Professor Claire Gwenlan

Abstract:

This article presents a new set of proton parton distribution functions, ATLASepWZVjet20, produced in an analysis at next-to-next-to-leading order in QCD. The new data sets considered are the measurements of W+ and W− boson and Z boson production in association with jets in pp collisions at 𝑠√ = 8 TeV performed by the ATLAS experiment at the LHC with integrated luminosities of 20.2 fb−1 and 19.9 fb−1, respectively. The analysis also considers the ATLAS measurements of differential W± and Z boson production at 𝑠√ = 7 TeV with an integrated luminosity of 4.6 fb−1 and deep-inelastic-scattering data from e±p collisions at the HERA accelerator. An improved determination of the sea-quark densities at high Bjorken x is shown, while confirming a strange-quark density similar in size to the up- and down-sea-quark densities in the range x ≲ 0.02 found by previous ATLAS analyses.

Abstract:

A bstract A search for long-lived particles, which have come to rest within the ATLAS detector, is presented. The subsequent decays of these particles can produce high-momentum jets, resulting in large out-of-time energy deposits in the ATLAS calorimeters. These de- cays are detected using data collected during periods in the LHC bunch structure when collisions are absent. The analysed dataset is composed of events from proton-proton collisions produced by the Large Hadron Collider at a centre-of-mass energy of $$ \sqrt{s} $$ s = 13 TeV and recorded by the ATLAS experiment during 2017 and 2018. The dataset used for this search corresponds to a total live time of 579 hours. The results of this search are used to derive lower limits on the mass of gluino R -hadrons, assuming a branching fraction $$ \mathcal{B}\left(\overset{\sim }{g}\to q\overline{q}{\chi}_1^0\right) $$ B g ~ → q q ¯ χ 1 0 = 100%, with masses of up to 1 . 4 TeV excluded for gluino lifetimes of 10 − 5 to 10 3 s.

Abstract:

A bstract A search for supersymmetry in events with four or more charged leptons (electrons, muons and τ -leptons) is presented. The analysis uses a data sample corresponding to 139 fb − 1 of proton-proton collisions delivered by the Large Hadron Collider at $$ \sqrt{s} $$ s = 13 TeV and recorded by the ATLAS detector. Four-lepton signal regions with up to two hadronically decaying τ -leptons are designed to target several supersymmetric models, while a general five-lepton signal region targets any new physics phenomena leading to a final state with five charged leptons. Data yields are consistent with Standard Model expectations and results are used to set upper limits on contributions from processes beyond the Standard Model. Exclusion limits are set at the 95% confidence level in simplified models of general gauge-mediated supersymmetry, excluding higgsino masses up to 540 GeV. In R -parity-violating simplified models with decays of the lightest supersymmetric particle to charged leptons, lower limits of 1 . 6 TeV, 1 . 2 TeV, and 2 . 5 TeV are placed on wino, slepton and gluino masses, respectively.

Abstract:

The results of a search for gluino and squark pair production with the pairs decaying via the lightest charginos into a final state consisting of two W bosons, the lightest neutralinos (χ~10), and quarks, are presented: the signal is characterised by the presence of a single charged lepton (e± or μ±) from a W boson decay, jets, and missing transverse momentum. The analysis is performed using 139 fb- 1 of proton–proton collision data taken at a centre-of-mass energy s=13 delivered by the Large Hadron Collider and recorded by the ATLAS experiment. No statistically significant excess of events above the Standard Model expectation is found. Limits are set on the direct production of squarks and gluinos in simplified models. Masses of gluino (squark) up to 2.2 (1.4) are excluded at 95% confidence level for a light χ~10.

Abstract:

Abstract This article documents the muon reconstruction and identification efficiency obtained by the ATLAS experiment for 139 $$\hbox {fb}^{-1}$$ fb-1 of pp collision data at $$\sqrt{s}=13$$ 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 $$Z\rightarrow \mu \mu $$ Z→μμ and $$J/\psi \rightarrow \mu \mu $$ J/ψ→μμ 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 $$|\eta |<2.7$$ |η|<2.7 .