Dr Benjamin Hodkinson

Abstract:

Processes with τ$$\tau $$-leptons in the final state are important for Standard Model measurements and searches for physics beyond the Standard Model. The ATLAS experiment at the Large Hadron Collider observes τ$$\tau $$-leptons produced in proton–proton collisions only through their decay products. Data analyses involving hadronically decaying τ$$\tau $$-leptons face challenges due to backgrounds from jets misidentified as τ$$\tau $$-leptons that are not modelled reliably by Monte Carlo simulations. Data-driven methods such as the fake-factor method allow such misidentified backgrounds to be predicted by measuring transfer factors, known as fake factors, in data from dedicated regions. This paper describes a refined technique for determining the fake factors, the Universal Fake Factor method. It evaluates the fake factors for a signal region by using fake factors from samples enriched in different sources of jets misidentified as τ$$\tau $$-leptons (light-quark, gluon, b-quark, and pile-up jets). Each fake factor is calculated as a linear combination of fake factors measured in these different enriched samples. For the full Run 2 data set, the systematic uncertainty of the calculated fake factors, evaluated using W(μν)$$W(\mu \nu )$$ enriched event sample, ranges from 15 to 35% depending on the τ$$\tau $$-lepton’s transverse momentum and charged-particle decay multiplicity.

Abstract:

Measurements of tt¯ℓ+ℓ-$$t\bar{t}\ell ^{+}\ell ^{-}$$ production in the region of high dilepton invariant mass with effective field theory (EFT) interpretations are presented. They are performed using final states with three isolated leptons (electrons or muons) and are based on s=13$$\sqrt{s} = 13$$ TeV$$\text {T}\text {e}\hspace{-1.00006pt}\text {V}$$ proton–proton collision data with an integrated luminosity of 140fb-1$$140\,\textrm{fb}^{-1}$$, recorded from 2015 to 2018 with the ATLAS detector at the Large Hadron Collider. Measurements of the tt¯ℓ+ℓ-$$t\bar{t}\,\ell ^+\ell ^-$$ signal strength and cross-section upper-limits are performed inclusively in lepton flavour and separately for electrons and muons. The study also aims to probe anomalous four-fermion interactions including to test for possible lepton flavor universality violation. No significant deviations from the Standard Model predictions are observed and the measurements are interpreted through the EFT formalism to provide new constraints on the relevant operators.

Abstract:

A search for R-parity-conserving supersymmetry in events with large missing transverse momentum, jets and at least one hadronically decaying τ$$\tau $$-lepton is presented. Both gluino and squark pair production are considered, with the cascade decay of each gluino or squark producing either a τ$$\tau $$-slepton or a τ$$\tau $$-sneutrino. Three channels are examined, requiring either exactly one hadronically decaying τ$$\tau $$-lepton and no other leptons, exactly one hadronically decaying τ$$\tau $$-lepton and at least one other lepton, or two or more hadronically decaying τ$$\tau $$-leptons. Analyses in the three channels are optimised independently and combined statistically. Two separate analysis strategies, either a cut-and-count or machine-learning approach, are used. The search uses 140 fb-1$$\textrm{fb}^{-1}$$ and 51.8 fb-1$$\textrm{fb}^{-1}$$ of pp collision data recorded by the ATLAS detector at the Large Hadron Collider during 2015–2018 at s=13$$\sqrt{s} = 13$$ TeV and 2022–2023 at s=13.6$$\sqrt{s} = 13.6$$ TeV, respectively. Gluino masses below 2.25 TeV and squark masses up to 1.7 TeV are excluded

Abstract:

Higgs boson production cross-sections via gluon–gluon fusion and vector-boson fusion in proton–proton collisions are measured in the H→WW∗→ℓνℓν$$H\rightarrow WW^*\rightarrow \ell \nu \ell \nu $$ decay channel. The Large Hadron Collider delivered proton–proton collisions at a centre-of-mass energy of 13TeV$$13\,\text {TeV}$$ between 2015 and 2018, which were recorded by the ATLAS detector, corresponding to an integrated luminosity of 140fb-1.$$140\,\text {fb}^{-1}.$$ The total cross-sections for Higgs boson production by gluon–gluon fusion and vector-boson fusion times the H→WW∗$$H\rightarrow WW^*$$ branching ratio are measured to be 12.4-1.2+1.3pb$$12.4^{+1.3}_{-1.2}\,\text {pb}$$ and 0.79-0.16+0.18pb,$$0.79^{+0.18}_{-0.16}\,\text {pb},$$ respectively, in agreement with the Standard Model predictions. Higgs boson production is further characterised through measurements of Simplified Template Cross-Sections in a total of fifteen kinematic fiducial regions. A new scheme of kinematic fiducial regions has been introduced to enhance the sensitivity to CP-violating effects in Higgs boson interactions. Both schemes are used to constrain CP-even and CP-odd dimension-six operators in the Standard Model effective field theory

Abstract:

ATLAS, a general-purpose experiment at the Large Hadron Collider (LHC), makes use of a large internationally-distributed computing infrastructure, including over 106$$10^6$$ TB of managed data on disk and tape and almost one million simultaneously running CPU cores. Upgrades for the High-Luminosity LHC (HL-LHC) will increase the required computing resources by a factor of 3–4 by the beginning of the 2030s, and by an order of magnitude before the conclusion of data taking at the beginning of the 2040s. These resources are spread over around 100 computing sites worldwide. Efforts are underway within the experiment to evaluate and mitigate various aspects of the environmental impact of the sites, with the additional long-term goal of making recommendations to the sites that will significantly reduce the total expected environmental impact in the HL-LHC era. These efforts take several forms: building awareness in the experiment community, adjusting aspects of the computing policy, and modifications of data center configurations, either in ways that take advantage of particular features of ATLAS workloads or in generic ways that reduce the environmental impact of the computing resources. This paper describes the ongoing investigations and approaches that have already provided useful and actionable outcomes.