Ramond-Ramond couplings of non-BPS D-branes

We study how non-BPS type II D-branes couple to R-R potentials. Upon tachyon condensation the couplings we find give rise to the Wess-Zumino action of BPS D-branes.Comment: LaTeX, 8 pages, no figures; minor changes, reference added, to be published in JHE

N=2 Instanton Calculus In Closed String Background

In this contribution we describe how to obtain instanton effects in four dimensional gauge theories by computing string scattering amplitudes in D3/D(-1) brane systems. In particular we study a system of fractional D3/D(-1) branes in a Z_2 orbifold and in a Ramond-Ramond closed string background, and show that it describes the gauge instantons of N=2 super Yang-Mills theory and their interactions with the graviphoton of N=2. Using string theory methods we compute the prepotential of the effective gauge theory exploiting the localization methods of the instanton calculus, showing that this leads to the same information given by the topological string.Comment: 12 pages, 2 figures; to appear in the proceedings of 21st Nishinomiya-Yukawa Memorial Symposium on Theoretical Physics: Noncommutative Geometry and Quantum Spacetime in Physics, Nishinomiya and Kyoto, Japan, 11-15 Nov 200

Deformed N=2 theories, generalized recursion relations and S-duality

We study the non-perturbative properties of N=2 super conformal field theories in four dimensions using localization techniques. In particular we consider SU(2) gauge theories, deformed by a generic epsilon-background, with four fundamental flavors or with one adjoint hypermultiplet. In both cases we explicitly compute the first few instanton corrections to the partition function and the prepotential using Nekrasov's approach. These results allow to reconstruct exact expressions involving quasi-modular functions of the bare gauge coupling constant and to show that the prepotential terms satisfy a modular anomaly equation that takes the form of a recursion relation with an explicitly epsilon-dependent term. We then investigate the implications of this recursion relation on the modular properties of the effective theory and find that with a suitable redefinition of the prepotential and of the effective coupling it is possible, at least up to the third order in the deformation parameters, to cast the S-duality relations in the same form as they appear in the Seiberg-Witten solution of the undeformed theory.Comment: 33 pages, no figures, LaTeX2

On the TTˉT\bar T deformation of the compactified boson and its interpretation in Lattice Gauge Theory

We study the effective string description of space-like Polyakov loop correlators at finite temperature with the goal of describing the behaviour of the space--like string tension in the vicinity of the deconfinement transition. To this end we construct the partition function of the Nambu-Goto effective string theory in presence of a compact transverse direction of length $L$ equal to the inverse temperature. We then show that, under particular conditions, our result can be interpreted as the partition function of the $T\bar T$ deformation of the 2d quantum field theory describing a compactified bosonic field and that this mapping allows a deeper insight on the behaviour of the space-like observables of the theory. In particular we show, by imposing that the spectrum of the model obeys the inviscid Burgers equation, that the $T\bar T$ deformations follow well defined trajectories in the parameter space $(\sigma,L)$ of the model, where $\sigma$ is the string tension, which are characterized by a constant value of the dimensionless compactification radius $\rho=L\sqrt{\sigma/2\pi}$. We discuss the potential usefulness of these results for studying the space--like string tension of the underlying Lattice Gauge Theory and its behaviour across the deconfinement transition.Comment: 28 pages, 2 figures. V2: expanded comments in sections 2.2, 2.3 and