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Measurements of top quark pair relative differential cross-sections with ATLAS in pp collisions at root s=7 TeV
Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). (Elementarpartikelfysik)
Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
Stockholm University, Faculty of Science, Department of Physics. (Systemfysik)
Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). (Elementarpartikelfysik)
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2013 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 73, no 1, 2261- p.Article in journal (Refereed) Published
Abstract [en]

Measurements are presented of differential cross-sections for top quark pair production in pp collisions at root s = 7 TeV relative to the total inclusive top quark pair production cross-section. A data sample of 2.05 fb(-1) recorded by the ATLAS detector at the Large Hadron Collider is used. Relative differential cross-sections are derived as a function of the invariant mass, the transverse momentum and the rapidity of the top quark pair system. Events are selected in the lepton (electron or muon) + jets channel. The background-subtracted differential distributions are corrected for detector effects, normalized to the total inclusive top quark pair production cross-section and compared to theoretical predictions. The measurement uncertainties range typically between 10 % and 20 % and are generally dominated by systematic effects. No significant deviations from the Standard Model expectations are observed.

Place, publisher, year, edition, pages
2013. Vol. 73, no 1, 2261- p.
National Category
Physical Sciences
URN: urn:nbn:se:su:diva-91046DOI: 10.1140/epjc/s10052-012-2261-1ISI: 000318288100002OAI: diva2:630474
Swedish Research CouncilKnut and Alice Wallenberg Foundation


Available from: 2013-06-19 Created: 2013-06-18 Last updated: 2013-12-12Bibliographically approved
In thesis
1. Top Quark Pair Production in ATLAS
Open this publication in new window or tab >>Top Quark Pair Production in ATLAS
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Large Hadron Collider (LHC) at the international particle physics laboratory CERN in Switzerland is currently the most powerful particle accelerator on earth. This thesis presents analyses of proton-proton collisions at the energy √s = 7 TeV, recorded by ATLAS, one of the detectors at the LHC. The goal of the LHC and its detectors is to find new phenomena not described by the Standard Model (SM) of particle physics.The top quark is the heaviest known elementary particle and it is produced in very large numbers at the LHC. Measuring the production cross-section of top pairs (ttbar) is important for many reasons: for validating the strong production mechanism of the SM, for commissioning and calibration of the detector and analysis software and because several scenarios for physics beyond the SM predict changes to the ttbar production cross-section.Five different measurements of the ttbar cross-section will be presented in this thesis. The first three are measurements of the total cross-section, the fourth is a simultaneous measurement of the ttbar, Zττ and WW cross-sections and the fifth is a measurement of the relative differential ttbar cross-section. The most accurate measurement of the total cross-section is 176 pb with a total uncertainty of 9%, and the relative differential cross-section for ttbar-masses above ~ 1 TeV is 0.007 1/TeV with an uncertainty of 43%. Both values agree with the SM predictions.Measurements or searches in particle physics often have to be conducted in the presence of uninteresting background processes.  Reducing and providing estimates of these backgrounds is one of the main analysis tasks. Many backgrounds can be simulated with sufficiently good accuracy. However, the background due to mis-identified leptons cannot be accurately simulated. This thesis presents and evaluates a method for estimating this background from data, and this is then used in the total ttbar cross-section measurements.

Abstract [sv]

LHC (Large Hadron Collider) vid det internationella partikelfysiklaboratoriet CERN i Schweiz är för närvarande världens mest kraftfulla partikleaccelerator. I den här avhandlingen presenteras anlyser av proton-protonkollisioner vid $\sqrt{s}=7\TeV{}$, registrerade med ATLAS som är en av detektorerna vid LHC. Målet med LHC och dess detektorer är att upptäcka nya fenomen som inte kan beskrivas av partikelfysikens nuvarande standardmodell (SM).

Toppkvarken är den tyngsta elementarpartikeln man känner till och den produceras i stort antal vid LHC. Att mäta produktionstvärsnittet för par av topkvarkar (\ttbar{}) är viktigt av flera skäl: för att validera den starka produktionsmekanismen i SM, och för att testa och kalibrera detektorn och analysverktygen. Dessutom förutsäger flera teoretiska utvidgningar av SM ändringar av tvärsnittet för \ttbar{}.

Fem olika mätningar av tvärsnittet för \ttbar{} presenteras i den här avhandlingen. De första tre är mätningar av det totala tvärsnittet, den fjärde är en simultan mätning av tvärsnitten för produktion av \ttbar{}, $WW$ och \Ztt{} och den femte är en mätning av det \emph{relativa differentiella} tvärsnittet. Det mest noggranna mätningen av det totala tvärsnittet är $176 $ pb med $9 \%$ total osäkerhet, och det relativa differentiella tvärsnittet för \ttbar{}-massor över $\sim 1 \TeV{}$ är $0.007 \TeV^{-1}$ med $43\%$ osäkerhet. Båda mätresultaten stämmer med respektive förutsägelse från standardmodellen.

Mätningar eller sökningar inom partikelfysiken måste ofta utföras i närvaro av ointressanta bakgrundsprocesser. Att reducera och skatta dessa bakgrunder är en av huvuduppgifterna inom en dataanalys. Många bakgrunder kan simuleras med tillräcklig precision. Men bakgrunden av felidentifierade leptoner kan däremot inte simuleras tillräckligt noggrant. Den här avhandlingen presenterar och utvärderar en metod för att skatta bakgrunden av felidentifierade leptoner från data, och resultatet används i mätningarna av det totala tvärsnittet för \ttbar{}.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2012. 124 p.
particle physics, top quark, LHC, ATLAS
National Category
Subatomic Physics
Research subject
urn:nbn:se:su:diva-79130 (URN)978-91-7447-572-2 (ISBN)
Public defence
2012-10-05, Svedenbergsalen (FD5), AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 5: Manuscript.

Available from: 2012-09-13 Created: 2012-08-28 Last updated: 2013-12-12Bibliographically approved

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Åsman, BarbroBendtz, KatarinaBohm, ChristianClément, ChristopheEriksson, DanielGellerstedt, KarlHellman, StenHolmgren, Sven-OlofJon-And, KerstinKim, HyeonKlimek, PawelLundberg, JohanLundberg, OlofMilstead, David A.Moa, TorbjörnOhm, Christian C.Papadelis, ArasSellden, BjörnSilverstein, Samuel B.Sjölin, JörgenStrandberg, SaraTylmad, MajaYang, Zhaoyu
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