Massimo Porrati

Pair creation of open strings in an electric field

Abstract We calculate exactly the rate of pair production of open bosonic and supersymmetric strings in a constant electric field. The rate agrees with Schwingers classic result in the weak-field limit, but diverges when the electric field approaches some critical value of the order of the string tension.

Superstrings with spontaneously broken supersymmetry and their effective theories

We discuss superstring models with spontaneous breaking of N=1, 2 or 4 space-time supersymmetry, via coordinate-dependent compactifications from five to four dimensions. We provide a description of the supersymmetry breaking mechanism in the formulation with complex world-sheet fermions, and we reinterpret it in terms of special deformations of the lorentzian charge lattice. Considering a representative string model with spontaneously broken N=1 supersymmetry and massless chiral fermions, we show that the knowledge of the spectrum of states and of the flat directions completely specifies its low-energy effective lagrangian, which turns out to be a new no-scale supergravity model. We outline the qualitative difference between these models and other scenarios of gaugino (gravitino) condensation or general non-perturbative phenomena.

Spontaneous supersymmetry breaking in string theory

Abstract We present a construction of string theories, with spontaneously broken supersymmetry and chiral fermion families, obtained by a suitable choice of boundary conditions in toroidal compactifications of ten-dimensional (1,1) or heterotic models. The crystallographic groups in four-dimensions determine the possible values of the supersymmetry-breaking parameters, which therefore are discrete.

General dimensional reduction of ten-dimensional supergravity and superstring☆

Abstract Dimensional reductions of supergravity theories are shown to yield to specific classes of four-dimensional no-scale models with N =4, 2 or 1 residual supersymmetry. N =1 “maximal” supergravity lagrangian, corresponding to the “untwisted” sector of orbifold compactification of superstrings, contains nine families and has a no-scale structure based on the Kahler manifold [SU(3, 3+3 n )/SU(3) ×SU(3+3 n )]×[SU(1,1)/U(1)]. The quantum consistency of the resulting theories give information on the non Kaluza-Klein (string) “twisted” sector.

NEW MINIMAL HIGHER DERIVATIVE SUPERGRAVITY COUPLED TO MATTER

We consider quadratic invariants in the new minimal formulation of N = 1, 4D supergravity, and obtain a simpler form of the Gauss-Bonnet combination. When added to the Einstein lagrangian, these new multiplets propagate different states than those in the old minimal formulation. In particular, the R + αR2 theory is shown to be equivalent to standard supergravity coupled to a massive vector multiplet. Our result shows the non-uniqueness of minimal supersymmetrizations of higher derivative theories.

Effective Lagrangians for Four-dimensional Superstrings

We derive effective supergravity lagrangians for the massless sector of new four-dimensional superstrings. The untwisted sector is obtained by suitable Z2 or Z2 ⊗ Z2′ truncations of N = 4, D = 4 lagrangians. For the N = 1 “minimal” models, related to the N = 4, SO(44) superstrings, we derive the Kahler potential, the superpotential and the ƒ gauge kinetic function. The underlying nonlinear sigma model is based on the Kahler manifold [SU(1,1)/U(1)] ⊗ Πi = 13 {SO(2, ni)/[SO(2) ⊗ SO(ni)]}. For models without massless twisted sector, this result provides the full superstring low-energy effective action. For models with massless twisted sector, the N = 1 theory may be obtained once the quaternionic manifold of the corresponding N = 2 hypermultiplets is specified.

Confinement and condensates without fine tuning in supergravity duals of gauge theories

We discuss a solution of the equations of motion of five-dimensional gauged type IIB supergravity that describes confining SU(N) gauge theories at large N and large t Hooft parameter. We prove confinement by computing the Wilson loop, and we show that our solution is generic, independent of most of the details of the theory. In particular, the Einstein-frame metric near its singularity, and the condensates of scalar, composite operators are universal. Also universal is the discreteness of the glueball mass spectrum and the existence of a mass gap. The metric is also identical to a generically confining solution recently found in type 0B theory.

RG fixed points in supergravity duals of 4-d field theory and asymptotically AdS spaces

Abstract Recently, it has been conjectured that supergravity solutions with two asymptotically AdS5 regions describe the RG flow of a 4-d field theory from a UV fixed point to an interacting IR fixed point. In this paper we lend support to this conjecture by showing that, in the UV (IR) limit, the two-point function of a minimally coupled scalar field depends only on the UV (IR) region of the metric, asymptotic to AdS5. This result is consistent with the interpretation of the radial coordinate of Anti de Sitter space as an energy scale, and it may provide an analog of the Callan–Symanzik equation for supergravity duals of strongly coupled field theories.

Duality and gaugino condensation in superstring models

Abstract The issue of gaugino condensation in superstring generated N=1 supergravity models is reconsidered. We show that gaugino condensation related to the breaking of an R-symmetry is consistent with broken supersymmetry, zero cosmological constant and duality transformation. The latter implies the existence of a massless pseudoscalar, related to a non-compact symmetry, which reflects the gauge invariance of the antisymmetric tensor field of superstring theories.

N = 1 superstrings with spontaneously broken symmetries

Abstract We construct N = 1 chiral superstrings with spontaneously broken gauge symmetry in four space-time dimensions. These new string solutions are obtained by a generalized coordinate-dependent Z 2 orbifold compactification of some non-chiral five-dimensional N = 1 and N = 2 superstrings. The scale of symmetry breaking is arbitrary (at least classically) and it can be chosen hierarchically smaller than the string scale (α′) − 1 2 .