[en] The authors show that the anomalies of the Virasoro algebra are due to the asymmetric behavior of raising and lowering operators with respect to the ground state of the string. With the adoption of a symmetric vacuum they obtain a non-anomalous theory in any number of dimensions, in particular for D = A

[en] This thesis is devoted to the study of various aspects of heterotic string compactification. The emphasis is on methods for reconstructing the effective low-energy field theory which results from compactifying the heterotic string on a smooth Calabi-Yau manifold. Chapters 1 and 3 are devoted left-right asymmetric, so-called (2,0) compactifications. Various results on the existence and properties of such compactifications are proved. Chapter 2 is devoted to reconstructing the cubic part of the superpotential in a certain left-right symmetric (2,2) compactification

[en] Over the past two years bosonic string field theory has become fairly well understood. By defining a string functional Φ that depends on the first-quantized ghosts, one is lead to a field equation Q Φ = 0 (based on the BRST physical state condition) and an action ∫ Φ Q Φ. In this paper, after reviewing the bosonic case, this procedure is applied to the Ramond string. A new feature of the first-quantized Ramond theory is the presence of commuting ghost zero modes that cause an infinite degeneracy in the BRST ground state. Once a suitable inner product is defined in this sector, it is found that the canonical action above cannot work, but a modified version ∫ΦGQΦ is gauge-invariant and yields the correct equations of motion for a carefully chosen operator G. This form is then compared to a Ramond action found by Witten, which has similar features but is defined using bosonized ghosts. A possible mapping between the two formalisms is suggested

[en] The vacuum structure of the Heterotic String is investigated. Methods from fleld theory and critical systems are being used to map out part of the moduli space of the (2,2)-configuration space of the Heterotic String. This configuration space breaks up into different Multidimensional spaces, each leading to a different physical particle spectrum. After explicitly constructing parts of the subspace of all (2,2)-vacua corresponding to complete intersection Calabi-Yau manifolds and tensor models of the N = 2 superconformal discrete minimal series, the spectrum of these models is computed and a search for phenomenological viable models is conducted. It turns out that there are only very few such models. In the second part of the thesis the construction of a new threegeneration model is explained and a detailed phenomenological analysis is presented

[en] The implications of string theory for the dimension of spacetime are investigated by two methods. First, a new potential class of string theories is studied, which have critical dimensions 3, 4 and 6. In particular, the partition functions of these theories are derived and interpreted using a generalized GSO projection. The possible uniqueness of field assignments, as well as the bosonization of the K = 4 model are also addressed. Second, using recent ideas in string cosmology, a new model is proposed to explain why three spatial dimensions grew large. Unlike the original work of Brandenberger and Vafa, this paradigm uses the theory of random walks. A simple computer model is developed to test the implications of this new approach. It is found that a four-dimensional spacetime can be explained by the proper choice of initial conditions

[en] By identifying the chiral fields and related operators in Gepner's models and Vafa's twisted N = 2 Landau-Ginzburg models, the authors find that they also have the same E₆ singlets spectra and therefore correspond to the same specific points of superstring vacua. This paper shows explicitly how to identify generations, anti-generations and the E₆ singlets in Vafa's class-A and E_τ models with Gepner's models

[en] String theory is believed to be a consistent quantum theory which unifies gravity and other forces and matter fields. Since, according to Einstein, gravity tells us the geometry of space-time, string theory should be formulated in a way that is independent of the background geometry. An attempt is made to analyze one of the proposals on geometric formulation of string theory. Geometric quantization is explained in detail. The loop space of background geometry is studied also to see whether we get consistent string theories. An extension to superstring is given, showing that the phase space of open superstring is also a Kaehler manifold. The geometrical meaning of ghosts of string theory is studied to construct a string field theory where ghost fields, c(σ) and string coordinates x^μ(σ) stand on a same footing. The BRST formalism for the open string field is rederived, while the Bowick and Rajeev proposal is modified. Namely, it is conjectured that the closed string field is a Kaehler potential of the extended loop space, the set of maps, S¹ → (x^μ, ρ), where ρ is the conformal factor of a string worldsheet

[en] A detailed calculation is presented of the generation of large-scale streaming velocities by nonsuperconducting cosmic strings. A spherical infall model is used to calculate the peculiar velocity induced by an individual loop of cosmic string at rest in the comoving frame. The effects of many uncorrelated loops are combined using both a semianalytic method and Monte Carlo simulations. The calculations are modified to include loop correlations. The results show that cosmic strings and cold dark matter are in apparent conflict with observations, but cosmic strings and hot dark mattter offer a hope of explaining the reported large-scale streaming velocities. 37 references

[en] Several perturbative calculations in string theories are presented. In part I, we calculate the complete low energy effective action to quartic order in the bosonic fields for the heterotic string. We find that, in addition to the term involving ζ(3) found previously, there are terms of the form Tr(F² - R²)² and four and six derivative terms. We also show that the heterotic effective action reduces to that of the superstring when the gauge connection is equal to the generalized spin connection. In part II, we use unitarity to calculate the width of states in the first massive level in the type II and heterotic superstring theories. We find that the unitarity cuts which arise in string loop amplitudes correspond to those which are predicted by the low energy effective field theory. In part III, we use functional integral techniques to calculate the scattering amplitude for four open off shell tachyons in Witten's string field theory and show that the residues of the first three poles agree with those obtained using oscillator methods

[en] We propose a method to investigate the conservation of brane charges at the intersection of two or more branes using the Thom classes of their normal bundles. In particular we find a relation between the charge of the branes involved in the configuration and the charge of the defects on the branes due to the intersection. We also explore the applications of our method for various brane intersections in type II strings and M-theory. (author)