We report on the observation of single top quark production by the D0 collaboration using a dataset of 2.3 fb^-1 collected at the Fermilab Tevatron ppbar collider. Several multivariate techniques are combined to separate the single top signal from backgrounds. The measured single top cross section is 3.94+-0.88pb. The probability to measure a cross section at this value or higher in the absence of signal is 2.9*10^-7, corresponding to a 5.0 standard deviation significance for the presence of signal. The lower limit at the 95% C.L. on the CKM matrix element Vtb is |Vtb|>0.78. A separate measurement of the t-channel cross section gives 3.14 +0.94 -0.80 pb.
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