Complete Measurement of the Top-quark Polarization in T-channel Single Top-quark Production Using Pp Collisions at 13 TeV with the ATLAS Detector

The top quarks are undoubtedly one of the most promising and experimentally relevant probes into finding new physics. They can be produced in charged-current electroweak processes via a W tb vertex. Its unique mass scale led to a late discovery in experiment — until 1995 at the Tevatron proton-antiproton collider at Fermilab on the events from top pair production. The observation of the electroweak single top process was established even later — in 2009, also at Fermilab based on 2.3 fb-1 and 3.2 fb-1 of CDF/DØ data. Nowadays, the high energy proton-proton collider — the Large Hardon Collider (LHC), with a data set of 139 fb-1 from the ATLAS detector, makes it possible to perform precision measurements on top quarks using both tt ̄and single top channels. At the LHC, electroweak production of single top quarks in the t-channel leads, in the standard model, to a high degree of top quark polarization. Two subprocesses, ub ! dt and db ! u ̄t contribute to t-channel production of single top, while the charge-conjugate processes contribute to production of antitop. The top (antitop) quark spin is expected to be polarized along(opposite to) the direction of the light-quark momentum. In this thesis I present a measurement of the top quark polarization produced within a fiducial region of acceptance, using an integrated luminosity 139 fb-1 of proton-proton collisions at 13 TeV, collected by the ATLAS detector. The top decay chain: t->W+b-> l+mu+b, include a lepton, a neutrino and a b quark in the final state, which interact with the ATLAS detector, allowing the top quark to be fully reconstructed. From the angular distribution of the top quark decay products, we obtain all three components of the polarization of both top quarks and top anti-quarks

Angular analyses of t-channel single top production and decay

Angular analyses of t-channel single top production and decay Electroweak production of single top in the t-channel at the LHC yields large samples of polarized top quarks which can be used to accurately constrain the Wtb vertex. This vertex can be parameterized in terms of complex anomalous couplings $V_L$, $V_R$, $g_L$, and $g_R$, which enter into an effective Lagrangian. A series of analyses carried out with the ATLAS detector aims at improving constraints on these parameters. These are generally based on Fourier techniques in angular variables of the top decay kinematics. Four angles completely describe the kinematics of production of single top in the t-channel and its subsequent semileptonic decay. An analysis of a double-differential single top decay rate based on 4.6 fb$^{-1}$ of proton-proton collisions at 7 TeV uses spherical harmonics as the basis functions in an angular analysis. A followup analysis based on 20.2 fb$^{-1}$ of data at 8 TeV of the triple differential decay rate uses higher orthogonal functions designated as $M$-functions of three angles for a more complete characterization of the Wtb vertex. This poster compares these analyses and discusses the phenomenological basis of an analysis of the quadruple differential rate of production and decay of top quarks in the t-channel, using $M$-functions of four angles, which can be applied to the 13 TeV datasets presently available

A detailed map of Higgs boson interactions by the ATLAS experiment ten years after the discovery

The standard model of particle physics$^{1–4}$ describes the known fundamental particles and forces that make up our Universe, with the exception of gravity. One of the central features of the standard model is a field that permeates all of space and interacts with fundamental particles$^{5–9}$. The quantum excitation of this field, known as the Higgs field, manifests itself as the Higgs boson, the only fundamental particle with no spin. In 2012, a particle with properties consistent with the Higgs boson of the standard model was observed by the ATLAS and CMS experiments at the Large Hadron Collider at CERN$^{10,11}$. Since then, more than 30 times as many Higgs bosons have been recorded by the ATLAS experiment, enabling much more precise measurements and new tests of the theory. Here, on the basis of this larger dataset, we combine an unprecedented number of production and decay processes of the Higgs boson to scrutinize its interactions with elementary particles. Interactions with gluons, photons, and W and Z bosons—the carriers of the strong, electromagnetic and weak forces—are studied in detail. Interactions with three third-generation matter particles (bottom (b) and top (t) quarks, and tau leptons (τ)) are well measured and indications of interactions with a second-generation particle (muons, μ) are emerging. These tests reveal that the Higgs boson discovered ten years ago is remarkably consistent with the predictions of the theory and provide stringent constraints on many models of new phenomena beyond the standard model.The Standard Model of particle physics describes the known fundamental particles and forces that make up our universe, with the exception of gravity. One of the central features of the Standard Model is a field that permeates all of space and interacts with fundamental particles. The quantum excitation of this field, known as Higgs field, manifests itself as the Higgs boson, the only fundamental particle with no spin. In 2012, a particle with properties consistent with the Higgs boson of the Standard Model was observed by the ATLAS and CMS experiments at the Large Hadron Collider at CERN. Since then, more than 30 times as many Higgs bosons have been recorded by the ATLAS experiment, allowing much more precise measurements and new tests of the theory. Here, on the basis of this larger dataset, we combine an unprecedented number of production and decay processes of the Higgs boson to scrutinize its interactions with elementary particles. Interactions with gluons, photons, and $W$ and $Z$ bosons -- the carriers of the strong, electromagnetic, and weak forces -- are studied in detail. Interactions with three third-generation matter particles (bottom ($b$) and top ($t$) quarks, and tau leptons ($\tau$)) are well measured and indications of interactions with a second-generation particle (muons, $\mu$) are emerging. These tests reveal that the Higgs boson discovered ten years ago is remarkably consistent with the predictions of the theory and provide stringent constraints on many models of new phenomena beyond the Standard Model

Operation and performance of the ATLAS semiconductor tracker in LHC Run 2

Abstract The semiconductor tracker (SCT) is one of the tracking systems for charged particles in the ATLAS detector. It consists of 4088 silicon strip sensor modules. During Run 2 (2015–2018) the Large Hadron Collider delivered an integrated luminosity of 156 fb-1 to the ATLAS experiment at a centre-of-mass proton-proton collision energy of 13 TeV. The instantaneous luminosity and pile-up conditions were far in excess of those assumed in the original design of the SCT detector. Due to improvements to the data acquisition system, the SCT operated stably throughout Run 2. It was available for 99.9% of the integrated luminosity and achieved a data-quality efficiency of 99.85%. Detailed studies have been made of the leakage current in SCT modules and the evolution of the full depletion voltage, which are used to study the impact of radiation damage to the modules.</jats:p