Measurement of the Higgs boson coupling with tau leptons and search for an additional neutral MSSM Higgs boson with the ATLAS detector

El Modelo Estándar de física de partículas (SM), la teoría que describe el comportamiento de las partículas subatómicas, ha sido un éxito en su precisión y exactitud. Uno de sus pilares básicos es el mecanismo de Higgs, que regula los términos de masa de las partículas fundamentales. La partícula responsable de este mecanismo es el bosón de Higgs, que fue descubierto en 2012 en los experimentos ATLAS y CMS del colisionador de partículas LHC del CERN, mediante el análisis de su acoplamiento a partículas bosónicas (fotones y bosones W y Z). Tras su descubrimiento, es necesario confirmar que el resto sus propiedades se corresponden con las predichas por el SM, como, por ejemplo, su acoplamiento a fermiones. El primer objetivo de esta tesis es el estudio del acoplamiento del bosón de Higgs a leptones tau y su comparación con el valor predicho por el SM. Eso se hace mediante el análisis de colisiones protón-protón producidas por el LHC y recogidas por el detector ATLAS durante el Run 1, con una luminosidad acumulada de 4.5 ifb a una energía en el centro de masas de 7 TeV y 20.3 ifb a una energía en el centro de masas de 8 TeV. La tesis describe el análisis de eventos di-tau y la optimización de su significancia mediante el uso de un tipo de algoritmo multivariante (MVA), un BDT, entrenado en los modelos de fondo y señal predichos por el SM. El resultado final muestra un exceso de eventos respecto al fondo con una significancia estadística observada (esperada) de 4.5 sigma (3.4 sigma), que representa evidencia del acoplamiento directo del bosón de Higgs a fermiones. La fuerza de señal normalizada medida, mu = 1.43 (+0.43,-0.37), es compatible con el SM. El SM, sin embargo, no está completo, ya que hay fenómenos naturales para los que no provee de solución, mientras que la naturalidad de algunos de sus mecanismos está en entredicho. Para solucionar estos problemas se han propuesto varias extensiones del SM, de entre las que destaca la supersimetría (SUSY), en especial en su mínima implementación (MSSM). En este modelo, el mecanismo de Higgs está compuesto de cinco bosones, uno de los cuales sería el descubierto en 2012. Del resto, dos de ellos podrían tener las propiedades adecuadas de masa y acoplamiento a fermiones para ser detectados por los experimentos del LHCacon un análisis análogo al descrito para el SM. El segundo objetivo de esta tesis es la búsqueda de los bosones de Higgs adicionales predichos por el MSSM en el rango de alta masa (200 GeV-1.2 TeV) mediante el análisis de eventos di-tau obtenidos durante el primer año del Run 2 del LHC por el detector ATLAS, con una luminosidad acumuladada de 3.2 ifb a una energía en el centro de masas de 13 TeV. Como resultado del análisis, no se observa ningún exceso significativo, los datos obtenidos son compatibles con el fondo esperado por el SM. Por tanto, se establecen límites a la sección eficaz de producción de los bosones de Higgs, y se restringe el espacio de parámetros (tan beta–mA) permitido para diferentes escenarios fenomenológicos del MSSM.The Standard Model of particle physics (SM), the theory that describes the behaviour of the subatomic particles, has been a success in precision and accuracy. One of its basic key foundations is the Higgs mechanism, which controls the mass terms of the fundamental particles. The responsible particle of this mechanism is the Higgs boson, which was discovered in 2012 by the LHC experiments, ATLAS and CMS, at CERN. This was done by analyzing its coupling to bosonic particles, i.e. photons, and the W and Z bosons. After the discovery, it is necessary to check if all the properties of the observed particle correspond to their SM prediction. One of this properties is the coupling to fermions (Yukawa coupling). The first objective of this thesis is the study of the coupling of the Higgs boson to tau leptons and its comparison to the value predicted by the SM. This is done by the analysis of proton-proton collisions produced at the LHC and recorded by the ATLAS detector during the Run 1, which accounts for 4.5 ifb collected at an energy in the center of mass of 7 TeV and 20.3 ifb at an energy in the center of mass of 8 TeV. The thesis describes the analysis of di-tau events and the enhancing of the significance by the use of a multivariate algorithm (MVA), a BDT, which was trained using the signal and background models predicted by the SM. The final results shows an excess of events respect to the expected background with an observed (expected) statistical significance of 4.5 sigma (3.4 sigma), which provides evidence for the direct coupling of the Higgs boson to fermions. The measured signal strength normalized to the SM expectation of mu = 1.43 (+0.43,-0.37) is consistent with the predicted Yukawa coupling strength in the SM. The SM, however, is not complete, as it lacks a proper explanation for several natural phenomena, while the naturalness of some of its mechanisms is contested. In order to solve these issues, several extensions of the SM have been proposed, such as supersymmetry (SUSY), especially in its minimal implementation (MSSM). In this model, the Higgs mechanism is composed of five Higgs bosons, one of which would be the particle discovered in 2012. In addition, two of the remaining bosons could have the properties, such as mass and coupling to fermions, suitable for being detected at the LHC with an analysis analogous to the SM. The second aim of this thesis is the search for additional Higgs bosons predicted by the MSSM in the high-mass range (200 GeV–1.2 TeV). This is done by analyzing di-tau events obtained by the ATLAS detector during the first year of the Run 2 of the LHC, which accounts for an integrated luminosity of 3.2 ifb at an energy in the center of mass of 13 TeV. No significant excess is observed as a result of the analysis, the data is fully compatible with the background expected by the SM. Therefore, exclusion limits are set on the production cross-sections of the Higgs bosons and on the (tan \u1d6fd–mA) parameter space of different phenomenological scenarios of the MSSM

Measurements of the Total and Differential Higgs Boson Production Cross Sections Combining the H??????? and H???ZZ*???4??? Decay Channels at s\sqrt{s}=8??????TeV with the ATLAS Detector

Measurements of the total and differential cross sections of Higgs boson production are performed using 20.3~fb$^{-1}$ of $pp$ collisions produced by the Large Hadron Collider at a center-of-mass energy of $\sqrt{s} = 8$ TeV and recorded by the ATLAS detector. Cross sections are obtained from measured $H \rightarrow \gamma \gamma$ and $H \rightarrow ZZ ^{*}\rightarrow 4\ell$ event yields, which are combined accounting for detector efficiencies, fiducial acceptances and branching fractions. Differential cross sections are reported as a function of Higgs boson transverse momentum, Higgs boson rapidity, number of jets in the event, and transverse momentum of the leading jet. The total production cross section is determined to be $\sigma_{pp \to H} = 33.0 \pm 5.3 \, ({\rm stat}) \pm 1.6 \, ({\rm sys}) \mathrm{pb}$. The measurements are compared to state-of-the-art predictions.Measurements of the total and differential cross sections of Higgs boson production are performed using 20.3  fb-1 of pp collisions produced by the Large Hadron Collider at a center-of-mass energy of s=8  TeV and recorded by the ATLAS detector. Cross sections are obtained from measured H→γγ and H→ZZ*→4ℓ event yields, which are combined accounting for detector efficiencies, fiducial acceptances, and branching fractions. Differential cross sections are reported as a function of Higgs boson transverse momentum, Higgs boson rapidity, number of jets in the event, and transverse momentum of the leading jet. The total production cross section is determined to be σpp→H=33.0±5.3 (stat)±1.6 (syst)  pb. The measurements are compared to state-of-the-art predictions.Measurements of the total and differential cross sections of Higgs boson production are performed using 20.3 fb$^{-1}$ of $pp$ collisions produced by the Large Hadron Collider at a center-of-mass energy of $\sqrt{s} = 8$ TeV and recorded by the ATLAS detector. Cross sections are obtained from measured $H \rightarrow \gamma \gamma$ and $H \rightarrow ZZ ^{*}\rightarrow 4\ell$ event yields, which are combined accounting for detector efficiencies, fiducial acceptances and branching fractions. Differential cross sections are reported as a function of Higgs boson transverse momentum, Higgs boson rapidity, number of jets in the event, and transverse momentum of the leading jet. The total production cross section is determined to be $\sigma_{pp \to H} = 33.0 \pm 5.3 \, ({\rm stat}) \pm 1.6 \, ({\rm sys}) \mathrm{pb}$. The measurements are compared to state-of-the-art predictions