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Title: Observation of s-channel single top quark production at the Tevatron
Author: Cremonesi, Matteo
ISNI:       0000 0004 8507 1712
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2014
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Testing the Standard Model (SM) and looking for new phenomena have been the focus of generations of particle physicists in the last decades. Following this spirit, this thesis presents two searches. The first is the search for single top quark production from the exchange of an s-channel virtual W boson using events with an imbalance in the total transverse energy, b-tagged jets, and no identified leptons. Assuming the electroweak production of top quarks of mass 172.5 GeV/c2 in the s-channel, a cross section of 1.12(+0.61)-0.57 (stat+syst) pb, with a significance of 1.9 standard deviations, is measured. This measurement is combined with the result obtained from events with an imbalance in total transverse momentum, b-tagged jets, and exactly one identified lepton, yielding a cross section of 1.36<(+0.37)-0.32 (stat+syst) pb, with a significance of 4.2 standard deviations. The first observation of single-top-quark production in the s channel through the combination of the CDF and D0 measurements is also reported. The measured cross section is σs = 1.29(+0.26)-0.24 pb. The probability of observing a statistical fluctuation of the background to a cross section of the observed size or larger is 1.8 × 10-10, corresponding to a significance of 6.3 standard deviation. The second is the search for W'-like resonances decaying to tb. No significant excess above the SM prediction is found. Using a benchmark W' → tb left-right symmetric model, 95% C.L. mass-dependent upper limits are placed on the W' boson production cross section times branching ratio to tb. Assuming a W' boson with SM-like couplings and allowed (forbidden) decay to leptons, W' → tbis excluded with 95% C.L. for W' boson masses below 860 (880) GeV/c2. Relaxing the hypothesis on SM-like couplings, we exclude W' boson coupling strength values as a function of the W' boson mass above 10% of the SM coupling strength for MW0 = 300 GeV/c2. The constraints obtained with the present analysis are the most stringent for charged resonance masses below 570 GeV/c2 decaying to a top and a bottom quark.
Supervisor: Hays, Chris Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available