Person: Spearman, William
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Spearman
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William
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Spearman, William
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Publication Measurement of the Centrality and Pseudorapidity Dependence of the Integrated Elliptic Flow in Lead–lead Collisions at √sNN=2.76TeV With the ATLAS Detector(Springer Nature, 2014) Franklin, Melissa; Huth, John; Ippolito, Valerio; Lopez Mateos, David; Mercurio, Kevin; Morii, Masahiro; Spearman, William; Yen, Andy; Zevi Della Porta, GThe integrated elliptic flow of charged particles produced in Pb+Pb collisions at sNN=2.76 TeV has been measured with the ATLAS detector using data collected at the Large Hadron Collider. The anisotropy parameter, v2, was measured in the pseudorapidity range |η|≤2.5 with the event-plane method. In order to include tracks with very low transverse momentum pT, thus reducing the uncertainty in v2 integrated over pT, a 1~ b-1 data sample recorded without a magnetic field in the tracking detectors is used. The centrality dependence of the integrated v2 is compared to other measurements obtained with higher pT thresholds. The integrated elliptic flow is weakly decreasing with |η|. The integrated v2 transformed to the rest frame of one of the colliding nuclei is compared to the lower-energy RHIC data.Publication Measurement of the Top-Quark Mass in the Fully Hadronic Decay Channel From ATLAS Data at √s= 7TeV(Springer Nature, 2015) Franklin, Melissa; Huth, John; Ippolito, Valerio; Lopez Mateos, David; Mercurio, Kevin; Spearman, William; Sun, Siyuan; Yen, Andy; Zevi Della Porta, GThe mass of the top quark is measured in a data set corresponding to 4.6 fb-1 of proton–proton collisions with centre-of-mass energy √ = 7TeV collected by the ATLAS detector at the LHC. Events consistent with hadronic decays of top–antitop quark pairs with at least six jets in the final state are selected. The substantial background from multijet production is modelled with data-driven methods that utilise the number of identified b-quark jets and the transverse momentum of the sixth leading jet, which have minimal correlation. The top-quark mass is obtained from template fits to the ratio of three-jet to dijet mass. The three-jet mass is calculated from the three jets produced in a top-quark decay. Using these three jets the dijet mass is obtained from the two jets produced in the W boson decay. The top-quark mass obtained from this fit is thus less sensitive to the uncertainty in the energy measurement of the jets. A binned likelihood fit yields a top-quark mass of mt = 175.1 ± 1.4 (stat.) ± 1.2 (syst.) GeV.Publication Evidence for Light-by-Light Scattering in Heavy-Ion Collisions With the ATLAS Detector at the LHC(Springer Science and Business Media LLC, 2017-08-14) Morii, Masahiro; Huth, John; Franklin, Melissa; Spearman, WilliamLight-by-light scattering (γγ→γγ) is a quantum-mechanical process that is forbidden in the classical theory of electrodynamics. This reaction is accessible at the Large Hadron Collider thanks to the large electromagnetic field strengths generated by ultra-relativistic colliding lead (Pb) ions. Using 480 μb−1 of Pb+Pb collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector, the ATLAS Collaboration reports evidence for the γγ→γγ reaction. A total of 13 candidate events are observed with an expected background of 2.6±0.7 events. After background subtraction and analysis corrections, the fiducial cross section of the process Pb+Pb(γγ)→Pb(∗)+Pb(∗)γγ, for photon transverse energy ET>3 GeV, photon absolute pseudorapidity |η|<2.4, diphoton invariant mass greater than 6 GeV, diphoton transverse momentum lower than 2 GeV and diphoton acoplanarity below 0.01, is measured to be 70 ± 24 (stat.) ± 17 (syst.) nb, which is in agreement with Standard Model predictions.Publication Precision Measurement and Interpretation of Inclusive W+ , W− and Z/γ∗ Production Cross Sections With the ATLAS Detector(Springer Science and Business Media LLC, 2017-06) Morii, Masahiro; Huth, John; Franklin, Melissa; Spearman, WilliamHigh-precision measurements by the ATLAS Collaboration are presented of inclusive ��+→ℓ+��, ��−→ℓ−��¯ and ��/��∗→ℓℓ (ℓ=��,��) Drell–Yan production cross sections at the LHC. The data were collected in proton–proton collisions at ��√=7TeV with an integrated luminosity of 4.6fb−1. Differential ��+ and ��− cross sections are measured in a lepton pseudorapidity range |��ℓ|<2.5. Differential ��/��∗ cross sections are measured as a function of the absolute dilepton rapidity, for |��ℓℓ|<3.6, for three intervals of dilepton mass, ��ℓℓ, extending from 46 to 150GeV. The integrated and differential electron- and muon-channel cross sections are combined and compared to theoretical predictions using recent sets of parton distribution functions. The data, together with the final inclusive ��±�� scattering cross-section data from H1 and ZEUS, are interpreted in a next-to-next-to-leading-order QCD analysis, and a new set of parton distribution functions, ATLAS-epWZ16, is obtained. The ratio of strange-to-light sea-quark densities in the proton is determined more accurately than in previous determinations based on collider data only, and is established to be close to unity in the sensitivity range of the data. A new measurement of the CKM matrix element |������| is also provided.Publication Measurement of the W + W − Production Cross Section in Pp Collisions at a Centre-of-Mass Energy ofs=13TeV With the ATLAS Experiment(Elsevier BV, 2017-10) Morii, Masahiro; Huth, John; Franklin, Melissa; Spearman, WilliamThe production of opposite-charge W-boson pairs in proton–proton collisions at TeV is measured using data corresponding to 3.16 fb−1 of integrated luminosity collected by the ATLAS detector at the CERN Large Hadron Collider in 2015. Candidate W-boson pairs are selected by identifying their leptonic decays into an electron, a muon and neutrinos. Events with reconstructed jets are not included in the candidate event sample. The cross-section measurement is performed in a fiducial phase space close to the experimental acceptance and is compared to theoretical predictions. Agreement is found between the measurement and the most accurate calculations available.Publication Search for Squarks and Gluinos in Events With Hadronically Decaying Tau Leptons, Jets and Missing Transverse Momentum in Proton–proton Collisions at √s=13 TeV Recorded With the ATLAS Detector(Springer Science and Business Media LLC, 2016-12) Morii, Masahiro; Franklin, Melissa; Ippolito, Victoria; Huth, John; Mercurio, K; Spearman, WilliamA search for supersymmetry in events with large missing transverse momentum, jets, and at least one hadronically decaying tau lepton has been performed using 3.2 fb−1 of proton–proton collision data at ��√=13 TeV recorded by the ATLAS detector at the Large Hadron Collider in 2015. Two exclusive final states are considered, with either exactly one or at least two tau leptons. No excess over the Standard Model prediction is observed in the data. Results are interpreted in the context of gauge-mediated supersymmetry breaking and a simplified model of gluino pair production with tau-rich cascade decays, substantially improving on previous limits. In the GMSB model considered, supersymmetry-breaking scale (Λ) values below 92 TeV are excluded at the 95% confidence level, corresponding to gluino masses below 2000 GeV. For large values of tan��, values of Λ up to 107 TeV and gluino masses up to 2300 GeV are excluded. In the simplified model, gluino masses are excluded up to 1570 GeV for neutralino masses around 100 GeV. Neutralino masses below 700 GeV are excluded for all gluino masses between 800 and 1500 GeV, while the strongest exclusion of 750 GeV is achieved for gluino masses around 1450 GeV.Publication Measurement of Forward-Backward Multiplicity Correlations in Lead-Lead, Proton-Lead, and Proton-Proton Collisions With the ATLAS Detector(American Physical Society (APS), 2017-06-28) Morii, Masahiro; Franklin, Melissa; Huth, John; Spearman, William; Skottowe, H; Mercurio, KTwo-particle pseudorapidity correlations are measured in √sNN = 2.76 TeV Pb + Pb, √sNN = 5.02 TeV p+Pb, and √s = 13 TeV pp collisions at the Large Hadron Collider (LHC), with total integrated luminosities of approximately 7μb−1, 28 nb−1, and 65 nb−1, respectively. The correlation function CN(η1,η2) is measured as a function of event multiplicity using charged particles in the pseudorapidity range |η| < 2.4. The correlation function contains a significant short-range component, which is estimated and subtracted. After removal of the short-range component, the shape of the correlation function is described approximately by 1 + a21 1/2 η1η2 in all collision systems over the full multiplicity range. The values of a2 1 1/2 are consistent for the opposite-charge pairs and same-charge pairs, and for the three collision systems at similar multiplicity. The values of a21 1/2 and the magnitude of the short-range component both follow a power-law dependence on the event multiplicity. The short-range component in p + Pb collisions, after symmetrizing the proton and lead directions, is found to be smaller at a given η than in pp collisions with comparable multiplicity.Publication Measurement of the Relative Width Difference of the B0-B0 System With the ATLAS Detector(Springer Science and Business Media LLC, 2016-06) Morii, Masahiro; Franklin, Melissa; Huth, John; Lopez Mateos, D; Mercurio, Kevin; Skottowe, Hugh Philip; Spearman, WilliamAbstract: This paper presents the measurement of the relative width difference ∆Γd/Γd of the B0-B ̄0 system using the data collected by the ATLAS experiment at the LHC in pp √√ collisions at s = 7 TeV and s = 8 TeV and corresponding to an integrated luminosity of 25.2fb−1. The value of ∆Γd/Γd is obtained by comparing the decay-time distributions of B0 → J/ψKS and B0 → J/ψK∗0(892) decays. The result is ∆Γd/Γd = (−0.1±1.1 (stat.)± 0.9 (syst.)) × 10−2. Currently, this is the most precise single measurement of ∆Γd/Γd. It agrees with the Standard Model prediction and the measurements by other experiments.Publication Search for Lepton-Flavour-Violating Decays of the Higgs and Z Bosons With the ATLAS Detector(Springer Science and Business Media LLC, 2017-02) Morii, Masahiro; Huth, John; Franklin, Melissa; Spearman, WilliamDirect searches for lepton flavour violation in decays of the Higgs and Z bosons with the ATLAS detector at the LHC are presented. The following three decays are considered: H → eτ , H → μτ , and Z → μτ . The searches are based on the data sample of proton–proton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 20.3 fb−1 at a centre-of-mass energy of √s = 8 TeV. No significant excess is observed, and upper limits on the lepton-flavour-violating branching ratios are set at the 95% confidence level: Br(H → eτ) < 1.04%, Br(H → μτ ) < 1.43%, and Br(Z → μτ ) < 1.69 × 10−5.Publication Search for Charged Higgs Bosons Produced in Association With a Top Quark and Decaying via H±→τν Using Pp Collision Data Recorded at S=13 TeV by the ATLAS Detector(Elsevier BV, 2016-08) Morii, Masahiro; Huth, John; Franklin, Melissa; Spearman, WilliamCharged Higgs bosons produced in association with a single top quark and decaying via H± → τν are searched for with the ATLAS experiment at the LHC, using proton–proton collision data at √s = 13 TeV corresponding to an integrated luminosity of 3.2 fb−1. The final state is characterised by the presence of a hadronic τ decay and missing transverse momentum, as well as a hadronically decaying top quark, resulting in the absence of high-transverse-momentum electrons and muons. The data are found to be consistent with the expected background from Standard Model processes. A statistical analysis leads to 95% confidence-level upper limits on the production cross section times branching fraction, σ(pp → [b]t H±) × BR(H± → τν), between 1.9 pb and 15 fb, for charged Higgs boson masses ranging from 200 to 2000 GeV. The exclusion limits for this search surpass those obtained with the proton–proton collision data recorded at √s = 8 TeV.