OSTI.GOV Thesis/Dissertation: Measurement of the electroweak top quark production cross section and the CKM matrix element Vtb with the D0 experiment
Title: Measurement of the electroweak top quark production cross section and the CKM matrix element Vtb with the D0 experiment
At particle accelerators the Standard Model has been tested and will be tested further to a great precision. The data analyzed in this thesis have been collected at the world's highest energetic-collider, the Tevatron, located at the Fermi National Accelerator Laboratory (FNAL) in the vicinity of Chicago, IL, USA. There, protons and antiprotons are collided at a center-of-mass energy of {radical}s = 1.96 TeV. The discovery of the top quark was one of the remarkable results not only for the CDF and D0 experiments at the Tevatron collider, but also for the Standard Model, which had predicted the existence of the top quark because of symmetry arguments long before already. Still, the Tevatron is the only facility able to produce top quarks. The predominant production mechanism of top quarks is the production of a top-antitop quark pair via the strong force. However, the Standard Model also allows the production of single top quarks via the electroweak interaction. This process features the unique opportunity to measure the |V tb| matrix element of the Cabbibo-Kobayashi-Maskawa (CKM) matrix directly, without assuming unitarity of the matrix or assuming that the number of quark generations is three. Hence, the measurement of the cross section ofmore » electroweak top quark production is more than the technical challenge to extract a physics process that only occurs one out of ten billion collisions. It is also an important test of the V-A structure of the electroweak interaction and a potential window to physics beyond the Standard Model in the case where the measurement of |V{sub tb}| would result in a value significantly different from 1, the value predicted by the Standard Model. At the Tevatron two production processes contribute significantly to the production of single top quarks: the production via the t-channel, also called W-gluon fusion, and the production via the s-channel, known as well as W* process. This analysis searches for the combined s+t channel production cross section, assuming the ratio of s-channel production over t-channel production is realized in nature as predicted by the Standard Model. A data set of approximately 1 fb -1 is analyzed, the data set used by the D0 collaboration to claim evidence for single top quark production. Events with two, three, and four jets are used in the analysis if they contain one or two jets that were tagged as originating from the decay of a b hadron, an isolated muon or electron, and a significant amount of missing transverse energy. This selection of events follows the signature that the single top quark events are expected to show in the detector. In the meantime, both collaborations D0 and CDF have analyzed a larger data set and have celebrated the joint observation of single top quark production. The novelty of the analysis presented here is the way discriminating observables are determined. A so-called Multi-Process Factory evaluates each event under several hypotheses. A common analysis technique for example in top quark properties studies is to reconstruct the intermediate particles in the decay chain of the signal process from the final state objects measured in the various subdetectors. An essential part of such a method is to resolve the ambiguities that arise in the assignment of the final state objects to the partons of the decay chain. In a Multi-Process Factory this approach is extended and not only the decay chain of the signal process is reconstructed, but also the decay chains of the most important background processes. From the numerous possible event configurations for each of the signal and background decay chains the most probable configuration is selected based on a likelihood measure. Properties of this configuration, such as mass of the reconstructed top quark, are then used in a multivariate analysis technique to separate the expected signal contribution from the background processes. The technique which is used is called Boosted Decision Trees and has only recently been introduced in high energy physics analyses. A Bayesian approach is used to finally extract the cross section from the discriminant output of Boosted Decision Trees.« less
Kirsch, Matthias [1]
RWTH Aachen Univ. (Germany)
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Report Number(s):
FERMILAB-THESIS-2009-28
TRN: US0903481
Thesis/Dissertation
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; ACCELERATORS; ANTIPROTONS; CROSS SECTIONS; DECAY; FERMILAB ACCELERATOR; FERMILAB COLLIDER DETECTOR; FERMILAB TEVATRON; HIGH ENERGY PHYSICS; MATRIX ELEMENTS; MULTIVARIATE ANALYSIS; MUONS; PROTONS; QUARKS; S CHANNEL; STANDARD MODEL; SYMMETRY; T CHANNEL; T QUARKS; TRANSVERSE ENERGY; UNITARITY; Experiment-HEP
Kirsch, Matthias. Measurement of the electroweak top quark production cross section and the CKM matrix element Vtb with the D0 experiment. United States: N. p., 2009. Web. doi:10.2172/964428.
Kirsch, Matthias. Measurement of the electroweak top quark production cross section and the CKM matrix element Vtb with the D0 experiment. United States. https://doi.org/10.2172/964428
Kirsch, Matthias. Mon . "Measurement of the electroweak top quark production cross section and the CKM matrix element Vtb with the D0 experiment". United States. https://doi.org/10.2172/964428. https://www.osti.gov/servlets/purl/964428.
@article{osti_964428,
title = {Measurement of the electroweak top quark production cross section and the CKM matrix element Vtb with the D0 experiment},
author = {Kirsch, Matthias},
abstractNote = {At particle accelerators the Standard Model has been tested and will be tested further to a great precision. The data analyzed in this thesis have been collected at the world's highest energetic-collider, the Tevatron, located at the Fermi National Accelerator Laboratory (FNAL) in the vicinity of Chicago, IL, USA. There, protons and antiprotons are collided at a center-of-mass energy of {radical}s = 1.96 TeV. The discovery of the top quark was one of the remarkable results not only for the CDF and D0 experiments at the Tevatron collider, but also for the Standard Model, which had predicted the existence of the top quark because of symmetry arguments long before already. Still, the Tevatron is the only facility able to produce top quarks. The predominant production mechanism of top quarks is the production of a top-antitop quark pair via the strong force. However, the Standard Model also allows the production of single top quarks via the electroweak interaction. This process features the unique opportunity to measure the |Vtb| matrix element of the Cabbibo-Kobayashi-Maskawa (CKM) matrix directly, without assuming unitarity of the matrix or assuming that the number of quark generations is three. Hence, the measurement of the cross section of electroweak top quark production is more than the technical challenge to extract a physics process that only occurs one out of ten billion collisions. It is also an important test of the V-A structure of the electroweak interaction and a potential window to physics beyond the Standard Model in the case where the measurement of |V{sub tb}| would result in a value significantly different from 1, the value predicted by the Standard Model. At the Tevatron two production processes contribute significantly to the production of single top quarks: the production via the t-channel, also called W-gluon fusion, and the production via the s-channel, known as well as W* process. This analysis searches for the combined s+t channel production cross section, assuming the ratio of s-channel production over t-channel production is realized in nature as predicted by the Standard Model. A data set of approximately 1 fb-1 is analyzed, the data set used by the D0 collaboration to claim evidence for single top quark production. Events with two, three, and four jets are used in the analysis if they contain one or two jets that were tagged as originating from the decay of a b hadron, an isolated muon or electron, and a significant amount of missing transverse energy. This selection of events follows the signature that the single top quark events are expected to show in the detector. In the meantime, both collaborations D0 and CDF have analyzed a larger data set and have celebrated the joint observation of single top quark production. The novelty of the analysis presented here is the way discriminating observables are determined. A so-called Multi-Process Factory evaluates each event under several hypotheses. A common analysis technique for example in top quark properties studies is to reconstruct the intermediate particles in the decay chain of the signal process from the final state objects measured in the various subdetectors. An essential part of such a method is to resolve the ambiguities that arise in the assignment of the final state objects to the partons of the decay chain. In a Multi-Process Factory this approach is extended and not only the decay chain of the signal process is reconstructed, but also the decay chains of the most important background processes. From the numerous possible event configurations for each of the signal and background decay chains the most probable configuration is selected based on a likelihood measure. Properties of this configuration, such as mass of the reconstructed top quark, are then used in a multivariate analysis technique to separate the expected signal contribution from the background processes. The technique which is used is called Boosted Decision Trees and has only recently been introduced in high energy physics analyses. A Bayesian approach is used to finally extract the cross section from the discriminant output of Boosted Decision Trees.},
doi = {10.2172/964428},
url = {https://www.osti.gov/biblio/964428}, journal = {},
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Observation of Electroweak Single Top-Quark Production with the CDF II Experiment
Thesis/Dissertation Lueck, Jan
The standard model of elementary particle physics (SM) predicts, besides the top-quark pair production via the strong interaction, also the electroweak production of single top-quarks [19]. Up to now, the Fermilab Tevatron proton-antiproton-collider is the only place to produce and study top quarks emerging from hadron-hadron-collisions. Top quarks were directly observed in 1995 during the Tevatron Run I at a center-of-mass energy of √s = 1.8 TeV simultaneously by the CDF and D0 Collaborations via the strong production of top-quark pairs. Run II of the Tevatron data taking period started 2001 at √s = 1.96 TeV after a five yearmore » upgrade of the Tevatron accelerator complex and of both experiments. One main component of its physics program is the determination of the properties of the top quark including its electroweak production. Even though Run II is still ongoing, the study of the top quark is already a successful endeavor, confirmed by dozens of publications from both Tevatron experiments. A comprehensive review of top-quark physics can be found in reference. The reasons for searching for single top-quark production are compelling. As the electroweak top-quark production proceeds via a Wtb vertex, it provides the unique opportunity of the direct measurement of the CKM matrix element |V tb|, which is expected to be |V tb| ~ 1 in the SM. Significant deviations from unity could be an indication of a fourth quark generation, a production mode via flavor-changing neutral currents, and other new phenomena, respectively. There are two dominating electroweak top-quark production modes at the Fermilab Tevatron: the t-channel exchange of a virtual W boson striking a b quark and the s-channel production of a timelike W boson via the fusion of two quarks. In proton-antiproton-collisions the third electroweak production mode, the associated Wt production of an on-shell W boson in conjunction with a top quark has a comparatively negligible small predicted cross section. Therefore, the vast majority of the CDF and D0 single top-quark analyses search for the combined s- and t-channel signal, with the production ratio to be given by the SM. In Tevatron Run I, several limits on the single top-quark production cross section were set by CDF and D0, whereas in Run II, even stronger limits followed by both collaborations. Furthermore, limits on the non-SM production of single top-quarks via flavor-changing neutral currents could be obtained. The electroweak production of single top-quarks has not yet been observed up to the time of this thesis, although the D0 and shortly thereafter the CDF Collaborations found first evidence. The experimental challenge of the search for single top-quark production is the tiny expected signal beneath a large and imprecisely known amount of background processes. The relative fraction of background events is at the order of about ten times higher compared to the top-quark pair production. Consequently, the expected signal amounts to about 5% of the full candidate event sample whose background contribution is only known to a level at the order of 20%. Furthermore, the signal events themselves are expected to be not as distinct from the background as the top-quark pair production since there is only one heavy object present in the event. Thus, experimental methods like simple counting experiments are not sufficiently sensitive and the development of more sophisticated analysis techniques is required to distinguish such small signals from alike and inaccurately known background processes. Neural networks comply with those requirements. They can be used to distinguish between signal and background processes by combining the information contained in several variables into a powerful discriminant, while each variable has a rather low separation capability. The application of those neural network discriminants to collision data provide a method for the extraction of the signal fraction and its significance. This thesis presents a neural network search for combined as well as separate s- and t-channel single top-quark production with the CDF II experiment at the Tevatron using 3.2 fb -1 of collision data. It is the twelfth thesis dealing with single top-quark production performed within the CDF Collaboration, whereas three have been done in Run I and eight in Run II.« less
Search for Electroweak Single-Top Quark Production with the CDF II Experiment
Technical Report Buehler, Matthias ; /Karlsruhe U., EKP
The CDF II experiment and the Tevatron proton-antiproton collider are parts of the Fermi National Laboratories (Fermilab). The Fermilab is located in the vicinity of Chicago, USA. Today, the Tevatron is the only collider which is able to produce the heaviest known elementary particle, the top quark. The top quark was discovered at the Tevatron by the CDF and the D0 collaborations in 1995 [1]. So far, all the top quarks found are produced via the strong interaction as top-antitop pairs. The Standard Model of elementary particle physics also predicts single-top quark production via the electroweak interaction. This production modemore » has not yet been observed. The CDF and the D0 collaborations have set upper limits on the cross section for that process in Run I [2, 3] and improved those results in Run II [4, 5]. Single-top quark production is one of the major interests in Run II of the Tevatron as it offers several ways to test the Standard Model and to search for potential physics beyond the Standard Model. The measurement of the cross section of singly produced top quarks via the electroweak interaction offers the possibility to determine the Cabbibo-Kobayashi-Maskawa (CKM) matrix element V{sub tb} directly. The CKM matrix defines the transformation from the eigenstates of the electroweak interactions to the mass eigenstates of the quarks. V{sub tb} gives the strength of the coupling at the Wtb vertex. The single-top quark is produced at this vertex and therefore the cross section of the single-top quark production is directly proportional to |V{sub tb}|{sup 2}. In the Standard Model, three generations of quarks and the unitarity of the CKM matrix are predicted. This leads to V{sub tb} {approx} 1. Up to now, there is no possibility to measure V{sub tb} without using the assumption that there are a certain number of quark generations. Since the measurement of the cross section of single-top quark production is independent of this assumption it could verify another prediction of the Standard Model or give hints towards physics beyond the Standard Model such as a fourth generation of quarks. In addition, electroweak single-top quark production is an important background for the Higgs boson search in the mass range of 90 GeV/c{sup 2} to 130 GeV/c{sup 2} at the Tevatron in the WH channel. Two single-top quark production modes are dominant at the Tevatron, the t-channel or W-gluon fusion and the s-channel or W* process. Since it is challenging to separate the signal from the various background events we use a neural network to combine several variables into one powerful discriminant. The simulated Monte Carlo sample outputs of the neural networks are used as templates for a likelihood fit to the outputs of the neural networks of the data. In this thesis CDF II data corresponding to an integrated luminosity of 695 pb{sup -1} is discussed. As this analysis yields no significant evidence of electroweak single-top production it is not possible to measure any cross sections. Consequently we determine upper limits on the cross sections of the t- and s-channel production separately and combined.« less
Measurement of the single top production cross section in proton-antiproton collisions at 1.96 TeV
Thesis/Dissertation Tanasijczuk, Andres Jorge
This thesis describes a search for singly produced top quarks via an electroweak vertex in head-on proton-antiproton collisions at a center of mass energy of √s = 1.96 TeV. The analysis uses a total of 2.3 fb -1 of data collected with the D0 detector at Fermilab, corresponding to two different run periods of the Tevatron collider. Two channels contribute to single top quark production at the Tevatron, the s-channel and the t-channel. In the s-channel, a virtual W boson is produced from the aniquilation of a quark and an antiquark and a top and a bottom quarks are producedmore » from the W decay. The top quark decays almost exclusively into a W boson and a bottom quark. Final states are considered in which the W boson decays leptonically into an electron or a muon plus a neutrino. Thus, at the detector level, the final state characterizing the s-channel contains one lepton, missing energy accounting for the neutrino, and two jets from the two bottom quarks. In the t-channel, the final state has an additional jet coming from a light quark. Clearly, a precise reconstruction of the events requires a precise measurement of the energy of the jets. A multivariate technique, Bayesian neural networks, is used to extract the single top signal from the overwhelming background still left after event selection. A Bayesian likelihood probability is then computed to measure the single top cross section. Assuming the observed excess is due to single top events, the measured single top quark production cross section is σ(p$$\bar{p}$$ → tb + X, tqb + X) = 4.70 +1.18 -0.93 pb. The observed excess is associated with a p-value of (3.2 ± 2.3) x 10 -8, assuming the background-only hypothesis. This p-value corresponds to an excess over background of 5.4 standard deviations for a Gaussian density. The p-value computed using the standard model signal cross section of 3.46 pb is (22.7 ± 0.6) x 10 -6, corresponding to an expected significance of 4.08 standard deviations.« less
Measurement of the top quark mass in the dilepton final state using the matrix element method
Thesis/Dissertation Grohsjean, Alexander
The 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 Elementmore » 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 -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 top Run IIa = 170.6 ± 6.1(stat.) -1.5 +2.1(syst.)GeV; m top Run IIb = 174.1 ± 4.4(stat.) -1.8 +2.5(syst.)GeV; m top 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.« less
Search for Third Generation Squarks in the Missing Transverse Energy plus Jet Sample at CDF Run II
Thesis/Dissertation Marono, Miguel Vidal
The twentieth century leaves behind one of the most impressive legacies, in terms of human knowledge, ever achieved. In particular the StandardModel (SM) of particle physics has proven to be one of the most accurate descriptions of Nature. The level of accuracy of some theoretical predictions has never been attained before. It includes the electromagnetic interaction, and the weak and strong force, developing the Lagrangian from symmetry principles. There are two different types of fundamental constituents of Nature, in the framework of the Standard Model: bosons and fermions. Bosons are those particles responsible for carrying the interactions among the fermions,more » which constitute matter. Fermions are divide into six quarks and six leptons, forming a three-folded structure. All these fermions and bosons have an antimatter partner. However, several difficulties point along with the idea that the Standard Model is only an effective low energy theory. These limitations include the difficulty to incorporate gravity and the lack of justification to fine tuning of some perturbative corrections. Moreover, some regions of the theory are not understood, like the mass spectrum of the Standard Model or the mechanism for electroweak symmetry breaking. Supersymmetry is a newer theoretical framework, thought to adress the problems found in the Standard Model, while preserving all its predictive power. It introduces a new symmetry that relates a new boson to each SM fermion and a new fermion to each SM boson. In this way, for every existing boson in the SM it must exist a fermionic super-partner (named with a sufix ino), and likewise, for every fermion a bosonic super-partner (named with a prefix s) must also exist. Moreover, another symmetry called R-parity is introduced to prevent baryon and lepton number violating interactions. If R-parity is conserved, super-particles can only be pair-produced and they cannot decay completely in SM particles. This implies the existence of a lightest SUSY particle (LSP) which would provide a candidate for cold dark matter, that account for 23% of the universe content, as strongly suggested by recent astrophysical data [1]. The Tevatron is a hadron collider operating at Fermilab, USA. This accelerator provides proton-antiproton (p$$\bar{p}$$) collisions with a center of mass energy of √s = 1.96 TeV. CDF and D0 are the detectors built to analyse the products of the collisions provided by the Tevatron. Both experiments have produced a very significant scientific output in the last few years, like the discovery of the top quark or the measurement of the B s mixing. The Tevatron experiments are also reaching sensitivity to the SM Higgs boson. The scientific program of CDF includes a broad spectrum on searches for physics signatures beyond the Standard Model. Tevatron is still the energy frontier, what means an unique opportunity to produce a discovery in physic beyond the Standard Model. The analyses presented in this thesis focus on the search for third generation squarks in the missing transverse energy plus jets final state. The production of sbottom ($$\tilde{b}$$) and stop ($$\tilde{t}$$) quarks could be highly enhanced at the Tevatron, giving the possibility of discovering new physics or limiting the parameter space available in the theory. No signal is found over the predicted Standard Model background in both searches. Instead, 95% confidence level limits are set on the production cross section, and then translated into the mass plane of the hypothetical particles. This thesis sketches the basic theory concepts of the Standard Model and the Minimal Supersymmetric Extension in Chapter 2. Chapter 3, describes the Tevatron and CDF. Based on the CDF subsystems information, Chapter 4 and 5 describe the analysis objet reconstruction and the heavy flavor tagging tools. The development of the analyses is shown in Chapter 6 and Chapter 7. Finally, Chapter 8 is devoted to discuss the results and conclusions of this work, and future prospects.« less