Search for vector-like quarks with the ATLAS experiment

Dissertação de mestrado em Física (área de especialização em Física Aplicada)In the present thesis, data originated from proton-proton collisions at the Large Hadron Collider and collected by the ATLAS experiment in 2015 was used for the search of pair-produced vector-like quarks T and B decaying to a Z boson and a third generation quark, focusing on the dilepton channel. A strategy built upon the 8 TeV analysis was further developed. Control regions were defined in order to model the Monte Carlo simulation of the background. The data in the considered signal region is not yet unblinded by the ATLAS collaboration and, therefore, the expected sensitivity of the search was evaluated using pseudo-data built under the background only hypothesis. The vector-like B has 95% confidence level expected mass limits of 766 GeV for the singlet hypothesis and 873 GeV for the doublet hypothesis. The vector-like T singlet and doublet hypotheses have expected limits of 687 GeV and 797 GeV, respectively, representing an encouraging improvement with respect to the currently published mass limits. This text is complementary to the dilepton boosted topology analysis being developed by a group at the TU Dortmund, Germany, and the trilepton channel being developed by a group at the University of Arizona, USA. Orthogonality between the analysis presented in the present thesis and the trilepton analysis is ensured by definition, but with the boosted topology analysis it has to be constructed. The first steps in that direction are described at the end of this text, based on the multiplicity of large-R jets.Nesta tese, dados originados em colisões de protão-protão no Large Hadron Collider e coletados pela experiência ATLAS em 2015 foram usados para a pesquisa de quarks vetoriais T e B, produzidos em pares, decaindo para um bosão Z e um quark de terceira geração, com foco no canal dileptónico. Um estratégia construída a partir da análise a 8 TeV foi desenvolvida. Regiões de controlo foram definidas, por forma a modular a simulação de Monte Carlo do fundo. Os dados na região de sinal considerada ainda estão ocultados pela colaboração de ATLAS, e, portanto, a sensibilidade esperada para esta pesquisa foi avaliada usando pseudo-dados gerados a partir da hipótese da existência de apenas fundo. O quark vectorial B , num limite de confiança de 95%, tem limites esperados de 766 GeV para o caso em que é singleto, e 873 GeV para dobleto. O quark vectorial T, singleto e dobleto, têm limites esperados de 687 GeV e 797 GeV, respectivamente, o que representa uma melhoria encorajadora em relação aos limites de massa atualmente publicados. Esta tese é complementar com a topologia dileptónica boosted que está a ser desenvolvida por um grupo na TU Dortumund, Alemanha, e com o canal trileptónico, que está a ser desenvolvido por um grupo na Universidade do Arizona, EUA. A ortogonalidade entre a análise aqui apresentada e a análise trileptónica está assegurada por definição, mas no caso da topologia boosted esta tem de ser construída. Os primeiros passos nessa direcção são descritos no final deste texto, com base na multiplicidade de jatos com largo raio.LIP (Laboratório de Instrumentação e física experimental de Partículas), FCT/MEC (Fundação Ciência e Tecnologia / Ministério da Educção e da Ciência), FEDER (Fundo Europeu de Desenvolvimento Regional) for funding my activities this past year, as established by the partnership PT2020 partnership with COMPETE2020 (Autoridade de Gestão do Programa Operacional Competitividade e Internacionalização), by providing me with a research scholarship (Reference LIP/BI - 25/2015)

Trends in studies of Brazilian stream fish assemblages

Studies about fish assemblages in Brazilian streams have grown in recent years, however, it remains unclear whether this increase is followed by increments in the diversity of addressed topics and theoretical frameworks adopted by researchers. We performed a systematic search for Brazilian studies on stream fish assemblages recording study region, publication year, objectives, and spatial and temporal scales adopted. The number of studies is unevenly distributed among regions. Most papers describe the general structure of local fish assemblages and their scientific objectives have not varied through time. Studies have been conducted mainly at small temporal and spatial scales, though the latter is increasing over time. We argue for the need of focusing on recently developed ecological theories and frameworks, and expanding the temporal and spatial scales of studies. These changes will improve regional and local conservation policies, and the visibility of aquatic Brazilian research in the global scientific community. © 2016 Associação Brasileira de Ciência Ecológica e Conservaçã

Observation of Higgs boson production in association with a top quark pair at the LHC with the ATLAS detector

The observation of Higgs boson production in association with a top quark pair ( tt¯H ), based on the analysis of proton–proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider, is presented. Using data corresponding to integrated luminosities of up to 79.8 fb −1 , and considering Higgs boson decays into bb¯ , WW⁎ , τ+τ− , γγ , and ZZ⁎ , the observed significance is 5.8 standard deviations, compared to an expectation of 4.9 standard deviations. Combined with the tt¯H searches using a dataset corresponding to integrated luminosities of 4.5 fb −1 at 7 TeV and 20.3 fb −1 at 8 TeV, the observed (expected) significance is 6.3 (5.1) standard deviations. Assuming Standard Model branching fractions, the total tt¯H production cross section at 13 TeV is measured to be 670 ± 90 (stat.) −100+110 (syst.) fb, in agreement with the Standard Model prediction.Peer Reviewe

Measurement of photon–jet transverse momentum correlations in 5.02 TeV Pb + Pb and pppp collisions with ATLAS

Jets created in association with a photon can be used as a calibrated probe to study energy loss in the medium created in nuclear collisions. Measurements of the transverse momentum balance between isolated photons and inclusive jets are presented using integrated luminosities of 0.49 nb$^{-1}$ of Pb+Pb collision data at $\sqrt{s_\mathrm{NN}}=5.02$ TeV and 25 pb$^{-1}$ of $pp$ collision data at $\sqrt{s}=5.02$ TeV recorded with the ATLAS detector at the LHC. Photons with transverse momentum $63.1 31.6$ GeV and pseudorapidity $\left|\eta^\mathrm{jet}\right| 7\pi/8$. Distributions of the per-photon jet yield as a function of $x_\mathrm{J\gamma}$, $(1/N_\gamma)(\mathrm{d}N/\mathrm{d}x_\mathrm{J\gamma})$, are corrected for detector effects via a two-dimensional unfolding procedure and reported at the particle level. In $pp$ collisions, the distributions are well described by Monte Carlo event generators. In Pb+Pb collisions, the $x_\mathrm{J\gamma}$ distribution is modified from that observed in $pp$ collisions with increasing centrality, consistent with the picture of parton energy loss in the hot nuclear medium. The data are compared with a suite of energy-loss models and calculations.Peer Reviewe

Pervasive gaps in Amazonian ecological research

Biodiversity loss is one of the main challenges of our time,1,2 and attempts to address it require a clear un derstanding of how ecological communities respond to environmental change across time and space.3,4 While the increasing availability of global databases on ecological communities has advanced our knowledge of biodiversity sensitivity to environmental changes,5–7 vast areas of the tropics remain understudied.8–11 In the American tropics, Amazonia stands out as the world’s most diverse rainforest and the primary source of Neotropical biodiversity,12 but it remains among the least known forests in America and is often underrepre sented in biodiversity databases.13–15 To worsen this situation, human-induced modifications16,17 may elim inate pieces of the Amazon’s biodiversity puzzle before we can use them to understand how ecological com munities are responding. To increase generalization and applicability of biodiversity knowledge,18,19 it is thus crucial to reduce biases in ecological research, particularly in regions projected to face the most pronounced environmental changes. We integrate ecological community metadata of 7,694 sampling sites for multiple or ganism groups in a machine learning model framework to map the research probability across the Brazilian Amazonia, while identifying the region’s vulnerability to environmental change. 15%–18% of the most ne glected areas in ecological research are expected to experience severe climate or land use changes by 2050. This means that unless we take immediate action, we will not be able to establish their current status, much less monitor how it is changing and what is being lostinfo:eu-repo/semantics/publishedVersio

Pervasive gaps in Amazonian ecological research

Biodiversity loss is one of the main challenges of our time,1,2 and attempts to address it require a clear understanding of how ecological communities respond to environmental change across time and space.3,4 While the increasing availability of global databases on ecological communities has advanced our knowledge of biodiversity sensitivity to environmental changes,5,6,7 vast areas of the tropics remain understudied.8,9,10,11 In the American tropics, Amazonia stands out as the world's most diverse rainforest and the primary source of Neotropical biodiversity,12 but it remains among the least known forests in America and is often underrepresented in biodiversity databases.13,14,15 To worsen this situation, human-induced modifications16,17 may eliminate pieces of the Amazon's biodiversity puzzle before we can use them to understand how ecological communities are responding. To increase generalization and applicability of biodiversity knowledge,18,19 it is thus crucial to reduce biases in ecological research, particularly in regions projected to face the most pronounced environmental changes. We integrate ecological community metadata of 7,694 sampling sites for multiple organism groups in a machine learning model framework to map the research probability across the Brazilian Amazonia, while identifying the region's vulnerability to environmental change. 15%–18% of the most neglected areas in ecological research are expected to experience severe climate or land use changes by 2050. This means that unless we take immediate action, we will not be able to establish their current status, much less monitor how it is changing and what is being lost

Search for pair-production of vector-like quarks in final states with at least one Z boson decaying into a pair of electrons or muons in pp collision data collected with the ATLAS detector at √s = 13 TeV

A search for pair-produced vector-like quarks decaying into a Z boson and a third generation quark using 2015 and 2016 pp collision data at √s = 13 TeV collected by the ATLAS experiment at the Large Hadron Collider, corresponding to an integrated luminosity of 36.1 fb −1 , is presented. No significant excess over the Standard Model expectation was found so lower exclusion limits at 95% confidence level were set on the vector-like quark masses at 1030 (1210) GeV for singlet (doublet) T and 1010 (1140) GeV for singlet (doublet) B

Search for pair-produced vector-like quarks decaying to a\\ ZZ boson with the ATLAS experiment

Vector-like quarks are predicted by many beyond the Standard Model theories. They have the same left- and right-handed SU(2) quantum numbers and can have Flavor-Changing Neutral Currents at tree level. They can be isospin singlets, doublets or triplets. This analysis uses the 2015 and 2016 dataset collected by the ATLAS detector, corresponding to 36.1 fb$^{-1}$, targeting pair-production of VLQ decaying to a $Z$ boson. Channels design, signal regions and results interpretation are shown