1,720,984 research outputs found
Top Quark Physics at ATLAS: From Cross-Section Measurements to Search for New Particles
No full textThis thesis presents experimental studies of the top quark within the ATLAS experiment at the Large Hadron Collider (LHC). The top quark, with its mass of
172.5 GeV, is the heaviest known elementary particle and possesses unique properties: it has the strongest coupling to the Higgs boson among all SM particles and decays before hadronization, allowing its study as an almost free quark.
Two complementary physics analyses are presented in this thesis. First, the measurement of the single top quark production cross section in the t-channel at a
centre-of-mass energy of √s = 5.02 TeV, using pp collision data corresponding to an integrated luminosity of 255 pb−1 is detailed. After event selections a Boosted
Decision Tree is employed for signal-to-background discrimination, and a Profile Likelihood fit is used to extract the total t-channel cross-section, the individual top-
and antitop-quark production cross-sections, and their ratio. The t-channel single top quark process is observed for the first time at √s = 5.02 TeV with a significance of 6.1σ. The inclusive cross-section for the t-channel single-top production is measured to be σ(tq + ̄tq) = 27.1+4.4 −4.1 (stat.) +4.4 −3.7 (syst.) pb, while the individual cross-sections are measured to be σ(tq) = 19.8+3.9 −3.1 (stat.) +2.9 −2.2 (syst.) pb and σ( ̄tq) = 7.3+3.2 −2.1 (stat.) +2.8 −1.5 (syst.) pb respectively. The ratio between top- and antitop-quark production is found to be Rt = 2.73+1.43 −0.82 (stat.) +1.01 −0.29 (syst.). All measurements are in good agreement with the Standard Model (SM) predictions.
Second, a search for new heavy resonances decaying to top-antitop quark pairs is conducted using the full Run 2 dataset, consisting of 140 fb−1 if pp collisions at √s = 13 TeV. The analysis targets three specific Beyond the Standard Model scenarios: a leptophobic Z′ boson from Top-colour-assisted technicolour, and two variants
of Randall-Sundrum models predicting Kaluza-Klein excitations of the graviton and gluon. The search is performed in the semileptonic decay channel, targeting both re-
solved and boosted topologies, implementing a specialized overlap-removal technique to enhance the sensitivity at high invariant masses. No significant deviation from the SM prediction is observed. Exclusion limits are set on the production cross-section times branching ratio for the considered signal models, obtaining a higher sensitivity in the extracted limits with respect to the previous ATLAS and CMS searches in the same decay channel.
The thesis also includes technical contributions to the ATLAS collaboration combined performance effort, focusing on electron identification in environments with
nearby hadronic jets. This study examines the electron identification process and its efficiencies in challenging scenarios where electron showers overlap with jet constituents. This work contributes to the development of current and especially future physics analyses in the ATLAS collaboration.This thesis presents experimental studies of the top quark within the ATLAS experiment at the Large Hadron Collider (LHC). The top quark, with its mass of
172.5 GeV, is the heaviest known elementary particle and possesses unique properties: it has the strongest coupling to the Higgs boson among all SM particles and decays before hadronization, allowing its study as an almost free quark.
Two complementary physics analyses are presented in this thesis. First, the measurement of the single top quark production cross section in the t-channel at a
centre-of-mass energy of √s = 5.02 TeV, using pp collision data corresponding to an integrated luminosity of 255 pb−1 is detailed. After event selections a Boosted
Decision Tree is employed for signal-to-background discrimination, and a Profile Likelihood fit is used to extract the total t-channel cross-section, the individual top-
and antitop-quark production cross-sections, and their ratio. The t-channel single top quark process is observed for the first time at √s = 5.02 TeV with a significance of 6.1σ. The inclusive cross-section for the t-channel single-top production is measured to be σ(tq + ̄tq) = 27.1+4.4 −4.1 (stat.) +4.4 −3.7 (syst.) pb, while the individual cross-sections are measured to be σ(tq) = 19.8+3.9 −3.1 (stat.) +2.9 −2.2 (syst.) pb and σ( ̄tq) = 7.3+3.2 −2.1 (stat.) +2.8 −1.5 (syst.) pb respectively. The ratio between top- and antitop-quark production is found to be Rt = 2.73+1.43 −0.82 (stat.) +1.01 −0.29 (syst.). All measurements are in good agreement with the Standard Model (SM) predictions.
Second, a search for new heavy resonances decaying to top-antitop quark pairs is conducted using the full Run 2 dataset, consisting of 140 fb−1 if pp collisions at √s = 13 TeV. The analysis targets three specific Beyond the Standard Model scenarios: a leptophobic Z′ boson from Top-colour-assisted technicolour, and two variants
of Randall-Sundrum models predicting Kaluza-Klein excitations of the graviton and gluon. The search is performed in the semileptonic decay channel, targeting both re-
solved and boosted topologies, implementing a specialized overlap-removal technique to enhance the sensitivity at high invariant masses. No significant deviation from the SM prediction is observed. Exclusion limits are set on the production cross-section times branching ratio for the considered signal models, obtaining a higher sensitivity in the extracted limits with respect to the previous ATLAS and CMS searches in the same decay channel.
The thesis also includes technical contributions to the ATLAS collaboration combined performance effort, focusing on electron identification in environments with
nearby hadronic jets. This study examines the electron identification process and its efficiencies in challenging scenarios where electron showers overlap with jet constituents. This work contributes to the development of current and especially future physics analyses in the ATLAS collaboration
First t-channel single top measurement in pp collisions at an energy in the centre-of-mass of √s = 5.02 TeV with the ATLAS experiment
Full text linkA single top quark production measurement in proton-proton (pp) collisions at the Large Hadron Collider (LHC) at an energy in the centre-of-mass √s = 5.02 TeV with data collected by the ATLAS experiment is reported. This analysis uses a Boosted Decision Tree (BDT) to separate signal from background events and its output distribution is used for a Profile-Likelihood fit. Some of the calibrations used in this analysis are taken from a recent ATLAS top quark pair production cross-section measurement at the same energy of √s = 5.02 TeV (ATLAS Collaboration, JHEP, 06 (2023) 138). The analysis takes into account forward jets to enhance the t-channel process, with an ad-hoc calibration for their energy
First measurement of the t-channel single top production in pp collisions at √s = 5.02$ TeV with the ATLAS experiment
Full text linkA measurement of single top quark production t−channel cross-section in proton-proton collisions at an energy in the centre-of-mass of √s=5.02 TeV with 257 pb−1 of data collected by the ATLAS experiment at LHC is reported.
This analysis applies event selections to identify t−channel single top candidate events decaying semi-leptonically. Selected events are then used to train a Boosted Decision Tree (BDT) to optimize signal versus background separation. A Profile-Likelihood (PL) fit is performed to measure the total cross-section σ(tq+t ̄q) and the ratio between the top quark (tq), and anti-top quark (t ̄q) cross-sections Rt. The analysis uses forward jets to enhance the t−channel significance (jet η<4.0). Measured values of the cross-section σ(tq+t ̄q)=6.6+4.3−4.0(stat.) +4.4−3.6(syst.) and of the ratio Rt=2.74+1.44−0.83(stat.) +1.04−0.29(syst.) are well in agreement with Standard Model (SM) predictions
Top-quark pair cross-section at √s = 13.6 TeV with the ATLAS experiment
Full text linkThe ATLAS detector started collecting data in 2022 at the centre-of-mass energy √s = 13.6 TeV. In this report, an analysis of proton-proton collision
data collected in August 2022 by the ATLAS detector at the LHC is shown. The analysis aim is to measure the top-quark pair production cross-section and its ratio to the Z boson production cross-section. Some of the first plots showing a comparison between Run 3 data and predictions in the eμ final state are presented
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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