Matteo Bertolini: Bio SISSA Home

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Quantum field theory is the language in which all of modern physics is formulated. It represents the marriage of quantum mechanics with special relativity and provides the mathematical framework in which to describe particle physics. A particular class of quantum field theories known as gauge theories have the great virtue of being able to describe interactions between different particles.

In Nature we believe there are four fundamental interactions which, eventually, are responsible for the physical phenomena we observe: the electromagnetic, weak, strong and gravitational interactions. A (gauge) quantum field theory model, known as the Standard Model, has proven to be the correct way to describe, to an incredible high level of precision against experiments, the first three of them. Unfortunately, gravity seems to escape a consistent quantum field theory description. String theory is an extension of ordinary quantum field theory which, besides other things, has the great virtue of being able to include also gravity into the game. For this reason, string theory is believed to be an all-encompassing theory of the Universe, unifying all forces of Nature. The price to pay is quite high, though. First, it is quite hard. Second, you must believe a number of weird things: that fundamental objects are not point-like but rather one-dimensional strings, that we may live not in four but in ten or eleven space-time dimensions, etc... This could seem science fiction, at first sight. However, once you start entering its misteries, you get convinced string theory is not so exotic. In fact, I believe string theory is a quite conservative (and reasonable) way to extend ordinary quantum field theory, after all. \

I now briefly sketch those aspects of string theory I am interested in, these days. I am a son of the so-called Second String Revolution and my research interests have always being centred, in a way or another, on D-branes, these being undoubtedly the stars of this revolution.


D-branes - The Gauge/String Correspondence One of the biggest achievements of the second string revolution was to realize that in string theory, besides fundamental strings, there are also higher dimensional defects, called D-branes. These objects have a two-fold nature. On the one hand their dynamics, at low energy, can be described in terms of gauge theory degrees of freedom. On the other hand, they are solitons in string theory and, as such, they arise as classical solutions of the low-energy effective theory and curve space-time. The above complementarity of descriptions is the central ingredient of the so-called gauge/string correspondence. \ Its original version, the AdS/CFT duality, states the equivalence between a string theory defined on five-dimensional Anti-de Sitter space times a five-sphere and a very peculiar four dimensional gauge theory, N=4 supersymmetric Yang-Mills theory. The AdS/CFT duality is a very deep and successful conjecture, but also has some limitations. In particular, it deals with a highly supersymmetric and conformal gauge theory, thus very far from describing the physical world. Soon after the discovery of the AdS/CFT duality many efforts were aimed trying to overcome the above limitations and find new dual pairs where the gauge theories under consideration were a) less supersymmetric and b) non-conformal. For instance, it is now clear that many information can be gained on the perturbative and non-perturbative properties of N=1 supersymmetric gauge theories, with and without matter, from suitable (super)gravity backgrounds. Progress have also been made for the more ambitious goal of describing N=0, i.e. non supersymmetric gauge theories. Indeed, the final (and very difficult) goal is to find a way to describe QCD via a suitable string dual, being able to understand qualitatively and quantitatively the strong coupling dynamics of QCD via these new tools.


D-branes - Brane Worlds Imagine our Universe is a ten dimensional string world with D-branes. In this world, gauge degrees of freedom are confined on the branes while gravity degrees of freedom live in the full ten dimensional space-time. Still, weak (as we like them to be) gravity effects may be felt on the brane. In string theory there are D-branes of different dimensions: a Dp-brane is a p+1-dimensional defect whose world volume is described in terms of p spatial dimensions and one time dimension. This raises the crazy idea to describe the Standard Model of fundamental interactions on a 3-brane and understand four dimensional gravity as "pulled-back" from the ten dimensional world. This is often summarized saying that we live on a brane. This idea has become very popular and different lines of research are been developed. One of them consists in trying to embed the Standard Model (or supersymmetric extensions thereof) into string theory via a bottom-up approach, engineering suitable D-brane configurations living in some given ten dimensional geometric background so to reproduce the known particle spectrum and interactions, at the most possible high level of accuracy. Intersecting and magnetized brane worlds are examples of these models. This also opened up the possibility of addressing old but crucial issues in string compactifications as the problem of moduli stabilization, in a novel and controllable way. Indeed, via complicated enough magnetized brane world models (and some additional closed string background fluxes) one can in principle achieve a Standard Model-like Lagrangian, at low energy, with all string moduli being fixed.