Person: Lazovich, Tomo
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Lazovich
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Tomo
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Lazovich, Tomo
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Publication Luminosity Determination in Pp Collisions at S√ S = 8 TeV Using the ATLAS Detector at the LHCMorii, Masahiro; Chan, S. K.; Clark, B; Giromini, Paolo; Franklin, Melissa; Huth, John; Ippolito, Victoria; Lazovich, Tomo; Lopez Mateos, D; Rogan, CharlotteThe luminosity determination for the ATLAS detector at the LHC during pp collisions at s√= 8 TeV in 2012 is presented. The evaluation of the luminosity scale is performed using several luminometers, and comparisons between these luminosity detectors are made to assess the accuracy, consistency and long-term stability of the results. A luminosity uncertainty of δℒ/ℒ = ±1.9% is obtained for the 22.7 fb-1 of pp collision data delivered to ATLAS at s√= 8 TeV in 2012.Publication Search for New Phenomena in Different-Flavour High-Mass Dilepton Final States in Pp Collisions at S√=13 S = 13 Tev With the ATLAS Detector(Springer Verlag) Morii, Masahiro; Chan, S; Clark, Beatrice; Giromini, Paolo; Franklin, Melissa; Huth, John; Ippolito, Victoria; Lazovich, Tomo; Lopez Mateos, DA search is performed for a heavy particle decaying into different flavour dilepton pairs (𝑒𝜇, 𝑒𝜏 or 𝜇𝜏), using 3.2 fb−1 of proton–proton collision data at 𝑠√=13 TeV collected in 2015 by the ATLAS detector at the Large Hadron Collider. No excess over the Standard Model prediction is observed. Limits at the 95 % credibility level are set on the mass of a 𝑍′ boson with lepton-flavour-violating couplings at 3.0, 2.7 and 2.6 TeV, and on the mass of a supersymmetric 𝜏 sneutrino with R-parity-violating couplings at 2.3, 2.2 and 1.9 TeV, for 𝑒𝜇, 𝑒𝜏 and 𝜇𝜏 final states, respectively. The results are also interpreted as limits on the threshold mass for quantum black hole production.Publication Search for the Standard Model Higgs Boson Produced by Vector-Boson Fusion and Decaying to Bottom Quarks in S√=8 S = 8 TeV Pp Collisions With the ATLAS DetectorChan, Stephen; Clark, Brian Lee; Franklin, Melissa; Giromini, Paolo; Huth, John; Ippolito, Valerio; Lazovich, Tomo; Lopez Mateos, David; Morii, Masahiro; Rogan, Christopher S.; Skottowe, Hugh Philip; Sun, Siyuan; Tolley, Emma; Tong, Baojia; Tuna, Alexander; Yen, Andy; Zambito, StefanoA search with the ATLAS detector is presented for the Standard Model Higgs boson produced by vector-boson fusion and decaying to a pair of bottom quarks, using 20.2 fb−1 of LHC proton-proton collision data at √s = 8 TeV. The signal is searched for as a resonance in the invariant mass distribution of a pair of jets containing b-hadrons in vector-boson-fusion candidate events. The yield is measured to be −0.8 ± 2.3 times the Standard Model cross-section for a Higgs boson mass of 125 GeV. The upper limit on the cross-section times the branching ratio is found to be 4.4 times the Standard Model cross- section at the 95% confidence level, consistent with the expected limit value of 5.4 (5.7) in the background-only (Standard Model production) hypothesis.Publication Search for the Production of Single Vector-Like and Excited Quarks in the Wt Final State in Pp Collisions at s√=8 TeV TeV With the ATLAS Detector(Springer Science and Business Media LLC, 2016-02) Morii, Masahiro; Franklin, Melissa; Huth, John; Lazovich, TomoA search for vector-like quarks and excited quarks in events containing a top quark and a W boson in the final state is reported here. The search is based on 20.3 fb−1 of proton-proton collision data taken at the LHC at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector. Events with one or two leptons, and one, two or three jets are selected with the additional requirement that at least one jet contains a b-quark. Single-lepton events are also required to contain at least one large-radius jet from the hadronic decay of a high-p T W boson or a top quark. No significant excess over the expected background is observed and upper limits on the cross-section times branching ratio for different vector-like quark and excited-quark model masses are derived. For the excited-quark production and decay to Wt with unit couplings, quarks with masses below 1500 GeV are excluded and coupling-dependent limits are set.Publication Measurement of the tt⎯⎯W T T ¯ W and tt⎯⎯Z T T ¯ Z Production Cross Sections in Pp Collisions at S√=8 S = 8 TeV With the ATLAS Detector(Springer) Morii, Masahiro; Barreiro Guimaraes da Costa, J.; Catastini, P; Zambito, S; Yamaguchi, D; Franklin, Melissa; Huth, John; Ippolito, Victoria; Lazovich, Tomo; Lopez Mateos, D; Mercurio, K.M.The production cross sections of top-quark pairs in association with massive vector bosons have been measured using data from pp collisions at ��√=8 TeV. The dataset corresponds to an integrated luminosity of 20.3 fb−1 collected by the ATLAS detector in 2012 at the LHC. Final states with two, three or four leptons are considered. A fit to the data considering the ����⎯⎯�� and ����⎯⎯�� processes simultaneously yields a significance of 5.0σ (4.2σ) over the background-only hypothesis for ����⎯⎯�� (����⎯⎯��) production. The measured cross sections are ������⎯⎯��=369+100−91 fb and ������⎯⎯��=176+58−52 fb. The background-only hypothesis with neither ����⎯⎯�� nor ����⎯⎯�� production is excluded at 7.1σ. All measurements are consistent with next-to-leading-order calculations for the ����⎯⎯�� and ����⎯⎯�� processes.Publication Search for New Phenomena in Events With at Least Three Photons Collected in Pp Collisions at √s = 8 TeV With the ATLAS Detector(Springer Nature, 2016) Clark, Brian Lee; Franklin, Melissa; Huth, John; Ippolito, Valerio; Lazovich, Tomo; Lopez Mateos, David; Mercurio, Kevin; Morii, Masahiro; Spearman, William; Sun, Siyuan; Tolley, Emma; Tuna, Alexander; Yen, Andy; Zambito, StefanoResults of a search for new phenomena in events with at least three photons are reported. Data from proton–proton collisions at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb −1−1 , were collected with the ATLAS detector at the LHC. The observed data are well described by the Standard Model. Limits at the 95 % confidence level on new phenomena are presented based on the rate of events in an inclusive signal region and a restricted signal region targeting the rare decay Z→3γZ→3γ , as well as di-photon and tri-photon resonance searches. For a Standard Model Higgs boson decaying to four photons via a pair of intermediate pseudoscalar particles (a), limits are found to be σ× BR (h→aa)× BR (a→γγ)2<10−3σSMσ× BR (h→aa)× BR (a→γγ)2<10−3σSM for 10 GeV mH> 125 GeV, and for a Z′Z′ decaying to three photons via Z′→a+γ→3γZ′→a+γ→3γ . Additionally, the observed limit on the branching ratio of the Z boson decay to three photons is found to be BR (Z→3γ)<2.2×10−6(Z→3γ)<2.2×10−6 , a result five times stronger than the previous result from LEP.Publication Measurement of the transverse momentum and ϕ∗η distributions of Drell–Yan lepton pairs in proton–proton collisions at √s=8 TeV with the ATLAS detector(Springer Nature, 2016) Clark, Brian Lee; Franklin, Melissa; Huth, John; Ippolito, Valerio; Lazovich, Tomo; Lopez Mateos, David; Mercurio, Kevin; Morii, Masahiro; Spearman, William; Sun, Siyuan; Tolley, Emma; Tuna, Alexander; Yen, Andy; Zambito, StefanoDistributions of transverse momentum p T and the related angular variable φ ∗ η of Drell–Yan lepton pairs are measured in 20.3 fb−1 of proton–proton collisions at √ s = 8 TeV with the ATLAS detector at the LHC. Measurements in electron-pair and muon-pair final states are corrected for detector effects and combined. Compared to previous measurements in proton–proton collisions at √ s = 7 TeV, these new measurements benefit from a larger data sample and improved control of systematic uncertainties. Measurements are performed in bins of lepton-pair mass above, around and below the Z-boson mass peak. The data are compared to predictions from perturbative and resummed QCD calculations. For values of φ ∗ η < 1 the predictions from the Monte Carlo generator ResBos are generally consistent with the data within the theoretical uncertainties. However, at larger values of φ ∗ η this is not the case. Monte Carlo generators based on the parton-shower approach are unable to describe the data over the full range of p T while the fixed-order prediction of Dynnlo falls below the data at high values of p T . ResBos and the parton-shower Monte Carlo generators provide a much better description of the evolution of the φ ∗ η and p T distributions as a function of lepton-pair mass and rapidity than the basic shape of the data.Publication Measurements of the Higgs boson production and decay rates and coupling strengths using pp collision data at √s=7 and 8 TeV in the ATLAS experiment(Springer Nature, 2016) Clark, Brian Lee; Franklin, Melissa; Huth, John; Ippolito, Valerio; Lazovich, Tomo; Lopez Mateos, David; Mercurio, Kevin; Morii, Masahiro; Spearman, William; Sun, Siyuan; Tolley, Emma; Yen, AndyCombined analyses of the Higgs boson production and decay rates as well as its coupling strengths to vector bosons and fermions are presented. The combinations include the results of the analyses of the H → γγ, ZZ*, WW*, Zγ, bb̅, ττ and μμ decay modes, and the constraints on the associated production with a pair of top quarks and on the off-shell coupling strengths of the Higgs boson. The results are based on the LHC proton-proton collision datasets, with integrated luminosities of up to 4.7 fb⁻¹ at √s̅ =7 TeV and 20.3 fb⁻¹ at √s̅ = 8 TeV, recorded by the ATLAS detector in 2011 and 2012. Combining all production modes and decay channels, the measured signal yield, normalised to the Standard Model expectation, is 1.18_{-0.14} ^{+0.15}. The observed Higgs boson production and decay rates are interpreted in a leading-order coupling framework, exploring a wide range of benchmark coupling models both with and without assumptions on the Higgs boson width and on the Standard Model particle content in loop processes. The data are found to be compatible with the Standard Model expectations for a Higgs boson at a mass of 125.36 GeV for all models considered.Publication Reconstruction of primary vertices at the ATLAS experiment in Run 1 proton–proton collisions at the LHC(Springer Nature, 2017) Chan, Stephen; Clark, Brian Lee; Franklin, Melissa; Giromini, Paolo; Huth, John; Ippolito, Valerio; Lazovich, Tomo; Lopez Mateos, David; Morii, Masahiro; Rogan, Christopher S.; Sun, Siyuan; Tolley, Emma; Tong, Baojia; Tuna, Alexander; Yen, Andy; Zambito, StefanoThis paper presents the method and performance of primary vertex reconstruction in proton-proton collision data recorded by the ATLAS experiment during Run 1 of the LHC. The studies presented focus on data taken during 2012 at a centre-of-mass energy of s√=8 TeV. The performance has been measured as a function of the number of interactions per bunch crossing over a wide range, from one to seventy. The measurement of the position and size of the luminous region and its use as a constraint to improve the primary vertex resolution are discussed. A longitudinal vertex position resolution of about 30 micrometers is achieved for events with high multiplicity of reconstructed tracks. The transverse position resolution is better than 20 micrometers and is dominated by the precision on the size of the luminous region. An analytical model is proposed to describe the primary vertex reconstruction efficiency as a function of the number of interactions per bunch crossing and of the longitudinal size of the luminous region. Agreement between the data and the predictions of this model is better than 3% up to seventy interactions per bunch crossing.Publication Evidence for single top-quark production in the s-channel in proton-proton collisions at s√=8 TeV with the ATLAS detector using the Matrix Element Method(Elsevier BV, 2016) Clark, Brian Lee; Franklin, Melissa; Huth, John; Ippolito, Valerio; Lazovich, Tomo; Lopez Mateos, David; Mercurio, Kevin; Morii, Masahiro; Spearman, William; Sun, Siyuan; Tolley, Emma; Tuna, Alexander; Yen, Andy; Zambito, StefanoThis Letter presents evidence for single top-quark production in the s-channel using proton-proton collisions at a centre-of-mass energy of 8 TeV with the ATLAS detector at the CERN Large Hadron Collider. The analysis is performed on events containing one isolated electron or muon, large missing transverse momentum and exactly two b-tagged jets in the final state. The analysed data set corresponds to an integrated luminosity of 20.3 fb−1. The signal is extracted using a maximum-likelihood fit of a discriminant which is based on the matrix element method and optimized in order to separate single-top-quark s-channel events from the main background contributions, which are top-quark pair production and W boson production in association with heavy-flavour jets. The measurement leads to an observed signal significance of 3.2 standard deviations and a measured cross-section of σs=4.8±0.8(stat.)+1.6−1.3(syst.) pb, which is consistent with the Standard Model expectation. The expected significance for the analysis is 3.9 standard deviations.