Supergravity, D-brane Probes and thermal super Yang-Mills: a comparison

A D3-brane probe in the context of AdS/CFT correspondence at finite temperature is considered. The supergravity predictions for the physical effective couplings of the world-volume gauge theory of the probe brane are compared to those calculated in one-loop perturbation theory in the thermal gauge theory. It is argued that when the Higgs expectation value is much larger than the temperature, the supergravity result must agree with perturbative thermal Yang-Mills. This provides a perturbative test of the Maldacena conjecture. Predictions for the running electric and magnetic effective couplings, beyond perturbation theory are also obtained. Phenomenological applications for universe-branes are discussed. In particular mechanisms are suggested for reducing the induced cosmological constant and naturally obtaining a varying speed of light and a consequent inflation on the universe brane.Comment: LaTeX, 23 pages, 4 eps figures; v2: Comparison with Tseytlin-Yankielowicz ammended. Extra discussion on reduced supersymmetry on branes added; v3: A few minor changes to improve presentation. Version published in JHEP10(1999)1

Dissecting the string theory dual of QCD

Input from QCD and string theory is used in order to elucidate basic features of the string theory dual of QCD, It is argued that the relevant string theory is a five-dimensional version of the type-0 superstring. The vacuum solution is asymptotically AdS$_5$, and the geometry near the boundary is stringy. The structure of YM perturbation theory however emerges near the boundary. In the IR, the theory is argued to be well-approximated by a two-derivative truncation that takes into account strong coupling effects. This explains the success of previously proposed five-dimensional Eistein-dilaton gravity with an appropriate potential to describe salient features of the strong YM dynamics.Comment: LateX 33 pages, no figures. Based on presentations at various meertings. To appear in the proceedings of the 4th RTN-EU conference, Varna, Bulgaria (v2) Various misprints corrected. Added discussion on the definition of the 't Hooft coupling and issues of scheme dependenc

Gravity and axions from a random UV QFT

It is postulated that the UV QFT is enormous and random. The coupling of the Standard Model to such QFT is analyzed. It is argued that massless 4d gravity and axions are general avatars of the postulate. The equivalence principle emerges naturally as well as a concrete set of sources for its breaking. The axion scale is related to the 4d Planck scale as $f=M_P/N$, where $N$ is the "number of colors" of the (almost) hidden UV CFT.Comment: Latex, 39 page

On novel string theories from 4d gauge theories

We investigate strings theories as defined from four dimensional gauge theories. It is argued that novel (super)string theories exist up to 26 dimensions. Some of them may support weakly curved geometries.Comment: LateX, 17 pages. (v2) Removed showkeys command and added references (v3) Added new references and updated the discussion on space-time dependent couplings (v4) Added reference

Introduction to Non-perturbative String Theory

These are a set of lectures presented at the CERN-La Plata-Santiago de Compostella School of Physics, La Plata, May 1997. A brief introduction to the non-perturbative structure of string theory is presented. Various non-perturbative dualities in ten and six dimensions as well as D-branes are discussed.Comment: 60 pages, LaTeX, 4 eps figures include

Brane-bulk energy exchange and cosmological acceleration

The consequences for the brane cosmological evolution of energy exchange between the brane and the bulk are analyzed. A rich variety of brane cosmologies is obtained, depending on the precise mechanism of energy transfer, the equation of state of brane-matter and the spatial topology. An accelerating era is generically a feature of the solutions. (Prepared for 36th International Symposium Ahrenshoop on the Theory of Elementary Particles: Recent Developments in String M Theory and Field Theory, Wernsdorf, Germany, 26-30 Aug 2003.)Comment: 14 pages, 2 figures, Late

Duality Symmetries and Topology Change in String Theory

Duality symmetries for strings moving in non-trivial spacetime backgrounds are analysed. It is shown that for backgrounds generated from WZW and coset CFT models such duality symmetries are exact to all orders in string perturbation theory. Their implications for string dynamics in non-trivial/singular spacetimes are discussed. (Talk given at the EPS 93 Conference, held at Marseille, July 22-27. To appear in the Proceedings.)Comment: lateX file, 6pp. CERN-TH.7003/9

Four-Dimensional N=2 Superstring Constructions and their (Non-) Perturbative Duality Connections

We investigate the connections between four-dimensional, N=2 M-theory vacua constructed as orbifolds of type II, heterotic, and type I strings. All these models have the same massless spectrum, which contains an equal number of vector multiplets and hypermultiplets, with a gauge group of the maximal rank allowed in a perturbative heterotic string construction. We find evidence for duality between two type I compactifications recently proposed and a new heterotic construction that we present here. This duality allows us to gain insight into the non-perturbative properties of these models. In particular we consider gravitational corrections to the effective action.Comment: 18 pages, LaTex, 1 figur

Solving the Decompactification Problem in String Theory

We investigate heterotic ground states in four dimensions in which N=4 supersymmetry is spontaneously broken to N=2. N=4 supersymmetry is restored at a decompactification limit corresponding to $m_{3/2}\to 0$. We calculate the full moduli dependent threshold corrections and confirm that they are supressed in the decompactification limit $m_{3/2}\to 0$ as expected from the restoration of N=4 supersymmetry. This should be contrasted with the behavior of the standard N=2 groundstates where the coupling blow up linearly with the volume of the decompactifying manifold. This mechanism provides a solution to the decompactification problem for the gauge coupling constants. We also discuss how the mechanism can be implemented in ground states with lower supersymmetry.Comment: 14pp, LaTeX some typos correcte

Non-perturbative triality in heterotic and type II N=2 strings

The non-perturbative equivalence of four-dimensional N=2 superstrings with three vector multiplets and four hypermultiplets is analysed. These models are obtained through freely acting orbifold compactifications from the heterotic, the symmetric and the asymmetric type II strings. The heterotic scalar manifolds are (SU(1,1) / U(1))^3 for the S,T,U moduli sitting in the vector multiplets and SO(4,4)/ (SO(4) X SO(4)) for those in the hypermultiplets. The type II symmetric duals correspond to a self-mirror Calabi-Yau threefold compactification with Hodge numbers h(1,1)=h(2,1)=3, while the type II asymmetric construction corresponds to a spontaneous breaking of the N=(4,4) supersymmetry to N=(2,0). Both have already been considered in the literature. The heterotic construction instead is new and we show that there is a weak/strong coupling S-duality relation between the heterotic and the asymmetric type IIA ground state with S(Het)=-1/S(As); we also show that there is a partial restoration of N=8 supersymmetry in the heterotic strong-coupling regime. We compute the full (non-)perturbative R2 and F2 corrections and determine the prepotential.Comment: Latex, no figures, 24 page