We present measurements of the cross sections for the two main production modes of single top quarks
in pp¯ collisions at √s = 1.96 TeV in the Run II data collected with the D0 detector at the Fermilab
Tevatron Collider, corresponding to an integrated luminosity of 9.7 fb−1. The s-channel cross section is
measured to be σ(pp¯ →tb+ X)=1.10+0.33
−0.31 pb with no assumptions on the value of the t-channel cross
section. Similarly, the t-channel cross section is measured to be σ(pp¯ →tqb + X) = 3.07+0.54
−0.49 pb. We
also measure the s+t combined cross section as σ(pp¯ →tb+X,tqb+X)=4.11+0.60
−0.55 pb and set a lower
limit on the CKM matrix element |Vtb|>0.92 at 95% C.L., assuming mt =172.5 GeV. The probability to
measure a cross section for the s channel at the observed value or higher in the absence of signal is
1.0×10−4, corresponding to a significance of 3.7 standard deviations.