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The High Luminosity upgrade of the Large Hadron Collider (HL-LHC), necessitates that the ATLAS experiment replace their current Inner Detector (ID) system. The new Inner Tracker (ITk) will be an all silicon detector, utilising both pixel and strip sensors, with the aim of performing as well, or better than the current system - but in a much more challenging environment. The ITk Strip detector will consist of 17888 modules, ∼ 700 of which will be produced in the Scandinavian ITk Cluster - a collaboration of Copenhagen, Lund, Oslo and Uppsala university and our industrial partner NOTE.This work encompasses the journey from individual components through industrial scale module assembly and on to performance evaluation studies at the DESY II testbeam facility. Optimisation studies were performed of the correlated multi-variable calibration necessary for a glue robot to precisely and reliably dispense the two component epoxy used in the bonding of front-end electronics to the silicon sensor. Procedures and tools were developed for integrating this process into an industrial workflow, and to account for future fundamental changes, such as a switch in the epoxy utilised.To demonstrate sufficient tracking performance of ITk strip modules, even at end-of-life, testbeam campaigns of pre-irradiated modules are conducted. These campaign serve as vi-tal feasibility studies for the ITk as a whole. Reconstruction of end-cap type modules have been historically tricky, due to their complex geometry. This work presents the full integration of semi-automated end-cap type module reconstruction in the Corryvreckan testbeam analysis framework. This represent a major improvement in turnover time from raw data to final result, making the previously impossible concept of live reconstruction during testbeam campaigns within reach.

A search is presented for a heavy scalar (H) or pseudo-scalar (A) predicted by the two-Higgs-doublet models, where the H/A is produced in association with a top-quark pair (t t<over bar>H/A), and with the H/A decaying into a tt<over bar> pair. The full LHC Run 2 proton-proton collision data collected by the ATLAS experiment is used, corresponding to an integrated luminosity of 139 fb -1 . Events are selected requiring exactly one or two opposite-charge electrons or muons. Data-driven corrections are applied to improve the modelling of the tt<over bar> +jets background in the regime with high jet and b-jet multiplicities. These include a novel multi-dimensional kinematic reweighting based on a neural network trained using data and simulations. An H/A-mass parameterised graph neural network is trained to optimise the signal-to-background discrimination. In combination with the previous search performed by the ATLAS Collaboration in the multilepton final state, the observed upper limits on the tt<over bar>H/A → tt<over bar>tt<over bar> production cross-section at 95% confidence level range between 14 fb and 5.0 fb for an H/A with mass between 400 GeV and 1000 GeV, respectively. Assuming that both the H and A contribute to the tt<over bar>tt<over bar> cross-section, tan β values below 1.7 or 0.7 are excluded for a mass of 400 GeV or 1000 GeV, respectively. The results are also used to constrain a model predicting the pair production of a colour-octet scalar, with the scalar decaying into a tt<over bar> pair.

Inclusive and differential cross-sections are measured at particle level for the associated production of a top quark pair and a photon (tt<over bar>γ). The analysis is performed using an integrated luminosity of 140 fb -1 of proton-proton collisions at a centre-of-mass energy of 13TeV collected by the ATLAS detector. The measurements are performed in the single-lepton and dilepton top quark pair decay channels focusing on tt<over bar>γ topologies where the photon is radiated from an initial-state parton or one of the top quarks. The absolute and normalised differential cross-sections are measured for several variables characterising the photon, lepton and jet kinematics as well as the angular separation between those objects. The observables are found to be in good agreement with the Monte Carlo predictions. The photon transverse momentum differential distribution is used to set limits on effective field theory parameters related to the electroweak dipole moments of the top quark. The combined limits using the photon and the Z boson transverse momentum measured in tt<over bar> production in associations with a Z boson are also set.

Measurements of both the inclusive and differential production cross sections of a top-quark-top-antiquark pair in association with a Z boson (t (t) over barZ) are presented. Final states with two, three or four isolated leptons (electrons or muons) are targeted. The measurements use the data recorded by the ATLAS detector in pp collisions at root s = 13 TeV at the Large Hadron Collider during the years 2015-2018, corresponding to an integrated luminosity of 140 fb(-1). The inclusive cross section is measured to be sigma(t (t) over barZ) = 0.86 +/- 0.04 (stat.) +/- 0.04 (syst.) pb and found to be in agreement with the most advanced Standard Model predictions. The differential measurements are presented as a function of a number of observables that probe the kinematics of the t (t) over barZ system. Both the absolute and normalised differential cross-section measurements are performed at particle level and parton level for specific fiducial volumes, and are compared with NLO+NNLL theoretical predictions. The results are interpreted in the framework of Standard Model effective field theory and used to set limits on a large number of dimension-6 operators involving the top quark. The first measurement of spin correlations in t (t) over barZ events is presented: the results are in agreement with the Standard Model expectations, and the null hypothesis of no spin correlations is disfavoured with a significance of 1.8 standard deviations.

A search for a heavy CP-odd Higgs boson, A, decaying into a Z boson and a heavy CP-even Higgs boson, H, is presented. It uses the full LHC Run 2 dataset of pp collisions at root s = 13TeV collected with the ATLAS detector, corresponding to an integrated luminosity of 140 fb(-1). The search for A -> ZH is performed in the l(+)l(-)t (t) over bar t and nu(nu) over barb (b) over bar final states and surpasses the reach of previous searches in different final states in the region with m(H) > 350 GeV and m(A) > 800 GeV. No significant deviation from the Standard Model expectation is found. Upper limits are placed on the production cross-section times the decay branching ratios. Limits with less model dependence are also presented as functions of the reconstructed m(t (t) over bar) and m(b (b) over bar) distributions in the l(+)l(-)t (t) over bar and nu(nu) over barb (b) over bar channels, respectively. In addition, the results are interpreted in the context of two-Higgs-doublet models.

This paper reports the observation of top-quark pair production in proton-lead collisions in the ATLAS experiment at the Large Hadron Collider. The measurement is performed using 165 nb − 1 of p +Pb data collected at √sNN = 8 . 16 TeV in 2016. Events are categorised in two analysis channels, consisting of either events with exactly one lepton (electron or muon) and at least four jets, or events with two opposite-charge leptons and at least two jets. In both channels at least one b -tagged jet is also required. Top-quark pair production is observed with a significance over five standard deviations in each channel. The top-quark pair production cross-section is measured to be σ t¯t =58.1±2.0(stat.) +4.8 −4.4 (syst.) nb, with a total uncertainty of 9%. In addition, the nuclear modification factor is measured to be R pA =1.090±0.039(stat.) +0.094 −0.087 (syst.). The measurements are found to be in good agreement with theory predictions involving nuclear parton distribution functions.

The inclusive Higgs boson production crosssection is measured in the di-photon and the Z Z∗ → 4decay channels using 31.4 and 29.0 fb−1 of pp collision datarespectively, collected with the ATLAS detector at a centreof-mass energy of √s = 13.6 TeV. To reduce the modeldependence, the measurement in each channel is restrictedto a particle-level phase space that closely matches the channel’s detector-level kinematic selection, and it is correctedfor detector effects. These measured fiducial cross-sectionsare σfid,γ γ = 76+14−13 fb, and σfid,4 = 2.80 ± 0.74 fb, inagreement with the corresponding Standard Model predictions of 67.6±3.7 fb and 3.67±0.19 fb. Assuming StandardModel acceptances and branching fractions for the two channels, the fiducial measurements are extrapolated to the fullphase space yielding total cross-sections of σ (pp → H) =67+12−11 pb and 46±12 pb at 13.6 TeV from the di-photon andZ Z∗ → 4 measurements respectively. The two measurements are combined into a total cross-section measurement ofσ (pp → H) = 58.2±8.7 pb, to be compared with the Standard Model prediction of σ (pp → H)SM = 59.9 ± 2.6 pb.

This article presents a search for new resonances decaying into a Z or W boson and a 125 GeV Higgs boson h, and it targets the v (v) over barb (b) over bar, l(+)l(-) b (b) over bar, or l(+/-) vb (b) over bar final states, where l - e or mu, in proton-proton collisions at root s - 13 TeV. The data used correspond to a total integrated luminosity of 139 fb(-1) collected by the ATLAS detector during Run 2 of the LHC at CERN. The search is conducted by examining the reconstructed invariant or transverse mass distributions of Zh or Wh candidates for evidence of a localised excess in the mass range from 220 GeV to 5TeV. No significant excess is observed and 95% confidence-level upper limits between 1.3 pb and 0.3 fb are placed on the production cross section times branching fraction of neutral and charged spin-1 resonances and CP-odd scalar bosons. These limits are converted into constraints on the parameter space of the Heavy Vector Triplet model and the two-Higgs-doublet model.

This paper presents direct searches for lepton flavour violation in Higgs boson decays, H -> e tau and H -> mu tau, performed using data collected with the ATLAS detector at the LHC. The searches are based on a data sample of proton-proton collisions at a centre-of-mass energy root s = 13 TeV, corresponding to an integrated luminosity of 138 fb(-1). Leptonic (tau -> l nu(l)nu(tau)) and hadronic (tau -> hadrons nu(tau)) decays of the tau-lepton are considered. Two background estimation techniques are employed: the MC-template method, based on data-corrected simulation samples, and the Symmetry method, based on exploiting the symmetry between electrons and muons in the Standard Model backgrounds. No significant excess of events is observed and the results are interpreted as upper limits on lepton-flavour-violating branching ratios of the Higgs boson. The observed (expected) upper limits set on the branching ratios at 95% confidence level, B(H -> e tau) < 0.20% (0.12%) and B(H -> mu tau ) < 0.18% (0.09%), are obtained with the MC-template method from a simultaneous measurement of potential H -> e tau and H -> mu tau signals. The best-fit branching ratio difference, B(H -> mu tau) -> B(H -> e tau), measured with the Symmetry method in the channel where the tau-lepton decays to leptons, is (0.25 0.10)%, compatible with a value of zero within 2.5 sigma.

A search for a new heavy scalar or pseudo-scalar Higgs boson (H/A) produced in association with a pair of top quarks, with the Higgs boson decaying into a pair of top quarks (H/A -> t (t) over bar) is reported. The search targets a final state with exactly two leptons with same-sign electric charges or at least three leptons. The analysed dataset corresponds to an integrated luminosity of 139 fb(-1) of proton-proton collisions collected at a centre-of-mass energy of 13TeV with the ATLAS detector at the LHC. Two multivariate classifiers are used to separate the signal from the background. No significant excess of events over the Standard Model expectation is observed. The results are interpreted in the context of a type-II two-Higgs-doublet model. The observed (expected) upper limits at 95% confidence level on the t (t) over barH/A production cross-section times the branching ratio of H/A -> t (t) over bar range between 14 (10) fb and 6 (5) fb for a heavy Higgs boson with mass between 400 GeV and 1000 GeV, respectively. Assuming that only one particle, either the scalar H or the pseudo-scalar A, contributes to the t (t) over bart (t) over bar final state, values of tan beta below 1.2 or 0.5 are excluded for a mass of 400 GeV or 1000 GeV, respectively. These exclusion ranges increase to tan beta below 1.6 or 0.6 when both particles are considered.