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Measurement of The Single Top Quark Production Cross Section at sqrt(s)=1.96 TeV

Abstract

Within the standard model top quarks are predicted to be most often produced in pairs

via the strong interaction. However they can also be produced singly through the weak

interaction. This is a rarer process with many experimental challenges. It is interesting

because it provides a new window to search for evidence of physics beyond the standard

model picture, such as a fourth generation of quarks or to search for insight into the Higgs

Mechanism. Single top production also provides a direct way to calculate the CKM matrix

element Vtb.

This thesis presents new measurements for single top quark production in the s+t, s and

t channels using 5.4 fb-1 of data collected at the DØ detector at Fermilab in Batavia, IL. The analysis was performed using Boosted decision trees to separate signal from background and Bayesian statistcs to calculate all the cross sections. The results obtained are:

s channel : σ (ppbar → tb + X) = 0.68 +0.41 - 0.39 pb,

t channel : σ (ppbar → tqb + X) = 3.03 +0.78 - 0.66 pb,

Combined s+t channels : σ(ppbar → tb + X, tqb + X) = 3.01 +0.80 - 0.75 pb.

The s channel has a Gaussian significance of 1.6σ, the t channel 5.5σ, and the s+t 4.3σ. The results are consistent with the standard model predictions within one standard deviation. By combining these results with the results for two other analyses (using different MVA techniques) improved results of:

Combined s+t channels : σ(ppbar → tb + X, tqb + X) = 3.43 +0.73 - 0.74 pb,

t channel : σ(ppbar → tqb + X) = 2.90 +0.59 - 0.59 pb.

were obtained. These give a 5.6σ significance for the combined s + t channel and 5.5σ for the t channel.

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