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In 2011, the ATLAS detector recorded an integrated luminosity of over 5 fb−1 of proton-proton collisions delivered by the LHC at a centre-of-mass square root of s = 7 TeV. The first of two analyses is a test of the standard model and the world's most precise measurement of the top quark pair production cross section for final states which include a hadronically decaying tau lepton. The second analysis uses the same dataset to search for a charged Higgs boson, also resulting in the world's best limits for the search channel. In the cross section measurement, 2.1 fb−1 of ATLAS proton-proton collision data is used to measure the top quark pair production cross section in events containing an isolated electron or muon and a tau lepton decaying hadronically. After initial event requirements, the leading background comes from top quark pairs with jets faking tau leptons. A fit to a tau lepton identification variable is used to determine the signal yield. The measured cross section, [sigma][subscript{tt̄}] 186±13(stat.)±2019(syst.)±7(lumi.)pb, is in good agreement with the standard model prediction. Several extensions to the standard model predict the existence of at least one charged Higgs boson, H[superscript ±]. According to these extensions, the top quark can decay into a bottom quark and a light charged Higgs boson in addition to the standard model decay to a bottom quark and aW boson. In the second analysis, event yield ratios between different final states are measured using 4.6 fb−1 of ATLAS data. This is compared to simulation to search for a violation of lepton universality. This ratio-based method reduces the impact of systematic uncertainties in the analysis. No significant deviations from the standard model predictions are observed. With the assumption that the charged Higgs boson branching ratio to a tau lepton and a neutrino is 100%, upper limits in the range 3.2%-4.4% can be placed on the top quark to charged Higgs branching ratio for 90 less than or equal to m[subscript {H[superscript ±]}] less than or equal to 140 GeV. After combination with results from a search for charged Higgs bosons in tt̄ decays using the thad+jets final state, upper limits on this branching ratio can be set in the range 0.8%-3.4%, for 90 less than or equal to m[subscript {H[superscript ±]}] less than or equal to 140 GeV.
The first part of this thesis presents the measurement of the inclusive cross-section for electron production from heavy-flavour decays in the electron transverse momentum range 7 GeV
This thesis presents the search for New Physics in events with four top quarks using the data collected in proton-proton collisions by the ATLAS experiment at the LHC. The dataset corresponds to the one taken during all 2011 at √s = 7 TeV and a part of 2012 at √s = 8 TeV. The analysis focuses on a final state with two leptons (electrons and muons) with the same electric charge. This signature is experimentally favored since the presence of two same-sign leptons in the final state allows to reduce the background coming from Standard Model (SM) processes. The results are interpreted in the context of a low energy effective field theory, which assumes that New Physics at low energy can manifest itself as a four right-handed top contact interaction. In this context, this analysis allows testing a class of beyond-the-SM (BSM) theories which at low energy can manifest in this way. Backgrounds to this search have been estimated using simulated samples and data-driven techniques. Different sources of systematic uncertainties have been also considered. The final selection of events has been optimized by aiming at minimizing the expected upper limit on the four tops production cross-section in case of no signal events found. The signal region is then analyzed by looking for an excess of events with respect to the predicted background. No excess of events has been observed, and the observed upper limit on the four tops production cross-section has been computed. This limit is then translated to an upper limit on the coupling strength C=2 of the model. An upper limit on the four tops production cross-section in the SM has been also computed in the analysis performed at √s = 8 TeV. In addition to the physics analysis of the four tops signal, some studies about the LASER calibration system of the ATLAS Tile calorimeter are presented. In particular, they are related to the photodiodes system used to measure the intensity of the laser light in the LASER system.
The ATLAS Experiment at the CERN Large Hadron Collider is a particle physics experiment to study fundamental particles and their interactions at very high energies. Supersymmetry is a theory of new physics beyond the Standard Model of particle physics. A search for directly produced pairs of the supersymmetric partner of the top quark was performed using 20 fb^-1 of proton--proton collision data at a centre of mass energy of 8 TeV taken in 2012. The search targeted a model where the supersymmetric partner of the top quark (``scalar top'') decays via the supersymmetric partner of the tau lepton (``scalar tau'') into the supersymmetric partner of the graviton (``gravitino''). Scalar top candidates were searched for in pp collision events with either two hadronically decaying taus, two light leptons (electrons or muons), or one hadronically decaying tau and one light lepton. The numbers of events passing the analysis selection criteria agree with the Standard Model expectations. Exclusion limits at the 95% confidence level were set as a function of the scalar top and scalar tau masses. Depending on the scalar tau mass, ranging from the 87 GeV limit set by the LEP experiments to a few GeV below the scalar top mass, lower limits between 490 GeV and 640 GeV were placed on the scalar top mass within the model considered.
A search for undiscovered particles decaying into top-antitop quark pairs produced in proton-proton collisions with the ATLAS experiment at the Large Hadron Collider utilizing 20.3 fb$^-1$ of data collected at $\sqrt{s}=8$ \tev\ center-of-mass energy during the 2012 data taking period is presented. The invariant mass spectrum of events containing multiple jets, exactly one lepton, and missing transverse energy and which are consistent with the decay of a top-antitop quark pair is studied and found to be consistent with that predicted by the Standard Model. Upper limits on the production cross section times branching ratio of several benchmark signal models are set at a 95\% confidence level.
"This thesis describes my research work performed from 2010 to 2015 using 8 TeV proton-proton collision data collected with the ATLAS detector. The ATLAS detector is one of the four major experiments of the LHC collider at CERN. It is a general-purpose detector, designed to record proton-proton collisions. Through the use of various types of subdetectors, the trajectories and energy deposits of particles created in the interaction are recorded. The raw data is processed and then sent to the Grid for further reconstruction and analysis. The Grid is a distributed computing network, which connects many computing clusters located in various institutes worldwide. This gives the ATLAS collaboration the massive data-processing power that it needs in order to process the recorded data of the detector and simulated events. For this thesis, I heavily relied on the Nikhef Tier 1 Grid-site to analyse 50TB of data, using over a hundred CPU-years. The ATLAS scientific programme focuses on measurements in a new energy and luminosity domain to understand the generation of mass of elementary particles. Meanwhile, the long elusive Higgs boson has been discovered which confirms the Standard Model prediction. The Standard Model is however considered as incomplete and new physics may be around the corner. Known processes may show subtle deviations between prediction and actual measurements. The test-case which is pursuit in this thesis is the production of top-quarks. The top-quarks have a mass of about the electroweak scale and decay before the hadronization can start. Hence, the top-quark process offers a possibility to study a bare heavy quark for which accurate predictions exist which are confronted with data in this thesis."--Samenvatting auteur.
This paper reports a measurement of the cross section for the pair production of top quarks in p{bar p} collisions at √s = 1.96 TeV at the Fermilab Tevatron. The data was collected from the CDF II detector in a set of runs with a total integrated luminosity of 1.1 fb−1. The cross section is measured in the dilepton channel, the subset of t{bar t} events in which both top quarks decay through t → Wb → l[nu]b, where l = e, [mu], or [tau]. The lepton pair is reconstructed as one identified electron or muon and one isolated track. The use of an isolated track to identify the second lepton increases the t{bar t} acceptance, particularly for the case in which one W decays as W → [tau][nu]. The purity of the sample may be further improved at the cost of a reduction in the number of signal events, by requiring an identified b-jet. They present the results of measurements performed with and without the request of an identified b-jet. the former is the first published CDF result for which a b-jet requirement is added to the dilepton selection. In the CDF data there are 129 pretag lepton + track candidate events, of which 69 are tagged. With the tagging information, the sample is divided into tagged and untagged sub-samples, and a combined cross section is calculated by maximizing a likelihood. The result is [sigma]{sub t{bar t}} = 9.6 ± 1.2(stat.){sub -0.5}{sup +0.6}(sys.) ± 0.6(lum.) pb, assuming a branching ratio of BR(W → l[nu]) = 10.8% and a top mass of m{sub t} = 175 GeV/c2.
We present a measurement of the top quark pair (t{bar t}) production cross section ([sigma]{sub t{bar t}}) in p{bar p} collisions at √s = 1.96 TeV using 230 pb−1 of data collected by the D0 experiment at the Fermilab Tevatron Collider. We select events with one charged lepton (electron or muon), missing transverse energy, and jets in the final state. We employ lifetime-based b-jet identification techniques to further enhance the t{bar t} purity of the selected sample. For a top quark mass of 175 GeV, we measure [sigma]{sub t{bar t}} = 8.6{sub -1.5}{sup +1.6}(stat. + syst.) ± 0.6 (lumi.) pb, in agreement with the standard model expectation.
The latest of the 'Lepton Photon' symposium, one of the well-established series of meetings in the high-energy physics community, was successfully organized at the South Campus of Sun Yat-sen University, Guangzhou, China, from August 7-12, 2017, where physicists around the world gathered to discuss the latest advancements in the research field.This proceedings volume of the Lepton Photon 2017 collects contributions by the plenary session speakers and the posters' presenters, which cover the latest results in particle physics, nuclear physics, astrophysics, cosmology, and plans for future facilities.