1,530 research outputs found
Measurement of the Top Quark Mass in the Dilepton Final State using the Matrix Element Method
No full textThe top quark, discovered in 1995 by the CDF and D0 experiments at the Fermilab Tevatron Collider, is the heaviest known fundamental particle. The precise knowledge of its mass yields important constraints on the mass of the yet-unobserved Higgs boson and allows to probe for physics beyond the Standard Model. The first measurement of the top quark mass in the dilepton channel with the Matrix Element method at the D0 experiment is presented. After a short description of the experimental environment and the reconstruction chain from hits in the detector to physical objects, a detailed review of the Matrix Element method is given. The Matrix Element method is based on the likelihood to observe a given event under the assumption of the quantity to be measured, e.g. the mass of the top quark. The method has undergone significant modifications and improvements compared to previous measurements in the lepton+jets channel: the two undetected neutrinos require a new reconstruction scheme for the four-momenta of the final state particles, the small event sample demands the modeling of additional jets in the signal likelihood, and a new likelihood is designed to account for the main source of background containing tauonic Z decay. The Matrix Element method is validated on Monte Carlo simulated events at the generator level. For the measurement, calibration curves are derived from events that are run through the full D0 detector simulation. The analysis makes use of the Run II data set recorded between April 2002 and May 2008 corresponding to an integrated luminosity of 2.8 fb{sup -1}. A total of 107 t{bar t} candidate events with one electron and one muon in the final state are selected. Applying the Matrix Element method to this data set, the top quark mass is measured to be m{sub top}{sup Run IIa} = 170.6 {+-} 6.1(stat.){sub -1.5}{sup +2.1}(syst.)GeV; m{sub top}{sup Run IIb} = 174.1 {+-} 4.4(stat.){sub -1.8}{sup +2.5}(syst.)GeV; m{sub top}{sup comb} = 172.9 {+-} 3.6(stat.) {+-} 2.3(syst.)GeV. Systematic uncertainties are discussed, and the results are interpreted within the Standard Model of particle physics. As the main systematic uncertainty on the top quark mass comes from the knowledge of the absolute jet energy scale, studies for a simultaneous measurement of the top quark mass and the b jet energy scale are presented. The prospects that such a simultaneous determination offer for future measurements of the top quark mass are outlined
Assessing the limits of resolution in top quark pair reconstruction algorithms
No full textThis thesis studies the reconstruction of top quark-antiquark (t¯t) pairs in the dilepton decay channel, where both top quarks decay leptonically and two neutrinos escape detection. Since the event kinematics are underconstrained, the Sonnenschein algorithm, a fully analytical method used by the CMS and ATLAS experiments at the LHC, where quark-antiquark pairs of the heaviest elementary particle are produced in proton-proton collisions, is applied. The analysis focuses on the reconstructed invariant mass of the t¯t system and the factors limiting its resolution. A systematic, stepwise study is carried out using simulations of increasing realism: from parton-level events generated with Powheg, through showering and hadronization with Pythia 8, up to full detector simulation with Geant4. This approach quantifies how physics and detector effects degrade the reconstruction and highlights the intrinsic limitations of the Sonnenschein method.The results demonstrate that the usage of the Sonnenschein algorithm already introduces some mis-reconstruction even with idealized inputs, reaching a relative bias of 0.1% and a relative resolution of 2.5%. This can be attributed to numerical instabilities of the algorithm itself. Furthermore, its performance degrades significantly under realistic conditions. Transitioning from parton-level to detector-level inputs introduces substantial information loss, particularly from parton showering and jet formation, leading to resolutions exceeding 25% in high-mass regions and notable biases originating mostly from the constraints of the equations and event topologies. The standard practice of selecting the lowest-mass solution is shown to induce systematic biases, especially at higher masses.Mitigation strategies, including mass sampling from Breit–Wigner distributions and smearing techniques, improve the reconstruction efficiency and reduce resolution to an average of 23.7% at detector level, highlighting the algorithm’s sensitivity to input imperfections. These findings indicate that while the Sonnenschein algorithm provides a robust analytical foundation for top-pair reconstruction, further improvements, such as refined jet detection strategies and machine learning-assisted solution selection, are essential for exploiting its full potential in realistic experimental contexts
Messung des inklusiven tt-Wirkungsquerschnitts und Suche nach zusätzlichen Skalaren in tt-Endzuständen mit dem CMS-Experiment
No full textTwo key measurements and two phenomenological studies of top quark pair (tt) production with the CMS experiment at the CERN Large Hadron Collider are presented. They include the first measurement of the tt cross section at the new energy frontier, a detailed analysis of the tt threshold culminating in the first observation of tt bound state effects, and searches for new physics in tt final states, such as heavy scalars, pseudoscalars or Axion-Like Particles.
The inclusive tt production cross section σ(tt) is measured for the first time at the world-record energy of √s = 13.6 TeV, using 1.21 fb^(-1) of early LHC Run 3 data. By combining the dilepton and lepton+jets (l+jets) decay channels and constraining the lepton and b tagging efficiencies in situ, a precision of 3.4%, comparable to previous σ(tt) measurements, is achieved. This constitutes the first measurement of proton-proton scattering at √s = 13.6 TeV worldwide.
Furthermore, a search for heavy spin-0 states decaying to tt using 138 fb^(-1) of LHC Run 2 data at √s = 13 TeV in the dilepton channels is presented. The invariant tt mass m(tt) is combined with spin correlation observables to gain sensitivity to the spin and CP structure of possible new intermediate states.
The analysis is supported by a detailed modeling study of off-shell tt production and of the interference between tt and tW production.
An excess of events over the tt continuum background is observed at low values of m(tt), with spin correlations consistent with a pseudoscalar state. It is interpreted in terms of a pseudoscalar tt bound state ηt, and its cross section is measured to be σ(ηt) = 8.7 ± 1.1 pb using a simplified model inspired by non-relativistic quantum chromodynamics.
The excess is statistically significant at more than five standard deviations, constituting the first observation of tt bound state effects.
Other interpretations of the observed excess are similarly possible. In particular, scenarios with generic pseudoscalar or scalar bosons are explored, and exclusion regions on their coupling to the top quark are derived both for the dilepton channels alone as well as in a combination with a separate analysis of the l+jets channels. In addition, Axion-Like Particles (ALPs) decaying to tt are considered in the case of vanishing tree-level ALP-gluon couplings, while the more generic case is investigated phenomenologically in simulation.Es werden zwei essentielle Messungen und zwei phänomenologische Studien zur Produktion von Top-Quark-Paaren (tt) mit dem CMS-Experiment am CERN Large Hadron Collider vorgestellt.
Sie umfassen die erste Messung des tt-Wirkungsquerschnitts bei der weltweit höchsten Schwerpunktsenergie, eine detaillierte Analyse der tt-Produktionsschwelle, die in die erste Beobachtung von einem gebundenen Zustand des tt-Systems mündet, sowie Suchen nach neuer Physik in tt-Endzuständen, wie etwa schwere Skalar- oder Pseudoskalarbosonen oder Axion-Like Particles.
Der inklusive tt-Produktionsquerschnitt σ(tt) wird zum ersten Mal bei √s = 13.6 TeV gemessen, unter Verwendung von frühen LHC Run 3-Daten mit integrierter Luminosität von 1.21 fb^(-1). Durch Kombination der Dilepton- und Lepton+Jets (l+jets)-Zerfallskanäle von tt und simultane Bestimmung der Lepton- und b-tagging-Effizienzen in situ wird eine Präzision von 3.4% erreicht, die mit früheren σ(tt)-Messungen vergleichbar ist.
Dies stellt die weltweit erste Messung von Proton-Proton-Streuprozessen bei √s = 13.6 TeV dar.
Darüber hinaus wird eine Suche nach schweren Spin-0-Zuständen, die zu tt zerfallen, in den Dilepton-Kanälen mit 138 fb^(-1) Daten von LHC Run 2 bei √s = 13 TeV vorgestellt. Die invariante Masse von tt (m(tt)) wird mit Spinkorrelations-Observablen kombiniert, um die Sensitivität gegenüber dem Spin und der CP-Struktur möglicher neuer intermediärer Zustände zu erhöhen.
Die Analyse wird durch detaillierte Studien zur Modellierung der Off-Shell-tt-Produktion sowie zur Interferenz zwischen tt- und tW-Produktion untermauert.
Ein Überschuss von Ereignissen gegenüber dem tt-Kontinuums-Hintergrund wird bei niedrigen Werten von m(tt) und mit Spinkorrelationen konsistent mit einem pseudoskalaren Zustand beobachtet. Er wird als pseudoskalarer gebundener tt-Zustand ηt interpretiert, und dessen Produktionsquerschnitt wird mithilfe eines vereinfachten, von nichtrelativistischer Quantenchromodynamik inspirierten Modells zu σ(ηt) = 8.7 ± 1.1 pb gemessen.
Der Überschuss ist mit einer Signifikanz von mehr als fünf Standardabweichungen statistisch belegt und stellt somit die erste Beobachtung von gebundenen Zuständen im tt-System dar.
Weitere Interpretationen des beobachteten Überschusses sind ebenfalls möglich.
Insbesondere werden Szenarios mit generischen pseudoskalaren oder skalaren Bosonen untersucht, und Ausschlussregionen
hinsichtlich ihrer Kopplungen an das Top-Quark werden sowohl für die Dilepton-Kanäle allein als in Kombination mit einer separaten Analyse der l+jets-Kanäle berechnet. Zusätzlich werden zu tt zerfallende Axion-Like Particles (ALPs) betrachtet: einerseits im Fall verschwindender ALP-Gluon-Kopplungen, andererseits im allgemeinen Fall, der phänomenologisch in Simulationen untersucht wird
Effects of model parameter variations forthe bbHZall process in theperformance of multivariate classifiers
No full textThe first goal of this thesis was to check whether the parameter variations of the 2HDM+a model,in the process of bbHZall, have an impact on multivariate methods. The second goalwas to check the feasibility of using a singular deep neural network trained on all given masspoints of the heavy Higgs and dark matter mediator.It was found that the variation of model parameters unrelated to mass of particles had no sig-nificant impact on the performance of multivariate methods. Also it was ascertained that it canbe sensible to use a singular deep neural network trained on all mass points when comparing itsdiscrimination power to the case of training a network for every single mass point
Measurement of the inclusive production cross section and search for additional scalars in final states at CMS
No full textTwo key measurements and two phenomenological studies of top quark pair () production with the CMS experiment at the CERN Large Hadron Collider are presented. They include the first measurement of the cross section at the new energy frontier, a detailed analysis of the threshold culminating in the first observation of bound state effects, and searches for new physics in final states, such as heavy scalars, pseudoscalars or Axion-Like Particles.The inclusive production cross section is measured for the first time at the world-record energy of , using of early LHC Run 3 data. By combining the dilepton and lepton+jets () decay channels and constraining the lepton and b tagging efficiencies \textit{in situ}, a precision of , comparable to previous measurements, is achieved. This constitutes the first measurement of proton-proton scattering at worldwide.Furthermore, a search for heavy spin-0 states decaying to using of LHC Run 2 data at in the dilepton channels is presented. The invariant mass () is combined with spin correlation observables to gain sensitivity to the spin and structure of possible new intermediate states. The analysis is supported by a detailed modeling study of off-shell production and of the interference between and tW production.An excess of events over the continuum background is observed at low values of , with spin correlations consistent with a pseudoscalar state. It is interpreted in terms of a pseudoscalar bound state , and its cross section is measured to be using a simplified model inspired by non-relativistic quantum chromodynamics. The excess is statistically significant at more than five standard deviations, constituting the first observation of bound state effects.Other interpretations of the observed excess are similarly possible. In particular, scenarios with generic pseudoscalar or scalar bosons are explored, and exclusion regions on their coupling to the top quark are derived both for the dilepton channels alone as well as in a combination with a separate analysis of the channels. In addition, Axion-Like Particles (ALPs) decaying to are considered in the case of vanishing tree-level ALP-gluon couplings, while the more generic case is investigated phenomenologically in simulation
Study of Interferences Between Resonant Heavy-Higgs-Boson Production and Top-Antitop-Quark Production in the nStandard Model
No full textRecent discoveries in the area of cosmology, like the proposition of dark matter, haveindicated that there may be additional particles beyond the Standard Model which mayexplain the phenomena of Dark Matter in the Universe. Among many suggestions ofwhat Dark Matter actually is, there are models such as two-Higgs doublet Models thatpredict the existence of additional scalar (H) or pseudoscalar (A) Higgs bosons whichcould function as mediators between Standard Model and Dark Matter particles. If theheavy H and A Higgs bosons couple via Yukawa interactions to Standard Model particles,their coupling to top quarks will be largest of all Standard Model particles. Therefore,at the LHC searches for the production of top quark pairs via resonant production ofH and A Higgs bosons are performed. Such searches are challenging because signal andbackground processes interfere, and therefore a peak-dip structure is searched for, ratherthan performing the classical hunt for a bump in the invariant top-antitop quark massdistribution.Since simulating particle collisions with the Monte Carlo method is computationallyexpensive, a new technique with less computation effort would be great progress for investigatingthe Higgs boson hypotheses, in particular because of the large signal-backgroundinterferences. In this thesis, a new approach using a reweighting method is tested forcreating simulated data of the signal process pp ! A/H ! t ¯t including interference withthe Standard Model pp ! t ¯t background process. Such a reweighting method would allowto effectively generate many different signal samples involving different H and A masses,widths and couplings, and would therefore save a huge amount of CPU used for genuinelysimulating signal Monte Carlo samples. In this thesis, simulated data of the heavy Higgssignal utilizing the reweighting method are compared to genuinely generated MonteCarlo samples of that signal, with the aim to validate the reweighting method. Angularcorrelations sensitive to the spin of the heavy Higgs bosons are investigated togetherwith the invariant top-antitop quark mass distribution. While for the spin variables thereweighted data matches the genuinely simulated samples well, the method still has tobe improved to be able to describe the invariant top-antitop quark mass distribution
Suche nach pseudoskalaren Dunkle-Materie-Mediator-Teilchen in Signaturen mit zwei Bottom-Quarks, einem leptonisch zerfallenden - Boson und fehlender Transversalenergie
No full textObwohl etwa 85% der gesamten Materie im Universum aus Dunkler Materie besteht (vgl.Eidemüller 2017: 5), ist ihre Natur bisher noch ungeklärt. Lediglich die Gravitationswirkung, die Dunkle Materie auf sichtbare Materie zu hab en scheint, ist erwiesen. Dasgegenwärtig etablierte Standardmo dell der Teilchenphysik gelangt hinsichtlich einer Erklärung an seine Grenzen. Möglicherweise muss dieses für eine Beschreibung der Dunklen Materie durch die Theorie der WIMPs und des 2-Higgs-Dublett-Mo dells ergänzt werden(vgl. Blum 2008/2009: 3).Ziel dieser Bachelorarbeit ist es, in diesem Zusammenhang nach der Produktion vonDunkler Materie (χ) im Kontext eines Higgs-Zerfalls (H) in ein leptonisch zerfallendesZ0-Bosonen und einen pseudoskalaren Dunkle Materie - Mediator (a) sowie einem zu-sätzlichen Bottom-Quark-Paar zu suchen. Dafür soll der Nicht-Standardmodell-Prozess ̄bbH,H → Z0a(Z0 → `+`−,a → ̄χχ) (vgl. Tunney/No/Fair-bairn 2017: 1) mit der Top-Quark-Paar-Produktion mit dileptonischen Endzuständen im Standardmodell unddem Drell-Yan-Prozess, die zusammen den Hauptuntergrund bilden, mittels Monte-Carlo-Simulation verglichen werden. Auf diesem Weg sollen Observablen identifiziert werden,die die Nicht-Standardmodell-Hypothese bei Messungen am LHC (CMS-Detektor) verifizieren können.Im Zuge des Vergleichs der Prozesse konnten folgende Messgrößen für eine Unterscheidung dieser herausgestellt werden: Transversalimpuls, Summen der Massen sowie verschiedene Winkelobservablen (Azimutwinkel, Pseudorapidität, Winkeldistanz) bezüglich der den Prozessen gemeinsamen Endzustandsteilchen. Ein weiterer aussagekräftiger Unterscheidungsparameter ist der fehlende Transversalimpuls als Folge nicht im Detektorregistrierbarer Teilchen.Even though dark matter accounts approximately 85% of the matter in the universe(see DESY 2019), its nature is still a riddle, only the gravitational effect dark matterseems to have on visible matter is proven. Our current Standard Model of particle physicshas reached its limits regarding an explanation. Potentially it has to be completed by thetheory of WIMPs and the two-Higgs-doublet model (see Blum 2008/2009: 3).This bachelor thesis aims to search for the production of dark matter (χ) in the contextof a Higgs boson decay (H) into a leptonic decaying Z0 boson and a pseudoscalar darkmatter-mediator a as well as an additional bottom-quark pair. Therefore the non-StandardModel decay ̄bbH,H → Z0a(Z0 → `+`−,a → ̄χχ)(see Tunney/No/Fairbairn 2017: 1)should be compared to the dileptonic Standard Model-top-quark decay and the Drell-Yanprocess, together both decays form the main background, using Monte Carlo Simulation.In this way observables should b e identifed, which could verify the non-Standard Model-hypothesis regarding measurements at the LHC (CMS detector).In the course of comparison the following measurement parameters were found, whichallow a differentiation of the decays: transverse momentum, sums of masses as well asdifferent angular observables (azimuth angle, pseudorapidity, angular distance) regardingthe four final state particles both decays have in common. Another significant parameteris the missing transverse momentum, which is the consequence of non-registered particles in the detector
Measurement of the top pair production cross section and properties of top quark production and decay in proton-antiproton collisions at 1.96 TeV center-of-mass energy using the D\O Detector
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