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Los rayos cósmicos de energía extrema son partículas subatómicas que llegan a la tierra con energías mayores a 10¹⁸ eV. Su flujo es muy reducido (menos de 1 rayo cósmico primario por kilómetro cuadrado y por siglo) y sólo pueden ser detectados a través de las cascadas de partículas que producen tras el impacto con un núcleo de la atmósfera. La determinación del origen, la composición y el espectro energético de los rayos cósmicos de energía extrema representa uno de los mayores retos para la física de astropartículas en la actualidad. Hace ya más de 15 años un grupo de científicos comenzó una colaboración internacional para hacer frente a este reto. Hoy, la Colaboración Pierre Auger reúne a más de 450 autores de 17 países y gestiona la construcción y la operación del observatorio de rayos cósmicos más grande del mundo. En la primera parte de esta Tesis (capítulos 1 y 2) presento una reseña de las características de los rayos cósmicos, el estado actual de conocimiento en este campo y las técnicas empleadas para su detecci ón, haciendo especial hincapié en el Observatorio Pierre Auger y en los resultados publicados por la Colaboración Pierre Auger, de la cual formo parte desde 2004. La detección de rayos cósmicos de energía extrema plantea un enorme desafío científico tecnológico, en el que existen aún muchos problemas que resolver y otros tantos que mejorar. En el inicio del doctorado me interioricé en las simulaciones por computadora de las cascadas de partículas, y un viaje de especialización a Roma me llevó a interesarme por los eventos recolectados por el detector de fluorescencia. En la segunda parte de esta Tesis (capítulos 3 y 4), reporto las mejoras logradas sobre diversos aspectos de los algoritmos utilizados para la simulación de eventos generados por rayos cósmicos de muy alta energía y sobre los algoritmos utilizados para la reconstrucción de los mismos a partir de los datos recolectados por el detector de fluorescencia. Estas mejoras brindan mayor precisión en las medidas experimentales y contribuyen a tener una mejor comprensión del funcionamiento del detector. En particular, en el capítulo 3 presento el desarrollo de un nuevo método de reconstrucci ón que hace uso del tamaño finito del spot óptico de los telescopios de fluorescencia y del tamaño finito de la imagen de la cascada, brindando una mejora importante en la calidad de la reconstrucción y avanzando en la mejor comprensión del funcionamiento de los detectores. En el capítulo 4 presento un estudio detallado del balance energético de las cascadas de partículas iniciadas por rayos cósmicos utilizando simulaciones Monte Carlo y cuál es el uso correcto de las mismas para la simulación y reconstrucción de eventos de fluorescencia. Mientras se finalizaba la construcción del Observatorio, comenzó a tomar forma la idea de disminuir su límite de detección para estudiar la zona del espectro de rayos cósmicos en la que se cree se encuentra la transición entre el origen galáctico y extragal áctico de los mismos. En la tercera parte de esta Tesis (capítulos 5 y 6) presento un nuevo software para la simulación de cascadas subterráneas que desarrolle para cubrir las necesidades de AMIGA, una de las mejoras planeadas para el Observatorio Pierre Auger que incluye la instalación de detectores de muones enterrados a 3m de profundidad. Así, en el capítulo 5 describo la estructura del software desarrollado y presento algunas de las pruebas realizadas para su validación y en el capítulo 6 presento los primeros resultados obtenidos para AMIGA. El software fue desarrollado buscando una gran versatilidad, de manera que pueda ser utilizado para tratar problemas planteados por otros experimentos. Algunos ejemplos de aplicación en otros escenarios, como la simulación de cascadas en hielo para la detección de neutrinos por la técnica de radio, son también presentados en ése capítulo. Para concluir, algunos comentarios generales sobre los resultados presentados en esta Tesis y las perspectivas que abren para futuros estudios fueron volcados en el capítulo 7.

En esta Tesis se investigan propiedades de dos sistemas fı́sicos cuyo denominador común es que su dinámica responde a la interacción fuerte, cuyo lagrangiano es el de la Cromodinámica Cuántica, pero que debido a que ambos sistemas se estudian en el régimen de acoplamiento fuerte,no es posible la utilización de las técnicas de la teorı́a de perturbaciones en el contexto de la Teorı́a Cuántica de Campos Relativista (TCC). Esto ofrece un escenario adecuado y muy interesante para el empleo de las técnicas basadas directamente en la dualidad entre las Teorı́as de Yang-Mills (TYM) por un lado, y la supergravedad (SUGRA) y la Teorı́a de Supercuerdas (TSC), por otro. Primeramente se investiga la estructura de glueballs de la Teorı́a de Yang-Mills supersimétrica (SYM) con N = 4 supersimetrı́as y grupo de gauge SU (Nc) en 3+1 dimensiones, ası́ como de mesones escalares de la teorı́a de gauge en 3+1 dimensiones con N = 2 supersimetrı́as. Eso se desarrolla en el contexto de la dispersión inelástica profunda (DIS) de leptones cargados por glueballs y mesones. Se calculan los tensores hadrónicos mediante el uso de sus correspondientes descripciones holográficas duales. Se desarrollan cálculos a one-loop en supergravedad del tipo IIB. En ambos casos se obtuvo un resultado central que consiste en que las expansiones en 1/Nc, por un lado, y en potencias de Λ^2 /q^2 , donde Λ es el cut-off IR de la TYM, mientras que q es el tetraimpulso transferido por el fotón virtual desde un leptón al hadrón (en el proceso DIS), por otro, no conmutan. Eso da como resultado que en el caso de los mesones escalares la comparación de los momentos de menor orden de las funciones de estructura mejoran sustancialmente el nivel de acuerdo con resultados desde Lattice QCD al considerar el lı́mite de altas energı́as y luego el lı́mite 1 << N c . En particular, para el pión los cálculos previos a nivel árbol cuya diferencia con Lattice QCD es del orden del 10 %, con los resultados de esta Tesis a nivel one-loop llegan al orden de aproximación del 1, 27 %. Asimismo, se obtuvieron nuevas relaciones del tipo de Callan-Gross. La segunda clase de sistemas fuertemente acoplados investigados corresponde a plasmas de quarks y gluones generados en colisiones ultra-relativistas de iones pesados en el Relativistic Heavy Ion Collider (RHIC, Broohaven, USA) y en el Large Hadron Collider (LHC, CERN, Suiza). Para el caso de las colisiones de iones de Au-Au y de Pb-Pb, en esta Tesis se desarrollan simulaciones numéricas considerando la descripción holográfica dual del plasma, desde la supergravedad del tipo IIB, en el caso de un agujero negro del tipo Schwarzschild asintóticamente AdS_5, considerando todas las correcciones en α'^3 , donde α' = l_s^2 es decir el cuadrado de la longitud de la cuerda fundamental, a partir de la TSC del tipo IIB. Con esto se han obtenido curvas correspondientes a la emisión de fotones en dichos procesos y se ha comparado estos resultados directamente con resultados de los experimentos observando un mejor ajuste del espectro. Se obtuvieron coeficientes del flujo anisotrópico para colisiones de Au-Au en el rango de centralidad 0-20 % y 20-40 % en el mismo orden que los perturbativos indicando que posiblemente sea otras fuentes no consideradas que resuelven la tensión con los datos experimentales.

En este trabajo de Tesis Doctoral se llevó a cabo un estudio de las características de los modelos con simetría de gauge SU(3)_C ⊗ SU(3)_L ⊗ U(1)_X, o modelos 331, que constituyen extensiones relativamente simples del Modelo Estándar de las interacciones electrodébiles y fuertes. El trabajo ha sido orientado a contribuir al conocimiento de la fenomenología de estos modelos, abarcando tres aspectos fundamentales: en primer lugar, se estudiaron en detalle los acoplamientos que pueden dar lugar a manifestaciones significativas de la nueva física contenida en estas teorías; en segundo lugar, se buscaron observables adecuados para que estas manifestaciones de nueva física puedan tener lugar a escalas alcanzables por los experimentos actuales, estableciendo cotas para los parámetros de la teoría a partir de los datos experimentales existentes; y por último, se estudiaron predicciones para las propiedades de nuevas partículas presentes en los modelos, que podrán ser detectadas en el futuro próximo en experimentos como el LHC o el ILC.

The role of communities associated to a substrate as fundamental components of an aquatic system has been recognized by many authors. Although periphyton has been considered of great importance in shallow lakes, its inclusion in different ecological models has been unusual. Phillips et al. (1978) were the first in considering epiphyton as an internal mechanism leading the system to a change in its state from greater water transparecy to more turbid conditions. In contrast, Scheffer et al. (1993) proposed the Alternative Equilibria Hypothesis (AEH), which considers that a shallow lake can alternate between two states, a Clear Water State (stabilized by the presence of submerged macrophytes) and a Turbid Water State (stabilized by an elevated phytoplankton biomass), excluding in this scheme the periphytic communities. Later only the conceptual model coined by Goldsborough and Robinson (1996, G&R model) emphasizes again the importance of epiphyton as well as that of epipelon and metaphyton by establishing the possibility of occurrence of four states according to the contributions to the total algal biomass of each of the algal taxocenosis. Some analyses of the system’s dynamics have been performed in pampean lakes in the context of the AEH. On the other hand, knowledge regarding periphytic and benthic communities is still poor. We propose as an hypothesis that phytoperiphyton will respond to the occurrence of different equilibria states, displaying, at the same time, strong interrelationships with the phytobenthos as well as with the phytoplankton. On the other hand, we consider the possibility that the phytoperiphytic and phytobenthic assemblages may function as internal mechanisms both, as state change or as state stability factors. Finally, we assess the importance of phytoperiphyton as regards its contributions to the total algal biomass in the context of the G&R model, comparing then the results with those obtained in the case of the AEH. The general objectives of the present investigation are to gather information about the structure and dynamics of the phytoperiphyton and its relationships with environmental factors as well as with the phytobenthon and phytoplankton in a pampean shallow lake; and to analyze phytoperiphytic dynamics under the AEH and the G&R model contexts, evaluating periphytic contributions to the total algal biomass. This investigation was carried out in an seepage shallow lake, Lacombe Lake, which is included in the “Grupo Marginal de lagunas” group in the pericoastal region of Provincia de Buenos Aires. Sampling was performed from July 2001 to June 2002 at four sites according to their exposure to environmental factors and the presence of macrophytes: bulrush beds, (JP and JC); and sites devoided of emergent vegetation but with stands of submerged macrophytes during the growing season (ALe and ALr). J sites and ALe were also representative of deeper zones and ALr of a site near the shore. Environmental variables were measured in situ and water samples were collected for chemical analysis and for plankton identification and quantification, following a vertical profile (excepting in ALr). In the case of phytoperiphyton, bulrush epiphyte samples (epiphyton) were taken following a vertical profile and those of submerged macrophytes were collected from the first 25 cm of the stems. Benthon (epipelon) was sampled using a corer. Aliquots for chemical analysis were also extracted from those samples. Vegetal substrate dynamics was also evaluated during sampling. At the laboratory, concentrations of algal nutrients, of the eight principal ions and of soluble polyphenols, as well as alkalinity and total hardness were measured. Phytoplanktonic, epiphytic and epipelic samples were treated for active a chlorophyll and pheopigment determinations. Organism identification and quantification were made according to standardized and own developed methods. First, variations in biotic and abiotic variables were analyzed for the determination of different equilibrium states occurrence. Three perturbation events were registered: precipitations in August, October and March. All of them involved water level increases and conductivity diminutions. Especially in the first two events, nutrient input from runoff was observed; meanwhile in March the input of water determined the dilution in nutrients concentrations. On the other hand, light conditions improved, particularly from October onwards, when stands of submerged macrophytes developed and a decrease of phytoplankton density and a change in its specific composition was detected. The latter being generated by the dilution effects of water input, the shading of the aquatic plants and algal consumption by cladocerans. March event also involved an improvement of light underwater climate but the increase in the water level affected macrophyte stands which mainly withdrawed to the shallowest zones. As regards phytoplankton, changes in the specific composition were recorded again. The statistic evaluation of these facts in conjunction with the analysis of permanence, resiliency and resistance properties of the dominating taxocenosis leads to interpret in the context of the AEH that two equilibria states occurred in the shallow lake. During the first three months a Turbid Water State developed, given by the phytoplankton domination given by an oscilatorial cyanophyte, minor transparency and relatively higher nutrient concentrations. In October and November the passage between states occurred, thus considering these months as transitional. From December onwards a Clear Water State established given by extended development of stands of submerged macrophytes, greater water transparency and minor nutrient concentrations. Finally, from March to May, given the submerged macrophyte withdrawal to shallowest sectors it can be interpreted that the system oscillated around a clear water state, or else, taking into account changes in the planktonic community, it is suggested that a third different state established, the “mixed phytoplankton” state. It is also suggested that states developed in Lacombe could be alternative. Next, the structure and dynamics of phytoperiphyton on bulrush and submerged macrophytes was analyzed. Vertical spatial and temporal variations of bulrush epiphyton showed to be more important than differences between sites. These variations involved smaller biomass values and a simpler physiognomy during turbid water phase and along the vertical gradient in all sampling occasions. In October, related to better light underwater conditions and the nutrient input, epiphytic biomass increased, principally promoted by species typical of the turbid phase. Nonetheless, a structural change started in this month, driven by herbivores but also by the modification of internal relationships between members of the same taxocenosis (shading and less bare substrate to be colonized). This resulted in the establishment of the characteristic assemblages of the clear water phase. During the first three months domination by stalked diatoms structured the community in a unique stratum, resistant to the less favourable light conditions and resilient to August perturbations. From December onwards Spirogyra species dominated, constituting the upper stratum, followed by Oedogonium species in the middle stratum and diatoms in the lower one. These assemblages were stabilized by mechanisms mainly displayed by the zignematalean taxon (shading, epifitism inhibition), allowing to considerer that they were resilient to March perturbation event (de novo establishment on non previously colonized surfaces). In face of the permanence, resistance and resilience properties showed by the taxocenosis, two different states of bulrush phytoperiphyton are considered: turbid water and clear water assemblages. This temporal pattern appeared less well delimited with depth and showed up later in time. Epiphyton data suggested that both states could be alternative. Differences between sites could be observed in respect to phytoperiphyton growing on submerged macrophytes, but the same temporal pattern showed by bulrush periphyton couldn’t be detected due to a great variability in the data gathered. In the next section, the analysis of the structure and dynamics of the phytobenton was carried out. It was showed that differences between the deepest sampling points and the shallowest site were greater than temporal ones. The composition of the assemblages was scarcely variable in time. The taxocenosis was principally related to the poor light conditions recorded on the sediment surface throughout the entire sampling period, even during the clear water phase. The examination of the specific composition in function of its origin evidenced the influence that the epipelon receives from other taxocenosis, constituting a refuge site for allochthonous taxa during unfavourable situations. The scarce variability of the epipelic fraction is also showed. In this way, it is considered that the taxocenosis maintained itself in only one state, indifferent to state changes in the lake and principally resisting the darkness and bearing to live in the unstable sediments. Subsequently, first the roles of the phytoperiphyton and complementary those of phytobenton, as state change or state stability agents were evaluated. The harmful effect of the phytoperiphyton on submerged macrophytes was probably counteracted by plant ramification production and allelopathy. The evidence suggested that its role as nutrient sequestrator is of minor importance in this case, due to other factors that might have disturbed phytoplankton more intensely than competition with phytoperiphyton. Much evidence supported the role of phytobenton as a source of epiphytic and planktonic species belonging either to turbid and clear water state assemblages. The results did not support the hypothesis that phytobenton may play a role as a sediment-stabilizing factor. On the contrary, it may be perturbating agent. Next, an analysis of the contributions of each community to the total algal biomass in the context of the G&R model was performed. Comparisons with AEH results were also carried out. This leaded to the consideration of the occurrence of the following states in Lacombe Lake: a Lake State consistent with the Turbid Water phase, an Open State consistent with the Clear Water phase and local Dry States in ALr site occurred both during the turbid water phase (July) or the clear water situation (March-April). The substantial contribution of epiphyton especially, that growing on submerged macrophytes, is emphasized. The importance of the discrimination of different fractions of epipelon material according to its origin when evaluating the contributions of each community to total algal biomass is also highlighted. Finally, a new interpretation is presented based on algal taxocenosis dynamics, which incorporates elements from both, the AEH and the G&R model. In conclusion, phytoperiphyton in this pampean shallow lake is strongly affected by the factors that determine the establishment or passage from one equilibrium state to another. These assemblages respond to changes through modifications in their own internal structure. These structures gradually alter with depth, remaining these changes more fully explained by the comparison with phytobenton dynamics. Besides the possible roles that the taxocenosis might undertake as an internal mechanism in the change or stabilization of systems states in different situations other than those encountered during this study, epiphyton importance in the systems context, is principally evidenced by the great contribution that these taxocenosis make to the total algal biomass, constituting the dominant compartment during clear water states.