Measurement of top-quark polarisation observables and a search for dark matter in single-top-quark production with the ATLAS detector at the LHC

Esta tesis presenta dos análisis diferentes basados en los datos resultantes de colisiones protón-protón a una energía en el centro de masas de 13 TeV recolectados con el detector ATLAS del LHC, correspondientes a una luminosidad integrada de 139 fb$^{-1}$. El primer análisis presenta una medida de secciones eficaces diferenciales normalizadas del canal de producción de quarks top en solitario $t$-channel, donde el quark top se desintegra leptónicamente, como función de los tres ángulos de polarización del leptón para eventos con quarks top y quarks antitop tanto separadamente como de forma inclusiva. Estos observables son particularmente sensibles a efectos de nueva física en el vértice $tWb$. Los eventos seleccionados contienen exactamente un electrón o muón aislado, un alto momento transverso perdido y exactamente dos jets, siendo uno de ellos identificado como $b$-jet. Requerimientos más estrictos se aplican para discriminar los eventos del $t$-channel con respecto a los fondos. Estas medidas están en acuerdo con las predicciones del Modelo Estándar, y han sido utilizadas para establecer límites en la parte real e imaginaria del operador dipolo $C_{tW} = 0.4$\pm$1.1$ y $C_{itW} = -0.3$\pm$0.4$ en un intervalo de confianza del 95\%. Los límites en la parte imaginaria, sensibles a componentes de violación CP, son los mejores límites obtenidos a partir de medidas de altas energías en el momento en que este trabajo es presentado. El segundo análisis presenta una búsqueda directa de partículas de materia oscura producidas junto a un quark top solitario que se desintegra hadrónicamente. Los eventos seleccionados contienen exactamente cero leptones, cero jets de baja calidad, al menos un jet grande identificado como procedente de un quark top, cero o un jet identificado como $b$-jet, una gran cantidad de momento transverso perdido, y una amplia separación entre la dirección de los jets y la dirección del momento transverso perdido. Además, este análisis se beneficia de técnicas innovadoras de análisis multivariado para mejorar la discriminación de señal sobre los diferentes fondos del Modelo Estándar. En ausencia de un exceso de datos importante, los resultados son utilizados para proporcionan cotas superiores en un intervalo de confianza del \95% para la sección eficaz de producción de dos modelos: la producción de materia oscura a través del modelo resonante y el modelo no-resonante. Los límites observados (esperados) en la masa del mediador escalar $m_{\phi}$ son 4.6 (4.7)~TeV para el modelo resonante, y 2.6 (2.7)~TeV para la masa del mediador vectorial $m_{V}$ para el model no-resonante. Estos resultados constituyen los límites más restrictivos en esta sección eficaz de producción de materia oscura en el momento en que este trabajo es presentado.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 fb$^{-1}$, 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 at the time this work has been presented

Search for tt¯ resonances in fully hadronic final states in pp collisions at √s = 13 TeV with the ATLAS detector

This paper presents a search for new heavy particles decaying into a pair of top quarks using 139 fb of proton-proton collision data recorded at a centre-of-mass energy of s = 13 TeV with the ATLAS detector at the Large Hadron Collider. The search is performed using events consistent with pair production of high-transverse-momentum top quarks and their subsequent decays into the fully hadronic final states. The analysis is optimized for resonances decaying into a tt¯ pair with mass above 1.4 TeV, exploiting a dedicated multivariate technique with jet substructure to identify hadronically decaying top quarks using large-radius jets and evaluating the background expectation from data. No significant deviation from the background prediction is observed. Limits are set on the production cross-section times branching fraction for the new Z′ boson in a topcolor-assisted-technicolor model. The Z′ boson masses below 3.9 and 4.7 TeV are excluded at 95% confidence level for the decay widths of 1% and 3%, respectively. [Figure not available: see fulltext.

Evidence for the production of three massive vector bosons with the ATLAS detector

A search for the production of three massive vector bosons in proton–proton collisions is performed using data at s=13 TeV recorded with the ATLAS detector at the Large Hadron Collider in the years 2015–2017, corresponding to an integrated luminosity of 79.8 fb. Events with two same-sign leptons ℓ (electrons or muons) and at least two reconstructed jets are selected to search for WWW→ℓνℓνqq. Events with three leptons without any same-flavour opposite-sign lepton pairs are used to search for WWW→ℓνℓνℓν, while events with three leptons and at least one same-flavour opposite-sign lepton pair and one or more reconstructed jets are used to search for WWZ→ℓνqqℓℓ. Finally, events with four leptons are analysed to search for WWZ→ℓνℓνℓℓ and WZZ→qqℓℓℓℓ. Evidence for the joint production of three massive vector bosons is observed with a significance of 4.1 standard deviations, where the expectation is 3.1 standard deviations.Peer Reviewe

Measurement of isolated-photon plus two-jet production in pp collisions at √s = 13 TeV with the ATLAS detector

The dynamics of isolated-photon plus two-jet production in pp collisions at a centre-of-mass energy of 13 TeV are studied with the ATLAS detector at the LHC using a dataset corresponding to an integrated luminosity of 36.1 fb. Cross sections are measured as functions of a variety of observables, including angular correlations and invariant masses of the objects in the final state, γ + jet + jet. Measurements are also performed in phase-space regions enriched in each of the two underlying physical mechanisms, namely direct and fragmentation processes. The measurements cover the range of photon (jet) transverse momenta from 150 GeV (100 GeV) to 2 TeV. The tree-level plus parton-shower predictions from Sherpa and Pythia as well as the next-to-leading-order QCD predictions from Sherpa are compared with the measurements. The next-to-leading-order QCD predictions describe the data adequately in shape and normalisation except for regions of phase space such as those with high values of the invariant mass or rapidity separation of the two jets, where the predictions overestimate the data. [Figure not available: see fulltext.

Search for resonances decaying into a weak vector boson and a Higgs boson in the fully hadronic final state produced in proton-proton collisions at s =13 TeV with the ATLAS detector

A search for heavy resonances decaying into a W or Z boson and a Higgs boson produced in proton-proton collisions at the Large Hadron Collider at s=13 TeV is presented. The analysis utilizes the dominant W→qq¯′ or Z→qq¯ and H→bb¯ decays with substructure techniques applied to large-radius jets. A sample corresponding to an integrated luminosity of 139 fb-1 collected with the ATLAS detector is analyzed and no significant excess of data is observed over the background prediction. The results are interpreted in the context of the heavy vector triplet model with spin-1 W′ and Z′ bosons. Upper limits on the cross section are set for resonances with mass between 1.5 and 5.0 TeV, ranging from 6.8 to 0.53 fb for W′→WH and from 8.7 to 0.53 fb for Z′→ZH at the 95% confidence level

Measurement of the Z(-> l(+)l(-))gamma production cross-section in pp collisions at root s=13 TeV with the ATLAS detector

The production of a prompt photon in association with a Z boson is studied in proton-proton collisions at a centre-of-mass energy s = 13 TeV. The analysis uses a data sample with an integrated luminosity of 139 fb collected by the ATLAS detector at the LHC from 2015 to 2018. The production cross-section for the process pp → ℓℓγ + X (ℓ = e, μ) is measured within a fiducial phase-space region defined by kinematic requirements on the photon and the leptons, and by isolation requirements on the photon. An experimental precision of 2.9% is achieved for the fiducial cross-section. Differential cross-sections are measured as a function of each of six kinematic variables characterising the ℓℓγ system. The data are compared with theoretical predictions based on next-to-leading-order and next-to-next-to-leading-order perturbative QCD calculations. The impact of next-to-leading-order electroweak corrections is also considered. [Figure not available: see fulltext.]

Measurement of fiducial and differential W+W- production cross-sections at √s=13 TeV with the ATLAS detector

A measurement of fiducial and differential cross-sections for WW production in proton–proton collisions at s=13 TeV with the ATLAS experiment at the Large Hadron Collider using data corresponding to an integrated luminosity of 36.1 fb is presented. Events with one electron and one muon are selected, corresponding to the decay of the diboson system as WW→ eνμν. To suppress top-quark background, events containing jets with a transverse momentum exceeding 35 GeV are not included in the measurement phase space. The fiducial cross-section, six differential distributions and the cross-section as a function of the jet-veto transverse momentum threshold are measured and compared with several theoretical predictions. Constraints on anomalous electroweak gauge boson self-interactions are also presented in the framework of a dimension-six effective field theory

Measurement of the ratio of cross sections for inclusive isolated-photon production in pp collisions at √s = 13 and 8 TeV with the ATLAS detector

The ratio of the cross sections for inclusive isolated-photon production in pp collisions at centre-of-mass energies of 13 and 8 TeV is measured using the ATLAS detector at the LHC. The integrated luminosities of the 13 TeV and 8 TeV datasets are 3.2 fb and 20.2 fb , respectively. The ratio is measured as a function of the photon transverse energy in different regions of the photon pseudorapidity. The predictions from next-to-leading-order perturbative QCD calculations are compared with the measured ratio. The experimental systematic uncertainties as well as the uncertainties affecting the predictions are evaluated taking into account the correlations between the two centre-of-mass energies, resulting in a reduction of up to a factor of 2.5 (5) in the experimental (theoretical) systematic uncertainties. The predictions based on several parameterisations of the proton parton distribution functions agree with the data within the reduced experimental and theoretical uncertainties. In addition, this ratio to that of the fiducial cross sections for Z boson production at 13 and 8 TeV using the decay channels Z → e e and Z → μ μ is made and compared with the theoretical predictions. In this double ratio, a further reduction of the experimental uncertainty is obtained because the uncertainties arising from the luminosity measurement cancel out. The predictions describe the measurements of the double ratio within the theoretical and experimental uncertainties.[Figure not available: see fulltext.].Peer Reviewe

Measurements of top-quark pair differential and double-differential cross-sections in the l plus jets channel with pp collisions at root s=13 TeV using the ATLAS detector

Single- and double-differential cross-section measurements are presented for the production of top-quark pairs, in the lepton + jets channel at particle and parton level. Two topologies, resolved and boosted, are considered and the results are presented as a function of several kinematic variables characterising the top and t t system and jet multiplicities. The study was performed using data from pp collisions at centre-of-mass energy of 13 TeV collected in 2015 and 2016 by the ATLAS detector at the CERN Large Hadron Collider (LHC), corresponding to an integrated luminosity of 36 fb-1. Due to the large tt cross-section at the LHC, such measurements allow a detailed study of the properties of top-quark production and decay, enabling precision tests of several Monte Carlo generators and fixed-order Standard Model predictions. Overall, there is good agreement between the theoretical predictions and the data

Search for heavy neutral Higgs bosons produced in association with b-quarks and decaying into b-quarks at s =13 TeV with the ATLAS detector

A search for heavy neutral Higgs bosons produced in association with one or two b-quarks and decaying to b-quark pairs is presented using 27.8 fb-1 of s=13 TeV proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider during 2015 and 2016. No evidence of a signal is found. Upper limits on the heavy neutral Higgs boson production cross section times its branching ratio to bb¯ are set, ranging from 4.0 to 0.6 pb at 95% confidence level over a Higgs boson mass range of 450 to 1400 GeV. Results are interpreted within the two-Higgs-doublet model and the minimal supersymmetric Standard Model