Non-perturbative Aspects of Quantum Field Theories from Holography

Abstract

In this thesis we have employed the holographic duality to study the non-perturbative regime of a one-parameter family of theories with multi-scale dynamics. Normally, this (super)string theory motivated duality identifies gauge theories in flat space with string theories in a certain curved spacetime. Its relevance roots in its ability to relate the strongly-coupled regime of gauge theories with classical gravity governed by Einstein's equations. In the Introduction of the thesis, we have reviewed the main string theory ingredients that we used throughout the thesis and revisited some of the previous results which are the starting point of out study.

In Chapter 2, we gather the explicit form of the supergravity solutions whose dual are the gauge theories of interest. These are three-dimensional gauge theories. Generically, they share the same physics at high energies, given by Yang-Mills and Chern-Simons interactions. Remarkably, when the low energy regime of the theories is studied, a rich variety of non-perturbative phenomenology is discovered.

In particular, for a generic value of the parameter distinguishing the theories, they develop a mass gap in their spectrum. However, the two theories which are obtained at the limiting values of the parameter are special. On the one hand, the theory flows towards an infrarred fixed point, dual to a Conformal Field Theory. On the other hand, a confining theory is obtained, in the sense that the potential felt between quarks grows linearly with the distance between them for large separations.

All these phenomena, together with the computation of the spectrum of spin-0 and spin-2 states, are studied in Chapter 3. The fact that in this system the Renormalisation Group flow can pass close to a Conformal Field Theory motivated the search of a light dilaton in the spectrum. But such light state was not found, the reason for that being that the values of the source and the vacuum expectation value that prevented the flow from finishing at the fixed point where of the same order.

On top of that, in this Chapter some entanglement entropy measures were studied. This last investigation was motivated by the fact that in the literature some quantities extracted from such magnitudes where proposed as a probe for confinement. Our results show that, when these quantities are considered in our system, they are not able to discriminate between confining and non-confining gapped theories.

Not only did we consider the theories at zero temperature case, but we also studied thermal states by constructing numerical black brane solutions in the gravity side. Black branes are very much like black holes, with the peculiarity that their surface extends in non-compact directions. Such solutions are discussed in Chapter 4. As a result, we understood their phase diagram, exhibiting a rich structure endowed with first and second order phase transitions, as well as a triple point where three phases coexist and a critical point where the second order phase transition takes place.

Intrigued by the effect that the proximity of a Conformal Field Theory could have in the Renormalisation Group flow of a field theory, in Chapter 5 we carried out a study on complex Conformal Field Theories. We proposed their holographic dual, and analysed some of their properties in the strongly-coupled case.

Finally, in Chapter 6, we studied transport coefficients in holographic theories which model Quantum Chromodynamics. We concluded that the holographic results are quite different from the ones obtained using perturbative techniques. These studies could have phenomenological consequences and find their application in astrophysical observations concerning neutron stars.

En esta tesis hemos utilizado la dualidad holográfica para entender el régimen no perturbativo de una familia uni-paramétrica de teorías con múltiples escalas. Primeramente, hemos repasado los ingredientes esenciales que necesitamos de teoría de cuerdas. A la vez, hemos introducimos algunos resultados previos que son el punto de partida de nuestras investigaciones.

Tras dicha introducción, se recogen todas las soluciones de supergravedad duales a las teorías en tres dimensiones que estudiamos. Genéricamente, comparten la misma física a altas energías pero a bajas energías muestran una rica fenomenología. En particular, desarrollan un salto de masa en su espectro. Curiosamente, las teorías correspondientes a tomar los valores límites del parámetro son especiales. En un caso, la teoría fluye a una teoría de campos conforme. En el otro se obtiene una teoría confinante, con potencial lineal entre quarks.

También se calcula el espectro de estados con espín 0 y espín 2. Además, se analizan diferentes medidas de entrelazamiento cuántico que en nuestro caso no son capaces de discriminar entre teorías con confinamiento y teorías con un salto de masa. Esto contrasta con algunas propuestas que se encuentran en la literatura.

Adicionalmente hemos construido numéricamente soluciones de branas negras, que describen estados térmicos de las teorías. Hemos descubierto un diagrama de fases muy rico, con transiciones de fase de primer y segundo orden, junto a un punto crítico y un punto triple.

Interesados por el efecto que una teoría conforme de campos pudiera tener si es cercana al flujo del grupo de renormalización de otra teoría, en el Capítulo 5 nos adentramos en el estudio de teorías conformes de campos complejas, dando su el dual holográfico.

Finalmente, se calculan coeficientes de transporte en teorías holográficas que modelan Cromodinámica Quántica y que podrían tener consecuencias fenomenológicas en observaciones referentes a estrellas de neutrones.