31 research outputs found
Refined Topological Vertex and Instanton Counting
It has been proposed recently that topological A-model string amplitudes for
toric Calabi-Yau 3-folds in non self-dual graviphoton background can be
caluculated by a diagrammatic method that is called the ``refined topological
vertex''. We compute the extended A-model amplitudes for SU(N)-geometries using
the proposed vertex. If the refined topological vertex is valid, these
computations should give rise to the Nekrasov's partition functions of N=2
SU(N) gauge theories via the geometric engineering. In this article, we verify
the proposal by confirming the equivalence between the refined A-model
amplitude and the K-theoretic version of the Nekrasov's partition function by
explicit computation.Comment: 22 pages, 6 figures, minor correction
Cosmology from Moduli Dynamics
We investigate moduli field dynamics in supergravity/M-theory like set ups
where we turn on fluxes along some or all of the extra dimensions. As has been
argued in the context of string theory, we observe that the fluxes tend to
stabilize the squashing (or shape) modes. Generically we find that at late
times the shape is frozen while the radion evolves as a quintessence field. At
earlier times we have a phase of radiation domination where both the volume and
the shape moduli are slowly evolving. However, depending on the initial
conditions and the parameters of the theory, like the value of the fluxes,
curvature of the internal manifold and so on, the dynamics of the internal
manifold can be richer with interesting cosmological consequences, including
inflation.Comment: 38 pages, 6 figures; references adde
Casimir Energies for 6D Supergravities Compactified on T_2/Z_N with Wilson Lines
We compute (as functions of the shape and Wilson-line moduli) the one-loop
Casimir energy induced by higher-dimensional supergravities compactified from
6D to 4D on 2-tori, and on some of their Z_N orbifolds. Detailed calculations
are given for a 6D scalar field having an arbitrary 6D mass m, and we show how
to extend these results to higher-spin fields for supersymmetric 6D theories.
Particular attention is paid to regularization issues and to the identification
of the divergences of the potential, as well as the dependence of the result on
m, including limits for which m^2 A> 1 where A is the volume of
the internal 2 dimensions. Our calculation extends those in the literature to
very general boundary conditions for fields about the various cycles of these
geometries. The results have potential applications towards Supersymmetric
Large Extra Dimensions (SLED) as a theory of the Dark Energy. First, they
provide an explicit calculation within which to follow the dependence of the
result on the mass of the bulk states which travel within the loop, and for
heavy masses these results bear out the more general analysis of the
UV-sensitivity obtained using heat-kernel methods. Second, because the
potentials we find describe the dynamics of the classical flat directions of
these compactifications, within SLED they would describe the present-day
dynamics of the Dark Energy.Comment: 40 pages, 7 figure
Phase Separation of Rigid-Rod Suspensions in Shear Flow
We analyze the behavior of a suspension of rigid rod-like particles in shear
flow using a modified version of the Doi model, and construct diagrams for
phase coexistence under conditions of constant imposed stress and constant
imposed strain rate, among paranematic, flow-aligning nematic, and log-rolling
nematic states. We calculate the effective constitutive relations that would be
measured through the regime of phase separation into shear bands. We calculate
phase coexistence by examining the stability of interfacial steady states and
find a wide range of possible ``phase'' behaviors.Comment: 23 pages 19 figures, revised version to be published in Physical
Review
On Smooth Time-Dependent Orbifolds and Null Singularities
We study string theory on a non-singular time-dependent orbifold of flat
space, known as the `null-brane'. The orbifold group, which involves only
space-like identifications, is obtained by a combined action of a null Lorentz
transformation and a constant shift in an extra direction. In the limit where
the shift goes to zero, the geometry of this orbifold reproduces an orbifold
with a light-like singularity, which was recently studied by Liu, Moore and
Seiberg (hep-th/0204168). We find that the backreaction on the geometry due to
a test particle can be made arbitrarily small, and that there are scattering
processes which can be studied in the approximation of a constant background.
We quantize strings on this orbifold and calculate the torus partition
function. We construct a basis of states on the smooth orbifold whose tree
level string interactions are nonsingular. We discuss the existence of physical
modes in the singular orbifold which resolve the singularity. We also describe
another way of making the singular orbifold smooth which involves a sandwich
pp-wave.Comment: 24 pages, one figur
Cosmological Spacetimes from Negative Tension Brane Backgrounds
We identify a time-dependent class of metrics with potential applications to
cosmology, which emerge from negative-tension branes. The cosmology is based on
a general class of solutions to Einstein-dilaton-Maxwell theory, presented in
{hep-th/0106120}. We argue that solutions with hyperbolic or planar symmetry
describe the gravitational interactions of a pair of negative-tension
-branes. These spacetimes are static near each brane, but become
time-dependent and expanding at late epoch -- in some cases asymptotically
approaching flat space. We interpret this expansion as being the spacetime's
response to the branes' presence. The time-dependent regions provide explicit
examples of cosmological spacetimes with past horizons and no past naked
singularities. The past horizons can be interpreted as S-branes. We prove that
the singularities in the static regions are repulsive to time-like geodesics,
extract a cosmological `bounce' interpretation, compute the explicit charge and
tension of the branes, analyse the classical stability of the solution (in
particular of the horizons) and study particle production, deriving a general
expression for Hawking's temperature as well as the associated entropy.Comment: 43 pages, 8 figures. Published versio
Prospects for e+e- physics at Frascati between the phi and the psi
We present a detailed study, done in the framework of the INFN 2006 Roadmap,
of the prospects for e+e- physics at the Frascati National Laboratories. The
physics case for an e+e- collider running at high luminosity at the phi
resonance energy and also reaching a maximum center of mass energy of 2.5 GeV
is discussed, together with the specific aspects of a very high luminosity
tau-charm factory. Subjects connected to Kaon decay physics are not discussed
here, being part of another INFN Roadmap working group. The significance of the
project and the impact on INFN are also discussed. All the documentation
related to the activities of the working group can be found in
http://www.roma1.infn.it/people/bini/roadmap.html.Comment: INFN Roadmap Report: 86 pages, 25 figures, 9 table
Accuracy Assessment of the ESA CCI 20M Land Cover Map: Kenya, Gabon, Ivory Coast and South Africa
This working paper presents the overall and spatial accuracy assessment of the European Space Agency (ESA) 20 m prototype land cover map for Africa for four countries: Kenya, Gabon, Ivory Coast and South Africa. This accuracy assessment was undertaken as part of the ESA-funded CrowdVal project. The results varied from 44% (for South Africa) to 91% (for Gabon). In the case of Kenya (56% overall accuracy) and South Africa, these values are largely caused by the confusion between grassland and shrubland. However, if a weighted confusion matrix is used, which diminishes the importance of the confusion between grassland and shrubs, the overall accuracy for Kenya increases to 79% and for South Africa, 75%. The overall accuracy for Ivory Coast (47%) is a result of a highly fragmented land cover, which makes it a difficult country to map with remote sensing. The exception was Gabon with a high overall accuracy of 91%, but this can be explained by the high amount of tree cover across the country, which is a relatively easy class to map
Missed Opportunities for Loco-Regional Treatment of Elderly Women with Breast Cancer
Background: Factors associated with surgery, adjuvant radiotherapy, and chemotherapy and whether there were missed opportunities for treatment in elderly patients were determined in an Asian setting. Methods: All 5616 patients, diagnosed with breast cancer in University Malaya Medical Centre from 1999 to 2013 were included. In 945 elderly patients (aged 65 years and above), multivariable logistic regression was performed to identify factors associated with treatment, following adjustment for age, ethnicity, tumor, and other treatment characteristics. The impact of lack of treatment on survival of the elderly was assessed while accounting for comorbidities. Results: One in five elderly patients had comorbidities. Compared to younger patients, the elderly had more favorable tumor characteristics, and received less loco-regional treatment and chemotherapy. Within stage I–IIIa elderly breast cancer patients, 10 % did not receive any surgery. These patients were older, more likely to be Malays, have comorbidities, and bigger tumors. In elderlies with indications for adjuvant radiotherapy, no irradiation (30 %) was associated with increasing age, comorbidity, and the absence of systemic therapy. Hormone therapy was optimal, but only 35 % of elderly women with ER negative tumors received chemotherapy. Compared to elderly women who received adequate treatment, those not receiving surgery (adjusted hazard ratio: 2.30, 95 %CI: 1.10–4.79), or radiotherapy (adjusted hazard ratio: 1.56, 95 %CI: 1.10–2.19), were associated with higher mortality. Less than 25 % of the survival discrepancy between elderly women receiving loco-regional treatment and no treatment were attributed to excess comorbidities in untreated patients. Conclusion: While the presence of comorbidities significantly influenced loco-regional treatment decisions in the elderly, it was only able to explain the lower survival rates in untreated patients up to a certain extent, suggesting missed opportunities for treatment
Systematic review of primary total hip arthroplasty using titanium-titanium modular-neck prostheses: the true risk of revision
Aims: Modular-neck femoral implants are used to enable more variability in femoral neck version, offset and length. It has been reported that these implants carry a higher rate of revision. The aim of this review was to assess the overall and cause-specific revision rate of titanium-titanium alloy modular-neck implants in primary total hip arthroplasty (THA). Methods: A systematic review was conducted following PRISMA guidelines and utilising multiple databases. All results were screened for eligibility. Studies published from 2000 onwards, using a current-generation, titanium-titanium, modular-neck implant were included. Overall and cause-specific revision rates were analysed, comparing to fixed-neck prostheses where applicable. Results: 920 studies were screened. After applying exclusion criteria, 23 were assessed in full and 14 included. These consisted of 12 case series and 2 joint registry analyses. 21,841 patients underwent a modular-neck implant with a weighted mean follow-up of 5.7 years, mean age of 62.4 years, and average body mass index (BMI) of 28.4kg/m2. The overall revision rate was 3.95% and 2.98% for modular and fixed-neck prostheses, respectively. For studies with >5 years follow-up the mean revision rate was 3.08%. There was no difference in cause-specific revision rates by implant design. Mean improvement in Harris Hip Score was 41.9. Conclusions: At medium-term, revision rates for titanium-titanium primary modular-neck THA are acceptable. These prostheses are a sensible management option in patients with considerable anatomical hip deformity not amenable to correction with standard fixed-neck implants. Patients of male gender, high BMI and requiring prostheses with a larger neck, offset or head are at higher risk of implant failure. © The Author(s) 2020