Aspects of the free field description of string theory on AdS_3

The near boundary limit of string theory in AdS_3 is analysed using the Wakimoto free field representation of SL(2,R). The theory is considered as a direct product of the SL(2,R)/U(1) coset and a free boson. Correlation functions are constructed generalizing to the non-compact case the integral representation of conformal blocks introduced by Dotsenko in the compact SU(2) CFT. Sectors of the theory obtained by spectral flow manifestly appear. The formalism naturally leads to consider scattering processes violating winding number conservation. The consistency of the procedure is verified in the factorization limit.Comment: 25 pages, typos corrected, references added, to appear in JHE

T-duality and α′\alpha'-corrections

We construct an $O(d,d)$ invariant universal formulation of the first-order $\alpha'$-corrections of the string effective actions involving the dilaton, metric and two-form fields. Two free parameters interpolate between four-derivative terms that are even and odd with respect to a $Z_2$-parity transformation that changes the sign of the two-form field. The $Z_2$-symmetric model reproduces the closed bosonic string, and the heterotic string effective action is obtained through a $Z_2$-parity-breaking choice of parameters. The theory is an extension of the generalized frame formulation of Double Field Theory, in which the gauge transformations are deformed by a first-order generalized Green-Schwarz transformation. This deformation defines a duality covariant gauge principle that requires and fixes the four-derivative terms. We discuss the $O(d,d)$ structure of the theory and the (non-)covariance of the required field redefinitions.Comment: 35 pages, v2: Minor corrections, published in JHE

Double Field Theory: A Pedagogical Review

Double Field Theory (DFT) is a proposal to incorporate T-duality, a distinctive symmetry of string theory, as a symmetry of a field theory defined on a double configuration space. The aim of this review is to provide a pedagogical presentation of DFT and its applications. We first introduce some basic ideas on T-duality and supergravity in order to proceed to the construction of generalized diffeomorphisms and an invariant action on the double space. Steps towards the construction of a geometry on the double space are discussed. We then address generalized Scherk-Schwarz compactifications of DFT and their connection to gauged supergravity and flux compactifications. We also discuss U-duality extensions, and present a brief parcours on world-sheet approaches to DFT. Finally, we provide a summary of other developments and applications that are not discussed in detail in the review.Comment: 121 pages, invited review for Class. Quantum Grav; v2: Updated reference

Fusion Rules in N=1 Superconformal Minimal Models

The generalization to N=1 superconformal minimal models of the relation between the modular transformation matrix and the fusion rules in rational conformal field theories, the Verlinde theorem, is shown to provide complete information about the fusion rules, including their fermionic parity. The results for the superconformal Tricritical Ising and Ashkin-Teller models agree with the known rational conformal formulation. The Coulomb gas description of correlation functions in the Ramond sector of N=1 minimal models is also discussed and a previous formulation is completed.Comment: 13 pages, latex, to appear in Phys. Lett.

Heterotic α\alpha'-corrections in Double Field Theory

We extend the generalized flux formulation of Double Field Theory to include all the first order bosonic contributions to the $\alpha '$ expansion of the heterotic string low energy effective theory. The generalized tangent space and duality group are enhanced by $\alpha'$ corrections, and the gauge symmetries are generated by the usual (gauged) generalized Lie derivative in the extended space. The generalized frame receives derivative corrections through the spin connection with torsion, which is incorporated as a new degree of freedom in the extended bein. We compute the generalized fluxes and find the Riemann curvature tensor with torsion as one of their components. All the four-derivative terms of the action, Bianchi identities and equations of motion are reproduced. Using this formalism, we obtain the first order $\alpha'$ corrections to the heterotic Buscher rules. The relation of our results to alternative formulations in the literature is discussed and future research directions are outlined.Comment: Published versio

Cyclic Permutations in Kazama-Suzuki String Models

Moddings by cyclic permutation symmetries are performed in 4-dimensional strings, built up from N=2 coset models of the type $CP_m=SU(m+1)/SU(m)\times U(1)$. For some exemplifying cases, the massless chiral and antichiral states of $E_6$ are computed. The extent of the equivalence between different conformal invariant theories which possess equal chiral rings is analyzed.Comment: 26 pages, LaTex fil

Correlators in AdS_3 string theory

The computation of two and three point functions in the Coulomb gas free field approach to string theory in the SL(2,R)/U(1) black hole background is reviewed. An interesting relation arises when comparing the results obtained using two different screening operators. The formalism is then modified to study string theory propagating in AdS$_3$ which is considered as the direct product of the SL(2)/U(1) coset times a timelike free boson. This representation allows to naturally include the spectral flow symmetry and winding number in vertex operators and correlation functions. Two and three point tachyon amplitudes are computed in this new scenario and the results coincide with previous reports in the literature. Novel expressions are found for processes violating winding number conservation.Comment: 25 pages; v2: typos corrected and references added; v3: minor change

Comprehensive partnering management model for highway construction projects delivered using traditional and alternative methods

This dissertation is designed to explore the use of partnering as a decision tool for improving highway construction project performance regarding the project delivery method used by the transportation agencies. Project partnering was implemented in response to project performance issues that are often caused by the adversarial relationships that characterize the industry. Partnering is a tool to improve relationships by providing a framework for open communication and joint problem solving whose goal is to obtain win/win outcomes. Despite the widespread use of partnering, the literature review did not reveal previous research that quantifies the costs and benefits of implementing partnering over an extended period of time. The following three issues are of primary concern to all public transportation agencies during project development and delivery: (1) selecting the appropriate project delivery method, (2) maximizing project cost/time certainty while minimizing disruptions due to disputes, and (3) ensuring proactive project quality management while creating a safe environment for both workers and the traveling public. The increased use of alternative project delivery methods has caused the above issues to become increasingly interrelated and created a project management challenge for state departments of transportation (DOT). Not only do these projects allow concurrent design and construction, but they also move at a faster pace, which demands a much higher degree of both integration and active collaboration to meet the demands of an aggressive schedule. Initially implemented by the Arizona DOT in 1996, partnering has been found to be an effective tool for creating the necessary atmosphere for honest, information-rich communication between the owner and its design consultants and construction contractors. However, the American Association of State Highway and Transportation Officials (AASHTO) Partnering Handbook was published in 1998 based only on traditional low bid project delivery and as such does not include guidance for partnering projects delivered using alternative contracting methods (ACM). This dissertation aims to contribute to the body of knowledge in partnering by extending the understanding of project level partnering, and documenting effective practice found in the research to the program level. This will be achieved by answering the following overarching research question: Are there quantifiable benefits for implementing project level partnering practices that can be accrued by institutionalizing the principles of partnering at the program level? The research findings will also have practical implications for transportation agencies which will address that gaps in current knowledge through the following objectives: develop decision-making procedures to select projects to be delivered using partnering based on their project performance metrics; develop a framework procedure to measure the impact of partnering practices; developing guidance for partnering projects delivered using ACMs; and develop a partnering organizational maturity model to identify potential areas of improvements at program level

Novel Insights into Membrane Transport from Computational Methodologies

Atomic-resolution imaging of the plasma membrane and its constituents has advanced significantly in recent years. However, membrane transport is profoundly reliant on dynamic processes ranging from highly concerted atomic fluctuations to large-scale conformational changes, which cannot be sufficiently described by static structural information. As a consequence, computational methodologies have become a prominent tool to investigate membrane organisation and dynamics. In particular, molecular dynamics simulation has proven to be a pertinent method to investigate how matter is transported through membranes, either directly through the membrane or via integral membrane proteins, in an appropriate level of detail. In this chapter, we will provide a brief overview of molecular dynamics simulations and related methodologies, and use prototypical biological systems to illustrate how these methods have contributed to our understanding of unassisted diffusion through membranes, passive diffusion through ion channels, signalling through receptors and active transport through transporters