2,449 research outputs found
Gauge Threshold Corrections for Local String Models
We study gauge threshold corrections for local brane models embedded in a
large compact space. A large bulk volume gives important contributions to the
Konishi and super-Weyl anomalies and the effective field theory analysis
implies the unification scale should be enhanced in a model-independent way
from M_s to R M_s. For local D3/D3 models this result is supported by the
explicit string computations. In this case the scale R M_s comes from the
necessity of global cancellation of RR tadpoles sourced by the local model. We
also study D3/D7 models and discuss discrepancies with the effective field
theory analysis. We comment on phenomenological implications for gauge coupling
unification and for the GUT scale.Comment: 30 pages; v2: references added, minor typos correcte
Unification, KK-thresholds and the top Yukawa coupling in F-theory GUTs
In a class of F-theory SU(5) GUTs the low energy chiral mass spectrum is
obtained from rank one fermion mass textures with a hierarchical structure
organised by U(1) symmetries embedded in the exceptional E_8 group. In these
theories chiral fields reside on matter `curves' and the tree level masses are
computed from integrals of overlapping wavefuctions of the particles at the
triple intersection points. This calculation requires knowledge of the exact
form of the wavefuctions. In this work we propose a way to obtain a reliable
estimate of the various quantities which determine the strength of the Yukawa
couplings. We use previous analysis of KK threshold effects to determine the
(ratios of) heavy mass scales of the theory which are involved in the
normalization of the wave functions. We consider similar effects from the
chiral spectrum of these models and discuss possible constraints on the
emerging matter content. In this approach, we find that the Yukawa couplings
can be determined solely from the U(1) charges of the states in the
`intersection' and the torsion which is a topological invariant quantity. We
apply the results to a viable SU(5) model with minimal spectrum which satisfies
all the constraints imposed by our analysis. We use renormalization group
analysis to estimate the top and bottom masses and find that they are in
agreement with the experimental values.Comment: 28 pages, 2 figure
Scavenger community response to the removal of a dominant scavenger
The alteration of scavenging communities can reduce basic ecosystem services and increase risks to human and wildlife health. Recent work demonstrated that scavenging communities in agricultural landscapes are extremely efficient: superabundant mesopredators sequestered system energy by dominating scavenging activity. To explore how the disturbance of these communities affects the stability of carrion removal as an ecosystem function, we experimentally manipulated a scavenging community within an agricultural landscape by reducing the abundance of the dominant scavenger, raccoons Procyon lotor. We then monitored the fates of 676 mouse Mus musculus carcasses placed in 13 control and 13 removal woodlots from June 2007 – May 2008. The diversity of vertebrate scavengers did not change between control and removal woodlots and scavenging by invertebrates was unaffected by our experiment. Although Virginia opossums Didelphis virginiana and other scavengers exhibited a functional response when raccoons were reduced in abundance, the increases did not change the proportional allocation of carcasses among scavengers. Finally, the reduced abundance of a major scavenger affected system efficiency. More carcasses remained un-scavenged at the end of trials in removal woodlots than in control woodlots. This experiment demonstrates the vulnerability of a critical ecosystem service, carrion removal, to perturbations of the scavenging community and serves to highlight the method by which scavenger communities may respond to perturbations
Quantum Invariants, Modular Forms, and Lattice Points II
We study the SU(2) Witten--Reshetikhin--Turaev invariant for the Seifert
fibered homology spheres with M-exceptional fibers. We show that the WRT
invariant can be written in terms of (differential of) the Eichler integrals of
modular forms with weight 1/2 and 3/2. By use of nearly modular property of the
Eichler integrals we shall obtain asymptotic expansions of the WRT invariant in
the large-N limit. We further reveal that the number of the gauge equivalent
classes of flat connections, which dominate the asymptotics of the WRT
invariant in N ->\infinity, is related to the number of integral lattice points
inside the M-dimensional tetrahedron
T-Branes and Monodromy
We introduce T-branes, or "triangular branes," which are novel non-abelian
bound states of branes characterized by the condition that on some loci, their
matrix of normal deformations, or Higgs field, is upper triangular. These
configurations refine the notion of monodromic branes which have recently
played a key role in F-theory phenomenology. We show how localized matter
living on complex codimension one subspaces emerge, and explain how to compute
their Yukawa couplings, which are localized in complex codimension two. Not
only do T-branes clarify what is meant by brane monodromy, they also open up a
vast array of new possibilities both for phenomenological constructions and for
purely theoretical applications. We show that for a general T-brane, the
eigenvalues of the Higgs field can fail to capture the spectrum of localized
modes. In particular, this provides a method for evading some constraints on
F-theory GUTs which have assumed that the spectral equation for the Higgs field
completely determines a local model.Comment: 110 pages, 5 figure
The Globular Cluster System of the Virgo Dwarf Elliptical Galaxy VCC 1087
We have analysed the globular cluster (GC) system of the nucleated dwarf
elliptical galaxy VCC 1087 in the Virgo cluster, based on Keck/LRIS
spectroscopy and archival HST/ACS imaging. We estimate VCC 1087 hosts a total
population of 77+/-19 GCs, which corresponds to a relatively high V-band
specific frequency of 5.8+/-1.4. The g-z color distribution of the GCs shows a
blue (metal-poor) peak with a tail of redder (metal-rich) clusters similar in
color to those seen in luminous ellipticals. Spectroscopy of a subsample of 12
GCs suggests that the GC system is old and coeval (~10 Gyr), with a fairly
broad metallicity distribution (-1.8<[m/H]<-0.8). In contrast, an integrated
spectrum of the underlying galaxy starlight reveals that its optical luminosity
is dominated by metal-rich, intermediate-aged stars. Radial velocities of the
GCs suggest rotation close to the major axis of the galaxy, and this rotation
is dynamically significant with (v/sigma)^* >1. A compilation of the kinematics
of the GC systems of 9 early-type galaxies shows surprising diversity in the
v/sigma parameter for GC systems. In this context, the GC system of VCC 1087
exhibits the most significant rotation to velocity dispersion signature.
Modeling the velocity dispersion profile of the GCs and galaxy stars suggest
fairly constant mass-to-light ratios of ~3 out to 6.5 kpc. The present
observations can entertain both baryonic and non-baryonic solutions, and GC
velocities at larger radii would be most valuable with regard to this issue. We
discuss the evolution of VCC 1087 in terms of the galaxy ``harassment''
scenario, and conclude that this galaxy may well be the remains of a faded,
tidally perturbed Sc spiral [abridged].Comment: 17 pages, 13 figures, to appear in the A
A SAURON study of dwarf elliptical galaxies in the Virgo Cluster: kinematics and stellar populations
Dwarf elliptical galaxies (dEs) are the most common galaxy type in nearby
galaxy clusters; even so, many of their basic properties have yet to be
quantified. Here we present the results of our study of 4 Virgo dwarf
ellipticals obtained with the SAURON integral field unit on the William
Herschel Telescope (La Palma, Spain). While traditional long-slit observations
are likely to miss more complicated kinematic features, with SAURON we are able
to study both kinematics and stellar populations in two dimensions, obtaining a
much more detailed view of the mass distribution and star formation histories.
What is visible even in such a small sample is that dEs are not a uniform
group, not only morphologically, but also as far as their kinematic and stellar
population properties are concerned. We find the presence of substructures,
varying degrees of flattening and of rotation, as well as differences in age
and metallicity gradients. We confirm that two of our galaxies are
significantly flattened, yet non-rotating objects, which makes them likely
triaxial systems. The comparison between the dwarf and the giant groups shows
that dEs could be a low-mass extension of Es in the sense that they do seem to
follow the same trends with mass. However, dEs as progenitors of Es seem less
likely as we have seen that dEs have much lower abundance ratios.Comment: 8 pages, 6 figures; to appear in the proceedings of the JENAM 2010
Symposium on Dwarf Galaxies (Lisbon, September 9-10, 2010); minor edits and
references adde
The sensitivity of harassment to orbit: Mass loss from early-type dwarfs in galaxy clusters
We conduct a comprehensive numerical study of the orbital dependence of harassment on early-type dwarfs consisting of 168 different orbits within a realistic, Virgo-like cluster, varying in eccentricity and pericentre distance. We find harassment is only effective at stripping stars or truncating their stellar discs for orbits that enter deep into the cluster core. Comparing to the orbital distribution in cosmological simulations, we find that the majority of the orbits (more than three quarters) result in no stellar mass loss. We also study the effects on the radial profiles of the globular cluster systems of early-type dwarfs. We find these are significantly altered only if harassment is very strong. This suggests that perhaps most early-type dwarfs in clusters such as Virgo have not suffered any tidal stripping of stars or globular clusters due to harassment, as these components are safely embedded deep within their dark matter halo. We demonstrate that this result is actually consistent with an earlier study of harassment of dwarf galaxies, despite the apparent contradiction. Those few dwarf models that do suffer stellar stripping are found out to the virial radius of the cluster at redshift = 0, which mixes them in with less strongly harassed galaxies. However when placed on phase-space diagrams, strongly harassed galaxies are found offset to lower velocities compared to weakly harassed galaxies. This remains true in a cosmological simulation, even when haloes have a wide range of masses and concentrations. Thus phase-space diagrams may be a useful tool for determining the relative likelihood that galaxies have been strongly or weakly harassed
Embedding Fractional Quantum Hall Solitons in M-theory Compactifications
We engineer U(1)^n Chern-Simons type theories describing fractional quantum
Hall solitons (QHS) in 1+2 dimensions from M-theory compactified on eight
dimensional hyper-K\"{a}hler manifolds as target space of N=4 sigma model.
Based on M-theory/Type IIA duality, the systems can be modeled by considering
D6-branes wrapping intersecting Hirzebruch surfaces F_0's arranged as ADE
Dynkin Diagrams and interacting with higher dimensional R-R gauge fields. In
the case of finite Dynkin quivers, we recover well known values of the filling
factor observed experimentally including Laughlin, Haldane and Jain series.Comment: Latex, 14 pages. Modified version, to appear in IJGMM
Field-Tuned Superconductor-Insulator Transition in BaPb1-xBixO3
BaPbBiO is found to exhibit a field-tuned superconductor to
insulator transition for Bi compositions 0.24 0.29. The
magnetoresistance of optimally doped samples manifests a
temperature-independent crossing point and scaling of the form
, where is the field determined
by the temperature-independent crossing point, and = 0.69 0.03.
High resolution transmission electron microscopy measurements reveal a complex
intergrown nanostructure comprising tetragonal and orthorhombic polymorphs.
Data are analyzed in terms of both a classical effective medium theory and a
field-tuned quantum phase transition, neither of which provides a completely
satisfactory explanation for this remarkable phenomenology
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