presence of normal breast fibroblasts constitute the minimal permissive microenvironment to induce near-complete tumor phenotype reversion. These human breast 3D tissue morphogenesis models promise to become reliable tools for studying tissue interactions, therapeutic screening and drug target validation
Isomeric states close to doubly magic Sn studied with JYFLTRAP
The double Penning trap mass spectrometer JYFLTRAP has been employed to
measure masses and excitation energies for isomers in Cd,
Cd, Cd and Te, for isomers in In and
In, and for isomers in Sn and Sb. These first
direct mass measurements of the Cd and In isomers reveal deviations to the
excitation energies based on results from beta-decay experiments and yield new
information on neutron- and proton-hole states close to Sn. A new
excitation energy of 144(4) keV has been determined for Cd. A good
agreement with the precisely known excitation energies of Cd,
Sn, and Sb has been found.Comment: 10 pages, 6 figures, submitted to Phys. Rev.
Note and Comment
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On the Duality between Perturbative Heterotic Orbifolds and M-Theory on T^4/Z_N
The heterotic string compactified on an orbifold T^4/\IZ_N
has gauge group with (massless) states in its twisted sectors
which are charged under both gauge group factors. In the dual M-theory on
(T^4/\IZ_N)\otimes(S^1/\IZ_2) the two group factors are separated in the
eleventh direction and the G and G' gauge fields are confined to the two
boundary planes, respectively. We present a scenario which allows for a
resolution of this apparent paradox and assigns all massless matter multiplets
locally to the different six-dimensional boundary fixed planes. The resolution
consists of diagonal mixing between the gauge groups which live on the
connecting seven-planes (6d and the eleventh dimension) and one of the gauge
group factors. We present evidence supporting this mixing by considering gauge
couplings and verify local anomaly cancellation. We also discuss open problems
which arise in the presence of U_1 factors.Comment: 45 pages, one figur
Resonance ionization spectroscopy of thorium isotopes - towards a laser spectroscopic identification of the low-lying 7.6 eV isomer of Th-229
In-source resonance ionization spectroscopy was used to identify an efficient
and selective three step excitation/ionization scheme of thorium, suitable for
titanium:sapphire (Ti:sa) lasers. The measurements were carried out in
preparation of laser spectroscopic investigations for an identification of the
low-lying Th-229m isomer predicted at 7.6 +- 0.5 eV above the nuclear ground
state. Using a sample of Th-232, a multitude of optical transitions leading to
over 20 previously unknown intermediate states of even parity as well as
numerous high-lying odd parity auto-ionizing states were identified. Level
energies were determined with an accuracy of 0.06 cm-1 for intermediate and
0.15 cm-1 for auto-ionizing states. Using different excitation pathways an
assignment of total angular momenta for several energy levels was possible. One
particularly efficient ionization scheme of thorium, exhibiting saturation in
all three optical transitions, was studied in detail. For all three levels in
this scheme, the isotope shifts of the isotopes Th-228, Th-229, and Th-230
relative to Th-232 were measured. An overall efficiency including ionization,
transport and detection of 0.6 was determined, which was predominantly limited
by the transmission of the mass spectrometer ion optics
Lack of c-kit receptor promotes mammary tumors in N-nitrosomethylurea-treated Ws/Ws rats
<p>Abstract</p> <p>Background</p> <p>c-<it>kit </it>is expressed in various cell types during development and it has been linked to the promotion of cellular migration, proliferation and/or survival of melanoblasts, hematopoietic progenitors and primordial germ cells. Several reports have proposed a role for the c-<it>kit </it>gene on carcinogenesis. Gain-of-function mutations are associated with diseases such as mastocytosis and gastrointestinal stromal tumors among others. However, very little is known about pathologies associated with loss-of-function mutations. Regarding breast cancer, c-kit protein and mRNA are highly expressed in normal breast but their expression decreases or is absent in the presence of breast cancer. We studied the role of <it>c-kit </it>in mammary carcinogenesis in the Ws/Ws rats carrying spontaneous lack-of-function mutation in the c-<it>kit </it>gene. Fifty day-old virgin female Ws/Ws rats and their wild type counterparts were injected with either 50 mg/kg body weight of the chemical carcinogen N-nitrosomethylurea or with vehicle. The animals were followed-up for 6 months. Fisher 344 rats were used as positive controls for tumor development.</p> <p>Results</p> <p>Eleven weeks after treatment, palpable tumors were detected in the Ws/Ws rats. The tumor incidence was 80% in Ws/Ws rats, while no tumors were observed in the wild type rats (p = 0.006). Our data show that the lack of c-kit is permissive for the development of mammary tumor in Ws/Ws rats treated with carcinogen.</p> <p>Conclusion</p> <p>We conclude that the lack of c-kit may contribute to an imbalanced homeostatic state in the mammary gland either by affecting signaling between stroma and epithelium, or through the lack of mast cells.</p
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