A search is performed for dark matter particles produced in association with a resonant pair of Higgs bosons using 140 fb⁻¹ of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the Large Hadron Collider. This signature is expected in some extensions of the Standard Model predicting the production of dark matter particles, and is interpreted in terms of a dark Higgs model containing a Z′ mediator in which the dark Higgs boson s decays into a pair of Higgs bosons. The dark Higgs boson is reconstructed through final states with at least three b-tagged jets, produced by the pair of Higgs boson decays, in events with significant missing transverse momentum consistent with the presence of dark matter. The observed data are found to be in good agreement with Standard Model predictions, constraining scenarios with dark Higgs boson masses within the range of 250 to 400 GeV and Z′ mediators up to 2.3 TeV.

Neural simulation-based inference (NSBI) is a powerful class of machine-learning-based methods for statistical inference that naturally handles high-dimensional parameter estimation without the need to bin data into low-dimensional summary histograms. Such methods are promising for a range of measurements, including at the Large Hadron Collider, where no single observable may be optimal to scan over the entire theoretical phase space under consideration, or where binning data into histograms could result in a loss of sensitivity. This work develops a NSBI framework for statistical inference, using neural networks to estimate probability density ratios, which enables the application to a full-scale analysis. It incorporates a large number of systematic uncertainties, quantifies the uncertainty due to the finite number of events in training samples, develops a method to construct confidence intervals, and demonstrates a series of intermediate diagnostic checks that can be performed to validate the robustness of the method. As an example, the power and feasibility of the method are assessed on simulated data for a simplified version of an off-shell Higgs boson couplings measurement in the four-lepton final states. This approach represents an extension to the standard statistical methodology used by the experiments at the Large Hadron Collider, and can benefit many physics analyses.

This report reviews the published results of searches for possible additional scalar particles and exotic decays of the Higgs boson performed by the ATLAS Collaboration using up to 140 fb−1 of 13 TeV proton–proton collision data collected during Run 2 of the Large Hadron Collider. Key results are examined, and observed excesses, while never statistically compelling, are noted. Constraints are placed on parameters of several models which extend the Standard Model, for example by adding one or more singlet or doublet fields, or offering exotic Higgs boson decay channels. Summaries of new searches as well as extensions of previous searches are discussed. These new results have a wider reach or attain stronger exclusion limits. New experimental techniques that were developed for these searches are highlighted. Search channels which have not yet been examined are also listed, as these provide insight into possible future areas of exploration.

A combination of searches for singly and doubly charged Higgs bosons, H± and H±±, produced via vector-boson fusion is performed using 140 fb−1 of proton–proton collisions at a centre-of-mass energy of 13 TeV, collected with the ATLAS detector during Run 2 of the Large Hadron Collider. Searches targeting decays to massive vector bosons in leptonic final states (electrons or muons) are considered. New constraints are reported on the production cross-section times branching fraction for charged Higgs boson masses between 200 GeV and 3000 GeV. The results are interpreted in the context of the Georgi-Machacek model for which the most stringent constraints to date are set for the masses considered in the combination.

A combination of searches for the single production of vectorlike top quarks (𝑇) is presented. These analyses are based on proton-proton collisions at √𝑠 =13  TeV recorded in 2015–2018 with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 139  fb−1. The 𝑇 decay modes considered in this combination are into a top quark and either a Standard Model Higgs boson or a 𝑍 boson (𝑇 →𝐻⁡𝑡 and 𝑇→𝑍⁢𝑡). The individual searches used in the combination are differentiated by the number of leptons (𝑒, 𝜇) in the final state. The observed data are found to be in good agreement with the Standard Model background prediction. Interpretations are provided for a range of masses and couplings of the vectorlike top quark for benchmark models and generalized representations in terms of 95% confidence level limits. For a benchmark signal prediction of a vectorlike top quark SU(2) singlet with electroweak coupling, 𝜅, of 0.5, masses below 2.1 TeV are excluded, resulting in the most restrictive limits to date.

Differential measurements of Higgs boson production in the tau-lepton-pair decay channel are presented in the gluon fusion, vector-boson fusion (VBF), VH and t (t) over barH associated production modes, with particular focus on the VBF production mode. The data used to perform the measurements correspond to 140 fb(-1) of proton-proton collisions collected by the ATLAS experiment at the LHC. Two methods are used to perform the measurements: the Simplified Template Cross-Section (STXS) approach and an Unfolded Fiducial Differential measurement considering only the VBF phase space. For the STXS measurement, events are categorized by their production mode and kinematic properties such as the Higgs boson's transverse momentum (p(T)(H)), the number of jets produced in association with the Higgs boson, or the invariant mass of the two leading jets (m(jj)). For the VBF production mode, the ratio of the measured cross-section to the Standard Model prediction for m(jj) > 1.5 TeV and p(T)(H) > 200 GeV (p(T)(H) < 200 GeV) is 1.29(-0.34)(+0.39) (0.12(-0.33)(+0.34)). This is the first VBF measurement for the higher-p(T)(H) criteria, and the most precise for the lower-p(T)(H) criteria. The fiducial cross-section measurements, which only consider the kinematic properties of the event, are performed as functions of variables characterizing the VBF topology, such as the signed Delta phi(jj) between the two leading jets. The measurements have a precision of 30%-50% and agree well with the Standard Model predictions. These results are interpreted in the SMEFT framework, and place the strongest constraints to date on the CP-odd Wilson coefficient c(H (W) over tilde).

The production of D^± and Ds± charmed mesons is measured using the D^±/Ds±→ ϕ(μμ)π^± decay channel with 137 fb⁻¹ of s = 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider during the years 2016–2018. The charmed mesons are reconstructed in the range of transverse momentum 12 < p<inf>T</inf> < 100 GeV and pseudorapidity |η| < 2.5. The differential cross-sections are measured as a function of transverse momentum and pseudorapidity, respectively, and compared with next-to-leading- order QCD predictions. The predictions are found to be consistent with the measurements in the visible kinematic region within the large theoretical uncertainties.

This thesis presents my contributions to measurements of the Higgs boson production cross-sections at the Large Hadron Collider, and a measurement of the luminosity. The data were recorded by the ATLAS experiment during the Run 2 of the LHC (2015-2018) at a center-of-mass energy of 13 TeV. Only the 2018 data is used for the luminosity measurement. The measurement of Higgs boson cross-sections considers two production modes: gluon-gluon fusion and vector-boson fusion. In both production modes, the analysis focuses on the decay channel in which the Higgs boson decays into a pair of W bosons, which both decay into a lepton and the corresponding neutrinos: H→WW*→ lνlν. The data selection requires events containing exactly one electron (or positron), one muon (or anti-muon), and significant missing transverse energy. The events are then divided into several categories using the simplified template cross section (STXS) framework. The cross-sections are measured across seven categories for gluon-gluon fusion production and eight categories for vector boson fusion production.

Furthermore, measurements are performed with an alternative event categorisation that offers sensitivity to the properties of Higgs boson couplings under charge-conjugation and parity inversion (CP). For this, events featuring multiple jets are categorised by the signed difference of the azimuthal angles of the two jets with the largest transverse momentum. 

The overall cross-section times branching ratio is measured to be 12.4+1.3(-1.2) pb for Higgs boson production via gluon-gluon fusion and 0.79+0.18(-0.16) pb for the vector boson fusion channel. These results align well with the Standard Model predictions of 10.4 ± 0.6 pb and 0.81 ± 0.02 pb, respectively, for a Higgs boson with a mass of 125 GeV. The results for each of the STXS framework categories, as well as for the CP-sensitive measurements, are all also consistent with the Standard Model predictions, albeit sometimes with large uncertainties.

A smaller part of this thesis describes my contribution to the luminosity measurement. Accurate luminosity measurements rely on various detectors and algorithms and are essential for the ATLAS physics program. The track counting algorithm is a key element for the offline luminosity measurements. It determines the luminosity by counting the average number of charged particle tracks produced at the interaction point during collisions. In 2018, the LHC began performing a new type of beam scans called emittance scans. These scans can be used as a tool to monitor the stability of the track-counting algorithm over time and ensure that the calibration does not drift.

Jets with different radius parameters R are an important tool for probing quantum chromodynamics processes at different angular scales. Jets with small R=0.2 are instrumental in measurements of the substructure of large-R jets resulting from collimated hadronic decays of energetic W, Z, and Higgs bosons, top quarks, and of potential new resonances. This paper presents measurements of the energy scale, resolution, and associated uncertainties of jets with radius parameters R=0.2 and 0.6, obtained using the ATLAS detector. The results are based on 37fb-1 of proton–proton collision data from the Large Hadron Collider at a centre-of-mass energy of s=13 TeV. A new in situ method for measuring jet energy scale differences between data and Monte Carlo simulations is presented. The systematic uncertainties in the jet energy scale for central jets (|η|<1.2) typically vary from 1% to about 5% as a function of |η| at very low transverse momentum, pT, of around 20 GeV for both R=0.2 and 0.6 jets. The relative energy resolution ranges from (35±6)% at pT=20 GeV to (6±0.5)% at pT=300 GeV for central R=0.2 jets, and is found to be slightly worse for R=0.6 jets. Finally, the effect of close-by hadronic activity on the jet energy scale is investigated and is found to be well modelled by the ATLAS Monte Carlo simulations.

This Letter reports the first evidence of electroweak production of same-sign W boson pairs where at least one of the W bosons is longitudinally polarized and the most stringent constraint to date for the production of two longitudinally polarized same-sign W bosons. The dataset used corresponds to an integrated luminosity of 140  fb^-1 of proton-proton collisions at a center-of-mass energy of 13 TeV, collected with the ATLAS detector during run 2 of the Large Hadron Collider. The study is performed in final states including two same-sign leptons (electrons or muons), missing transverse momentum, and at least two jets with a large invariant mass and a large rapidity difference. Two independent fits are performed targeting the production of same-sign W bosons with at least one, or two longitudinally polarized W bosons. The observed (expected) significance of the production with at least one longitudinally polarized W boson is 3.3 (4.0) standard deviations. An observed (expected) 95% confidence level upper limit of 0.45 (0.70) fb is reported on the fiducial production cross section of two longitudinally polarized same-sign W bosons.