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Top Quark Pair Production in ATLAS
Stockholm University, Faculty of Science, Department of Physics. (Elementary Particle Physics)
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.
Keyword [en]
particle physics, top quark, LHC, ATLAS
National Category
Subatomic Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-79130ISBN: 978-91-7447-572-2 (print)OAI: oai:DiVA.org:su-79130DiVA: diva2:548403
Public defence
2012-10-05, Svedenbergsalen (FD5), AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

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
List of papers
1. Measurement of the top quark-pair production cross section with ATLAS in pp collisions at $\sqrt{s}=7$ TeV
Open this publication in new window or tab >>Measurement of the top quark-pair production cross section with ATLAS in pp collisions at $\sqrt{s}=7$ TeV
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2011 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 71, no 3, 1577- p.Article in journal (Refereed) Published
Abstract [en]

A measurement of the production cross-section for top quark pairs (t (t) over bar) in pp collisions at root s = 7 TeV is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in two different topologies: single lepton (electron e or muon mu) with large missing transverse energy and at least four jets, and dilepton (ee, mu mu or e mu) with large missing transverse energy and at least two jets. In a data sample of 2.9 pb(-1), 37 candidate events are observed in the single-lepton topology and 9 events in the dilepton topology. The corresponding expected backgrounds from non-t (t) over bar Standard Model processes are estimated using data-driven methods and determined to be 12.2 +/- 3.9 events and 2.5 +/- 0.6 events, respectively. The kinematic properties of the selected events are consistent with SM t (t) over bar production. The inclusive top quark pair production cross-section is measured to be sigma(t (t) over bar) = 145 +/- 31 (stat.)(-27)(+42) (syst.) pb. The measurement agrees with perturbative QCD calculations.

National Category
Subatomic Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-62704 (URN)10.1140/epjc/s10052-011-1577-6 (DOI)000291698400004 ()
Projects
Atlas
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note
Publikationen har totalt 3083 författare, G. Aad et al.Available from: 2011-09-27 Created: 2011-09-27 Last updated: 2017-12-08Bibliographically approved
2. Measurement of the top quark pair production cross section with ATLAS in pp collisions at √s = 7 TeV in dilepton final states
Open this publication in new window or tab >>Measurement of the top quark pair production cross section with ATLAS in pp collisions at √s = 7 TeV in dilepton final states
2011 (English)Report (Other academic)
Abstract [en]

A measurement is presented of the production cross section of top quark pairs (σ_{\ttbar{}}) in proton-proton (pp) collisions at √s = 7 TeV recorded with the ATLAS detector at the CERN Large Hadron Collider. Using a data sample of 35 pb−1, candidate events are selected in the dilepton topology with large missing transverse energy, \met{}, and at least two jets.

A baseline analysis employing kinematic properties of the candidate events to separate the signal from background and using data-driven techniques to determine the most important backgrounds, results in a measurement ofσ_{\ttbar{}} = 173 ± 22(stat.)+18 −16(syst.)+8 −7(lum.) pb,

where the three uncertainties are from statistics, systematics and integrated luminosity, respectively.

We also perform a cross section measurement requiring at least one b-tagged jet and a looser kinematic selection that increases the signal-to-background ratio, yielding

σ_{\ttbar{}} = 171 ± 22(stat.)+21 -16(syst.)+7 −6(lum.) pb.

Additional studies are performed to corroborate these measurements; a technique that normalizes the \ttbar{} signal yield to the measured rate of Z decays, a two-dimensional template shape fit using the \met{} vs the number of jets to simultaneously measure the production cross sections of \ttbar{}, WW and Z → ττ final states, and a simultaneous measurement of σ_{\ttbar{}} and the b-tagging efficiency using the distribution of the number of tagged jets in each event.

All the measurements are in good agreement with each other and the Standard Model prediction.

Publisher
27 p.
National Category
Subatomic Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-79226 (URN)
Available from: 2012-08-30 Created: 2012-08-30 Last updated: 2012-08-31Bibliographically approved
3. Measurement of the Top Quark Pair Production Cross Section in pp Collisions at $\sqrt{s}=7$ TeV in Dilepton Final States with ATLAS
Open this publication in new window or tab >>Measurement of the Top Quark Pair Production Cross Section in pp Collisions at $\sqrt{s}=7$ TeV in Dilepton Final States with ATLAS
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2012 (English)In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 707, 459-477 p.Article in journal (Refereed) Published
Abstract [en]

A measurement of the production cross section of top quark pairs () in proton–proton collisions at a center-of-mass energy of 7 TeV recorded with the ATLAS detector at the Large Hadron Collider is reported. Candidate events are selected in the dilepton topology with large missing transverse energy and at least two jets. Using a data sample corresponding to an integrated luminosity of 35 pb−1, a production cross section is measured for an assumed top quark mass of mt=172.5 GeV. A second measurement requiring at least one jet identified as coming from a b quark yields a comparable result, demonstrating that the dilepton final states are consistent with being accompanied by b-quark jets. These measurements are in good agreement with Standard Model predictions.

National Category
Subatomic Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-74240 (URN)10.1016/j.physletb.2011.12.055 (DOI)000300211000004 ()
Projects
Atlas
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

Publikationen har totalt 3035 författare, G. Aad et al.

Available from: 2012-03-03 Created: 2012-03-03 Last updated: 2017-12-07Bibliographically approved
4. Measurement of the cross section for top-quark pair production in pp collisions at root s=7 TeV with the ATLAS detector using final states with two high-p(T) leptons
Open this publication in new window or tab >>Measurement of the cross section for top-quark pair production in pp collisions at root s=7 TeV with the ATLAS detector using final states with two high-p(T) leptons
Show others...
2012 (English)In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 5, 059- p.Article in journal (Refereed) Published
Abstract [en]

A measurement is reported of the production cross section of top-quark pairs (\ttbar{}) in proton-proton collisions at a center-of-mass energy of 7 TeV recorded with the ATLAS detector at the LHC. Candidate events have a signature consistent with containing two isolated leptons, large missing transverse momentum, and at least two jets. Using a data sample corresponding to an integrated luminosity of 0.70 fb−1, a \ttbar{} production cross section $\sigma{\ttbar{}}$ = 176 ± 5(stat.)+14 −11(syst.) ± 8(lum.) pb is measured for an assumed top-quark mass of mt = 172.5 GeV. This measurement is in good agreement with Standard Model predictions.

Keyword
Hadron-Hadron Scattering
National Category
Subatomic Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-79227 (URN)10.1007/JHEP05(2012)059 (DOI)000305236000059 ()
Available from: 2012-08-30 Created: 2012-08-30 Last updated: 2017-12-07Bibliographically approved
5. Measurements of top quark pair relative differential cross-sections with ATLAS in pp collisions at root s=7 TeV
Open this publication in new window or tab >>Measurements of top quark pair relative differential cross-sections with ATLAS in pp collisions at root s=7 TeV
Show others...
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.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-91046 (URN)10.1140/epjc/s10052-012-2261-1 (DOI)000318288100002 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

AuthorCount:2875;

Available from: 2013-06-19 Created: 2013-06-18 Last updated: 2017-12-06Bibliographically approved

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