Search for heavy particles decaying into a top-quark pair in the fully hadronic final state in pp collisions at s√= 13 TeV with the ATLAS detector

A search for new particles decaying into a pair of top quarks is performed using proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider at a center-of-mass energy of s=13  TeV corresponding to an integrated luminosity of 36.1  fb-1. Events consistent with top-quark pair production and the fully hadronic decay mode of the top quarks are selected by requiring multiple high transverse momentum jets including those containing b-hadrons. Two analysis techniques, exploiting dedicated top-quark pair reconstruction in different kinematic regimes, are used to optimize the search sensitivity to new hypothetical particles over a wide mass range. The invariant mass distribution of the two reconstructed top-quark candidates is examined for resonant production of new particles with various spins and decay widths. No significant deviation from the Standard Model prediction is observed and limits are set on the production cross-section times branching fraction for new hypothetical Z′ bosons, dark-matter mediators, Kaluza-Klein gravitons and Kaluza-Klein gluons. By comparing with the predicted production cross sections, the Z′ boson in the topcolor-assisted-technicolor model is excluded for masses up to 3.1?3.6 TeV, the dark-matter mediators in a simplified framework are excluded in the mass ranges from 0.8 to 0.9 TeV and from 2.0 to 2.2 TeV, and the Kaluza-Klein gluon is excluded for masses up to 3.4 TeV, depending on the decay widths of the particles.Fil: Arduh, Francisco Anuar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Cern - European Organization for Nuclear Research; SuizaFil: ATLAS Collaboration. No especifíca

Búsqueda de supersimetría en procesos con un fotón, electrón/muón y energía perdida en estado final con el detector ATLAS del LHC

El Modelo Estándar de la física de partículas ha demostrado ser una de las teorías más precisas en la actualidad, sin embargo, existen fenómenos en la naturaleza que no son posibles explicarlos con ella. Una de las extensiones posibles con mayor motivación teórica de física más allá del Modelo Estándar es supersimetría (SUSY), que resuelve, entre otras cosas, el problema de jerarquía. Para este trabajo se consideró SUSY en el contexto de General Gauge Mediated SUSY breaking model (GGM), en el que, SUSY está rota a una escala de energía mucho menor que la escala de Planck, resultando en el Gravitino como la partícula supersimétrica estable más liviana (LSP). La segunda partícula supersimétrica más liviana, denominada NLSP por sus siglas en inglés, define la fenomenología de estos modelos. En esta tesis se describe la búsqueda de SUSY en el marco de modelos GGM co-NLSP en colisiones protón-protón a una energía de centro de masa de 8 TeV y 13 TeV en el LHC , en procesos con un fotón, un leptón (electrón o muón) y energía faltante en el estado final. Se trata de modelos donde las masas de la NLSP, chargino y neutralino tipo-wino están prácticamente degeneradas. El análisis fue realizado utilizando todos los datos recolectados por el detector ATLAS durante el año 2012, 2015 y 2016 que corresponden a una luminosidad total integrada de 20.3 1/fb para los de 2012 y 36.1 1/fb para los colectados en 2015 y 2016.Facultad de Ciencias Exacta

Search for magnetic monopoles and stable particles with high electric charges in 8 TeV pp collisions with the ATLAS detector

A search for highly ionizing particles produced in proton-proton collisions at 8 TeV center-of-mass energy is performed by the ATLAS Collaboration at the CERN Large Hadron Collider. The data set used corresponds to an integrated luminosity of 7.0 fb-1. A customized trigger significantly increases the sensitivity, permitting a search for such particles with charges and energies beyond what was previously accessible. No events were found in the signal region, leading to production cross section upper limits in the mass range 200-2500 GeV for magnetic monopoles with magnetic charge in the range 0.5gD<|g|<2.0gD, where gD is the Dirac charge, and for stable particles with electric charge in the range 10<|z|<60. Model-dependent limits are presented in given pair-production scenarios, and model-independent limits are presented in fiducial regions of particle energy and pseudorapidity.La lista completa de autores que integran el documento puede consultarse en el archivoInstituto de Física La Plat

Search for magnetic monopoles and stable particles with high electric charges in 8 TeV pp collisions with the ATLAS detector

A search for highly ionizing particles produced in proton-proton collisions at 8 TeV center-of-mass energy is performed by the ATLAS Collaboration at the CERN Large Hadron Collider. The data set used corresponds to an integrated luminosity of 7.0 fb-1. A customized trigger significantly increases the sensitivity, permitting a search for such particles with charges and energies beyond what was previously accessible. No events were found in the signal region, leading to production cross section upper limits in the mass range 200-2500 GeV for magnetic monopoles with magnetic charge in the range 0.5gD<|g|<2.0gD, where gD is the Dirac charge, and for stable particles with electric charge in the range 10<|z|<60. Model-dependent limits are presented in given pair-production scenarios, and model-independent limits are presented in fiducial regions of particle energy and pseudorapidity.La lista completa de autores que integran el documento puede consultarse en el archivoInstituto de Física La Plat

Measurements of the top quark branching ratios into channels with leptons and quarks with the ATLAS detector

Measurements of the branching ratios of top quark decays into leptons and jets using events with tt (top antitop) pairs are reported. Events were recorded with the ATLAS detector at the LHC in pp collisions at a center-of-mass energy of 7 TeV. The collected data sample corresponds to an integrated luminosity of 4.6 fb-1. The measured top quark branching ratios agree with the Standard Model predictions within the measurement uncertainties of a few percent.La lista completa de autores que integran el documento puede consultarse en el archivoInstituto de Física La Plata (CONICET- Universidad Nacional de La Plata

Search for bottom squark pair production in proton–proton collisions at √s = 13 TeV with the ATLAS detector

The result of a search for pair production of the supersymmetric partner of the Standard Model bottom quark (˜b1) is reported. The search uses 3.2 fb−1 of pp collisions at √s = 13 TeV collected by the ATLAS experiment at the Large Hadron Collider in 2015. Bottom squarks are searched for in events containing large missing transverse momentum and exactly two jets identified as originating from bquarks. No excess above the expected Standard Model background yield is observed. Exclusion limits at 95 % confidence level on the mass of the bottom squark are derived in phenomenological supersymmetric R-parity-conserving models in which the ˜b1 is the lightest squark and is assumed to decay exclusively via ˜b1 → b˜χ01 , where ˜χ01 is the lightest neutralino. The limits significantly extend previous results; bottom squark masses up to 800 (840) GeV are excluded for the ˜χ01 mass below 360 (100) GeV whilst differences in mass above 100 GeV between the ˜b1 and the ˜χ01are excluded up to a ˜b1 mass of 500 GeV.Facultad de Ciencias Exacta

High-ET isolated-photon plus jets production in pp collisions at √s=8 TeV with the ATLAS detector

The dynamics of isolated-photon plus one-, two- and three-jet production in pp collisions at a centre-of-mass energy of 8 TeV are studied with the ATLAS detector at the LHC using a data set with an integrated luminosity of 20.2 fb−1. Measurements of isolated-photon plus jets cross sections are presented as functions of the photon and jet transverse momenta. The cross sections as functions of the azimuthal angle between the photon and the jets, the azimuthal angle between the jets, the photon–jet invariant mass and the scattering angle in the photon–jet centre-of-mass system are presented. The pattern of QCD radiation around the photon and the leading jet is investigated by measuring jet production in an annular region centred on each object; enhancements are observed around the leading jet with respect to the photon in the directions towards the beams. The experimental measurements are compared to several different theoretical calculations, and overall a good description of the data is found.La lista completa de autores que integran el documento puede consultarse en el archivoInstituto de Física La Plat

Measurement of the cross section for inclusive isolated-photon production in pp collisions at √s = 13 TeV using the ATLAS detector

Inclusive isolated-photon production in pp collisions at a centre-of-mass energy of 13 TeV is studied with the ATLAS detector at the LHC using a data set with an integrated luminosity of 3.2 fb−1. The cross section is measured as a function of the photon transverse energy above 125 GeV in different regions of photon pseudorapidity. Next-to-leading-order perturbative QCD and Monte Carlo event-generator predictions are compared to the cross-section measurements and provide an adequate description of the data.Facultad de Ciencias Exacta

Fiducial, total and differential cross-section measurements of t-channel single top quark production in pp collisions at 8TeV using data collected by the ATLAS detector

Detailed measurements of t-channel single topquark production are presented. They use 20.2 fb−1 of data collected by the ATLAS experiment in proton–proton collisions at a centre-of-mass energy of 8TeV at the LHC. Total, fiducial and differential cross-sections are measured for both top-quark and top-antiquark production. The fiducial crosssection is measured with a precision of 5.8% (top quark) and 7.8% (top antiquark), respectively. The total cross-sections are measured to be σtot(tq) = 56.7+4.3 −3.8 pb for top-quark production and σtot(¯tq) = 32.9+3.0 −2.7 pb for top-antiquark production, in agreement with the Standard Model prediction. In addition, the ratio of top-quark to top-antiquark production cross-sections is determined to be Rt = 1.72 ± 0.09. The differential cross-sections as a function of the transverse momentum and rapidity of both the top quark and the top antiquark are measured at both the parton and particle levels. The transverse momentum and rapidity differential crosssections of the accompanying jet from the t-channel scattering are measured at particle level. All measurements are compared to various Monte Carlo predictions as well as to fixed-order QCD calculations where available.Facultad de Ciencias Exacta

Search for doubly charged scalar bosons decaying into same-sign W boson pairs with the ATLAS detector

A search for doubly charged scalar bosons decaying into W boson pairs is presented. It uses a data sample from proton–proton collisions corresponding to an integrated luminosity of 36.1 fb−1 collected by the ATLAS detector at the LHC at a centre-of-mass energy of 13 TeV in 2015 and 2016. This search is guided by a model that includes an extension of the Higgs sector through a scalar triplet, leading to a rich phenomenology that includes doubly charged scalar bosons H±±. Those bosons are produced in pairs in proton–proton collisions and decay predominantly into electroweak gauge bosons H±± → W±W± . Experimental signatures with several leptons, missing transverse energy and jets are explored. No significant deviations from the Standard Model predictions are found. The parameter space of the benchmark model is excluded at 95% confidence level for H±± bosons with masses between 200 and 220 GeV.Facultad de Ciencias Exacta