New Physics in the Electroweak Sector Under Scrutiny at LHC

Abstract

For more than half a century, colliders have been in the forefront of studying the Standard Model (SM) predictions. The pinnacle of both SM and colliders occurred when on July the 4th 2012, at CERN Large Hadron Collider, the Higgs discovery was announced, almost 50 years after being postulated. Until 2012 the Higgs was the only missing piece of the SM, and its discov- ery was a milestone in the LHC. The complete analysis of Run 1 data and the preliminary ones from Run 2 data, indicate that this new particle is a scalar bo- son, with CP–even properties, as in the SM Higgs scenario. Furthermore, it seems that the observed state is directly connected to the electroweak symmetry breaking (EWSB), after analyzing interactions with gauge bosons. Until the moment there is no evidence of any physics BSM at the LHC in the form of new states. Clearly this disfavours any extension of the SM that predicts new particles at the TeV scale. This was the scenario when this Thesis was initiated. The Run 1 of LHC had reached its final luminosity, and the Run 2 was starting its operation. The lack of NP states at LHC served us as motivation to look for an alternative approach, instead of constraining ourselves to an specific SM completion. Here enters the model-independent philosophy, where through the use of an effective Lagrangian we start to confront all the existing available data and search for any possible de- viation of the SM predictions, by making use of the framework of Effective Field Theories (EFT). In the context of an EFT, we follow an atheist path: NP is expected to manifest directly at a scale Λ, which is higher than the scale at which the experiments are performed. Any effect of new physics (NP) at the low scale can be parametrized by a set of higher dimensional operators, that are suppressed by powers of the high energy scale. With this aim, in Chapter 2 we introduce the SM as an EFT at low energy. Our ignorance about the ultraviolet (UV) theory, and with no guidance of where the scale of NP could be laying, lead us to a bottom-up approach, where the higher dimensional operators used are driven by the existing data on the EW sector. This Chapter is supposed to set the roots of the analysis done in Chapters 3, 4 and 5. The results presented in these Chapters represent a step in the determination of the precision at which the different interactions in the EW sectors of the SM are being tested by the available data. The ultimate purpose of these analyses is to look for deviations that would be translated in the future on information regarding the UV completion of the SM. Up to this point our focus was on the scrutiny of the EWSB at LHC, and so the origin of the particles’ mass from a model-independent point of view. But still there are other alternatives to study physics beyond the SM, like direct searches at the LHC for some particular signature in a model. Clearly this approach only applies to neutrino models which are testable at LHC. In Chapter 6 we will study one such model of Type-III see-saw where the heavy states can live in the TeV region and, not less important, the couplings of these states are determined by the light neutrino masses and mixings.

Esta tesis se centra en la búsqueda de física mas allá del modelo estándar en el sector electrodébil, y en particular en el mecanismo de la rotura expontánea de la simetría electrodébil y de la generación de masas, usando datos recogidos por los experimentos del Large Hadron Collider. El zenit, tanto del modelo estándar como de la física experimental de colisionadores se alcanzó en 2012 cuando CERN anunció el descubrimiento de una nueva partícula que podría ser el boson de Higgs. Hasta 2012 el boson de Higgs era la única pieza que faltaba en la construcción del modelo estándard. Sin embargo, sabemos de forma feaciente que el modelo estándar no puede ser la teoría de la Naturaleza basandonos en hechos puramente observacionales tales como la existencia de materia oscura, de neutrinos masivos, y de la simetría materia-antimateria en el Universo. En particular, la falta de nuevos estados observados en el LHC nos sirvió como motivación para el uso de Lagrangianos efectivos como herramienta para testear en una forma independiente de modelo una amplia variedad de datos con el fin de buscar posibles desviaciones sobre las predicciones del modelo estándar. En este contexto, los efectos de nueva física, que se manifestarían de forma directa a una escala Lambda superior a aquella alcanzable en los experimentos estudiados, pueden parametrizarse en una serie de operadores de dimensión mayor que cuatro y que aparecen en el Lagrangiano suprimidos por potencias de Lambda. Para ello el capítulo 2 tras una breve introdución sobre como se construye una teoría efectiva. En los siguientes tres capítulos 3–5 presentamos un análysis en que el objetivo es buscar desviaciones de las predicciones del modelo estándar que pudieran darnos información sobre el modelo completo en el sector responsable para la generación de masas. Finalmente en el capítulo 6 presentamos los resultados de un estudio realizado en el contexto de un modelo de masa de neutrinos. Dado que el LHC es nuestra principal herramienta para testear las extensiones del modelo estándar, una cuestión obvia es si LHC puede darnos información acerca de las extensiones que permiten generar masas para los neutrinos.