Two-loop scalar diagrams from string theory

We show how to obtain correctly normalized expressions for the Feynman diagrams of $\Phi^3$ theory with an internal $U(N)$ symmetry group, starting from tachyon amplitudes of the open bosonic string, and suitably performing the zero--slope limit by giving an arbitrary mass $m$ to the tachyon. In particular we present explicit results for the two--loop amplitudes of $\Phi^3$ theory, in preparation for the more interesting case of the multiloop amplitudes of non--abelian gauge theories.Comment: 15 pages, plain Latex, 5 figures include

A unique theory of all forces

In discussing the construction of a consistent theory of quantum gravity unified with the gauge interactions we are naturally led to a string theory. We review its properties and the five consistent supersymmetric string theories in ten dimensions. We finally discuss the evidence that these theories are actually special limits of a unique 11-dimensional theory, called M-theory, and a recent conjecture for its explicit formulation as a supersymmetric Matrix theory.Comment: 11 pages, Latex. Talk given at the Conference `Beyond the standard model', V, Balholm, Norway, May 199

Non conformal gauge theories from D branes

We use fractional and wrapped branes to describe perturbative and nonperturbative properties of the gauge theories living on their worldvolume. (Talk given at the 35th International Symposium Ahrenshoop on the Theory of Elementary Particles, Wernsdorf, August 26-30, 2002.)Comment: 7 pages, Late

The Gauge/Gravity Correspondence for Non-Supersymmetric Theories

We review the string construction of the ``orientifold field theories'' and we show that for these theories the gauge/gravity correspondence is only valid for a large number of colours.Comment: 7 pages, Latex, Talk given at 37. International Symposium Ahrenshoop, Berlin, August 2004. Added references and acknowledgements; corrected typo

String techniques for the calculation of renormalization constants in field theory

We describe a set of methods to calculate gauge theory renormalization constants from string theory, all based on a consistent prescription to continue off shell open bosonic string amplitudes. We prove the consistency of our prescription by explicitly evaluating the renormalizations of the two, three and four-gluon amplitudes, and showing that they obey the appropriate Ward identities. The field theory limit thus performed corresponds to the background field method in Feynman gauge. We identify precisely the regions in string moduli space that correspond to different classes of Feynman diagrams, and in particular we show how to isolate contributions to the effective action. Ultraviolet divergent terms are then encoded in a single string integral over the modular parameter $\tau$. Finally, we derive a multiloop expression for the effective action by computing the partition function of an open bosonic string interacting with an external non-abelian background gauge field.Comment: 54 pages, Latex, uses FEYNMAN.te

String-derived renormalization of Yang-Mills theory

We review the application of bosonic string techniques to the calculation of renormalization constants and effective actions in Yang-Mills theory. We display the multiloop string formulas needed to compute Yang-Mills amplitudes, and we discuss how the renormalizations of proper vertices can be extracted in the field theory limit. We show how string techniques lead to the background field method in field theory, and indicate how the gauge invariance of the multiloop effective action can be inferred form the string formalism. (Proceedings of the 29th International Symposium on the Theory of Elementary Particles, Buckow (Germany), Aug.-Sept. 1995. Preprint DFTT 04/96)Comment: 11 pages. Latex, uses espcrc2.sty. Proceedings Buckow '9

Visualization of topological objects in the deconfinement phase of pure QCD

In the last years it turned out that instantons and monopoles have a certain local correlation in four-dimensional QCD. It was demonstrated by several groups independently and by different methods that at the locations of instantons also monopoles in the maximal abelian projection can be found with enhanced probability. Further we observed such nontrivial correlation functions in the deconfinement phase. We continue our visualization project to analyze several specific gauge-field configurations.Comment: 3 pages, 2 EPS figures, Poster presented at "Lattice'97", to appear in Nucl. Phys. B (Proc. Suppl.