Franco Pezzella

N-point g-loop vertex for a free bosonic theory with vacuum charge Q

Starting fron the N-point vertex on the sphere and using the sewing procedure we construct the N-point g-loop vertex for a free bosonic theory with vacuum charge Q. We then show that, when this vertex is saturated with N highest weight states, it gives their correlation function on an arbitrary Riemann surface of genus g. We also extend our formalism to the case of a free scalar field compactified on a circle, which is related to the Coulomb gas description of minimal models.

On the gauge/gravity correspondence and the open/closed string duality

In this paper we review the conditions for the validity of the gauge/gravity correspondence in both supersymmetric and nonsupersymmetric string models. We start by reminding what happens in type IIB theory on the orbifolds ℂ2/ℤ2 and ℂ3/(ℤ2 x ℤ2), where this correspondence beautifully works. In these cases, by performing a complete stringy calculation of the interaction among D3-branes, it has been shown that the fact that this correspondence works is a consequence of the open/closed duality and of the absence of threshold corrections. Then we review the construction of type 0 theories with their orbifolds and orientifolds having spectra free from both open and closed string tachyons and for such models we study the validity of the gauge/gravity correspondence, concluding that this is not a peculiarity of supersymmetric theories, but it may work also for nonsupersymmetric models. Also in these cases, when it works, it is again a consequence of the open/closed string duality and of vanishing threshold corrections.

Gauge/gravity correspondence from open/closed string duality

We compute the annulus diagram corresponding to the interaction of a fractional D3 brane with a gauge field on its world-volume and a stack of N fractional D3 branes on the orbifolds 2/2 and 3/(2 ? 2). We show that its logarithmic divergence can be equivalently understood as due either to massless open string states circulating in the loop or to massless closed string states exchanged between two boundary states. This follows from the fact that, under open/closed string duality, massless states in the open and closed string channels are matched into each other without mixing with massive states. This explains why the perturbative properties of many gauge theories living on the world-volume of less supersymmetric and nonconformal branes have been recently obtained from their corresponding supergravity solution.

N-point g-loop vertex for a fermionic theory with arbitrary spin

Abstract We use the sewing procedure of the operator formalism to construct explicitly the N -point g -loop vertex V N ; g for a free fermionic ( b , c )-system with conformal weight (λ, 1−λ). We show that this vertex has the structure we expect from geometrical arguments. We obtain also several geometrical objects, e.g. the holomorphic λ-differentials on an arbitrary Riemann surface, which turn out to be expressed as a Poincare θ-series over all elements of the Schottky group. From V N ; g we compute explicitly correlation functions for our system, finding agreement with the geometrical procedure.

Kahler Metrics and Yukawa Couplings in Magnetized Brane Models

Using the field theoretical approach introduced by Cremades, Ibanez and Marchesano for describing open strings attached to D9 branes having different magnetizations, we give a procedure for determining the Kahler metrics of those open strings in toroidal compactifications.

Open strings in the system D5/D9

We construct the six-dimensional Lagrangian for the massless twisted open strings with one end-point ending on a stack of D5 and the other on a stack of D9 branes, interacting with the gauge multiplets living respectively on the D5 and D9 branes. It is first obtained by uplifting to six dimensions the four-dimensional Lagrangian of the hypermultiplet and manifestly exhibits an SU(2) symmetry. We show by an explicit calculation that it is supersymmetric in six dimensions and then we check various terms of this Lagrangian by computing string amplitudes on the disk. Finally, starting from this Lagrangian and assuming the presence of non-zero magnetic fluxes along the extra compact dimensions, we determine the spectrum of the Kaluza–Klein states which agrees with the corresponding one obtained from string theory in the field theory limit.

Wrapped magnetized branes: two alternative descriptions?

We discuss two inequivalent ways for describing magnetized D-branes wrapped N times on a torus T2. The first one is based on a non-abelian gauge bundle U(N), while the second one is obtained by means of a Narain T-duality transformation acting on a theory with non-magnetized branes. We construct in both descriptions the boundary state and the open string vertices and show that they give rise to different string amplitudes. In particular, the description based on the gauge bundle has open string vertex operators with momentum dependent Chan-Paton factors.

N=1 Matter from Fractional Branes

We study a bound state of fractional D3-branes localized inside the world-volume of fractional D7-branes on the orbifold 3/(2 × 2). We determine the open string spectrum that leads to = 1 U(N1) × U(N2) × U(N3) × U(N4) gauge theory with matter having the number of D7-branes as a flavor index. We derive the linearized boundary action of the D7-brane on this orbifold using the boundary state formalism and we discuss the tadpole cancellation. After computing the asymptotic expression of the supergravity solution the anomalies of the gauge theory are reproduced.

Consistent off-shell tree string amplitudes

We construct off-shell tree bosonic string amplitudes, based on the operatorial formalism of the N-string vertex, with three external massless states both for open and closed strings by requiring them to be projective invariant. In particular our prescription leads, in the low-energy limit, to the three-gluon amplitude in the usual covariant gauge.

Comparing double string theory actions

A bstractAimed to a deeper comprehension of a manifestly T-dual invariant formulation of string theory, in this paper a detailed comparison between the non-covariant action proposed by Tseytlin and the covariant one proposed by Hull is done. These are obtained by making both the string coordinates and their duals explicitly appear, on the same footing, in the world-sheet action, so “doubling” the string coordinates along the compact dimensions. After a discussion on the nature of the constraints in both the models and the relative quantization, it results that the string coordinates and their duals behave like “non-commuting” phase space coordinates but their expressions in terms of Fourier modes generate the oscillator algebra of the standard bosonic string. A proof of the equivalence of the two formulations is given. Furthermore, open-string solutions are also discussed.