Attractions to radiation-like eras in superstring cosmologies

We review the cosmology induced by finite temperature and quantum effects on non-supersymmetric string models. We show the evolution is attracted to radiation-like solutions after the Hagedorn era and before the electroweak phase transition. This mechanism generates a hierarchy between the Planck mass and the supersymmetry breaking scale. A dynamical change of space-time dimension can take place.Comment: 8 page

Super no-scale models in string theory

We consider "super no-scale models" in the framework of the heterotic string, where the N=4,2,1 --> 0 spontaneous breaking of supersymmetry is induced by geometrical fluxes realizing a stringy Scherk-Schwarz perturbative mechanism. Classically, these backgrounds are characterized by a boson/fermion degeneracy at the massless level, even if supersymmetry is broken. At the 1-loop level, the vacuum energy is exponentially suppressed, provided the supersymmetry breaking scale is small, m_{3/2} << M_{string}. We show that the "super no-scale string models" under consideration are free of Hagedorn-like tachyonic singularities, even when the supersymmetry breaking scale is large, m_{3/2} ~ M_{string}. The vacuum energy decreases monotonically and converges exponentially to zero, when m_{3/2} varies from M_{string} to 0. We also show that all Wilson lines associated to asymptotically free gauge symmetries are dynamically stabilized by the 1-loop effective potential, while those corresponding to non-asymtotically free gauge groups lead to instabilities and condense. The Wilson lines of the conformal gauge symmetries remain massless. When stable, the stringy super no-scale models admit low energy effective actions, where decoupling gravity yields theories in flat spacetime, with softly broken supersymmetry.Comment: 1+46 pages, 1 figur

An N=1 Triality by Spectrum Matching

On promoting the type IIA side of the N=1 Heterotic/type IIA dual pairs of [1] to M-theory on a `barely G_2 Manifold' of [2], by spectrum-matching we show a possible triality between Heterotic on a self-mirror Calabi-Yau, M-theory on the above `barely G_2-Manifold' constructed from the Calabi-Yau on the type IIA side and $F$-theory on an elliptically fibered Calabi-Yau 4-fold fibered over a trivially rationally ruled CP^1 x E base, E being the Enriques surface. We raise an apparent puzzle on the F-theory side, namely, the Hodge data of the 4-fold derived can not be obtained by a naive freely acting orbifold of CY_3(3,243) x T^2 as one might have guessed on the basis of arguments related to dualities involving string, M and (definition of) F theories. There are some interesting properties of the antiholomorphic involution used in \cite{VW} for constructing the type IIA orientifold and by us in constructing the 'barely G_2 manifold', that we also study.Comment: 14 pages, LaTex; v3: journal versio

Duality of N=2 Heterotic -- Type I Compactifications in Four Dimensions

We discuss type I -- heterotic duality in four-dimensional models obtained as a Coulomb phase of the six-dimensional U(16) orientifold model compactified on T^2 with arbitrary SU(16) Wilson lines. We show that Kahler potentials, gauge threshold corrections and the infinite tower of higher derivative F-terms agree in the limit that corresponds to weak coupling, large T^2 heterotic compactifications. On the type I side, all these quantities are completely determined by the spectrum of N=2 BPS states that originate from D=6 massless superstring modes.Comment: 22 pages, LaTeX; typos corrected and references adde

Phases of supersymmetric gauge theories from M-theory on G_2 manifolds

We consider M-theory on compact spaces of G_2 holonomy constructed as orbifolds of the form (CY x S^1)/Z_2 with fixed point set \Sigma on the CY. This describes N=1 SU(2) gauge theories with b_1(\Sigma) chiral multiplets in the adjoint. For b_1=0, it generalizes to compact manifolds the study of the phase transition from the non-Abelian to the confining phase through geometrical S^3 flops. For b_1=1, the non-Abelian and Coulomb phases are realized, where the latter arises by desingularization of the fixed point set, while an S^2 x S^1 flop occurs. In addition, an extremal transition between G_2 spaces can take place at conifold points of the CY moduli space where unoriented membranes wrapped on CP^1 and RP^2 become massless.Comment: 21 pages, LaTeX, v2: mistake in spectra corrected, reference added, v3: one more reference added, version published in JHE

D-Instanton Corrections as (p,q)-String Effects and Non-Renormalization Theorems

We discuss higher derivative interactions in the type IIB superstring in ten dimensions. From the fundamental string point of view, the non-perturbative corrections are due to D-instantons. We argue that they can alternatively be understood as arising from $(p,q)$-strings. We derive a non-renormalization theorem for eight-derivative bosonic interactions, which states that terms involving either NS-NS or R-R fields occur at tree-level and one-loop only. By using the $SL(2, Z)$ symmetry of M-theory on $T^2$, we show that in order for the possible $R^{3m+1} (m=1,2,...)$ interactions in M-theory to have a consistent perturbative expansion in nine dimensions, $m$ must be odd. Thus, only $R^{6N+4} (N=0,1,...)$ terms can be present in M-theory and their string theory counterparts arise at $N$ and $2N+1$ loops. Finally, we treat an example of fermionic term.Comment: 24 pages, latex, additional arguments for the perturbative form of the eight-derivative interaction

Clinical Implementation of Hypofractionated Radiation therapy for Lung Malignancies

For patients with oligometastases, metastases limited in number and site, the use of radiation therapy treatment with a hypofractionated dose scheme has been proposed as a potential ablative approach. There are a limited number of prospective studies looking at hypofractionated radiation therapy (HRT) for lung oligometastasis. Normal lung tissue complication and radiation planning technique are significant limiting factors for the implementation of hypofractionated lung metastasis. The problem statement of this study is how to improve the clinical implementation of HRT for lung metastasis exploring lung toxicity predictors, and developing an efficient radiation planning method. In the first study, we analyzed the dose distribution for 28 patients with lung oligometastasis and treated with HRT to multiple metastases in the lungs. We identified several significant predictors for lung radiation pneumonitis (RP) including the mean lung dose (MLD), V13 and V20. In addition a dose-effect relation between the lung normalized total dose (NTD) and RP may exist up to 48 Gy in three fractions. The dose-response parameters derived in our study appear to agree with other hypofractionated results published in the literature. In the second study, we used an inverse planning algorithm to develop a new radiation planning method by limiting the number of segments per beam angle down to 1 segment. Single segment plans were able to significantly improve tumor coverage and conformality, reduce the risk of lung RP, while simplifying the planning process and delivery. Target conformality and normal lung tissue sparing did not gain much improvement from an increase of plan complexity to five segments over the simplified one segment technique. The automation of our method is a good alternative to more traditional methods and offers significant dosimetric benefits. In the third study, we verified the single segment planning technique via patient specific quality assurance (QA) in a motion phantom. We found good agreement between calculated and measured doses via thermoluminescent detectors (TLD) inside the target. A dose to distance agreement of 3%/3 mm and 2%/2 mm between calculation and film measurements for representative plans in a motion phantom was verified at 98.99% and 97.15%, respectively

On the equivalence of N=1 brane worlds and geometric singularities with flux

We consider Kaluza Klein reductions of M-theory on the Z_N orbifold of the spin bundle over S^3 along two different U(1) isometries. The first one gives rise to the familiar ``large N duality'' of the N=1 SU(N) gauge theory in which the UV is realized as the world-volume theory of N D6-branes wrapped on S^3, whereas the IR involves N units of RR flux through an S^2. The second reduction gives an equivalent version of this duality in which the UV is realized geometrically in terms of an S^2 of A_{N-1} singularities, with one unit of RR flux through the S^2. The IR is reached via a geometric transition and involves a single D6 brane on a lens space S^3/Z_N or, alternatively, a singular background (S^2\times R^4)/Z_N, with one unit of RR flux through S^2 and, localized at the singularities, an action of their stabilizer group in the U(1) RR gauge bundle, so that no massless twisted states occur. We also consider linear sigma-model descriptions of these backgrounds.Comment: 25 pages, LaTeX; v2: one reference added, published versio

Time-Varying Coefficients in a GMM Framework: Estimation of a Forward Looking Taylor Rule for the Federal Reserve.

This article deals with the estimation of a time-varying coefficients equation with endogenous regressors. A non-parametric approach is proposed, combining the Generalized Method of Moments (GMM) with the smoothing splines litterature as in Hodrick and Prescott (1981). This new method is used to analyze the evolution of a forward-looking Taylor rule for the Federal Reserve (FED) from 1960 until 2006. It suggests that monetary policy accommodated inflation during the 60s and the 70s whereas the chairmanship of P. Volcker was a turning point toward a more aggressive stance on inflation. In addition, monetary policy became more and more countercyclical.Monetary policy rules ; Generalized Method of Moments ; Time-varying coefficients ; Smoothing splines.