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Measurement of Spin Correlation in Top-Antitop Quark Events and Search for Top Squark Pair Production in pp Collisions at s/s = 8 TeV Using the ATLAS Detector *

G. Aad et al. (ATLAS Collaboration) (Received 16 December 2014; published 8 April 2015) A measurement of spin correlation in tt production is presented using data collected with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb- ! . The correlation between the top and antitop quark spins is extracted from dilepton ft events by using the difference in the azimuthal angle between the two charged leptons in the laboratory frame. In the helicity basis the measured degree of correlation corresponds to Ahelicity = 0.38 ± 0.04, in agreement with the standard model prediction. A search is performed for pair production of top squarks with masses close to the top quark mass decaying to predominantly right-handed top quarks and a light neutralino, the lightest supersymmetric particle. Top squarks with masses between the top quark mass and 191 GeV are excluded at the 95% confidence level. DOI: 10.1103/PhysRevLett. 114.142001

PACS numbers: 14.65.Ha, 12.38.Qk, 13.85.Qk, 14.80.Ly

Detailed studies of the correlation of the spin of top and antitop quarks in ft events produced at hadron colliders are of great interest; they provide important precision tests of the predictions of the standard model (SM) and are sensitive to many new physics scenarios [1-16]. The orientations of the top and antitop quark spins are trans­ ferred to the decay products and can be measured directly via their angular distributions [3,17,17-36]. The strength of their correlation has been studied previously by the CDF and DO collaborations in proton-antiproton scattering at 1.98 TeV [37-40] and by the ATLAS and CMS collabo­ rations in proton-proton scattering at 7 TeV [41-43], In this Letter the first measurement of ft spin correlation in proton-proton collisions at a center-of-mass energy of 8 TeV is presented. Because the polarization-analyzing power of the angular distributions of charged leptons from top and antitop quark decays is effectively 100% [44,45], dilepton final states of e e , p p , and e p are analyzed. An observable veiy sensitive to ft spin correlation is the azimuthal angle Atp between the charged leptons [34], which is also well measured by the ATLAS detector. First, the measurement of Atp is used to extract the spin Correlation strength At-ielicit\ (7 m ike ^ u n lik e Iike T" 'Winlike L where A like (7Vuniike) is the number of events where the top quark and top antiquark spins are parallel (antiparallel) with respect to the spin quantization axis. This axis is chosen to be that of the helicity basis, using the direction of flight of * Full author list given at the end of the article. Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distri­ bution o f this work must maintain attribution to the author(s) and the published articles title, journal citation, and DOI.

0031 -9007/15/114( 14)/142001(19)

the top quark in the center-of-mass frame of the tt system. Second, to study a specific model that predicts zero spin correlation, a search for supersymmetric (SUSY) top squark pair production is performed. At the Large Hadron Collider (LHC), the SUSY partners of the top quark, the top squarks, could be produced in pairs. Models with light top squarks are particularly attractive since they provide a solution to the hierarchy problem [46-49]. In such models, the mass m~h of the lighter top squark mass eigenstate f, could be close to the mass of the top quark m, [50,51], If the lightest SUSY particle, the neutralino/j (or alternatively the gravitino), is light and the top squark mass is only slightly larger than the top quark mass, two-body decays 7, -» tjff in which the momentum of j({\ is very small can predominate [16]. The masses of all other SUSY particles are assumed to be large. In SUSY models where R parity is conserved, such as the minimal supersymmetric standard model (MSSM) [52-56], this could lead to tix \x i intermediate states, appearing like SM ft production with additional missing transverse momentum carried away by the escaping neutralinos, making traditional searches exploiting kinematic differences as presented in Refs. [57-63] very difficult. ~tx~tx events can be distinguished from SM ft events through an increase of the measured ft cross section as analyzed in Ref. [64], and since top squarks have zero spin, through measuring angular correlations sensitive to spin correlation, as analyzed in this Letter. A description of the ATLAS detector can be found elsewhere [65], This analysis uses proton-proton collision data with a center-of-mass energy of = 8 TeV, corre­ sponding to an integrated luminosity of 20.3 fb-1. Monte Carlo (MC) simulation samples are used to evaluate the contributions, and shapes of distributions of

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kinematic variables, for signal ft events and for background processes not evaluated from complementary data samples. All MC samples are processed with the geant4 [66] simulation of the ATLAS detector [67] and are passed through the same analysis chain as data. The simulation includes multiple proton-proton interactions per bunch crossing (pileup). Events are weighted such that the distribution of the average number of interactions per bunch crossing matches that observed in data. Samples of ft events with SM spin correlation and without spin correlation are generated using [email protected] v4.06 [68,69]interfaced to Herwig v6.520 [70]for shower simulation and hadronization. Both samples are normalized to the NNLO cross section including next-to-next-toleading-logarithm corrections [71,72], The CT10 parton distribution function (PDF) set [73]is used. For the sample with no spin correlation, the parton shower simulation performs isotropic decays of the top quarks whereas the full matrix element is used for the generation of the SM spincorrelation sample. The top quark mass is set to 172.5GeV [74],The production of a ft pair in association with a Z or W boson is simulated using M adgraph 5 [75] interfaced to Pythia v6.426 [76] and is normalized to the next-toleading-order (NLO) quantum chromodynamics (QCD) cross sections [77], Backgrounds to ft events with same-flavor dilepton final states arise from the Drell-Yan Z/y* + jets production process with the Z/y* boson decaying into e +e~, p +p~ and x+x~, followed by leptonic decays of the x leptons. They are generated using the Alpgen v2.13 [77]generator including leading-order (LO) matrix elements with up to five additional partons. The CTEQ6L1 PDF set [78] is used, and the cross section is normalized to the next-to-next-toleading-order (NNLO) QCD prediction [79], Parton show­ ering and fragmentation are modeled by H erwig, and multiparton interactions are simulated by J im m y [80], The “MLM” parton-jet matching scheme [81] is employed. Correction factors are derived from data in Z/y* + jetsdominated control regions and applied to the predicted yields in the signal region, to account for the difference between the simulation prediction and data. Single top quark background from associated Wt pro­ duction is modeled with Powheg-B ox r2129 [82-85] interfaced with Pythia using the CT10 PDF set [73] and normalized to the approximate NNLO QCD theoretical cross section [86]. Single-top Zt and W Zt production is generated by M adgraph 5 interfaced with Pythia. The diboson (WW, WZ, ZZ) backgrounds are modeled using Sherpa v 1.4.1 [87] and are normalized to the theoretical calculation at NLO QCD [88]. The background arising from the misidentified and non­ prompt leptons (collectively referred to as “fake leptons”) is determined from a combination of MC simulation of W + jets events using Sherpa, single-top events via /-channel exchange using MC @NLO + HERWIG, ft events with

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single-lepton final states using [email protected] + HERWIG, and data using a technique known as the matrix method [89,90], Top squark pair-production samples are simulated using the HERWIG + + v2.6.1 [91] generator with the CTEQ6L1 PDFs [78]. The top squarks are assumed to decay exclusively viarf -* / / “.The corresponding mixing matrices for the top squarks and for the neutral inos are chosen such that the top quark has a right-handed polarization in 95% of the decays. Candidate events are selected in the dilepton topology. The analysis requires events selected on-line by inclusive single-lepton triggers (e or //). Electron candidates are reconstructed from an isolated electromagnetic calorimeter energy deposit matched to a charged-particle track in the inner detector and must pass “medium identification requirements” [92], Muon candidates were reconstructed by combining tracks reconstructed in both the inner detector and muon spectrometer [93]. Jets are reconstructed from clusters of adjacent calorimeter cells [65,94] using the antik, algorithm [95-97] with a radius parameter R = 0.4. Jets originating from b quarks were identified (“tagged”) using a multivariate discriminant employing the long lifetime, high decay multiplicity, hard fragmentation, and high mass of B hadrons [98,99], The missing transverse momentum (£™ss) is reconstructed as the magnitude of a vector sum of all calorimeter cell energies associated with topological clusters [100]. The following kinematic requirements are made: (i) Electron candidates are required to have transverse momemtum of p T > 25 GeV and pseudorapidity of |//| < 2.47, excluding electrons from the transition region between the barrel and end-cap calorimeters defined by l. 37 < \t]\ < 1.52. (The pseudorapidity r] is defined via the polar angle 0 as /; = - ln ta n (0 /2 ) [65].) Muon candidates are required to have p T > 25 GeV and \rj\ < 2.5. Events must have exactly two oppositely charged lepton candi­ dates (e+e~, p +p~, e±p f ) . (ii) Events must have at least two jets (after having removed the jet closest to the electron, if there are jets within a cone of AR = 0.2 around a selected electron) with p T > 25 GeV and \rj\ < 2.5. At least one jet must be identified as a b jet using a requirement in the multivariate discriminant corresponding to a 70% b-tagging efficiency. (iii) Events in the e+e~ and p +p~ channels must satisfy E j ' ss > 30 GeV to suppress backgrounds from Drell-Yan Z/y* + jets and W + je ts events. (iv) Events in the e+e~ and p +p~ channels are required to have m ee > 15 GeV (where € indicates e or //) to ensure compatibility with the simulated backgrounds and to remove contributions from T and J/xp production. In addition, must differ by at least 10 GeV from the Z boson mass (mz = 91 GeV) to further suppress the Z/y* + je ts background. (v) For the e±p f channel, no E Tmss or m f f requirements are applied. In this case, the remaining background from Z/y* rr) + jets production is further suppressed by

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16000

TABLE I. Observed dilepton yield in data and the expected SUSY and ft signals and background contributions. Systematic uncertainties due to theoretical cross sections and systematic uncertainties evaluated for data-driven backgrounds are included in the uncertainties.

14000

12000 5 10000 I

Process

(D

Yield

w

ft Z / f + jets tV (single top) ftV WW, WZ, ZZ Fake leptons

54000+™° 2800 ± 300 2600 ± 180 80 ± 11 180 ± 65 780 ± 7 8 0

Total non-f? Expected Observed

6400 ± 860 60000+™° 60424

Mi (m}1 = 180 GeV, my, = 1 GeV) X I

7100 ± 1100

requiring that the scalar sum of the p T of all selected jets and leptons is greater than 130 GeV. The expected numbers o f ft signal and background events are com pared to data in Table I. The expected yield for top squark pair production with a top squark mass of 180 GeV and a neutralino mass o f 1 GeV is also shown. Figure 1 shows the reconstructed A

distribution from the SM ft M C sim ulation with coefficient / SM, and from the ft sim ulation without spin correlation with coefficient (1 - / sM). The e +e~, p +p~ and e±/ 1 indicate a degree o f ft spin correlation larger than predicted by the SM. Systematic uncertainties are evaluated by applying the fit procedure to pseudoexperim ents created from simulated samples modified to reflect the systematic variations. The fit o f / sm is repeated to determ ine the effect o f each systematic uncertainty using the nom inal templates. The difference betw een the means of Gaussian fits to the results from many pseudoexperim ents using nom inal and modified pseudodata is taken as the systematic uncertainty on / SM [102]. The various systematic uncertainties are estimated in the same way as in Ref. [42] with the following exceptions:

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8000 6000 4000 2000

0 1.2

o to

cr

1.1

1 0.9 0.8

0

0.2

0.4 0.6 A(j) [rad] / Jt

0.8

1

FIG. I (color online). Reconstructed Atp distribution for the sum of the three dilepton channels. The prediction for back­ ground (blue histogram) plus SM ft production (solid black histogram) and background plus ft prediction with no spin correlation (dashed black histogram) is compared to the data and to the result of the fit to the data (red dashed histogram) with the orange band representing the total systematic uncertainty on / SM. Both the SM ft and the no spin correlation ft predictions are normalized to the NNLO cross section including next-to-next-toleading-logarithm corrections [71,72] (the theory uncertainty of 7% on this cross section is not displayed). The prediction for t j l production (m-, = 180 GeV and mfo = 1 GeV) normalized to the NLO cross section including next-to-leading-logarithm cor­ rections [101] plus SM ft production plus background is also shown (solid green histogram). The lower plot shows those distributions (except for background only) divided by the SM ft plus background prediction. since this analysis employs b tagging, the associated uncertainty is estimated by varying the relative normaliza­ tions of simulated b-jet, c-jet, and light-jet samples. The uncertainty due the choice of generator is determined by comparing the default to an alternative ft sample generated with the P o w h e g -B ox generator interfaced with P y t h ia . The uncertainty due to the parton shower and hadronization model is determined by comparing two ft samples generated by A l p g e n , one interfaced with P y t h ia and the other one interfaced with H e r w ig . The uncertainty on the amount of initial- and final-state radiation (ISR and FSR) in the simulated ft sample is assessed by comparing A l p g e n events, showered with P y t h ia , with varied amounts of ISR and FSR. As in Ref. [42], the size of the variation is compatible with the recent measurements of additional jet activity in ft events [103]. The W t normalization is varied within the theoretical uncertainties of the cross-section calculation [86], and the sensitivity to the interference between W t production and ft production at NLO is studied by comparing the predictions o f P o w h e g -B o x with the

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diagram-removal (baseline) and diagram-subtraction schemes [85,104]. As in Ref. [42], the uncertainty due to the top quark mass is evaluated but not included in the systematic unceitainties, since it would have no significant impact on the results. The sizes of the systematic uncertainties in terms of A / sm are listed in Table II. The total systematic uncertainty is calculated by combining all systematic uncertainties in quadrature. The measured value of / SM for the combined fit is 1.20 ± 0.05(stat) ± 0.13(syst). This agrees with previous results from ATLAS using data at a center-of-mass energy of 7 TeV [41,42], and compares to the best previous measurement using Acp of / SM = 1.19 ± 0.09(stat) ± 0.18(syst) [42], It also agrees with the SM prediction to within 2 standard deviations. This agrees with previous results from ATLAS using data at a center-of-mass energy of 7 TeV [41,42] and agrees with the SM prediction to within 2 standard deviations. An indirect extraction of 4 he|icity can be achieved by assuming that the ft sample is composed of top quark pairs as predicted by the SM, but with varying spin correlation. In that case, a change in the fraction / SM leads to a linear change of Ahelicity (see also Ref. [42]), and a value of the spin correlation strength in the helicity basis Ahelicity at a center-of-mass energy of 8 TeV is obtained by applying the measured value of / SM as a multiplicative factor to the SM prediction of Aj^ficity = 0.318 ± 0.005 [36]. This yields a measured value of Ahelicity = 0.38 ± 0.04.

TABLE II. Summary of systematic uncertainties on / SM in the combined dilepton final state. Source of uncertainty

A / sm

Detector modeling Lepton reconstruction Jet energy scale Jet reconstruction crniss tA T

Fake leptons b tagging

±0.01 ±0.02 ±0.01 aa A. Z am an,149 S. Zam bito,23 L. Zanello,

133a,133b

176

128

38a

23

130

D. Zanzi,88 C. Z eitnitz,1'0 M. Zem an,‘z8 A. Zem la,38a K. Zengel,z3 O. Z enin,13U T. Zenis, 57 ■

. 89

174

90

152 ■

145a

. 33b -

D. Z erw as,117 G. Zevi della Porta,3' D. Z hang,84 F. Z hang,1'4 H. Zhang,vu J. Zhang,0 L. Z hang,13Z R. Zhang,330 X. Zhang, 40 ,

33d ,

33b

65

33d

120 i

40

Z. Z hang,117 X. Zhao,40 Y. Zhao,330 Z. Zhao,330 A. Zhem chugov,03 J. Z hong,lzu B. Z hou,'" C. Zhou,43 L. Zhou,33 L. Zhou,‘ N. Z h o u ,164 C .G . Zhu,33d H. Zhu,33a J. Z hu,89 Y. Zhu,33b X. Zhuang,33a K. Zhukov,96 A. Zibell.175 D. Ziem inska,61 83

21

21

48

N. I. Zim ine,65 C. Zim m erm ann,83 R. Zim m erm ann,41 S. Zim m erm ann,zl S. Zim m erm ann,48 Z. Zinonos,

54

M. Z iolkow ski,14" G. Z obem ig,174 A. Zoccoli,20a'20b M. zur N edden,16 G. Z urzolo,104a,104b and L. Zw alinski30 (ATLAS Collaboration) 1Department o f Physics, University o f Adelaide, Adelaide, Australia "Physics Department, SUNY Albany, Albany, New York, USA 3Department o f Physics, University o f Alberta, Edmonton, Alberta, Canada ^Department o f Physics, Ankara University, Ankara, Turkey 4bDepartment o f Physics, Gazi University, Ankara, Turkey 4lIstanbul Aydin University, Istanbul, Turkey 4dDivision o f Physics, TOBB University o f Economics and Technology, Ankara, Turkey 5LAPP, CNRS/IN2P3 and Universite de Savoie, Annecy-le-Vieux, France bHigh Energy Physics Division, Argonne National Laboratory, Argonne, Illinois, USA 1Department o f Physics, University o f Arizona, Tucson, Arizona, USA 8Department o f Physics, The University o f Texas at Arlington, Arlington, Texas, USA 4Physics Department, University o f Athens, Athens, Greece 10Physics Department, National Technical University o f Athens, Zografou, Greece "institute o f Physics, Azerbaijan Academy o f Sciences, Baku, Azerbaijan 12Institut de Fisica d ’Altes Energies and Departament de Flsica de la Universitat Autdnoma de Barcelona, Barcelona, Spain ' 11Institute o f Physics, University of Belgrade, Belgrade, Serbia l3bVinca Institute o f Nuclear Sciences, University o f Belgrade, Belgrade, Serbia 14Department fo r Physics and Technology, University o f Bergen, Bergen, Norway 15Physics Division, Lawrence Berkeley National Laboratory and University o f California, Berkeley, California, USA 16Department o f Physics, Humboldt University, Berlin, Germany 17Albert Einstein Center fo r Fundamental Physics and Laboratory fo r High Energy Physics, University o f Bern, Bern, Switzerland 18School o f Physics and Astronomy, University o f Birmingham, Birmingham, United Kingdom l9jDepartment o f Physics, Bogazici University, Istanbul, Turkey '^Department o f Physics, Dogus University, Istanbul, Turkey Department o f Physics Engineering, Gaziantep University, Gaziantep, Turkey 20dINFN Sezione di Bologna, Italy "obDipartimento di Fisica e Astronomia, Universita di Bologna, Bologna, Italy 21Pltysikalisches Institut, University o f Bonn, Bonn, Germany 12Department o f Physics, Boston University, Boston, Massachusetts, USA 23Department o f Physics, Brandeis University, Waltham, Massachusetts, USA ~AaUniversidade Federal do Rio De Janeiro COPPE/EE/IF, Rio de Janeiro, Brazil -4bElectrical Circuits Department, Federal University o f Juiz de Fora (UFJF), Juiz de Fora, Brazil "4tFederal University o f Sao Joao del Rei (UFSJ), Sao Joao del Rei, Brazil ~ aInstituto de Fisica, Universidade de Sao Paulo, Sao Paulo, Brazil " Physics Department, Brookhaven National Laboratory, Upton, New York, USA 0dNational Institute o f Physics and Nuclear Engineering, Bucharest, Romania

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~6bNational Institute fo r Research and Development o f Isotopic and Molecular Technologies, Physics Department, Cluj Napoca, Romania ' LUniversity Politehnica Bucharest, Bucharest, Romania West University in Timisoara, Timisoara, Romania 27 Departamento de FIsica, Universidad de Buenos Aires, Buenos Aires, Argentina Cavendish Laboratory, University o f Cambridge, Cambridge, United Kingdom ‘ Department o f Physics, Carleton University, Ottawa, Ontario, Canada 30CERN, Geneva, Switzerland 31Enrico Fermi Institute, University o f Chicago, Chicago, Illinois, USA ''''Departamento de FIsica, Pontificia Universidad Catolica de Chile, Santiago, Chile 3_bDepartamento de FIsica, Universidad Tecnica Federico Santa Marla, Valparaiso, Chile aInstitute o f High Energy Physics, Chinese Academy o f Sciences, Beijing, China 33bDepartment o f Modern Physics, University o f Science and Technology o f China, Anhui, China 33cDepartment o f Physics, Nanjing University, Jiangsu, China 33dSchool o f Physics, Shandong University, Shandong, China 33ePhysics Department, Shanghai Jiao Tong University, Shanghai, China 33fPhysics Department, Tsinghua University, Beijing 100084, China 34Laboratoire de Physique Corpusculaire, Clermont Universite and Universite Blaise Pascal and CNRS/IN2P3, Clermont-Ferrand, France 35Nevis Laboratory, Columbia University, Irvington, New York, USA 36Niels Bohr Institute, University o f Copenhagen, Kobenhavn, Denmark 31''INFN Gruppo Collegato di Cosenza, Laboratori Nazionali di Frascati, Italy :'lbDipartimento di Fisica, Universitd della Calabria, Rende, Italy 38jAGH University o f Science and Technology, Faculty o f Physics and Applied Computer Science, Krakow, Poland m Marian Smoluchowski Institute o f Physics, Jagiellonian University, Krakow, Poland 39The Henryk Niewodniczanski Institute o f Nuclear Physics, Polish Academy o f Sciences, Krakow, Poland 40Physics Department, Southern Methodist University, Dallas, Texas, USA 41Physics Department, University o f Texas at Dallas, Richardson, Texas, USA 4~DESY, Hamburg and Zeuthen, Gennany 4iInstitut fur Experimentelle Physik TV, Technische Universitat Dortmund, Dortmund, Germany 44Institut fu r Kern- und Teilchenphysik, Technische Universitat Dresden, Dresden, Germany 45Department o f Physics, Duke University, Durham, North Carolina, USA 46SUPA-School o f Physics and Astronomy, University o f Edinburgh, Edinburgh, United Kingdom INFN Laboratori Nazionali di Frascati, Frascati, Italy 48 J Fakultat fu r Mathematik und Physik, Albert-Ludwigs-Universitdt, Freiburg, Germany 41Section de Physique, Universite de Geneve, Geneva, Switzerland 503INFN Sezione di Genova, Italy 5l}bDipartimento di Fisica, Universitd di Genova, Genova, Italy 5laE. Andronikashvili Institute o f Physics, Iv. Javakhishvili Tbilisi State University, Tbilisi, Georgia >lbHigh Energy Physics Institute, Tbilisi State University, Tbilisi, Georgia 52II Physikalisches Institut, Justus-Liebig-Universitdt Giessen, Giessen, Germany 53SUPA-School o f Physics and Astronomy, University o f Glasgow, Glasgow, United Kingdom 54II Physikalisches Institut, Georg-August-Universitat, Gottingen, Germany x Laboratoire de Physique Subatomique et de Cosmologie, Universite Grenoble-Alpes, CNRS/IN2P3, Grenoble, France 56Department o f Physics, Hampton University, Hampton, Virginia, USA 37Laboratory fo r Particle Physics and Cosmology, Harvard University, Cambridge, Massachusetts, USA 'Kirchhoff-Institut fur Physik, Ruprecht-Karls-Universitat Heidelberg, Heidelberg, Germany 58bPhysikalisches Institut, Ruprecht-Karls-Universitat Heidelberg, Heidelberg, Germany CZJTI Institut Jur technische Informatik, Ruprecht-Karls-Universitat Heidelberg, Mannheim, Germany Faculty o f Applied Information Science, Hiroshima Institute of Technology, Hiroshima, Japan JDepartment o f Physics, The Chinese University o f Hong Kong, Shatin, N.T., Hong Kong, China bobDepartment o f Physics, The University o f Hong Kong, Hong Kong, China ' LDepartment o f Physics, The Hong Kong University o f Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China 61Department o f Physics, Indiana University, Bloomington, Indiana, USA ■ Institut fu r Astro- und Teilchenphysik, Leopold-Franzens-Universitat, Innsbruck, Austria 63University o f Iowa, Iowa City, Iowa, USA 64Department o f Physics and Astronomy, Iowa State University, Ames, Iowa, USA 65Joint Institute fo r Nuclear Research, JINR Dubna, Dubna, Russia 11KEK, High Energy Accelerator Research Organization, Tsukuba, Japan 67Graduate School o f Science, Kobe University, Kobe, Japan 68Faculty o f Science, Kyoto University, Kyoto, Japan

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Kyoto University o f Education, Kyoto, Japan Department o f Physics, Kyushu University, Fukuoka, Japan 'instituto de Fisica La Plata, Universidad Nacional de La Plata and CONICET, La Plata, Argentina 12Physics Department, Lancaster University, Lancaster, United Kingdom 73a INFN Sezione di Lecce, Italy 73b Dipartimento di Matematica e Fisica, Universita del Salento, Lecce, Itcdy 74 Oliver Lodge Laboratory, University o f Liverpool, Liverpool, United Kingdom 75 Department o f Physics, Jozef Stefan Institute and University o f Ljubljana, Ljubljana, Slovenia 76 School o f Physics and Astronomy, Queen Mary University o f London, London, United Kingdom 77 Department o f Physics, Royal Holloway University o f London, Surrey, United Kingdom 78 Department o f Physics and Astronomy, University College London, London, United Kingdom 79Louisiana Tech University, Ruston, Louisiana, USA °Laboratoire de Physique Nucleaire et de Hautes Energies, UPMC and Universite Paris-Diderot and CNRS/IN2P3, Paris, France 81Fysiska institutionen, Lunds universitet, Lund, Sweden *~Departamento de Fisica Teorica C-15, Universidad Autonoma de Madrid, Madrid, Spain 83 Institut fu r Physik, Universitdt Mainz, Mainz, Germany 4School o f Physics and Astronomy, University o f Manchester, Manchester, United Kingdom 85 CPPM, Aix-Marseille Universite and CNRS/IN2P3, Marseille, France 86 'Department o f Physics, University o f Massachusetts, Amherst, Massachusetts, USA 1Department o f Physics, McGill University, Montreal, Quebec, Canada 88School o f Physics, University o f Melbourne, Victoria, Australia 89 Department o f Physics, The University o f Michigan, Ann Arbor, Michigan, USA 90 Department o f Physics and Astronomy, Michigan State University, East Lansing, Michigan, USA 91a ‘INFN Sezione di Milano, Italy BDipartimento di Fisica, Universita di Milano, Milano, Italy 92 B.I. Stepanov Institute o f Physics, National Academy o f Sciences o f Belarus, Minsk, Republic o f Belarus 93 'National Scientific and Educational Centre fo r Particle and High Energy Physics, Minsk, Republic o f Belarus 94Department o f Physics, Massachusetts Institute o f Technology, Cambridge, Massachusetts, USA 95Group o f Particle Physics, University o f Montreal, Montreal, Quebec, Canada 96 P.N. Lebedev Institute o f Physics, Academy o f Sciences, Moscow, Russia 97 Institute fo r Theoretical and Experimental Physics (ITEP), Moscow, Russia 98 National Research Nuclear University MEPhI, Moscow, Russia D.V.Skobeltsyn Institute o f Nuclear Physics, M.V.Lomonosov Moscow State University, Moscow, Russia 'Fakultat fu r Physik, Ludwig-Maximilians-Universitdt Miinchen, Miinchen, Germany 101 Max-Planck-Institut fu r Physik, Werner-Heisenberg-Institut, Miinchen, Gennany 102 "Nagasaki Institute o f Applied Science, Nagasaki, Japan 1Graduate School o f Science and Kobayashi-Maskawa Institute, Nagoya University, Nagoya, Japan 104a INFN Sezione di Napoli, Italy °Dipartimento di Fisica, Universita di Napoli, Napoli, Italy 105 'Department o f Physics and Astronomy, University o f New Mexico, Albuquerque, New Mexico, USA 'Institute for Mathematics, Astrophysics and Particle Physics, Radboud University Nijmegen/Nikhef, Nijmegen, Netherlands 107, Nikhef National Institute fo r Subatomic Physics and University o f Amsterdam, Amsterdam, Netherlands 108Department o f Physics, Northern Illinois University, DeKalb, Illinois, USA 109 Budker Institute o f Nuclear Physics, SB RAS, Novosibirsk, Russia no Department o f Physics, New York University, New York, New York, USA Ohio State University, Columbus, Ohio, USA 112 Faculty o f Science, Okayama University, Okayama, Japan Homer L. Dodge Department o f Physics and Astronomy, University o f Oklahoma, Norman, Oklahoma, USA 114 Department o f Physics, Oklahoma State University, Stillwater, Oklahoma, USA Palacky University, RCPTM, Olomouc, Czech Republic 116 Center for High Energy Physics, University o f Oregon, Eugene, Oregon, USA 117LAL, Universite Paris-Sud and CNRS/IN2P3, Orsay, France 118 Graduate School o f Science, Osaka University, Osaka, Japan 119 Department o f Physics, University o f Oslo, Oslo, Norway 120Department o f Physics, Oxford University, Oxford, United Kingdom ]~'dINFN Sezione di Pavia, Italy '~lbDipartimento di Fisica, Universita di Pavia, Pavia, Italy Department o f Physics, University o f Pennsylvania, Philadelphia, Pennsylvania, USA 123 Petersburg Nuclear Physics Institute, Gatchina, Russia i24dINFN Sezione di Pisa, Italy 70

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10 APRIL 2015

1Dipartimento di Fisica E. Fermi, Universita di Pisa, Pisa, Italy Department o f Physics and Astronomy, University o f Pittsburgh, Pittsburgh, Pennsylvania, USA 126a 'Laboratorio de Instrumentacao e Fisica Experimental de Particulas-LIP, Lisboa, Portugal 126b Faculdade de Ciencias, Universidade de Lisboa, Lisboa, Portugal 126c Department o f Physics, University o f Coimbra, Coimbra, Portugal Centro de Fisica Nuclear da Universidade de Lisboa, Lisboa, Portugal 1 Departamento de Fisica, Universidade do Minho, Braga, Portugal 126fr Departamento de Fisica Teorica y del Cosmos and CAFPE, Universidad de Granada, Granada (Spain) 126g Dep Fisica and CEFITEC o f Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, Caparica, Portugal 127Institute o f Physics, Academy o f Sciences o f the Czech Republic, Praha, Czech Republic ~ Czech Technical University in Prague, Praha, Czech Republic 129, 'Faculty o f Mathematics and Physics, Charles University in Prague, Praha, Czech Republic m State Research Center Institute fo r High Energy Physics, Protvino, Russia 131, Particle Physics Department, Rutherford Appleton Laboratory, Didcot, United Kingdom ~Ritsumeikan University, Kusatsu, Shiga, Japan maINFN Sezione di Roma, Italy 133b , Dipartimento di Fisica, Sapienza Universita di Roma, Roma, Italy lMaINFN Sezione di Roma Tor Vergata, Italy Dipartimento di Fisica, Universita di Roma Tor Vergata, Roma, Italy 135aINFN Sezione di Roma Tre, Italy 135b r Dipartimento di Matematica e Fisica, Universita Roma Tre, Roma, Italy 136ar 'Faculte des Sciences Ain Chock, Reseau Universitaire de Physique des Hautes Energies-Universite Hassan II, Casablanca, Morocco 136b Centre National de I’Energie des Sciences Techniques Nucleaires, Rabat, Morocco 136c Faculte des Sciences Semlalia, Universite Cadi Ayyad, LPHEA-Marrakech, Morocco Faculte des Sciences, Universite Mohamed Premier and LPTPM, Oujda, Morocco l36eFaculte des sciences, Universite Mohammed V-Agdal, Rabat, Morocco 137 DSM/IRFU, Institut de Recherches sur les Lois Fondamentales de VUnivers, CEA Saclay (Commissariat a l ’Energie Atomique et aux Energies Alternatives), Gif-sur-Yvette, France Santa Cruz Institute fo r Particle Physics, University o f California Santa Cruz, Santa Cruz, California, USA '^Department o f Physics, University o f Washington, Seattle, Washington, USA 140, Department o f Physics and Astronomy, University o f Sheffield, Sheffield, United Kingdom Department o f Physics, Shinshu University, Nagano, Japan 1x~Fachbereich Physik, Universitat Siegen, Siegen, Germany 143, Department o f Physics, Simon Fraser University, Burnaby, British Columbia, Canada '^SZ-AC National Accelerator Laboratory, Stanford, California, USA Faculty o f Mathematics, Physics & Informatics, Comenius University, Bratislava, Slovak Republic 145b Department o f Subnuclear Physics, Institute o f Experimental Physics o f the Slovak Academy o f Sciences, Kosice, Slovak Republic Department o f Physics, University o f Cape Town, Cape Town, South Africa Department o f Physics, University o f Johannesburg, Johannesburg, South Africa '^"School o f Physics, University o f the Witwatersrand, Johannesburg, South Africa '^"Department o f Physics, Stockholm University, Sweden The Oskar Klein Centre, Stockholm, Sweden 148Physics Department, Royal Institute o f Technology, Stockholm, Sweden Departments of Physics & Astronomy and Chemistry, Stony Brook University, Stony Brook, New York, USA 4 Department o f Physics and Astronomy, University o f Sussex, Brighton, United Kingdom School o f Physics, University o f Sydney, Sydney, Australia 152Institute o f Physics, Academia Sinica, Taipei, Taiwan 153 Department o f Physics, Technion: Israel Institute o f Technology, Haifa, Israel Raymond and Beverly Sackler School o f Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel 155 Department o f Physics, Aristotle University o f Thessaloniki, Thessaloniki, Greece International Center fo r Elementary Particle Physics and Department o f Physics, The University o f Tokyo, Tokyo, Japan 157 Graduate School o f Science and Technology, Tokyo Metropolitan University, Tokyo, Japan 8Department o f Physics, Tokyo Institute o f Technology, Tokyo, Japan Department o f Physics, University o f Toronto, Toronto, Ontario, Canada 'b0liTRIUMF, Vancouver, British Columbia, Canada 160b 'Department o f Physics and Astronomy, York University, Toronto, Ontario, Canada 161 Faculty o f Pure and Applied Sciences, University o f Tsukuba, Tsukuba, Japan 162 Department o f Physics and Astronomy, Tufts University, Medford, Massachusetts, USA 3Centro de Investigaciones, Universidad Antonio Narino, Bogota, Colombia 125

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week ending 10 APRIL 2015

164Department o f Physics and Astronomy, University o f California Irvine, Irvine, California, USA 165*INFN Gruppo Collegato di Udine, Sezione di Trieste, Udine, Italy 'bSbICTP, Trieste, Italy 1^Dipartimento di Chimica, Fisica e Ambiente, Universita di Udine, Udine, Italy 16(1Department o f Physics, University o f Illinois, Urbana, Illinois, USA 167Department o f Physics and Astronomy, University o f Uppsala, Uppsala, Sweden m Instituto de Fisica Corpuscular (IFIC) and Departamento de Fisica Atomica, Molecular y Nuclear and Departamento de Ingenierla Electronica and Instituto de Microelectronica de Barcelona (IMB-CNM), University o f Valencia and CSIC, Valencia, Spain 169Department o f Physics, University o f British Columbia, Vancouver, British Columbia, Canada 170Department o f Physics and Astronomy, University o f Victoria, Victoria, British Columbia, Canada 171Department o f Physics, University o f Warwick, Coventry, United Kingdom xllWaseda University, Tokyo, Japan 173Department o f Particle Physics, The Weizmann Institute o f Science, Rehovot, Israel 174Department o f Physics, University o f Wisconsin, Madison, Wisconsin, USA ]lbFakultat ftir Physik und Astronomie, Julius-Maximilians-Universitdt, Wurzburg, Germany 'lbFachbereich C Physik, Bergische Universitat Wuppertal, Wuppertal, Germany 177Department o f Physics, Yale University, New Haven, Connecticut, USA 178 Yerevan Physics Institute, Yerevan, Armenia 179Centre de Calcul de Flnstitut National de Physique Nucleaire et de Physique des Particules (IN2P3), Villeurbanne, France ■“Deceased. bAlso at Department of Physics, King’s College London, London, United Kingdom. cAlso at Institute of Physics, Azerbaijan Academy of Sciences, Baku, Azerbaijan. dAlso at Novosibirsk State University, Novosibirsk, Russia. "Also at TRIUMF, Vancouver BC, Canada. 'Also at Department of Physics, California State University, Fresno CA, USA. gAlso at Department of Physics, University of Fribourg, Fribourg, Switzerland. hAlso at Tomsk State University, Tomsk, Russia. ‘Also at CPPM, Aix-Marseille Universite and CNRS/IN2P3, Marseille, France. JAlso at Universita di Napoli Parthenope, Napoli, Italy. kAlso at Institute of Particle Physics (IPP), Canada. 'Also at Particle Physics Department, Rutherford Appleton Laboratory, Didcot, United Kingdom. "’Also at Department of Physics, St. Petersburg State Polytechnical University, St. Petersburg, Russia. "Also at Louisiana Tech University, Ruston LA, USA. "Also at Institucio Catalana de Recerca i Estudis Avancats, ICREA, Barcelona, Spain. pAlso at Department of Physics, The University of Texas at Austin, Austin TX, USA. qAlso at Institute of Theoretical Physics, Ilia State University, Tbilisi, Georgia. “Also at CERN, Geneva, Switzerland. “Also at Ochadai Academic Production, Ochanomizu University, Tokyo, Japan. 'Also at Manhattan College, New York, New York, USA. “Also at Institute of Physics, Academia Sinica, Taipei, Taiwan. vAlso at LAL, Universite Paris-Sud and CNRS/IN2P3, Orsay, France. wAlso at Academia Sinica Grid Computing, Institute of Physics, Academia Sinica, Taipei, Taiwan. “Also at Laboratoire de Physique Nucleaire et de Hautes Energies, UPMC and Universite Paris-Diderot and CNRS/IN2P3, Paris, France. yAlso at School of Physical Sciences, National Institute of Science Education and Research, Bhubaneswar, India. zAlso at Dipartimento di Fisica, Sapienza Universita di Roma, Roma, Italy. aaAlso at Moscow Institute of Physics and Technology State University, Dolgoprudny, Russia. bbAlso at Section de Physique, Universite de Geneve, Geneva, Switzerland. “ Also at International School for Advanced Studies (SISSA), Trieste, Italy. ddAlso at Department of Physics and Astronomy, University of South Carolina, Columbia SC, USA. “ Also at School of Physics and Engineering, Sun Yat-sen University, Guangzhou, China. "Also at Faculty of Physics, M.V.Lomonosov Moscow State University, Moscow, Russia. ggAlso at National Research Nuclear University MEPhI, Moscow. Russia. hbAlso at Institute for Particle and Nuclear Physics, Wigner Research Centre for Physics, Budapest. Hungary. "Also at Department of Physics, Oxford University, Oxford, United Kingdom. 11Also at Institut fiir Experimentaiphysik, Universitat Hamburg, Hamburg, Germany. kkAlso at Department of Physics, The University of Michigan, Ann Arbor, Michigan, USA. "Also at Discipline of Physics, University of KwaZulu-Natal, Durban, South Africa. mmAlso at University of Malaya, Department of Physics, Kuala Lumpur, Malaysia.

142001-19

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Measurement of spin correlation in top-antitop quark events and search for top squark pair production in pp collisions at √s=8  TeV using the ATLAS detector.

A measurement of spin correlation in tt[over ¯] production is presented using data collected with the ATLAS detector at the Large Hadron Collider in p...
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