Engineering topics
Publications and source records attributed to Alexander, G..
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Measurements of differential cross sections of top quark pair production in association with jets by the ATLAS experiment at the LHC are presented. The measurements are performed as functions of the top quark transverse momentum, the transverse momentum of the top quark-antitop quark system and the out-of-plane transverse momentum using data from pp collisions at s√=13 TeV collected by the ATLAS detector at the LHC in 2015 and corresponding to an integrated luminosity of 3.2 fb -1 . The top quark pair events are selected in the lepton (electron or muon) + jets channel. The measured cross sections, which are compared to several predictions, allow a detailed study of top quark production.
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Previous studies have shown that weighted angular moments derived from jet constituents encode the colour connections between partons that seed the jets. This paper presents measurements of two such distributions, the jet-pull angle and jet-pull magnitude, both of which are derived from the jet-pull angular moment. The measurement is performed in $t\bar{t}$ events with one leptonically decaying W boson and one hadronically decaying W boson, using 36.1fb -1 of pp collision data recorded by the ATLAS detector at s√=13 TeV delivered by the Large Hadron Collider. The observables are measured for two dijet systems, corresponding to the colour-connected daughters of the W boson and the two b-jets from the top-quark decays, which are not expected to be colour connected. To allow the comparison of the measured distributions to colour model predictions, the measured distributions are unfolded to particle level, after correcting for experimental effects introduced by the detector. While good agreement can be found for some combinations of predictions and observables, none of the predictions describes the data well across all observables.
An angular analysis of the decay B$0\atop{d}$ → K*μ + μ - is presented, based on proton-proton collision data recorded by the ATLAS experiment at the LHC. The study is using 20.3 fb -1 of integrated luminosity collected during 2012 at centre-of-mass energy of $\sqrt{s}=8 $ TeV. Measurements of the K* longitudinal polarisation fraction and a set of angular parameters obtained for this decay are presented. The results are compatible with the Standard Model predictions.
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CERN for the benefit of the ATLAS collaboration. A measurement of $J/\psi $ and $\psi (2\mathrm {S})$ production is presented. It is based on a data sample from Pb+Pb collisions at s NN =5.02 TeV and pp collisions at $ \sqrt{s}$=5.02 TeV recorded by the ATLAS detector at the LHC in 2015, corresponding to an integrated luminosity of 0.42 nb -1 and 25 pb -1 in Pb+Pb and pp, respectively. The measurements of per-event yields, nuclear modification factors, and non-prompt fractions are performed in the dimuon decay channel for 9 < p$\mathbb μμ\atop{T}$ < 40 GeV in dimuon transverse momentum, and -2 < y μμ < 2 in rapidity. Strong suppression is found in Pb+Pb collisions for both prompt and non-prompt $J/\psi $, increasing with event centrality. The suppression of prompt $\psi (2\mathrm {S})$ is observed to be stronger than that of $J/\psi $, while the suppression of non-prompt $\psi (2\mathrm {S})$ is equal to that of the non-prompt $J/\psi $ within uncertainties, consistent with the expectation that both arise from b-quarks propagating through the medium. Despite prompt and non-prompt $J/\psi $ arising from different mechanisms, the dependence of their nuclear modification factors on centrality is found to be quite similar.
A search for supersymmetric partners of top quarks decaying as t ~<!-- ~ --> 1 →<!-- → --> c χ<!-- χ --> ~<!-- ~ --> 1 0 and supersymmetric partners of charm quarks decaying as c ~<!-- ~ --> 1 →<!-- → --> c χ<!-- χ --> ~<!-- ~ --> 1 0 , where χ<!-- χ --> ~<!-- ~ --> 1 0 is the lightest neutralino, is presented. The search uses 36.1 fb -1 pp collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider and is performed in final states with jets identified as containing charm hadrons. Assuming a 100% branching ratio to c χ<!-- χ --> ~<!-- ~ --> 1 0 , top and charm squarks with masses up to 850 GeV are excluded at 95% confidence level for a massless lightest neutralino. For m t ~<!-- ~ --> 1 , c ~<!-- ~ --> 1 −<!-- - --> m χ<!-- χ --> ~<!-- ~ --> 1 0 < 100 GeV, top and charm squark masses up to 500 GeV are excluded.
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The efficiency to identify jets containing b-hadrons (b-jets) is measured using a high purity sample of dileptonic top quark-antiquark pairs ($t\bar{t}$) selected from the 36.1 fb -1 of data collected by the ATLAS detector in 2015 and 2016 from proton-proton collisions produced by the Large Hadron Collider at a centre-of-mass energy $\sqrt{s}=13$ TeV. Two methods are used to extract the efficiency from $t\bar{t}$ events, a combinatorial likelihood approach and a tag-and-probe method. A boosted decision tree, not using b-tagging information, is used to select events in which two b-jets are present, which reduces the dominant uncertainty in the modelling of the flavour of the jets. The efficiency is extracted for jets in a transverse momentum range from 20 to 300 GeV, with data-to-simulation scale factors calculated by comparing the efficiency measured using collision data to that predicted by the simulation. In conclusion, the two methods give compatible results, and achieve a similar level of precision, measuring data-to-simulation scale factors close to unity with uncertainties ranging from 2% to 12% depending on the jet transverse momentum.
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