Measurement of top-quark polarisation observables and a search for dark matter in single-top-quark production with the atlas detector at the LHC.
- Autores: Galo Gonzalvo Rodríguez
- Directores de la Tesis: M. José Costa Mezquita (dir. tes.), José Enrique García Navarro (codir. tes.)
- Lectura: En la Universitat de València ( España ) en 2022
- Idioma: inglés
- Tribunal Calificador de la Tesis: Andrea Knue (presid.), Marcel Vos (secret.), Héctor de la Torre Pérez (voc.)
- Programa de doctorado: Programa de Doctorado en Física por la Universitat de València (Estudi General)
- Materias:
- Enlaces
- Tesis en acceso abierto en: TESEO
- Resumen
This thesis describes two different analyses based on data from proton-proton collisions at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 139$^{-1}$fb, collected with the ATLAS detector at the LHC. The first analysis presents the measurement of the normalised differential cross-section of $t$-channel single-top-quark production, where the top quark decays leptonically, as a function of three polarisation angles of the lepton for top-quark and top-antiquark events both inclusively and separately. These observables are particularly sensitive to new physics effects in the $tWb$ vertex. Selected events contain exactly one isolated electron or muon, large missing transverse momentum and exactly two jets, one being $b$ tagged. Stringent selection requirements are applied to discriminate $t$-channel single-top-quark events from the background contributions. These measurements are in agreement with the Standard Model predictions and have been used to set limits in the real and imaginary part of the dipole operator $C_{tW} = 0.4 $\pm$ 1.1$ and $C_{itW} = -0.3 $\pm$ 0.4$ at 95\% confidence level. The limits on the imaginary part, which is sensitive to CP violation components, are the best limits from high-energy measurements at the time this work has been presented. The second analysis presents a direct search of dark matter particles produced in association with a single top quark which decays hadronically. Selected events contain exactly zero leptons, exactly zero loose jets, at least one top-tagged large-R jet, zero or one $b$ jets, large missing transverse momentum, and a big angular separation between the direction of the jets and the missing transverse momentum. In addition, this analysis benefits from cutting-edge multivariate analysis techniques to enlarge the signal discrimination over the Standard Model backgrounds. In the absence of any significant excess of data, the results are used to provide 95\% confidence level upper limits on the production cross-section for two models: the production of a dark matter signal from the resonant and the non-resonant models. The observed (expected) mass limits for the scalar mediator mass $m_{\phi}$ are 4.6 (4.7)~TeV for the resonant model, and 2.6 (2.7)~TeV for the vector mediator mass $m_{V}$ for the non-resonant model. These results constitute the most stringent constraints on this dark matter production cross-section.
