Observation of Electroweak Single Top-Quark Production with the CDF II Experiment

Thesis/Dissertation · Fri Jul 24 00:00:00 EDT 2009

DOI:https://doi.org/10.2172/966448· OSTI ID:966448

Lueck, Jan ^[1]

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1. Karlsruhe Inst. of Technology (Germany)

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 year 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.

• Research Organization: Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC02-07CH11359
• OSTI ID: 966448
• Report Number(s): FERMILAB-THESIS-2009-33; TRN: US0903971
• Country of Publication: United States
• Language: English

Related Subjects

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
ACCELERATORS
B QUARKS
CROSS SECTIONS
ELEMENTARY PARTICLES
FERMILAB COLLIDER DETECTOR
FERMILAB TEVATRON
INTERMEDIATE BOSONS
MATRIX ELEMENTS
NEURAL NETWORKS
NEUTRAL CURRENTS
PAIR PRODUCTION
PHYSICS
QUARKS
S CHANNEL
STANDARD MODEL
STRONG INTERACTIONS
T CHANNEL
T QUARKS
Experiment-HEP