Measurement of the associated production of a top quark pair and a Higgs boson (t¯tH) with boosted topologies

This thesis presents three studies focusing on boosted topologies that utilise machine learning techniques for boosted H → b¯b reconstruction using the ATLAS detector. The measurement of the t¯tH cross-section is a direct way of accessing the Higgs top Yukawa coupling (yt). Firstly, an all-hadronic feasibility study is shown, aimed at assessing boosted topologies in the all-hadronic t¯tH decay channel. It was found to have low statistical significance, with considerable efforts and data driven techniques required to reduce the QCD-multijet background. Secondly, the boosted contribution to the recent t¯tH, H → b¯b measurement using the full Run-2 ATLAS data set, 139f b−1 at √s = 13 TeV, is analysed. There is a considerable contribution from the boosted region to this result, particularly to the differential cross-section measurement of the Simplified Template Cross-Section (STXS) bins [300, 450) and [450, ∞) GeV. The result of the inclusive profile-likelihood fit is μ = 0.35+0.36−0.34, which corresponds to σ = 1.0(2.7) observed(expected) significance compared to the background-only hypothesis. Thirdly work on retraining the boosted H → b¯b reconstruction deep neural network (DNN) is shown for the Run-2 Legacy re-analysis. The bespoke DNN trained for the analysis showed some improvements over the previous round due to the updated analysis algorithms. It also outperformed the general purpose H → b¯b Xbb tagger. The final motivation for use of the bespoke DNN is that it allows the choice of boosted jet collection (RC-jets vs LR-jets). RC-jets re cluster “small” (∆R = 0.4) jets with ∆R = 1.0 while LR-jets directly cluster the calorimeter clusters with ∆R = 1.0, both using the anti-kt algorithm. The RC-jets jets are found to be advantageous. This is due to the ease of propagating systematics for combining with resolved regions and the good modelling observed using samples made with the Atlfast-2 detector simulation

ARMing HEP for the future Energy Efficiency of WLCG sites (ARM vs. x86)

We present a case for ARM chips as an alternative to standard x86 at WLCG sites to help reduce power consumption. New measurements are presented on the performance and energy consumption of two machines (one ARM and one x86), that were otherwise similar in specification and cost. The comparison was extended to a dual socket x86 node, representative of our site. These new results include the energy-efficiency and speed of singleand multithreaded jobs; the effect of hyper-threading; and an initial look at clock throttling as a way of shaping power-load. We observe significantly lower power consumption and often slightly better performance on the ARM machine and, noting the increased availability of ARM software builds from all LHC experiments and beyond, we plan to install a 2k-core ARM cluster at our WLCG Tier2 site at Glasgow in the summer of 2023. This will enable testing, physicsvalidation, and eventually an ARM production environment that will inform and influence other WLCG sites in the UK and worldwide

Inclusive and differential cross-section measurements of t\bartZ production in pp collisions at √s=13 TeV with the ATLAS detector, including EFT and spin-correlation interpretations

Measurements of both the inclusive and differential production cross sections of a top-quark-top-antiquark pair in association with a Z boson (tt¯Z) 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 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 σtt¯Z=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 tt¯Z 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 tt¯Z 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

Observation of quantum entanglement in top-quark pairs using the ATLAS detector

We report the highest-energy observation of entanglement, in top−antitop quark events produced at the Large Hadron Collider, using a proton−proton collision data set with a center-of-mass energy of s√=13 TeV and an integrated luminosity of 140 fb−1 recorded with the ATLAS experiment. Spin entanglement is detected from the measurement of a single observable D, inferred from the angle between the charged leptons in their parent top- and antitop-quark rest frames. The observable is measured in a narrow interval around the top−antitop quark production threshold, where the entanglement detection is expected to be significant. It is reported in a fiducial phase space defined with stable particles to minimize the uncertainties that stem from limitations of the Monte Carlo event generators and the parton shower model in modelling top-quark pair production. The entanglement marker is measured to be D=−0.547±0.002 (stat.)±0.021 (syst.) for 340<mtt¯<380 GeV. The observed result is more than five standard deviations from a scenario without entanglement and hence constitutes both the first observation of entanglement in a pair of quarks and the highest-energy observation of entanglement to date

Muon reconstruction and identification efficiency in ATLAS using the full Run 2 pp collision data set at \sqrt{s}=13 TeV

This article documents the muon reconstruction and identification efficiency obtained by the ATLAS experiment for 139 \hbox {fb}^{-1} of pp collision data at \sqrt{s}=13 TeV collected between 2015 and 2018 during Run 2 of the LHC. The increased instantaneous luminosity delivered by the LHC over this period required a reoptimisation of the criteria for the identification of prompt muons. Improved and newly developed algorithms were deployed to preserve high muon identification efficiency with a low misidentification rate and good momentum resolution. The availability of large samples of Z\rightarrow \mu \mu and J/\psi \rightarrow \mu \mu decays, and the minimisation of systematic uncertainties, allows the efficiencies of criteria for muon identification, primary vertex association, and isolation to be measured with an accuracy at the per-mille level in the bulk of the phase space, and up to the percent level in complex kinematic configurations. Excellent performance is achieved over a range of transverse momenta from 3 GeV to several hundred GeV, and across the full muon detector acceptance of |\eta |<2.7

Comparison of inclusive and photon-tagged jet suppression in 5.02 TeV Pb+Pb collisions with ATLAS

Parton energy loss in the quark–gluon plasma (QGP) is studied with a measurement of photon-tagged jet production in 1.7 nb−1 of Pb+Pb data and 260 pb−1 of pp data, both at sNN=5.02 TeV, with the ATLAS detector. The process pp →γ+jet+X and its analogue in Pb+Pb collisions is measured in events containing an isolated photon with transverse momentum (pT) above 50 GeV and reported as a function of jet pT. This selection results in a sample of jets with a steeply falling pT distribution that are mostly initiated by the showering of quarks. The pp and Pb+Pb measurements are used to report the nuclear modification factor, RAA, and the fractional energy loss, Sloss, for photon-tagged jets. In addition, the results are compared with the analogous ones for inclusive jets, which have a significantly smaller quark-initiated fraction. The RAA and Sloss values are found to be significantly different between those for photon-tagged jets and inclusive jets, demonstrating that energy loss in the QGP is sensitive to the colour-charge of the initiating parton. The results are also compared with a variety of theoretical models of colour-charge-dependent energy loss

Measurement of hadronic event shapes in high-p T multijet final states at √s = 13 TeV with the ATLAS detector

A measurement of event-shape variables in proton-proton collisions at large momentum transfer is presented using data collected at s = 13 TeV with the ATLAS detector at the Large Hadron Collider. Six event-shape variables calculated using hadronic jets are studied in inclusive multijet events using data corresponding to an integrated luminosity of 139 fb−1. Measurements are performed in bins of jet multiplicity and in different ranges of the scalar sum of the transverse momenta of the two leading jets, reaching scales beyond 2 TeV. These measurements are compared with predictions from Monte Carlo event generators containing leading-order or next-to-leading order matrix elements matched to parton showers simulated to leading-logarithm accuracy. At low jet multiplicities, shape discrepancies between the measurements and the Monte Carlo predictions are observed. At high jet multiplicities, the shapes are better described but discrepancies in the normalisation are observed. [Figure not available: see fulltext.

Measurement of the total cross section and ρ -parameter from elastic scattering in pp collisions at √s=13 TeV with the ATLAS detector

In a special run of the LHC with β⋆= 2.5 km, proton–proton elastic-scattering events were recorded at s=13 TeV with an integrated luminosity of 340μb-1 using the ALFA subdetector of ATLAS in 2016. The elastic cross section was measured differentially in the Mandelstam t variable in the range from - t= 2.5 · 10 - 4 GeV 2 to - t= 0.46 GeV 2 using 6.9 million elastic-scattering candidates. This paper presents measurements of the total cross section σtot , parameters of the nuclear slope, and the ρ -parameter defined as the ratio of the real part to the imaginary part of the elastic-scattering amplitude in the limit t→ 0 . These parameters are determined from a fit to the differential elastic cross section using the optical theorem and different parameterizations of the t-dependence. The results for σtot and ρ are σtot(pp→X)=104.7±1.1mb,ρ=0.098±0.011. The uncertainty in σtot is dominated by the luminosity measurement, and in ρ by imperfect knowledge of the detector alignment and by modelling of the nuclear amplitude