Cross-section measurements of top-quark pair production in association with a hard photon at 13 TeV with the ATLAS detector

25 years after the top quark's discovery, the Large Hadron Collider at CERN produces proton-proton collision data on unprecedented scales at unprecedented energies - and has heralded an era of top-quark precision measurements. The production of a top-quark pair in association with a photon ($t\bar{t}\gamma$) gives access to the electromagnetic top-photon coupling, one of the fundamental properties of the top quark. Various extensions of the Standard Model predict modifications of the coupling strength or structure, and deviations from the Standard Model prediction of the $t\bar{t}\gamma$ production cross-section would indicate new physics. With enough statistics available from the Large Hadron Collider, the electron-muon channel has gained particular interest due to its high signal purity and precise available theory predictions. This thesis presents results with the full Run 2 dataset collected with the ATLAS detector in proton-proton collisions at the Large Hadron Collider between 2015 and 2018 at 13 TeV centre-of-mass energy, corresponding to an integrated luminosity of 139 fb$^{-1}$. In order to compare the results to fixed-order calculations that include non-doubly-resonant diagrams, a combined measurement of $t\bar{t}\gamma + tW\gamma$ is performed. The focus is placed on a measurement of the fiducial inclusive cross-section in the electron-muon channel. Furthermore, the ATLAS data is unfolded to parton level and measurements of differential cross-sections as functions of several observables are presented. Both fiducial inclusive and differential results are compared to state-of-the-art fixed-order calculations at next-to-leading order in QCD. An additional focus of the thesis is placed on studies to use machine-learning techniques, in particular deep neural networks, for the identification of prompt photons.Comment: PhD dissertation (Univ. of G\"ottingen), 173 pages, 75 figures, 25 tables. Available on CERN CDS at https://cds.cern.ch/record/2725289

\nu-Flows: Conditional Neutrino Regression

We present $\nu$-Flows, a novel method for restricting the likelihood space of neutrino kinematics in high energy collider experiments using conditional normalizing flows and deep invertible neural networks. This method allows the recovery of the full neutrino momentum which is usually left as a free parameter and permits one to sample neutrino values under a learned conditional likelihood given event observations. We demonstrate the success of $\nu$-Flows in a case study by applying it to simulated semileptonic $t\bar{t}$ events and show that it can lead to more accurate momentum reconstruction, particularly of the longitudinal coordinate. We also show that this has direct benefits in a downstream task of jet association, leading to an improvement of up to a factor of 1.41 compared to conventional methods.Comment: 26 pages, 15 figure

ν2\nu^2-Flows: Fast and improved neutrino reconstruction in multi-neutrino final states with conditional normalizing flows

In this work we introduce $\nu^2$-Flows, an extension of the $\nu$-Flows method to final states containing multiple neutrinos. The architecture can natively scale for all combinations of object types and multiplicities in the final state for any desired neutrino multiplicities. In $t\bar{t}$ dilepton events, the momenta of both neutrinos and correlations between them are reconstructed more accurately than when using the most popular standard analytical techniques, and solutions are found for all events. Inference time is significantly faster than competing methods, and can be reduced further by evaluating in parallel on graphics processing units. We apply $\nu^2$-Flows to $t\bar{t}$ dilepton events and show that the per-bin uncertainties in unfolded distributions is much closer to the limit of performance set by perfect neutrino reconstruction than standard techniques. For the chosen double differential observables $\nu^2$-Flows results in improved statistical precision for each bin by a factor of 1.5 to 2 in comparison to the Neutrino Weighting method and up to a factor of four in comparison to the Ellipse approach.Comment: 20 pages, 16 figures, 5 table

Topological Reconstruction of Particle Physics Processes using Graph Neural Networks

We present a new approach, the Topograph, which reconstructs underlying physics processes, including the intermediary particles, by leveraging underlying priors from the nature of particle physics decays and the flexibility of message passing graph neural networks. The Topograph not only solves the combinatoric assignment of observed final state objects, associating them to their original mother particles, but directly predicts the properties of intermediate particles in hard scatter processes and their subsequent decays. In comparison to standard combinatoric approaches or modern approaches using graph neural networks, which scale exponentially or quadratically, the complexity of Topographs scales linearly with the number of reconstructed objects. We apply Topographs to top quark pair production in the all hadronic decay channel, where we outperform the standard approach and match the performance of the state-of-the-art machine learning technique.Comment: 25 pages, 24 figures, 8 table

Measurements of inclusive and differential cross-sections of tty production in the e+mu final state at 13 TeV with the ATLAS detector

The production of a top-antitop pair in association with a photon (tty) gives access to the electromagnetic top-photon coupling and, thus, a tty cross section measurement enables probing the coupling strength and structure. Deviations of the cross section would be an indication of physics beyond the Standard Model (BSM). While the process was first observed by ATLAS at the LHC at 7 TeV, the full Run 2 data collected with the ATLAS experiment now enables precise cross-section measurements. The e+mu channel is of particular interest due to high signal purity and its precise theory predictions. Inclusive and differential cross-section measurements are presented using proton-proton collision data corresponding to an integrated luminosity of 139/fb, collected with the ATLAS detector between 2015 and 2018 at 13 TeV centre-of-mass energy. Events with exactly one photon, one electron and one muon of opposite sign, and at least two jets, one of which must be b-tagged, are selected. Differential cross-sections as a function of several observables are unfolded to parton level and compared to state-of-the-art Monte Carlo simulations and next-to-leading order calculations

Recent results on associated top-quark production and searches for new top-quark phenomena with the ATLAS detector

The high center-of-mass energy of proton-proton collisions and the large available datasets at the CERN Large Hadron Collider allow to study rare processes of the Standard Model (SM) with unprecedented precision and search for new physics that might enhance extremely rare processes in the SM. Measurements of rare SM processes provide new tests of the SM predictions with the potential to unveil discrepancies with the SM predictions or provide important input for the improvement of theoretical calculations. Interesting processes are Flavour Changing Neutral Currents (FCNC): forbidden at tree level and highly suppressed at higher orders in the Standard Model (SM), FCNC processes can receive enhanced contributions in many extensions of the SM, so any measurable sign of such interactions is an indication of new physics. In addition, the recent observation of associated production of a single top quark with a photon completes the list of processes and adds sensitivity to the EW couplings of the top quark, while measurements of production asymmetries in various final states provide further precision tests of the SM. This talk covers two searches for FCNC processes with ATLAS, the observation of associated production of a single top quark with a photon, as well as measurements of production asymmetries in top-quark pair production in association with a photon and with a W boson

Using associated production of top quarks and neutral bosons to probe standard model couplings and search for new physics

The unprecedentedly large integrated luminosity accumulated by the ATLAS experiment from proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 13\,\mathrm{TeV}$ at the Large Hadron Collider (LHC) allows the study of rare Standard Model (SM) processes. The associated production of top-quark pairs or single top quarks and neutral bosons is such an example: it directly probes top-quark couplings to photons and $Z$ bosons and tests for deviations from the SM. Three such recent measurements are presented, all of which exploit the full dataset of the most recent Run 2 of the LHC, corresponding to $139\,\mathrm{fb}^{-1}$ of integrated luminosity. The cross-sections for the production of top-quark pairs in association with a photon ($t\bar{t}\gamma$) or with a $Z$ boson ($t\bar{t}Z$) are measured both inclusively and differentially as functions of kinematic observables characterising the $t\bar{t}$+boson system. The measurements are compared to predictions obtained by next-to-leading order (NLO) + parton-shower (PS) Monte Carlo simulations and fixed order NLO calculations, respectively. In a third measurement, the cross-section of single top-quark production in association with a $Z$ boson ($tZq$) probes two SM couplings in the same process, $t$-$Z$ and $W$-$Z$, and it is a background to the rare associated production of a single top quark and a Higgs boson. A neural network is used to improve the background rejection and to extract the signal. The measured cross-section is compared to the NLO SM prediction

Top quark production cross-section measurements

Measurements of the inclusive and differential cross-sections for top-quark pair and single top production cross sections in proton-proton collisions with the ATLAS detector at the Large Hadron Collider are presented at center-of-mass energies of 8 TeV and 13 TeV. The inclusive measurements reach high precision and are compared to the best available theoretical calculations. These measurements, including results using boosted tops, probe our understanding of top-pair production in the TeV regime. Measurements of the properties of the Wtb vertex in single top-quark production allow to set limits on anomalous couplings. All measurements are compared to state-of-the-art theoretical calculations

Cross-section measurements of top-quark pair production in association with a hard photon at 13 TeV with the ATLAS detector

25 years after the top quark's discovery, the Large Hadron Collider at CERN produces proton-proton collision data on unprecedented scales at unprecedented energies – and has heralded an era of top-quark precision measurements. The production of a top-quark pair in association with a photon ($t\bar{t}\gamma$) gives access to the electromagnetic top-photon coupling, one of the fundamental properties of the top quark. Various extensions of the Standard Model predict modifications of the coupling strength or structure, and deviations from the Standard Model prediction of the $t\bar{t}\gamma$ production cross-section would indicate new physics. With enough statistics available from the Large Hadron Collider, the electron-muon channel has gained particular interest due to its high signal purity and precise available theory predictions. This thesis presents results with the full Run 2 dataset collected with the ATLAS detector in proton-proton collisions at the Large Hadron Collider between 2015 and 2018 at 13 TeV centre-of-mass energy, corresponding to an integrated luminosity of 139 fb$^{-1}$. In order to compare the results to fixed-order calculations that include non-doubly-resonant diagrams, a combined measurement of $t\bar{t}\gamma + tW\gamma$ is performed. The focus is placed on a measurement of the fiducial inclusive cross-section in the electron-muon channel, where exactly one photon, one electron and one muon of opposite charge sign, and at least two jets, one of which must be $b$-tagged, are selected. Furthermore, the ATLAS data is unfolded to parton level and measurements of differential cross-sections as functions of several observables are presented. Both fiducial inclusive and differential results are compared to state-of-the-art fixed-order calculations at next-to-leading order in QCD. An additional focus of the thesis is placed on studies to use machine-learning techniques, in particular deep neural networks, for the identification of prompt photons.25 years after the top quark's discovery, the Large Hadron Collider at CERN produces proton-proton collision data on unprecedented scales at unprecedented energies - and has heralded an era of top-quark precision measurements. The production of a top-quark pair in association with a photon ($t\bar{t}\gamma$) gives access to the electromagnetic top-photon coupling, one of the fundamental properties of the top quark. Various extensions of the Standard Model predict modifications of the coupling strength or structure, and deviations from the Standard Model prediction of the $t\bar{t}\gamma$ production cross-section would indicate new physics. With enough statistics available from the Large Hadron Collider, the electron-muon channel has gained particular interest due to its high signal purity and precise available theory predictions. This thesis presents results with the full Run 2 dataset collected with the ATLAS detector in proton-proton collisions at the Large Hadron Collider between 2015 and 2018 at 13 TeV centre-of-mass energy, corresponding to an integrated luminosity of 139 fb$^{-1}$. In order to compare the results to fixed-order calculations that include non-doubly-resonant diagrams, a combined measurement of $t\bar{t}\gamma + tW\gamma$ is performed. The focus is placed on a measurement of the fiducial inclusive cross-section in the electron-muon channel. Furthermore, the ATLAS data is unfolded to parton level and measurements of differential cross-sections as functions of several observables are presented. Both fiducial inclusive and differential results are compared to state-of-the-art fixed-order calculations at next-to-leading order in QCD. An additional focus of the thesis is placed on studies to use machine-learning techniques, in particular deep neural networks, for the identification of prompt photons

Using associated production of top quarks and neutral bosons to probe standard model couplings and search for new physics

The unprecedentedly large integrated luminosity accumulated by the ATLAS detector at the highest proton-proton collision energy provided by LHC allows the study of rare SM processes. The associated production of top quarks with neutral bosons is such an example: it directly probes top-quark couplings to photons and Z bosons and tests for deviations from the standard model. Three measurements are presented. The cross sections for the production of top quark pairs in association to a photon (ttgamma) or to a Z boson (ttZ) are measured both inclusively and differentially as a function of kinematic variables characterizing the tt+boson system. Both sets of measurements use the full Run2 data set consisting of 139/fb of integrated luminosity. Final states with three and four leptons and b-jets are used to extract ttZ rates, while tt+gamma cross sections are derived from final states with one photon, one electron and one muon of opposite sign and at least two jets. The measurements are compared to predictions obtained by NLO+PS Monte Carlo and fixed order NLO calculations. The single top-quark production in association with a Z boson (tZq) probes two SM couplings in the same process (WWZ and tZ coupling) and it is a background to the rare associated production of a single top quark and a Higgs Boson. Using a total integrated luminosity of 139/fb collected in the LHC Run-2 from 2015 to 2018, events containing three isolated charged leptons (electrons or muons) and two or three jets, one of which is identified as containing a b-hadron are selected. A neural network is used to improve the background rejection and extract the signal. The measured cross section for tl+l-q production, including non-resonant dilepton pairs with dilepton mass larger than 30 GeV is presented and compared with the SM prediction