Quantum Field Theory is the framework that unifies Quantum Mechanics and Special Relativity. One application of this framework that is of paramount importance is the description of elementary particles and all their known interactions, except for the gravitational one, in the so-called Standard Model of Particle Physics.

Despite its enormous successes and its astounding range of applicability, the unification of Quantum Mechanics with the gravitational interaction seems to require going beyond the Quantum Field Theory framework. String Theory is the leading candidate for such an "ultimate theory". Although in its simplest, perturbative definition String Theory is a theory of one-dimensional objects, at the non-perturbative level it includes dynamical, higher-dimensional objects called "p-branes", with p the dimension of the object.

The study of the dynamics of these extended objects led in 1997 to the discovery of the so-called "gauge/string duality". This is a new connection between Quantum Field Theory and String Theory according to which a certain String Theory formulated in an appropriate spacetime in d+1 dimensions, typically an asymptotically anti-de Sitter spacetime, is completely equivalent to a gauge Quantum Field Theory formulated in flat spacetime in d-dimensions. Since this flat spacetime can be thought of as the boundary of the anti-de Sitter spacetime, the duality provides the most concrete implementation to date of the so-called "holographic principle", which is believed to be part of any consistent theory of quantum gravity. For this reason, the gauge/string duality is often referred to as "holography".

### OUR CONTRIBUTION

Research at the ICCUB covers many aspects of Quantum Field Theory, from the most phenomenological, as described in Particle Physics Phenomenology, to the most formal. The latter include the study of conformal and/or supersymmetric theories in different numbers of dimensions, both in their own right and in connection with the gauge/string duality.

In String Theory the research also encompasses a wide variety of topics, including the dynamics of branes; the study of the fundamental symmetries of String Theory; the determination of 1/N corrections in the gauge/string duality (with N the rank of the gauge group), and the identification of non-relativistic limits of the duality. As a theory of Quantum Gravity, String Theory also has profound implications for the physics of black holes. The ICCUB team in this area is described in Gravitation and Cosmology.

One fundamental aspect of the gauge/string duality is that it maps the strong coupling regime of the gauge theory to the regime on the string side in which the theory reduces to classical gravity. Exploiting this connection, researchers at the ICCUB use classical gravity to study the regimes of Quantum Chromodynamics that are inaccessible by conventional techniques. These include, for example, the far-from-equilibrium dynamics of the Quark-Gluon Plasma created in heavy ion collisions or the properties of dense quark matter that could be present at the core of neutron stars. More recently, they have started applying holography to aspects of Cosmology, such as the possible generation of primordial gravitational waves in cosmological phase transitions in the early Universe.

Holography is thus an excellent example of the synergetic nature of research at the ICCUB, since it connects fields as diverse as Quantum Field Theory, String Theory, Gravitation and Cosmology, Particle Physics and Nuclear Physics.

## LINES OF RESEARCH

- Conformal and Supersymmetric Field Theories
- Brane Dynamics
- Symmetries of String Theory
- Applications of Holography to Quantum Chromodynamics