Final Activity Report Summary - STRINGS VS. FIELDS (Gauge fields and strings)
The twentieth century has witnessed an impressive evolution in our knowledge of physics. In particular, very powerful and precise theories have been built in order to study the fundamental laws of nature and the world of particle physics. However, there are still many open puzzles whose ultimate solution remains elusive. The understanding of strongly coupled field theories can be considered one of them. All known fundamental interactions except gravity are described by so-called quantum field theories (QFT). A quite general feature of this kind of theories is that the different fundamental particles interact among themselves with varying strength, depending on the energy scale of the phenomenon one is studying. When such interactions become strong, the theory loses predictivity, i.e. it is not possible to make some computations using the standard QFT techniques and the theory cannot be fully understood, even if the basic principles are. This fact hinders, for instance, a more precise understanding of nuclear matter and the spectrum of states bounded by the strong interaction. In order to solve this problem, different approaches have been developed.
A powerful approach, which is maybe the most original, stems from pioneering work by Maldacena, who realised and made explicit that certain strongly coupled field theories in certain limits could be related to weakly coupled string theories (a duality sometimes called holographic, since the basic idea somehow resembles the holography used in optics). His remarkable observation has sparked a huge amount of activity in the theoretical physics community in the last decade, that has been quoted as the AdS/CFT duality.
In this framework, the general goal of this project has been to ameliorate the understanding of this connection between string theory and gravity and some features of the strong interaction (QCD), which describes nuclear matter.
In particular, two concrete topics have been analysed during this project: the first one corresponds to the study of the quantum effects produced by matter in the fundamental representation (in short, the quarks, the building blocks of protons and neutrons). They are known to produce different important physical phenomena. In the dual string language, one has to study stringy systems where the gravitational force of certain heavy objects called D-branes are taken into account. The method has been developed and thoroughly checked in different articles written within this project for several cases of interest (not really QCD, but, in a sense, similar field theories which are interesting by themselves or also may be thought as toy models for certain aspects of QCD).
A second topic has been the study of the highly non-trivial QCD vacuum from stringy language. It is known that such vacuum contains a non-trivial quark condensate which breaks the so-called chiral symmetry. We have shown in detail how this is described by the non-trivial physics of certain strings joining D-branes to anti-D-branes. This study may open the door for the holographic study of different questions of otherwise difficult answer. As a first example, a new view on the spectrum of heavy mesonic excitations may be developed.
A powerful approach, which is maybe the most original, stems from pioneering work by Maldacena, who realised and made explicit that certain strongly coupled field theories in certain limits could be related to weakly coupled string theories (a duality sometimes called holographic, since the basic idea somehow resembles the holography used in optics). His remarkable observation has sparked a huge amount of activity in the theoretical physics community in the last decade, that has been quoted as the AdS/CFT duality.
In this framework, the general goal of this project has been to ameliorate the understanding of this connection between string theory and gravity and some features of the strong interaction (QCD), which describes nuclear matter.
In particular, two concrete topics have been analysed during this project: the first one corresponds to the study of the quantum effects produced by matter in the fundamental representation (in short, the quarks, the building blocks of protons and neutrons). They are known to produce different important physical phenomena. In the dual string language, one has to study stringy systems where the gravitational force of certain heavy objects called D-branes are taken into account. The method has been developed and thoroughly checked in different articles written within this project for several cases of interest (not really QCD, but, in a sense, similar field theories which are interesting by themselves or also may be thought as toy models for certain aspects of QCD).
A second topic has been the study of the highly non-trivial QCD vacuum from stringy language. It is known that such vacuum contains a non-trivial quark condensate which breaks the so-called chiral symmetry. We have shown in detail how this is described by the non-trivial physics of certain strings joining D-branes to anti-D-branes. This study may open the door for the holographic study of different questions of otherwise difficult answer. As a first example, a new view on the spectrum of heavy mesonic excitations may be developed.