Estudios de fondos de electrones reconstruidos como fotones en búsquedas de supersimetría con el detector ATLAS

El Gran Colisionador de Hadrones (LHC) perteneciente al CERN, es el acelerador de hadrones más grande del mundo, diseñado para colisionar protones a una energía de centro de masa de \sqrt{s} = 14 TeV. El mismo consta de una serie de detectores cuyos diseños están orientados a distintos objetivos. Uno de ellos es ATLAS que tiene como uno de sus propósitos generales la búsqueda de nueva física. Supersimetría (SUSY) es una extensión del Modelo Estándar (SM) que postula la existencia de nuevas partículas relacionadas con las ya conocidas del SM, asociando a los fermiones (bosones) del mismo con nuevos bosones (fermiones). De esta forma se resuelven una serie de cuestiones que dejaban al SM incompleto, como por ejemplo los problemas de jerarquía y naturalidad, la ausencia de candidatos a materia oscura y la exclusión de la fuerza gravitatoria en la teoría. SUSY es una simetría rota y existen varios mecanismos propuesto de ruptura. Uno de ellos es el “Generalised Model of Gauge-Mediated Supersymmetry Breaking” (GGMSB) cuya ruptura está en la escala del TeV, pudiendo ser así detectable por el LHC. La estrategia general de búsqueda de SUSY consiste en encontrar en alguna región de señal, un exceso en el número de eventos con respecto al número predicho por el SM. Para ello es necesario tener un buen conocimiento de los procesos del SM que aportan eventos a esa región de señal. Estos procesos se denominan “fondos” y uno de ellos es el proveniente de la reconstrucción errónea de electrones como fotones en el detector ATLAS. La técnica para la estimación de ese fondo consiste básicamente en el conteo de eventos asociados al decaimiento del bosón Z → ee, donde uno de esos electrones puede ser erróneamente reconstruido como un fotón. De esta forma se calcula un factor de identificación errónea (“Fake Factor”) asociado a la proporción de electrones reconstruidos. Los resultados obtenidos en esta Tesis (compatibles con predicciones en base a datos y técnicas anteriores) fueron validados en nuevas regiones diseñadas específicamente para corroborar las contribuciones de este fondo. Los factores obtenidos en este trabajo son utilizados actualmente por la colaboración ATLAS para la estimación del fondo antes mencionado en búsquedas de SUSY con fotones en estado final.Facultad de Ciencias Exacta

Measurements of inclusive and differential fiducial cross-sections of t¯tγ production in leptonic final states at √s=13 TeV in ATLAS

Inclusive and differential cross-sections for the production of a top-quark pair in association with a photon are measured with proton-proton collision data corresponding to an integrated luminosity of 36.1 fb−1, collected by the ATLAS detector at the LHC in 2015 and 2016 at a centre-of-mass energy of 13 TeV . The measurements are performed in single-lepton and dilepton final states in a fiducial volume. Events with exactly one photon, one or two leptons, a channel-dependent minimum number of jets, and at least one b-jet are selected. Neural network algorithms are used to separate the signal from the backgrounds. The fiducial cross-sections are measured to be 521±9(stat.)±41(sys.) fb and 69±3(stat.)±4(sys.) fb for the single-lepton and dilepton channels, respectively. The differential cross-sections are measured as a function of photon transverse momentum, photon absolute pseudorapidity, and angular distance between the photon and its closest lepton in both channels, as well as azimuthal opening angle and absolute pseudorapidity difference between the two leptons in the dilepton channel. All measurements are in agreement with the theoretical predictions.Instituto de Física La Plat

Search for low-mass resonances decaying into two jets and produced in association with a photon using pp collisions at √s = 13 TeV with the ATLAS detector

A search is performed for localised excesses in dijet mass distributions of low-dijet-mass events produced in association with a high transverse energy photon. The search uses up to 79.8 fb−1 of LHC proton–proton collisions collected by the ATLAS experiment at a centre-of-mass energy of 13 TeV during 2015–2017. Two variants are presented: one which makes no jet flavour requirements and one which requires both jets to be tagged as b-jets. The observed mass distributions are consistent with multi-jet processes in the Standard Model. The data are used to set upper limits on the production cross-section for a benchmark Z' model and, separately, on generic Gaussian-shape contributions to the mass distributions, extending the current ATLAS constraints on dijet resonances to the mass range between 225 and 1100 GeV.La lista completa de autores que integran el documento puede verse en el archivo asociado.Facultad de Ciencias Exacta

Measurements of inclusive and differential fiducial cross-sections of t¯tγ production in leptonic final states at √s=13 TeV in ATLAS

Inclusive and differential cross-sections for the production of a top-quark pair in association with a photon are measured with proton-proton collision data corresponding to an integrated luminosity of 36.1 fb−1, collected by the ATLAS detector at the LHC in 2015 and 2016 at a centre-of-mass energy of 13 TeV . The measurements are performed in single-lepton and dilepton final states in a fiducial volume. Events with exactly one photon, one or two leptons, a channel-dependent minimum number of jets, and at least one b-jet are selected. Neural network algorithms are used to separate the signal from the backgrounds. The fiducial cross-sections are measured to be 521±9(stat.)±41(sys.) fb and 69±3(stat.)±4(sys.) fb for the single-lepton and dilepton channels, respectively. The differential cross-sections are measured as a function of photon transverse momentum, photon absolute pseudorapidity, and angular distance between the photon and its closest lepton in both channels, as well as azimuthal opening angle and absolute pseudorapidity difference between the two leptons in the dilepton channel. All measurements are in agreement with the theoretical predictions.Instituto de Física La Plat

Measurements of differential cross-sections in four-lepton events in 13 TeV proton-proton collisions with the ATLAS detector

Measurements of four-lepton differential and integrated fiducial cross-sections in events with two same-flavour, opposite-charge electron or muon pairs are presented. The data correspond to 139 fb−1 of s = 13 TeV proton-proton collisions, collected by the ATLAS detector during Run 2 of the Large Hadron Collider (2015–2018). The final state has contributions from a number of interesting Standard Model processes that dominate in different four-lepton invariant mass regions, including single Z boson production, Higgs boson production and on-shell ZZ production, with a complex mix of interference terms, and possible contributions from physics beyond the Standard Model. The differential cross-sections include the four-lepton invariant mass inclusively, in slices of other kinematic variables, and in different lepton flavour categories. Also measured are dilepton invariant masses, transverse momenta, and angular correlation variables, in four regions of four-lepton invariant mass, each dominated by different processes. The measurements are corrected for detector effects and are compared with state-of-the-art Standard Model calculations, which are found to be consistent with the data. The Z → 4ℓ branching fraction is extracted, giving a value of (4.41 ± 0.30) × 10−6. Constraints on effective field theory parameters and a model based on a spontaneously broken B − L gauge symmetry are also evaluated. Further reinterpretations can be performed with the provided information.La lista completa de autores que integran el documento puede consultarse en el archivo.Facultad de Ciencias Exacta

Search for pair production of third-generation scalar leptoquarks decaying into a top quark and a τ-lepton in pp collisions at root √s = 13 TeV with the ATLAS detector

A search for pair production of third-generation scalar leptoquarks decaying into a top quark and a τ-lepton is presented. The search is based on a dataset of pp collisions at √s = 13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb⁻¹. Events are selected if they have one light lepton (electron or muon) and at least one hadronically decaying τ -lepton, or at least two light leptons. In addition, two or more jets, at least one of which must be identified as containing b-hadrons, are required. Six final states, defined by the multiplicity and flavour of lepton candidates, are considered in the analysis. Each of them is split into multiple event categories to simultaneously search for the signal and constrain several leading backgrounds. The signal-rich event categories require at least one hadronically decaying τ-lepton candidate and exploit the presence of energetic final-state objects, which is characteristic of signal events. No significant excess above the Standard Model expectation is observed in any of the considered event categories, and 95% CL upper limits are set on the production cross section as a function of the leptoquark mass, for different assumptions about the branching fractions into tτ and bν. Scalar leptoquarks decaying exclusively into tτ are excluded up to masses of 1.43 TeV while, for a branching fraction of 50% into tτ, the lower mass limit is 1.22 TeV.La lista completa de autores que integran el documento puede consultarse en el archivo.Facultad de Ciencias ExactasInstituto de Física La Plat

Search for supersymmetry in events with four or more charged leptons in 139 fb⁻¹ of √s = 13 TeV pp collisions with the ATLAS detector

A search for supersymmetry in events with four or more charged leptons (electrons, muons and τ-leptons) is presented. The analysis uses a data sample corresponding to 139 fb⁻¹ of proton-proton collisions delivered by the Large Hadron Collider at √s = 13 TeV and recorded by the ATLAS detector. Four-lepton signal regions with up to two hadronically decaying τ-leptons are designed to target several supersymmetric models, while a general five-lepton signal region targets any new physics phenomena leading to a final state with five charged leptons. Data yields are consistent with Standard Model expectations and results are used to set upper limits on contributions from processes beyond the Standard Model. Exclusion limits are set at the 95% confidence level in simplified models of general gauge-mediated supersymmetry, excluding higgsino masses up to 540 GeV. In R-parity-violating simplified models with decays of the lightest supersymmetric particle to charged leptons, lower limits of 1.6 TeV, 1.2 TeV, and 2.5 TeV are placed on wino, slepton and gluino masses, respectively.La lista completa de autores que integran el documento puede consultarse en el archivo.Facultad de Ciencias ExactasInstituto de Física La Plat

Search for Dark Matter Produced in Association with a Dark Higgs Boson Decaying into W±W∓ or ZZ in Fully Hadronic Final States from √s=13 TeV pp Collisions Recorded with the ATLAS Detector

Several extensions of the Standard Model predict the production of dark matter particles at the LHC. An uncharted signature of dark matter particles produced in association with V V = W ± W ∓ or Z Z pairs from a decay of a dark Higgs boson s is searched for using 139 fb − 1 of pp collisions recorded by the ATLAS detector at a center-of-mass energy of 13 TeV. The s → V (q ¯ q) V (q ¯ q) decays are reconstructed with a novel technique aimed at resolving the dense topology from boosted V V pairs using jets in the calorimeter and tracking information. Dark Higgs scenarios with m s > 160 GeV are excluded.La lista completa de autores puede verse en el archivo asociado.Instituto de Física La Plat

Search for dark matter produced in association with a single top quark in √s = 13 TeV pp collisions with the ATLAS detector

This paper presents a search for dark matter in the context of a two-Higgs-doublet model together with an additional pseudoscalar mediator, a, which decays into the dark-matter particles. Processes where the pseudoscalar mediator is produced in association with a single top quark in the 2HDM+a model are explored for the first time at the LHC. Several final states which include either one or two charged leptons (electrons or muons) and a significant amount of missing transverse momentum are considered. The analysis is based on proton–proton collision data collected with the ATLAS experiment at √s = 13 TeV during LHC Run 2 (2015–2018), corresponding to an integrated luminosity of 139 fb−1. No significant excess above the Standard Model predictions is found. The results are expressed as 95% confidence-level limits on the parameters of the signal models considered.La lista completa de autores puede verse en el archivo asociado.Instituto de Física La Plat

Search for doubly and singly charged Higgs bosons decaying into vector bosons in multi-lepton final states with the ATLAS detector using proton-proton collisions at √s = 13 TeV

A search for charged Higgs bosons decaying into W±W± or W±Z bosons is performed, involving experimental signatures with two leptons of the same charge, or three or four leptons with a variety of charge combinations, missing transverse momentum and jets. A data sample of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider between 2015 and 2018 is used. The data correspond to a total integrated luminosity of 139 fb−1. The search is guided by a type-II seesaw model that extends the scalar sector of the Standard Model with a scalar triplet, leading to a phenomenology that includes doubly and singly charged Higgs bosons. Two scenarios are explored, corresponding to the pair production of doubly charged H±± bosons, or the associated production of a doubly charged H±± boson and a singly charged H± boson. No significant deviations from the Standard Model predictions are observed. H±± bosons are excluded at 95% confidence level up to 350 GeV and 230 GeV for the pair and associated production modes, respectively.Instituto de Física La Plat