Measurement of the ratio $$B(t \to W b)/B(t \to W q)$$ in $$t\bar{t}$$ dilepton channel at CDF
- Pisa U.
My analysis is based on the number of b-jets found in t¯t events using the dilepton sample with at least 2 jets in the final state. The charged leptons could be either electrons or muons. Tau leptons are not included. We use SecVtx algorithm, based on the reconstruction of a secondary vertex in the event, in order to identify a jet coming from b-quark fragmentation (b-tagging). Due to the high purity of the t¯t signal in dilepton events it is possible to perform a kinematic measurement of the t¯t cross section. Our strategy is to use this result to make prediction on the number of t¯t events. We divide our sample in subsets according to dilepton type (combination of the lepton type), number of jets in the final states and events with zero, one or two tags. The comparison between events and the prediction, given by the sum of the expected t¯t estimate and the background yield, in each subsample is made using a Likelihood function. Our measured value for R is the one which maximizes the Likelihood, i.e. gives the best match between our expectation and the observed data. We measure: p¯p!t¯t = 7.05±0.53stat±0.42lumi , R= 0.86±0.06 (stat+syst) and, in the hypothesis of CKM matrix unitarity with three quark generations, | Vtb | = 0.93 ± 0.03. Our analysis on the p¯p!t¯t was performed independently of the official dilepton analisys on the t¯t production cross section. So it represents also a valuable crosscheck for the official analysis. In chapter 1, a brief introduction to the theoretical framework is given. The standard model of elementary particles and the Quantum Cromodynamic theories are introduced. Then the top quark is presented, with a short descpription of its properties, as its mass, its production mode and its cross section. Some previous results on R are listed as well. Later we present the experiment that collected our data, both the collider (chapter 2) and the detector (CDF)(chapter 3). In chapter 4 we describe the physics object reconstruction, so how we collect the detector signals and translate those into physical particles traversing our detector. The event selection is described in chapter 5, where we report the complete list of the selection requirements and we estimate our sample composition. In chapters 6 and 7 we report our results for the t¯t production cross section and R. An indirect measurement for |Vtb| is given as well.
- Research Organization:
- Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), High Energy Physics (HEP)
- DOE Contract Number:
- AC02-07CH11359
- OSTI ID:
- 1250728
- Report Number(s):
- FERMILAB-MASTERS-2013-09; 1425874
- Country of Publication:
- United States
- Language:
- English
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