176 research outputs found
Fermion Representation Of The Rolling Tachyon Boundary Conformal Field Theory
A free fermion representation of the rolling tachyon boundary conformal field
theory is constructed. The representation is used to obtain an explicit,
compact, exact expression for the boundary state. We use the boundary state to
compute the disc and cylinder amplitudes for the half-S-brane.Comment: 27 page
Tachyon-Dilaton-induced Inflation as an alpha'-resummed String Background
Within the framework of a novel functional method on the world-sheet of the
string, we discuss simple but re-summed (in the Regge slope) inflationary
scenarios in the context of closed Bosonic strings, living in four target-space
dimensions, in the presence of non-trivial tachyon, dilaton and graviton
cosmological backgrounds. The inflationary solutions are argued to guarantee
the vanishing of the corresponding Weyl anomaly coefficients in a given
world-sheet renormalization scheme, thereby ensuring conformal invariance of
the corresponding sigma-model to all orders in the Regge slope. The key
property is the requirement of "homogeneity" of the corresponding Weyl anomaly
coefficients. Inflation entails appropriate relations between the dilaton and
tachyon field configurations, whose form can lead to either a de Sitter vacuum,
incompatible though (due to the cosmic horizons) with the perturbative string
scattering amplitudes, or to cosmic space-times involving brief inflationary
periods, interpolating smoothly between power-law and/or Minkowski Universes.
The latter situation is characterized by well-defined scattering amplitudes,
and is thus compatible with a perturbative string framework. It is this
scenario that we consider a self-consistent ground state in our framework,
which is based on local field redefinitions of background fields.Comment: 35 pages Latex, three eps figures incorporate
Tachyon Kinks on Unstable Dp-branes
In the context of tachyon effective theory coupled to Born-Infeld
electromagnetic fields, we obtain all possible singularity-free static flat
configurations of codimension one on unstable Dp-branes. Computed tension and
string charge density suggest that the obtained kinks are D(p-1) or
D(p-1)F1-branes.Comment: 22pages, LaTeX2e, 7figure
Closed String Field Theory with Dynamical D-brane
We consider a closed string field theory with an arbitrary matter current as
a source of the closed string field. We find that the source must satisfy a
constraint equation as a consequence of the BRST invariance of the theory. We
see that it corresponds to the covariant conservation law for the matter
current, and the equation of motion together with this constraint equation
determines the classical behavior of both the closed string field and the
matter. We then consider the boundary state (D-brane) as an example of a
source. We see that the ordinary boundary state cannot be a source of the
closed string field when the string coupling g turns on. By perturbative
expansion, we derive a recursion relation which represents the bulk
backreaction and the D-brane recoil. We also make a comment on the rolling
tachyon boundary state.Comment: 30 pages, LaTeX2e, no figures. Typos are correcte
Estimating the Cost of Executive Stock Options: Evidence from Switzerland
It is often argued that Black-Scholes (1973) values overstate the subjective NEWLINE value of stock options granted to risk-averse and under-diversified executives. NEWLINE We construct a “representative” Swiss executive and extend the certainty- NEWLINE equivalence approach presented by Hall and Murphy (2002) to assess NEWLINE the value-cost wedge of executive stock options. Even with low coefficients NEWLINE of relative risk aversion, the discount can be above 50% compared to the NEWLINE Black-Scholes values. Regression analysis reveals that the equilibrium level NEWLINE of executive compensation is explained by economic determinant variables NEWLINE such as firm size and growth opportunities, whereas the managers’ pay-forperformance NEWLINE sensitivity remains largely unexplained. Firms with larger NEWLINE boards of directors pay higher wages, indicating potentially unresolved NEWLINE agency conflicts. We reject the hypothesis that cross-sectional differences in NEWLINE the amount of executive pay vanish when risk-adjusted values are used as NEWLINE the dependent variable
Warped Tachyonic Inflation in Type IIB Flux Compactifications and the Open-String Completeness Conjecture
We consider a cosmological scenario within the KKLT framework for moduli
stabilization in string theory. The universal open string tachyon of decaying
non-BPS D-brane configurations is proposed to drive eternal topological
inflation. Flux-induced `warping' can provide the small slow-roll parameters
needed for successful inflation. Constraints on the parameter space leading to
sufficient number of e-folds, exit from inflation, density perturbations and
stabilization of the Kahler modulus are investigated. The conditions are
difficult to satisfy in Klebanov-Strassler throats but can be satisfied in T^3
fibrations and other generic Calabi-Yau manifolds. This requires large volume
and magnetic fluxes on the D-brane. The end of inflation may or may not lead to
cosmic strings depending on the original non-BPS configuration. A careful
investigation of initial conditions leading to a phenomenologically viable
model for inflation is carried out. The initial conditions are chosen on the
basis of Sen's open string completeness conjecture. We find time symmetrical
bounce solutions without initial singularities for k=1 FRW models which are
correlated with an inflationary period. Singular big-bang/big-crunch solutions
also exist but do not lead to inflation. There is an intriguing correlation
between having an inflationary universe in 4 dimensions and 6 compact
dimensions or a big-crunch singularity and decompactification.Comment: 43 pages, 9 figures. v3: Typos correcte
Development and validation of Prediction models for Risks of complications in Early-onset Pre-eclampsia (PREP):a prospective cohort study
Background: The prognosis of early-onset pre-eclampsia (before 34 weeks’ gestation) is variable. Accurate prediction of complications is required to plan appropriate management in high-risk women. Objective: To develop and validate prediction models for outcomes in early-onset pre-eclampsia. Design: Prospective cohort for model development, with validation in two external data sets. Setting: Model development: 53 obstetric units in the UK. Model transportability: PIERS (Pre-eclampsia Integrated Estimate of RiSk for mothers) and PETRA (Pre-Eclampsia TRial Amsterdam) studies. Participants: Pregnant women with early-onset pre-eclampsia. Sample size: Nine hundred and forty-six women in the model development data set and 850 women (634 in PIERS, 216 in PETRA) in the transportability (external validation) data sets. Predictors: The predictors were identified from systematic reviews of tests to predict complications in pre-eclampsia and were prioritised by Delphi survey. Main outcome measures: The primary outcome was the composite of adverse maternal outcomes established using Delphi surveys. The secondary outcome was the composite of fetal and neonatal complications. Analysis: We developed two prediction models: a logistic regression model (PREP-L) to assess the overall risk of any maternal outcome until postnatal discharge and a survival analysis model (PREP-S) to obtain individual risk estimates at daily intervals from diagnosis until 34 weeks. Shrinkage was used to adjust for overoptimism of predictor effects. For internal validation (of the full models in the development data) and external validation (of the reduced models in the transportability data), we computed the ability of the models to discriminate between those with and without poor outcomes (c-statistic), and the agreement between predicted and observed risk (calibration slope). Results: The PREP-L model included maternal age, gestational age at diagnosis, medical history, systolic blood pressure, urine protein-to-creatinine ratio, platelet count, serum urea concentration, oxygen saturation, baseline treatment with antihypertensive drugs and administration of magnesium sulphate. The PREP-S model additionally included exaggerated tendon reflexes and serum alanine aminotransaminase and creatinine concentration. Both models showed good discrimination for maternal complications, with an optimism-adjusted c-statistic of 0.82 [95% confidence interval (CI) 0.80 to 0.84] for PREP-L and 0.75 (95% CI 0.73 to 0.78) for the PREP-S model in the internal validation. External validation of the reduced PREP-L model showed good performance with a c-statistic of 0.81 (95% CI 0.77 to 0.85) in PIERS and 0.75 (95% CI 0.64 to 0.86) in PETRA cohorts for maternal complications, and calibrated well with slopes of 0.93 (95% CI 0.72 to 1.10) and 0.90 (95% CI 0.48 to 1.32), respectively. In the PIERS data set, the reduced PREP-S model had a c-statistic of 0.71 (95% CI 0.67 to 0.75) and a calibration slope of 0.67 (95% CI 0.56 to 0.79). Low gestational age at diagnosis, high urine protein-to-creatinine ratio, increased serum urea concentration, treatment with antihypertensive drugs, magnesium sulphate, abnormal uterine artery Doppler scan findings and estimated fetal weight below the 10th centile were associated with fetal complications. Conclusions: The PREP-L model provided individualised risk estimates in early-onset pre-eclampsia to plan management of high-or low-risk individuals. The PREP-S model has the potential to be used as a triage tool for risk assessment. The impacts of the model use on outcomes need further evaluation
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