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<strong>MEASUREMENT OF TOP QUARK POLARIZATIONS AND t ̄t SPIN CORRELATIONS USING DILEPTON FINAL STATES AT </strong>√<strong>s </strong>= <strong>13 TEV WITH THE LHC AND PROJECTIONS FOR THE HL-LHC</strong>Amandeep Singh Bakshi (16642605) 26 July 2023 (has links)
<p>The top quark is the most massive known elementary particle, and plays a pivotal role in our understanding of particle physics. Its unique properties offer valuable insights into the Standard Model and potential hints for physics beyond the Standard Model. In this thesis we present a precision measurement of the polarization of top quarks and spin correlations between top-antitop (ttbar) pairs using Run-II datasets collected from the Compact </p>
<p>Muon Solenoid detector at the Large Hadron Collider. In the first part of this thesis we introduce the theoretical framework of the Standard Model and its predictions regarding top quark spin polarization and ttbar spin correlations. Next, we describe the experimental setup, reconstruction techniques, and Monte Carlo simulations used in this research. Subsequently, details of the measurement, including event selection, top quark reconstruction, and unfolding are described. The analysis achieves exceptional signal purity and precision with respect to previous measurements. The measured values of coefficients are in agreement with Standard Model expected values as well as theoretical predictions at NLO in QCD. In a first, we provide provide double-differential cross sections of top quark spin polarizations and ttbar spin correlations, as a function of the invariant mass of the ttbar system. We expect the results from the final CMS publication to supersede any results presented in the measurement part of this thesis document. In the final section of this thesis, we present a projection study of top quark spin polarization and tt ̄ spin correlations at the upcoming High Luminosity LHC. We present projections of the fraction of SM-like events and show that the upcoming detector could reduce uncertainties by as much as a factor 2. An alternative to the SM scenario is also considered in the form of Super Symmetry, and it is shown that we can significantly increase the ultimate reach of the LHC to discover top squarks in the degenerate mass corridor in the top squark-neutralino plane, or in the absence of a discovery exclude top squarks up to 600 GeV. </p>
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Top Quark Spin Correlations with the CMS DetectorJason R Thieman (15354421) 27 April 2023 (has links)
<p>This dissertation presents precision measurements of top quark polarizations and top quark pair spin correlations, which probe the independent coefficients of the top-spin components of the top quark pair production density matrix, targeting all channels (ee, eµ, µµ) of the top quark pair dileptonic decay mode with final states containing two oppositely charged leptons, and using 137.7 fb<strong>⁻¹</strong> of data recorded by the CMS experiment at the LHC with 13 TeV center-of-mass energy, during 2016, 2017, and 2018.</p>
<p>All measured observables are corrected for detector efficiencies, acceptances, and migrations, unfolded to parton-level, and extrapolated to the full phase space using a regularized unfolding procedure.</p>
<p>Spin-density coefficients are extracted from the unfolded distributions and compared to theoretical predictions and predictions from Monte Carlo simulations with next-to-leading-order matrix element accuracy interfaced with parton-shower algorithms.</p>
<p>The measurements are performed both in the full phase-space and differentially as a function of top quark pair invariant mass.</p>
<p>The measured coefficients showed decent agreement with the MC predictions, and better agreement when compared to QCD perturbative calculations for top quark pair production at NLO with electroweak corrections, and the measurement precision for one-dimensional normalized unfolded cross-sections and extracted coefficients were improved by as much as a factor of two compared to previous measurements.</p>
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Phenomenology of Charged Higgs Bosons and B-meson DecaysEriksson, David January 2009 (has links)
For more than 30 years the Standard Model has been the theoretical foundation for particle physics. The theory has been verified successfully by experimental tests. Its biggest shortcoming is the non-discovery of the Higgs boson,responsible for giving the other particles masses. Despite its success there are hints that the Standard Model is not the complete theory and many extensions of it, such as supersymmetry, have been proposed. Extended theories often predict the existence of a charged Higgs boson and its detection will be a clear sign of physics beyond the Standard Model. The main focus in this thesis is on various phenomenological aspects of the charged Higgs boson. For favorable mass and couplings direct detection is shown to be possible at the Large Hadron Collider in production with an associated W boson. It is also shown how a light charged Higgs can have measurable effects on spin correlations in decays of pair-produced top quarks. The charged Higgs boson can also be seen indirectly, in for example B-meson decays, which can be used to put constraints on its mass and fermion couplings. Exclusion limits in two supersymmetric models are given together with a comparison with the discovery potentials for the LHC experiments. A tool for calculating properties, such as masses and decays, of both charged and neutral Higgs bosons in the Two-Higgs-Doublet Model is also presented. B-meson decays can also be used to test aspects of the strong interaction. Part of this thesis deals with improving and applying phenomenological models to B-meson decays. Although these models are not derived from first principles, their success shows that they capture important features of non-perturbative strong interactions.
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Effects of heavy Higgs bosons in the hadronic production of top-quark pairs including QCD correctionsGaller, Peter 13 February 2018 (has links)
In dieser Disseratation wird eine mögliche Erweiterung des Standardmodells der
Elementarteilchen (SM) im Higgs-Sektor mithilfe von Topquarkpaarproduktion am
Large Hadron Collider untersucht. Insbesondere wird dabei auf das sogenannte
Zwei-Higgs-Duplettmodell eingegangen. Dieses Modell führt mehrere Spin-0
Bosonen (auch Higgsbosonen genannt) zusätzlich zum SM-Higgsboson ein. Dabei
wird in dieser Arbeit von der Annahme ausgegangen, dass diese zusätzlichen
Higgsbosonen schwer genug sind um in ein Top-Antitop-Paar zu zerfallen. Somit
können die experimentellen Signaturen dieser neuen Teilchen mit Hilfe von
Observablen der Topquarkpaarproduktion untersucht werden. Dazu wird die
resonante Erzeugung von schweren Higgsbosonen und deren Zerfall in
Topquarkpaare bis einschließlich Quantenkorrekturen in der nächst-zu-führenden
Ordnung (NLO) in der QCD-Kopplungskonstanten berechnet. Weiterhin wird die
volle Spininformation des Top-Antitop-Paares beibehalten, welche die Analyse
von spinabhängigen Observablen erlaubt. Diese können, insbesondere in Falle von
Top-Antitop-Spinkorrelationen, sehr sensitiv auf Effekte schwerer Higgsbosonen
sein. Dies zeigt sich besonders in Vergleich zu spinunabhängigen Observablen.
Die Sensitivität von spinabhängigen Observablen kann zudem noch durch
entsprechende Schnitte auf den Phasenraum von Top- und Antitopquark verstärkt
werden. In dieser Dissertation wird ein Verfahren vorgestellt, mit dessen Hilfe
sich die Spinkorrelationen identifizieren lassen, welche die größte
Sensitivität auf die Effekte schwerer Higgsbosonen aufweisen. Außerdem wird
durch die Berechnung der Beiträge zur NLO u.a. gezeigt, dass diese Beiträge
wichtig sind um aussagekräftige und robuste Observablen zu definieren. Die
Ergebnisse der NLO, die in dieser Arbeit vorgestellt werden, sind die ersten
ihrer Art für die resonante Erzeugung von schweren Higgsbosonen und deren
Zerfall in Topquarkpaare. / In this dissertation a possible extension of the standard model of particle
physics (SM) in the Higgs sector is investigated using top-quark pair
production at the Large Hadron Collider as a probe. In particular, the
so-called two-Higgs-doublet model (2HDM) is studied. The 2HDM introduces several spin-0
bosons (which are also called Higgs bosons) in addition to the SM Higgs boson.
In this thesis these additional Higgs bosons are assumed to be heavy enough to decay into a
top-antitop quark pair. Thus, the experimental signatures of these new
particles can be studied through observables of top-quark pair production.
To this end the resonant production of heavy neutral Higgs bosons and their decay into
top-quark pairs in calculated up to next-to-leading order corrections in the
QCD coupling constant retaining the full spin information of the top-antitop
pair. This allows to analyse spin dependent observables which can be more
sensitive to effects of heavy Higgs bosons than spin independent ones
especially in the case of top-antitop spin correlations. The additional
application of kinematical cuts on the phase space of top and antitop quarks
can enhance the sensitivity further. In this thesis a method is presented that
can be used to construct the spin correlation which is most sensitive to the
effects of heavy Higgs bosons on top-quark pair production.
Furthermore, it is shown that the next-to-leading order
corrections are required to construct observables which entail robust
predictions. The results for the next-to-leading order in the QCD coupling
constant presented in this thesis were the first ones given for resonant heavy
Higgs production and decay into top-quark pairs.
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