Observation of top quark pair production in ATLAS at sqrt{s}=7 TeV Collaboration ATLAS 2010 The observation of ttbar production in pp collisions at a centre-of-mass energy sqrt(s) = 7 TeV is presented. In a data sample of 2.9 pb−1 recorded by the ATLAS detector at the Large Hadron Collider, 37 candidate events are observed in the single lepton topology with electrons or muons, and 9 in the dilepton topology. The corresponding backgrounds from non-ttbar Standard Model processes are estimated largely from data control samples. When all channels are combined, the background-only hypothesis is excluded at a significance level of 4.9 standard deviations. Kinematic properties of the selected events are consistent with Standard Model ttbar production. The ttbar production cross-section is measured to be sigma(ttbar) = 146 +37-33 +49-30 pb, where the first uncertainty is statistical, the second is systematic. INTNOTEATLASPRIV

Observation of top quark pair production in the semileptonic decay channel at sqrt(s) = 7 TeV with the ATLAS detector Working group Top 2010 The first cross section measurements of ttbar production in pp collisions at a centre-of-mass energy sqrt(s) = 7 TeV are presented. In a data sample of 2.9 pb−1 recorded by the ATLAS detector at the Large Hadron Collider, 37 candidate events are observed in the semileptonic topology with electrons or muons. The corresponding backgrounds from non-ttbar Standard Model processes are estimated largely from data control samples, including the most important one: W+jets. Three different analyses have been developed to measure the ttbar production cross section: a simple counting method and two dfferent fitting techniques. The background-only hypothesis is excluded at a significance level of 4.2 standard deviations. Kinematic properties of the selected events are consistent with Standard Model ttbar production. INTNOTEATLASPRIV

Analysis of ttbar production in the dilepton decay channel at 7 TeV with 2.9 pb−1 Working group Top 2010 We present a first measurement of ttbar production with the ATLAS detector at the LHC in pp collisions at sqrt(s) = 7 TeV in final states with two oppositely-charged leptons (ee, mm and em), significant missing transverse energy (ETmiss) and two or more jets. In a dataset with integrated luminosity of 2.9 pb−1 we observe nine events in the dilepton channel. Using data-driven background techniques, we find good agreement in nearby control regions. The significance of the measurement, excluding the ttbar production, is estimated to be equivalent to 2.8 standard deviations. The kinematic properties of the excess events are consistent with the ttbar hypothesis. Interpreting this excess as ttbar production, we measure a cross section of sigma(ttbar) = 151 +85−68(stat) +39−26(syst) pb. INTNOTEATLASPRIV

Lepton Trigger and Identification for the first {\it Top quark} observation Benekos N University of Illinois Urbana-Champaign Benslama K University of Regina Bosman M IFAE Cerrito L Queen Mary University of London Derue F LPNHE Ferrara V Humboldt-Universitaet zu Berlin Ferrari P Nikhef Fiorini L IFAE Heelan L University of Carleton Heim S Michigan State University Helsens C IFAE Henrichs A Georg-August-Universitaet Goettingen Husegawa S Nagoya University Ju X University of Regina Juste A IFAE Kaplan B Yale Krasznahorkay A New York University Kumar Jana D University of Oklahoma Lessard J University of Victoria Loginov A Yale Ming Y University of Regina Mir L IFAE Nadal J IFAE Okumura Y Nagoya University Papadelis A Stockholm University Poll J Queen Mary University of London Ryan P Michigan State University Roe A Georg-August-Universitaet Goettingen Salamanna G Nikhef Schwienhorst R Michigan State University Shabalina E Georg-August-Universitaet Goettingen Sherman D Yale Sidoti A Humboldt-Universitaet zu Berlin Theveneaux-Pelzer T LPNHE Tipton P Yale Vorwerk V IFAE Zhu H DESY 2010 In this note we present the selections applied to identify leptons (electrons and muons) for the first Top WG \ttbar~cross section measurements in the single and di-lepton channels. We measure the associated efficiencies at the trigger and offline level using the {\it Tag and probe} method, on the same dataset using for the cross section measurement, corresponding to an integrated luminosity of $\mathscr{L} = 2.89~\ipb$. We describe the procedure to rescale the MC-based acceptance used for the cross section measurements to represent data accurately; and we provide the value and uncertainties on the Scale Factors at all selection stages. The scale and the resolution difference in the determination of the lepton momentum, with respect to MC expectations, is also provided. INTNOTEATLASPRIV

Jet selection for top physics Arguin J F Lawrence Berkeley National Lab. Boumediene D LPC, Clermont Ferrand Bondioli M University of California, Irvine Cortiana G Max-Planck-Institut fuer Physik, Muenchen Costa Mezquita M Instituto de Fisica Corpuscular (IFIC), Centro Mixto UVEG-CSIC Cowden C University of Cambridge DeViveiros P-O University of Toronto Doglioni C University of Oxford Erdmann J University of Goettingen Ferrando J University of Oxford Gaponenko A Lawrence Berkeley National Lab. Ghodbane N LPC, Clermont Ferrand Issever C University of Oxford Liao H LPC, Clermont Ferrand Kovesarki P University of Bonn Kroeninger K University of Goettingen Lemmer B University of Goettingen Lessard J-R University of Victoria Losada M Antonio Narino University, Bogota Marti I Garcia S Instituto de Fisica Corpuscular (IFIC), Centro Mixto UVEG-CSIC Loureiro Mattioli K Antonio Narino University, Bogota Moreno Llacer M Instituto de Fisica Corpuscular (IFIC), Centro Mixto UVEG-CSIC Nisius R Max-Planck-Institut fuer Physik, Muenchen Okawa H University of California, Irvine Petteni M Simon Fraser University Piegaia R University of Buenos Aires Psoroulas S University of Bonn Romeo G University of Buenos Aires Ruehr F University of Arizona Spanò F Columbia University Urbaniec D Columbia University Lopez Mateos D Columbia University, California Institute of Technology Adomeit S Ludwig-Maximilians-University of Munich Biebel O Ludwig-Maximilians-University of Munich Pleier M-A Brookhaven National Lab. Kroseberg J University of Bonn Hellmich D University of Bonn Yang U University of Manchester Martyniuk A University of Manchester Pallin D LPC, Clermont Ferrand 2010 This note summarizes the current status of the definition of jets for the top-rediscovery effort. Studies on the data quality, jet cleaning and calibration are shown before more analysis-related topics are discussed. These include studies on the jet energy scale, resolution and efficiency. The note ends with a preliminary proposal for the jet definition which is done in accordance with the Jet/ETmiss-group. INTNOTEATLASPRIV

Missing Transverse Energy for Top Physics analyses with early ATLAS data at $sqrt{s}$=7~TeV Canepa A TRIUMF Cinca D University Blaise Pascal Clermont-Ferrand II Farrell S University of California Irvine Garitaonandia Elejabarrieta H Nikef Grundler U Iowa State University Johns K University of Arizona Kaushik V University of Arizona Khandanya H University of Illinois Urbana Champaign Lei X University of Arizona Lessard JR University of Victoria Liss T University of Illinois Urbana Champaign Nugent I TRIUMF Okawa H University of California Irvine Pallin D University Blaise Pascal Clermont-Ferrand II Taffard A University of California Irvine Toggerson B University of California Irvine Watson M University of Birmingham 2010 This note describes the object based missing transverse energy reconstruction and performance used in Top Physics analyses performed with 2.89ipb{} of data recorded by the ATLAS detector at ts{}=7~TeV. INTNOTEATLASPRIV

B-tagging for top physics analyses with early ATLAS data at sqrt(s) = 7 TeV Alvarez B Bachacou H Boudreau J Bousson N Cerrito L Clement B Cooper B Delemontex T Donini J Feligioni L Filthaut F Garberson F Golling T Guindon S Guo B Helsens C Hirsch F Holzbauer J Juste A Kendziorra C Khanov A Kroninger K Liu C Lapoire C Lucotte A zur Nedden M Oakham G Pinamonti M Pope B Rizatdinova F Saleem M Schwienhorst R Schwindling J Shabalina E Sinervo P Skubic P Suruliz K Talby M Tannoury N Vacavant L Walsh B Wang J Yu J 2010 Algorithms distinguishing jets originating from $b$ quarks from other jet flavors are important tools in top physics to improve purity and combinatorical background. This document describes the algorithms to identify $b$-quark jets in ATLAS and the calibration techniques based on LHC collider data. The performance of these algorithms is discussed for tbar{} cross section measurements at a centre of mass energy of 7~TeV{} for an integrated luminosity of about 3~ipb{}. INTNOTEATLASPRIV

Mis-identified lepton backgrounds to top quark pair production Abi B Adelman J Almond J Arguin J F Besana I Bordoni S Bosman M Brau J Cortes-Gonzalez A Doxiadis A Feligioni L Fiorini L Gellerstedt K Golling T Helsens C Hickman M Hirschbuehl D Husemann U Juste A Khanov A Khoriauli G Lange C Lari T Liss T Meloni F Mir L Nadal J Owen M Radics B Rajagopalan S Ridel M Rizatdinova F Saleem M Searcy M Shabalina E Sjolin J Sturm P Talby M Tannoury N Tardif D Tipton P Trincaz-Duvoid S Troncon C van~Vulpen I Vorwerk V Wall R Wagner W Whiteson D Yang U K Zhou N 2010 The note presents an analysis of the mis-identified lepton (``fake'') backgrounds to top quark pair production in the single-lepton and dilepton channels. Several data driven methods are used in each channel to estimate these backgrounds and assess the corresponding systematic uncertainties. This draft describes the various methods and their performance in simulated events; we present an estimate of the fake lepton contribution to the top-mix sample, as well as the estimates of the fake background in the 2.9 pb-1 dataset. INTNOTEATLASPRIV

Estimation of the W+Jets Background for Top Quark Re-Discovery in the Single Lepton+Jets Channel Acharya B Arguin J-F Barisonzi M Besana IM Bosman M Brock IC Calkins R Charkaborty D Cobal M Cristinziani M Ferrando J Fiorini L Henrichs A Heinemann B Helsens C Khoriauli G Kuhl T Juste A Lari T Meloni F Mir L Nadal J Nuncio-Quiroz E Pinamonti M Radics B de Sanctis U Shabalina E Shan L-Y Suruliz K Ta DB Troncon C Vorwerk V 2010 W boson production in association with jets is predicted to be the dominant background to $t ar{t}$ events in the single lepton + jets channel. This note discusses the W+jets background to top pair production for two different luminosity scenarios. Firstly we discuss the issues relevant for the top observation for $int mathcal{L} dt$ = 2.9 pb$^{-1}$. We first describe a data-driven estimate of the W+jets background before b-tagging using the Berends-Giele scaling. The final estimate of the W+jets background after b-tagging is extracted by combining this pretag W+jets estimate with the W+jets tagged fraction obtained in the top group b-tagging note. The predicted number of W+$ge$4jet after b-tagging is 1.7$pm$1.1 and 2.9$pm$1.6 in the electron and muon channels, respectively. This section also contains detailed studies of the systematic uncertainties associated with the W+jets shape prediction, which is relevant to top cross-section analyses using a kinematic fit. Secondly we discuss the prospects for future data-driven methods which are applicable for datasets of 10 pb$^{-1}$ or more. In particular we discuss how to use the W charge asymmetry and W-to-Z ratios to extract the W+jets background in top analyses. INTNOTEATLASPRIV

Study of Z/g+jets background for ttbar to 2 leptons analysis Cranmer K Cristinziani M Khandanyan H Lewis G Liss T Papadelis A Poghosyan T Sherman D Sjoelin J Werth M Whiteson D 2010 This note is a support note dedicated to Z/g+jets production as a background to the ttbar cross-section measurement in the dilepton channel. The output from the note is a baseline set of cuts for Z/g+jets rejection and an estimated yield, with uncertainty, of Z/g+jets events in the ttbar signal region. The estimate has been improved with respect to pure Monte Carlo using data driven techniques, and we present results for 2.9/pb of lepton-triggered ATLAS data. INTNOTEATLASPRIV

Technical aspects of the first top pair analyses Adelman J Barisonzi M Bell W H Brooijmans G Cortiana G Farrel S Federic P Ferrari A Finelli K Gaponenko A Husemann U Kumar Jana D Lessard J-R Lleres A Lucotte A Okawa H Pleier M-A Radics B Searcy J Taffard A Urbaniec D 2010 This note is dedicated to the technical aspects of the first ATLAS top pair analyses. It describes derived physics data samples, the software used to produce and analyze the samples, steps taken to ensure the correctness of the software and reproducibility of the results. The overall data quality requirements and closely related calculation of the integrated luminosity are also presented here. INTNOTEATLASPRIV

Monte Carlo samples used for top physics Allwood-Spires S Barisonzi M Beauchemin H Bruneliere R Buchanan J Castro N F Devetak E Feng C Ferrando J Hawkings R Hirschbuehl D Husemann U Kersevan B P Khoriauli G Kuhl T Lenz T Mijovc L Papadelis A Sanders M P Shan L-Y Sturm P Veloso F Verkerke W Vlasov N Wasicki C 2010 This note documents the standard Monte Carlo samples to be used for top rediscovery analyses with 7 TeV data. The dataset numbers, cross sections and statistics are detailed. In addition the justification of k-factors for the different signal and background processes is discussed. A new tool for matching heavy flavour production events in Alpgen samples is described. INTNOTEATLASPRIV